NOTE:  The unit described on this page is functional, but still in its prototype stage and improvements are gradually being made.

If you have questions and/or wish to obtain pre-programmed chips, you can find more info at the bottom of this web page.

Overview:

This page describes a simple microprocessor ("PIC") based TDOA (Time Difference of Arrival) "Homing" type of radio direction-finding (RDF) unit.  These types of direction-finding units work by switching two antennas back-and-forth rapidly, thus imparting a phase modulation on the received signal.  By observing the amplitude and phase of this phase modulation and manually moving the antenna back-and-forth, one can infer the bearing to the signal being received.

Note that while this scheme works with either FM or AM transmissions, only FM receivers may be used for reception:   In particular it works best with narrowband FM receivers such as those typically used in 2-meter FM amateur radio communications.

Recently added:

• An "Audible" Field-Strength meter.  This unit (version 2) can provide field-strength indications that can aid in the detection of a nearby transmitter and a receiver is not needed for this function.
  

Figure 1:
The as-built PIC TDOA RDF prototype
Click on image for a larger version

General comments on "Homing-Type" (TDOA) RDF units:

The circuits described on this page are of the "Homing" or "TDOA" type in that they switch two antennas rapidly and, by observing phase changes in the incoming RF signal, allow the user to determine the bearing of the transmitter.  Note that this unit does not indicate the bearing of the received signal, but rather the user must sweep the antenna back-and-forth and in this way, he/she can determine the direction from which the signal seems to be coming.

TDOA-type circuits have several advantages and disadvantages:

Advantages:

• Simple and cheap to build.  TDOA circuits need not be particularly complicated.  At their simplest, they can take the form of a simple oscillator (such as a 555) driving a switched antenna array, or one may construct a fancier version that provide some sort of "left/right" indication.
• They can be used with practically any narrowband FM receiver.  All that is required is that one connect to the receiver's audio (earphone) and antenna jacks.
• Intuitively easy to operate.  By knowing if the signal is to your left or right, you can generally find your way toward the signal source.
• Ability to work in the presence of strong signals.  Its "homing" ability can still work even when very close to the signal source - even when your radio's S-meter may be "pegged" with a strong signal, whereas some radio-attenuator combinations may become overloaded when very near the transmitter.
  

• Susceptible to multipath effects.  Although this affects all radio-direction finding systems, this system depends entirely on the phase of the incoming wave front and reflections can cause confusing results.  It takes practice to develop the skill to effectively recognize and deal with multipath situations.  Again, this can be said for ANY type of direction-finding activity using ANY gear!
• Reversal of antenna sense.  Although using this sort of system is intuitive, care must be taken to note when the transmitter is now behind the user, as evidenced by the reversal of the TDOA system's response (e.g. left becomes right, and right becomes left.)
  
• No signal strength reading.  Typically, these sorts of units give no indication as to the strength of a signal:  It may not be obvious to the user whether the transmitter is some distance away, or very close-by.  While many radios have "S-Meters", these tend to "peg" on even very weak signals, making them useless if you are anywhere near the transmitter!  Version 2 of this unit includes a rudimentary field-strength meter to help find very nearby transmitters.
  
• Bearing degradation due to modulation.  Usability can be affected by heavy modulation on the signal:  Because the unit determines the relative position of the transmitter mostly through the phase of the tone imparted on the receive audio by the antenna switching, audio on the signal being sought may diminish the efficacy of the unit's operation.  This unit, with its narrow sampling and "intelligent" averaging is more resistant to this affect than most other units.
  
• Cannot be used for pulse-type transmission.  These types of devices are generally unsuitable for very brief transmissions from sources such as wildlife tags.
• Cannot be used for all types of continuously-modulated signals.  These devices may be of limited usefulness when searching for certain types of signals, such as those emitted by Aircraft ELTs, which may be unstable in terms of amplitude and frequency.
  

• Signal bearing.  By virtue of the directional antenna, one naturally gets a bearing that may indicate the direction of source of the signal.
  
• Signal strength.  If the receiver is equipped with a good signal strength meter, it can provide a relative indication of the distance to the transmitter.
  

A PIC-based TDOA system:
 
To better-understand how this TDOA system works, first read the UARC Homing Circuit DF page.  This and related pages provide a general explanation as to how all similar TDOA systems operate.  This unit simulates, in software, much of the functionality of the "Metered" circuit - and provides a few other features as well.

Two schematics, two versions:

A quick glance at Figure 2 (below) shows that there are two schematics.  The older version (2008 and earlier) is shown in the TOP half of Figure 2 and it is capable of TDOA functions ONLY.  December 2008, I happened to run out of 12F675 chips, but I had some 12F683's onhand - devices with more program memory and a few additional hardware features.  While I was in the process of "porting" the code from the '675 to the '683 (a fairly simple process) I decided to start adding more features and, hopefully, improve performance.

With this version 2, the following changes were made:

• Improved filtering of the switching pulse to better-resist the effects of modulation on the received carrier.
• The addition of an "Audible Field Strength Meter" capability.  As mentioned previously, one of the downsides of this sort of TDOA system is that it tells the user nothing about the signal strength of the received signal and whether or not one is very nearby the transmitter, but the addition of field-strength indication, which can have sensitivity to -30dBm, can (hopefully) mitigate this shortcoming to some extent.

• http://www.utaharc.org/rptr/DF_ANT.GIF - This is a simple array that I use.  It can be built using a variety of materials - such as PVC pipe and brass or stainless-steel whips.
• Joe, WB2HOL, has several simple TDOA projects as well and the antennas described on these pages will work with this unit as well:
• A very simple TDOA unit based on a 555 timer.  This is about as simple a TDOA unit as you can get.
  
• This is Joe's version of a "metered" TDOA unit.
  

About the hardware:

In the "Metered" Circuit, a counter and some electronic switches are used to select a brief "window" during which the phase information of the signal being received will be sampled.  In so-doing, only that portion of the audio containing the "pulse" caused by the antenna switching is used to determine the bearing of the signal.

In this PIC-based circuit, all of the sampling and analyzing of the signal is done by a PIC microprocessor using simple DSP techniques.  This chip has an onboard 10-bit A/D (Analog-to-digital) converter, a built-in oscillator, and several timers - not to mention input/output pins to read the status of the pushbutton and to drive the antennas and LEDs.

The audio enters through C1 which, along with R1 and R2 form a simple highpass filter, R1 and R2 also bias the input at mid-supply, and R3 and C2 form a lowpass filter to lower the effective impedance on the A/D input (during the sample-and-hold charge transfer) as well as to removing some high-frequency noise and possible RF that may be present on the input.

The GP1 (pin 6) output is a square wave that, through C3 and R4 drives the PIN diodes on the antenna.  This output also works in conjunction with GP2 (pin 5) to drive the LEDs used to indicate direction:  Since GP1 is always a square-wave, one LED may be selected simply by setting GP2 high and the other may be selected by setting GP2 low.  The "brightness" of the LEDs (and, thus, the meter movement) is accomplished by providing a "duty cycle" setting to GP2:  If it's "on" for a short period, the LED will be dim, but if it's on for a longer time, it will be bright.

In version 1 (and the "Version 1 compatibility mode" of the version 2 firmware) the GP4 (pin 3) output drives LED2 that, if the A/D converter is being overdriven with too much audio from the receiver, it will flash.  This LED is not present in "Version 2" modes.  In Version 1, Pins 3 and 4 are tied together because with this device, pin 4 can only be used as an input - and it has no internal pull up resistor available:  It was easier just to tie it to adjacent pin 3 than to connect it to either pin 1 or 8, the power supply pins.  GP5 (pin 2) is an input with an internal weak pull-up and connected across it is PB1 which used to initiate a "calibrate" procedure. 

J1 is a stereo earphone jack wired such that when a pair of stereo headphones is plugged in, the internal speaker is muted:  In one channel may be heard the receiver audio, while the other channel contains only the switching tone.  In this way, the tone can always be heard - even if it is nulled out or buried in modulation or noise and cannot be heard via the receiver's speaker directly.  This latter function is helpful with version 2 firmware if one uses the field-strength indication function with an antenna that does not have switching diodes to modulate a tone on the received signal - such as a Yagi.

Indication of direction using LED(s):

In addition to the pitch of tone, one may also use LEDs to indicate relative direction.  LED1 could be a dual-color LED or, if the builder chooses, separate LEDs, with one each for left and right that could be of different colors to indicate whether the signal being sought is to the left or right:  Exactly how the LED(s) is/are connected will determine which color is associated with which direction and a high/low pitched tone.

Indication of direction using a meter:

In addition to (or instead of) LEDs, you may add a zero-center meter to indicate left or right.  Practically any zero-center type of meter may be used, but it is recommended that a meter movement of 1 milliamp or less (for full deflection) be used, with R9 being selected to provide appropriate current limiting to the meter.  Zero-center meters may be difficult to find in some cases, but they may often be found surplus as "tuning meters" for FM receivers.

To adjust the meter, first set R9 to maximum resistance to avoid "slamming" the needle and possibly causing damage.  Next, calibrate TDOA unit as described below, using a clean, unmodulated signal.  Then, listening to the signal, steer the antenna off to the left or right, causing the meter to deflect, adjusting R9 so that the meter is deflected a maximum amount.  Once this is done, move the antenna back and forth to verify that it deflects fully left and right, adjusting R9 as necessary.  If it turns out that the meter deflects backwards, simply reverse the leads to the meter - after first making certain that the signal source isn't behind you or that your antenna isn't upside-down!

Comment:  Personally, I use the audio tones almost exclusively.  Doing so allows one to keep an eye out for hazards, obstacles, and even the transmitter itself!

Audible Field Strength Meter functions:

Version 2 (using a PIC12F683) has the capability of an audible field-strength meter.  The "Mode" of the unit is selected using SW2, a center-off SPDT switch and pin 4, GP3:

• Pin 4 grounded:  TDOA/Field Strength Meter mode.  In this mode, the unit provides normal TDOA direction-finding tones, but the frequency of the switching tone is briefly interrupted, twice-per-second, with another tone that is indicative of the RF field strength (higher tone = stronger signal.)
• Pin 4 connected to +5V:  Field-Strength only mode.  In this mode, the field strength is indicated by a short tone burst (occurring 4 times-per-second) that is indicative of the field strength.  The "beep" rate is faster so that the user can readily discern the he/she is in the "Field Strength Only" mode.
• Pin 4 connected to Pin 6 (through R10):  TDOA-Only mode.  In this mode, all Field Strength Meter functions are disabled, so it performs ONLY the "normal" TDOA direction-finding functions.  In this mode, +5 volts is output via pin 3 to bias the detection diode to the "off" state to minimize the possibility of intermodulation distortion.
  

Figure 2:
Schematics of the PIC-based TDOA unit.
Top:  The original "Version 1" of the PIC TDOA.
Bottom:  The new "Version 2" circuit - which includes provisions for an audible Field-Strength Meter
Click on image for a larger version.

About the firmware:

The code in this processor generates a square wave that, upon each transition, couples through C3 and applies an AC signal to the diodes in the Antenna Array.  When the diodes are switched, a brief pulse appears in the demodulated audio (if the two antennas are NOT equidistant from the transmitter) a short time after the antenna switch occurs.  Because this amount of delay varies with each radio, one uses the "calibrate" button on a clean, unmodulated test signal.  During the calibration procedure, the precise delay between the antenna's being switched and the appearance of the pulse in the audio is measured and then stored in the processor's EEPROM.  Unless one uses a different radio, this calibration procedure need not be done again.

Also stored in EEPROM is the amplitude of the pulse measured during the "calibrate" procedure:  This allows the LED's brightness to be in some way indicative of the magnitude of the error of the user's antenna bearings.  While the radio's volume will not affect the bearing accuracy (unless the audio output is high enough to cause significant distortion - or so low that readings cannot be taken) if the "brightness versus bearing" feature is to be used, it is recommended that the user use the same volume setting every time after calibration with the radio being used.  If you do not wish to use the "brightness versus bearing" feature, simply turn the volume most of the way down, but have it high enough to get a reliable reading, do a calibration, and then turn volume back up to normal levels.

At the precise time delay after the antenna switch occurs (the delay having been determined using the "calibrate" procedure) a sample of the voltage from the receiver audio is taken - and this brief sample (the "window" of the sample is only a few hundred nanoseconds wide) contains the pulse resulting from the antenna switching.  Each time the antenna switches (there are two antenna switches per cycle) a pulse appears in the audio if the two antennas are not equidistant from the transmitter.  Once a pair of pulses has been accumulated (one from the positive antenna switch, and another from the negative antenna switch) the relative polarity and amplitude of those two pulses is calculated:  By knowing the polarity, one now knows whether the signal being sought is to the user's left or right.

To make the unit more resistant to noise modulation that might appear on the audio, multiple samples are averaged together to form one reading.  To further improve the user interface, the switching rate of the antennas (which produces a tone audible to the user through the receiver) is varied, with a lower tone indicating that the signal source is to the left, and a higher tone indicating that the signal is to the right.  Furthermore, the LED will not only change from red to green to as a left/right indication, but it will glow brighter/dimmer to represent the amount of error in the antenna's heading (e.g. brighter = more error.)  Of course, whether a signal to the left is indicated by a lower tone and/or a green LED is entirely up to the builder!

In version 2, analog voltages representative of field strength are input to pin 3:  When in a mode that provides S-Meter readings, a tone is produces that increases with voltage.  It should be noted that internally, the firmware "de-linearizes" the voltage-versus-frequency of the "Signal Strength Tone" so that even very slight changes of voltage cause obvious differences in the pitch of the tone.  This was done to allow the use of a simple diode detector (D2) and provide as much sensitivity as possible - even if the detected voltage is varying by only a few millivolts!  This "de-linearizing" has the effect of the pitch of the tone seeming to be more proportional to the distance to the transmitter and without this, very weak signals would cause almost no detectable variation in the tone's pitch, with all of the change seeming to occur only when one was right next to the transmitter.

Schematic and component information:

Figure 2 shows the schematic of the PIC-Based TDOA.  U1 is an 8-pin PIC microprocessor with built-in peripherals, such as an A/D converter and onboard CPU clock:  Because it is internally clocked, there is no need for an external crystal in this application - something that saves two pins.

There are many ways that a builder can construct their own version and the schematic shown is just a suggestion.

J1 - This is a "disconnect-type" stereo jack.  As shown, a built-in speaker (SPKR) is used to allow the user to hear the pitch of the tone as well as its nulling-out:  Remember, the speaker in the radio itself will be muted when you plug something into the radio's headphone jack!  When a pair of stereo headphones is plugged into J1, the internal speaker is muted and into one channel is patched the receiver's audio and into the other channel is the switching tone from the microprocessor - but without any receive audio.  This tone may be useful to the user as unlike the same tone as heard via the receiver, it is always the same value, as it doesn't get nulled out, and it doesn't get mixed in with the audio of the signal being received - something that could drown out the tone from the receiver. If one is using the Field Strength meter function, this tone is still present even if one is using an antenna that does not have switching diodes (such as a Yagi) - or even if one isn't using a radio at all!  Variable resistor R8 is used to adjust this tone to a comfortable level.

J2 - To this is connected the antenna array mentioned above.  This connector can be any sort suitable for the frequency involved:  For 2 meters, a BNC connector is typically used.

J3 - To this is connected the receiver and the comments about connector type for J2 also apply.  Make sure that you clearly label the two connectors as the unit will not work (but will not be damaged) if they are inadvertently swapped.

J4 - Audio is input from the receiver via this connector:  A stereo jack is shown, but this could also be a simple cable with a hardwired plug to match the receiver that you plan to use.

B1 - This circuit can be operated from any voltage from about 6.5 to 20 volts, but for portability a 9 volt transistor-type battery is typically used.  Note the presence of D1, a 1N4001 (or practically any diode capable of at least 1 amp):  This prevents U2 and/or U1 from being damaged should reverse-polarity accidentally be applied - something easy to do momentarily if one is trying to attach a 9 volt battery in the dark!

SW1 - This is a simple on/off switch for power.

PB1 - This is a normally-open pushbutton switch.  This is used to initiate a calibrate sequence that allows the microcontroller to adapt to the characteristics of the radio being used.

LED1A, LED1B - This could be one or two LEDs.  In the prototype, a single "dual-color" LED was used with one color (Red) indicating right and the other color (Green) indicating left.  If so-desired, separate LEDs could be used to do the same function and their location (left or right) could also indicate the apparent direction of the signal.

LED2 - This is a single LED (optional) used to indicate the status of the unit.  This is primarily used to indicate an overload condition due to excessive audio input levels.  You may wish to choose to omit this LED.

M1 - This is an optional meter that can be a method of indicating direction.  Often, surplus "Zero-Center" meters can be found and one of these may be used to provide a left/right indication.  This could be used in addition to the LED or instead of it.  Variable resistor R9 is used to set the "maximum" reading of the meter.  Note:  It is possible that a standard non-zero-centered meter could also be used - contact the author for more details.

Additional information for the Version 2 circuit:

D2 - This is used to detect field strength via the antenna.  For best sensitivity, use a germanium diode (such as a 1N34) or a microwave Schottky diode (e.g. HP2835 or similar.)  A small-signal silicon diode (such as a 1N914 or 1N4148) may also be used, but its use results in significantly lower sensitivity!  It may also be possible to use an active RF detector such as a logarithmic amplifier (see below) or a simple RF amplifier in front of the detector diode.
SW2 - This should be a center-off SPDT switch if one wishes to have a "TDOA-Only" mode available (recommended!)

Figure 3:
Top:  The as-built PIC TDOA prototype installed in an enclosure.  Note that the "Overload" LED was not used, as most Handie-Talkies cannot output enough audio to overdrive the PIC.
Bottom:  Inside the enclosure.
Click on image for a larger version

Additional comments:

Minor changes were recently made to the "Version 1" schematic, both to correct schematic errors/omissions and to improve performance.  These changes include:

• Capacitor C1 changed to 0.01uF:  This compensates for seemingly "excessive" high-frequency rolloff and ringing observed in some radios and better-preserves the antenna switching pulse.  C2 was changed from 0.002 to 0.001uF as well.
• C8, a bypass capacitor to be mounted very close to the processor, had inadvertently been omitted.
  
• One end of R8 (the "tone volume" control) is now grounded to allow a greater range of adjustment - including "off."
• D1, while mentioned in the text, wasn't on the old version:  Now it is.
• R5 has been changed to 150 ohms to increase LED drive.
• C3 and LED2 were shown with backwards polarity - not anymore...
• Most Handie-Talkies cannot output enough audio to overdrive the PIC, so one probably need not bother installing the OVERLOAD LED.  (This comment applicable only to "Version 1" operations.)
• For version 2, a PIC12F683 was used instead of the PIC12F675.

Building the unit:

As can be seen from the pictures, this unit was built into a piece of prototype perforated board:  While a printed circuit board could easily be designed, this has not been done.

Most aspects of construction are not critical, although there is the obvious recommendation of using an 8-pin socket for the PIC and to be neat in wiring as much as practical.  What is rather critical is the wiring of the components associated with J2 and J3, the antenna and receiver jacks!

Figure 3 shows the prototype after having been repackaged in an enclosure.  The enclosure (A Serpac M/N:  H-67, 9V which is Digi-Key P/N:  SRH67-9VG-ND) contains a separate compartment for 9 volt batteries and comes with spring contacts.  The power switch, speaker, LEDs and BNC connectors were held in place using either epoxy glue and/or rubber adhesive.

A small speaker was installed and a disconnect-type 3.5mm stereo audio jack (just visible on the lower-left) to allow the use of headphones:  The jack was wired so that the left channel contains only the tone while the right channel contains the audio from the receiver.

For this enclosure, separate left (green) and right (red) LEDs were used and I chose not to implement the "Overload" LED, as the volume required to make it illuminate is rather deafening!

A pair of BNC connectors for connection to the radio and antenna array may be seen along the right side of the enclosure and the "Calibrate/Mode" A pushbutton switch can be seen along the left side, just above the speaker:  Most of the "button" portion was removed, making an accidental operation less likely and to reduce the probability of the button snagging and being broken off.  The power switch can just be seen protruding from the lower-right side.

The Antenna and Radio Jacks:

As can be seen from the the bottom photograph of Figure 3, there are some components wired directly to J2 and J3 - namely C4 and R4 and, for Version 2, C9, R11, D2 and C10.  The important point to be made here is to minimize lead length!  Keep in mind that at J2 and J3 we have the RF from our antenna and it is best to keep very short (centimeter) lead lengths to minimize signal losses and to prevent stray pickup.

J2 and J3 have been purposely placed very close to each other, and in so-doing, the ground (shield) connections of these two jacks are connected to each other with a short wire and the various components may be soldered directly to these points.  There are a total of three connections made to the J2 and J3 circuit, namely a DC ground, the antenna drive signal from C3, and, in the case of the version 2 circuit, the connection from D2/C10.

Operational notes:

STARTUP:

DISABLE YOUR RADIO'S TRANSMITTER!

First, disable the transmit function if you are using a transceiver!  If your radio doesn't allow this, set it to the lowest power and take care to avoid accidentally transmitting and destroying the antenna switching diodes and/or D2.  This is important enough to say twice!

When the unit is powered up, the OVERLOAD indicator (Version 1 only) and one of the Left/Right LEDs will light for about a second and the antenna switching frequency will change several times, indicating  that the unit is restarting.  At this time, the unit will load the stored calibration, if one was present.  Note that the packaged prototype shown in Figure 3 does not include the "Overload" LED:  This is because most Handie-Talkies simply cannot output enough audio to overdrive the input of the TDOA unit, so there was no real need to install it - plus this LED isn't used with "Version 2" firmware anyway.

CALIBRATION:

Before anything else is done, the unit MUST be calibrated to the radio being used.  This is done by tuning in a strong, stable, UNMODULATED carrier and adjusting the antenna array for the loudest, cleanest tone (the two elements of the antenna array inline with the signal source, as if it were a Yagi) and then pressing the CALIBRATE button for about a second.  Once the pitch of the tone changes, release the CALIBRATE button - but keep the antenna steady until calibration is complete!

Once calibration has started, a low-pitch tone is heard and once it is complete (it takes about 3 seconds) the unit will restart as indicated by the changing audible tones.

Note:  Avoid accidentally pressing the calibration button!  In later firmware, a button-press of about a second is required to initiate a calibration sequence, although brief "accidental" presses of the button will cause the unit to "freeze" for an instant (with the unit immediately returning to normal operation) as the button-press is being timed.

Important notes about calibration:

LED Brightness:

The brightness of the direction LEDs is related to the loudness of the received tone:  The stronger the tone (or, the farther off-point the antenna) the brighter the LEDs and/or greater the meter deflection.  When the unit is calibrated, the loudness of the tone at the time of calibration is taken to be the level of "full" brightness.  This means that if the volume is later increased, it won't "dim" as easily when the antenna is aimed.  Conversely, if the volume is later reduced, the LEDs will be dimmer.

If, for whatever reason, you want the LEDs to run at full brightness all of the time, simply perform the calibration with the radio's volume set very low - but high enough to get a reading.:  When set to "normal" volume, the LED will be brighter.

LED Response to signals:

Antenna nulls:

If there is sufficient "switching tone" in the received audio, either the "Left" or "Right" LED will be illuminated.  Note, however, that is the antenna is aimed exactly at the signal source (and there is no multipath present) that the switching tone will null out (disappear) from the speaker audio:  This is a normal behavior for TDOA antenna sets and this is, in fact, an additional audio cue that your antenna is aimed.

When the tone disappears into a normal antenna null, the TDOA unit may not have sufficient signal to detect the switching tone.  When this happens, BOTH the left and right LEDs will illuminate and the switching tone will seek a "middle" frequency.

Remember:

• If you are using headphones and have built the circuit as shown in the schematic, one channel will contain the speaker audio in which the tone may null out, but the other channel will contain only the tone.
• Even if you use the "Field-Strength Only" mode in Version 2, you can still use the "null" of the tone (when using the switched antenna array, of course!) to determine the bearing to the signal - it's just that you will not get a left/right indication!
  

The effects of signal modulation:

When the signal is modulated, the switching tone - which is used to determine the signal bearing - gets mixed in with that audio being modulated.  This can "dilute" the switching tone and make it harder to detect by the TDOA unit in addition to making it more difficult to hear by the user!

To reduce the effects of such "dilution" the TDOA unit applies several filtering techniques to recover the "buried" switching tone.  Nevertheless, expect the Left/Right indications (both the tone and LEDs) to be affected by such on-channel audio.  What usually happens is that the unit becomes a bit slower to respond to left/right indications, and the left/right LEDs may randomly flicker with the modulated audio.  In general, however, one can easily spot which left/right LED is on most of the time, despite the flickering.

The effects of modulation on the receive signal are most severe when the antenna is aimed in the direction of the signal and a null in the tone occurs.  Because the switching tone will naturally null out anyway, it becomes harder to detect and the left/right indications (either via LED or tone) become less-distinct.  It is these situations that teach one the value of constantly sweeping the antenna back-and-forth as one walks:  Moving the antenna off to the side causes the tone amplitude to increase, allowing one to hear it and judge where the "center" would be if one could hear it!

In most cases, the TDOA unit can actually detect the switching tone more readily than one's ear.  It is for this reason that, through the headphone jack, one can hear only the switching tone and no receive audio on one of the audio channels.  With this, one can still hear the switching tone's frequency even if it is too weak to hear with the "naked ear" and it allows the tone to remain audible even if there is a lot of on-channel modulation!

Comments about the antenna array and switching tone:

• When constructing the antenna array, the antennas must NOT be spaced any farther than 1/2 wavelength at the highest frequency of operation!  If you do so, the antenna array will not work properly!
• If you find that the switching tone is too "loud" and causing audio distortion in your receiver, consider moving the two elements of the antenna array slightly closer together:  The farther they are apart, the louder the tone can be - if it isn't being "nulled" of course.  Practically speaking, most builders space their antennas only about 1/4 wavelength apart (or slightly less) on 2 meters as this makes for a smaller, more-compact array!
  

The use of different radios:

If you use several different radios, please be aware that the unit will have to be re-calibrated each time you change radios!

Also be aware that some radios may have "inverted" audio phase as compared to others.  This can cause the Left/Right indicators to become reversed:  For this reason, re-check to verify that your "left-right" indication is correct.  Simply reverse or turn the array upside-down to correct this!.

Changing frequency range:

This unit has two tone frequency ranges available in the TDOA mode:  Approximately 400-525 Hz (the default) for the "low" range, or about 640-770 Hz for the "upper" range.

The range of tone frequencies may be switched by holding the CALIBRATE button down for about 10-12 seconds.  After holding it down for about 4 seconds, the unit will "diddle" and do a calibration and then restart as indicated by the OVERLOAD LED lighting up.   After this, continue to hold the CALIBRATE button and after another 3-4 seconds, tone will "diddle" some more with the OVERLOAD indicator lit.  Continue holding the button until after the "diddle" stops and the frequency range of the switching tone will switch from low to high or vice-versa and at this time the button may be released.  This selected mode will be saved in EEPROM.

• AFTER CHANGING FREQUENCY RANGES, ALWAYS DO A CALIBRATION!!!
• Under conditions of severe multipath, the LOW frequency range (which is the default) is likely to work a bit more reliably owing to the fact that the distortion and ensuing bandwidth-spreading caused by the multipath is less-severe at the lower frequencies.
• When the TDOA is switched to a different frequency range, the pitch range of the Field-Strength meter will also change.
  
• In the "field-strength meter only" mode of version 2, the CALIBRATE button is disabled, so it is necessary to switch to a different mode in order to change the frequency range.

The "Overload" LED (Version 1 only):

Remember:  Not all radios can output enough audio to cause the CLIP LED to illuminate!  The OVERLOAD LED does NOT indicate whether or not the audio output from the radio itself is distorting!  If the OVERLOAD LED indicates, turn down the volume slightly.  On most radios, volume settings should not affect calibration, unless the calibration was done with the volume high enough to cause the radio's audio amplifier to clip.  The "OVERLOAD" LED is not implemented in the "Version 2" (field-strength meter) modes.

• Field-Strength/TDOA mode:  In this mode, normal TDOA operations are possible (e.g. high/low tone with left/right indications) but the switching tone is briefly interrupted, twice-per-second, with another tone that indicates relative field strength, with the higher-pitch tone indicating a stronger signal.  It should be noted that in this mode, both the "direction" tone and "field strength" tone can change - which some users might find confusing!
  
• Field-Strength ONLY mode:  In this mode, TDOA operations are disabled.  A low-pitch "reference" tone is sounded and briefly interrupted, four times per second, with another tone that indicates relative field strength - again, with a higher-pitch tone indicating a stronger signal.  This mode is provided to assist the user when one is very close to the transmitter and you don't want to be distracted by the "left/right" tones!  As mentioned above, you can still get a bearing on the signal when using this mode (if you are using the switched antenna array) by observing the "null" - you just won't get any "left/right" indication!
  

• It should be noted that the pitch of the is changed to indicate field strength:  If, for whatever reason, you cannot hear the switching tone through the radio's speaker (because the receiver is squelched, there is too much modulation on the received signal, you are using an antenna that does NOT have switching diodes, or your antenna position has caused the tone to be nulled) you should consider using headphones where one of the channels is wired to always provide the tone!
• As with any non-selective field-strength meter, this will indicate the presence of ANY RF field, not just that of the transmitter for which you are looking!  If you are near a broadcast station, a cell/mobile telephone site, another amateur transmitter, or some other transmitting source, you will have to verify that a signal that you are detecting with the field-strength meter is really that of the transmitter being sought!  Note that because a simple diode detector isn't very sensitive, only very nearby or high-power transmitters are likely to be detected.
  
• The standard two-antenna TDOA antenna array isn't directional at all and has a remarkably omnidirectional pattern - see more comments below.
• Again, If you use an antenna other than the TDOA antenna (such as a Yagi) you will not be able to hear the switching tone through the radio due to the lack of switching diodes in the antenna!  In this case, the use of a stereo headset, which allows the switching tone from the unit to be heard in one channel and the radio's audio to be heard in the other, will solve this problem!
• When in "TDOA Only" mode, 5 volts is outputted by pin 3 to bias the detector diode off.  This was done to minimize the likelihood of D2 causing intermodulation distortion in this mode.
• In lieu of a simple diode detector (D2) it is possible to use a different detector, such as an Analog Devices AD8307 logarithmic amplifier.  This device has far greater dynamic range and sensitivity than a simple diode detector, but the same warnings about other "nearby" transmitters apply - and it may be too sensitive, causing false indications on unrelated signals!  If an AD8307 or a similar device is used, keep in mind that when in the TDOA-Only mode, +5 volts is present on pin 3, so a series resistor (1k or so) should be put between pin 3 and the output of your detector. Also note that if such a device is used, it may also be necessary to provide additional high-pass filtering on its input to prevent it from detecting the low-level leakage of antenna switching pulses!  Important:  Always assure that C10, a 0.047uF capacitor, is present at pin 3 for proper operation!  Finally, be aware that the reading taken on pin 3 is highly "de-linearized" to better-accommodate the characteristics of a diode detector, so the response of the TDOA unit to the voltage output by the AD8307 (for example) will also be skewed somewhat.
  
• It may be possible to construct a modified TDOA antenna that, through the addition of some switches, could be transformed into a simple 2-element Yagi.  This has not been tried, so contact the author for details.  For ideas on how one may convert this sort of antenna array into one with some directionality, see http://www.homingin.com/hfinderfix.html.  Please note that the antenna recommended for the PIC TDOA isn't exactly like the one in the aforementioned article and it is likely that two switches would be needed to switch modes.
  
• While some attempt has been made to have the LEDs indicate something about field strength, their range or resolution of indication isn't anywhere near as great as that indicated by the tone.  Because of this, the LED(s) are only potentially useful as "weak/strong" indicators.  Note also that a meter, if used, may or may not provide a useful indication of signal strength, either!
  
• There is no calibration for the field-strength meter modes.  If the CALIBRATE button is pressed when in the "Field-Strength/TDOA" or "TDOA Only" modes, it will do a normal calibration for the TDOA functions.  Note that the CALIBRATE button is completely disabled when in the "Field-Strength Only" mode.
• It is not even necessary to use a receiver with the field strength meter function.  When doing this, it is recommended that the "Field Strength Only" mode is used, and to hear the tone, headphones will be required.  Note that without a receiver connected to J3, there might be some additional "bias" caused by low-level leakage of the antenna switching signal which results in a slightly higher-pitched "no signal" tone, but this shouldn't affect ultimate sensitivity.

• No tone is heard through the unit's speaker, or even the receiver when the earphone jack is disconnected.
• You will only hear a tone through the receiver if you are receiving a signal!  Make sure that:
• The signal is really there!  It may have stopped transmitting.
• That the receiver is tuned to the proper frequency.
• That the receiver's squelch isn't turned up too high.
  
• That the "tone squelch" of the receiver is disabled.
• That the antenna is connected.
• When you open the squelch, you hear noise squelch noise?  If not, receiver audio may not be getting to the RDF unit, so check to see if you hear the receiver's squelch noise when you unplug the cable from the radio's speaker jack.
  
• You will only hear a tone if you are using a "switched antenna" array!
• If you are using an antenna other than a switched antenna array similar to that described here, you will not hear a switching tone!  This type of antenna is necessary to be able to detect the phase of the received signal.
• If you are using the proper antenna array, make sure that you have properly connected the antenna and receiver to J2 and J3:  If you accidentally reversed these two connections - which is very easy to do in the dark - it will not work properly!
• If you are using the proper antenna array, and everything else seems fine, there could be a problem with the antenna.  If both of the switching diodes are burned out, you will not hear a switching tone.
• Make sure that the TDOA unit is turned on and that you are using a good battery.
• Did you switch to a directional antenna, such as a Yagi, to get better field-strength readings?  If so, make sure that you remember to use the switched antenna array for TDOA operations!
• If you are in "Field Strength Only" mode and using a Yagi (for better directivity and increased range) be aware that you'll hear a tone only if you use headphones:  In one channel, you'll hear the audio from the receiver, but in the other channel you will hear the tone generated by the PIC.
  
• Both left and right LEDs are on and they don't change.
• There may be no audio from the receiver.
• This unit requires audio from the receiver.  If you haven't plugged it into the receiver's speaker/earphone jack - something that may be obviated if you still hear audio from the receiver's speaker - it will not work!  Some receivers have more than one jack of the same type, so make sure that you have plugged into the correct one!
• Receiver audio may be too low.  Make sure that you have the volume turned up loud enough!
• Your squelch may be closed - or too tight.
  
• The receiver's audio output jack may be set to the "headphone" mode.  Some receivers, such as the Yaesu FT-817, have a switch - usually next to the jack - that is used to select between "headphone" and "external speaker" use.  If it is in the "headphone" mode, there will be very low output from the jack, possibly insufficient to drive the external speaker in the TDOA unit.  In this case, either use headphones with the TDOA unit, or switch the receiver to "speaker" mode.
• Is there a signal being received?  If you don't hear a tone from either the receiver or the speaker in the TDOA unit, re-check according to the steps above!
• You hear tone, but it doesn't change and both left/right LEDs are on.
• First, make sure that you are hearing tone from the TDOA unit and not your receiver:  If you are hearing it from your receiver, you probably didn't plug in the audio cable from the TDOA unit!
• If the volume is very low, it may not be enough for the TDOA unit to work.
• If the TDOA unit has not been calibrated, it may be very insensitive to the switching tone and you should recalibrate.  Even in this case, however, a high volume level should cause the LEDs to flicker somewhat.
• I'm trying to use the unit, but the readings make no sense at all!
• There may be multipath/reflections.
• It is a fact that multiplath/reflections greatly complicate the task of obtaining sensible bearings when trying to find a hidden transmitter.  The very best way to deal with these sorts of conditions is to practice finding transmitters.  Even if you hide your own transmitter and know where it is, you may observe how reflections from buildings, vehicles and terrain affect signals - especially when you are very close to the transmitter.  Unfortunately, TDOA-types of units are arguably more sensitive to reflection than pure Signal-Strength units - which is exactly why I added an S-Meter to the latest version.
  
• It is possible that the hidden transmitter uses a horizontally-polarized antenna!  If the transmitter has the "wrong" polarization (a favorite trick of those placing hidden transmitters!) then your readings will be affected.  By partially rotating the antenna (30-45 degrees or so) one way or the other you may be able to still get a bearing on the signal:  Do not rotate the antenna array  by a full 90 degrees, as it will no longer be able to discern left and right!  It may be possible to make an antenna array with elements that can be re-arranged to pick up some horizontal components.  Don't be afraid to practice and experiment on your own to get the "feel" of how things work!
  
• You may have the antenna upside-down or backwards.
• With a TDOA array, where you have left/right indications, it is vitally important that the antenna is oriented correctly.  If the antenna is back-to-front OR it is upside-down - both very easy things to do in the dark - the left/right indications will be backwards!
• The unit may not be calibrated to the radio that you are using.
• Because this unit uses "window" sampling, it is absolutely necessary that the unit be calibrated to the receiver that you are planning to use!  If you don't properly calibrate the unit, it may randomly vary its response to the signal in a way that seems to have nothing to do with where the antenna array is pointed.
• Remember:
• Recalibrate using a clean, unmodulated signal with the antenna oriented to provide the loudest, clearest switching tone!
• Hold the antenna steady while it's recalibrating - a process that takes less than two seconds.  If you are in a vehicle, make sure you stop it before recalibrating.
• After recalibrating, re-check to make sure that it provides the proper left/right indication:  If your TDOA unit was not calibrated before, the left/right indications may have been backwards or confused.
• The antenna array may be damaged:
• The switched antenna array requires that both elements be properly switched!  Note that when one element is turned on, the other element is to be turned off and vice-versa.  This might not happen if:
• Both diodes were accidentally installed the same way when it was built.  This would prevent the two elements from being alternately selected, but you would still hear a tone.  (Note on the schematic of the antenna that the diodes are connected opposite each other in terms of polarity.)
  
• One of the two diodes has failed - possibly damaged by accidentally transmitting.  Even if only one diode is working, you will still hear a tone, but your readings will be confusing!
• The two elements of the antenna array may be too far apart for the frequency you are planning to use!
• In order for a system like this to work, the two antenna elements MUST be LESS THAN 1/4 wavelength apart from each other!  If they are 1/4 wavelength apart (or greater) then the reading's will be confusing and seem to be "multi-lobed", giving false "left/right" readings that will leave you scratching your head!  It is recommended that they be spaced at least 20% closer than 1/4 wavelength at the highest frequency for which you intend to use the array.
  
• For the 2-meter amateur band, the maximum spacing should be about 16 inches (40cm)
• For the 70cm amateur band, the maximum spacing should be about 6.5 inches (16cm)
• Spacing the two antennas closer than the above is permissible if you want to make a more-compact antenna array.  The closer they are space, the "quieter" the tone will become on the received signal:  This can make the "null" a bit harder to determine and, if the signal that you are listening to is being modulated, the "quieter" tone can be harder to hear via the listener's ear and more-difficult for the unit to detect and give left/right positions.
• Shorter elements can also work:  You can actually use a 70cm array on 2 meters, but because they are shorter and much closer together, the switching tone and left/right sensitivity will be noticeably reduced!
  
• Does your radio have fancy DSP filtering?
• This probably doesn't apply to very many older handie-talkies, but if you are using it with an all-mode mobile rig or a Software-Defined Radio, it might!  If there is DSP filtering - such as noise filtering - in the receiver you are using, make sure that it is turned off!  Such a filter (or signal processing) may alter the phase of the received switching pulse - or remove it completely - or cause slightly varying amounts of delay, scrambling readings.  If your radio has DSP and you still can't get it to work, try substituting another receiver, remembering to re-calibrate!
• Some of the newest handie-talkies do have digital audio processing inside them that may make them unusable with this sort of unit.  If you can't get it to work, try a different (and older) radio to see if it works!.
  
• The unit is making a beeping noise, but I can't seem to get any direction indication.
• You may have selected the "Field Strength Only" mode.  In this mode, you'll hear a low-pitched tone, possibly interspersed with a very short higher-pitched tone, with the interruption rate of about 3-4 times per seconds.  In this mode, only the field-strength reading is provided with no left/right indications.  Note that even in this mode, when using the switched antenna array, you can still achieve a "tone null" in the direction of the signal.
• Don't forget that the "Field Strength/TDOA" mode also beeps, but it can provide bearing and field strength information simultaneously.  Only the "TDOA Only" mode does not beep.
  

DISABLE YOUR RADIO'S TRANSMITTER!!!

If you are using a transceiver - such as a handie-talking, you should disable the transmitter if possible and if not, set it to the lowest power.  You should then calibrate the unit as described above unless you know for absolute certain that you have already calibrated it to the radio that you are using.  (If you aren't completely sure, re-calibrate!)

Again:  If you do not know for certain that the unit has been calibrated with your radio, you should do so now!  Failure to do so may cause poor performance and/or erroneous/misleading readings!

Then, the user should, with a known signal toward the front, verify proper operation of the unit by moving the antenna left and right, making sure that the tone goes up or down (and the LED goes red or green) with left or right movement of the antenna.  Note that it is the user's preference that determines whether or not a "high" tone indicates that the signal is to the left or to the right.  Note that if there is no audio input or if there is equal or conflicting phase information from each antenna, the "direction" LED(s) will flicker quickly between red and green.

NOTE:  As with most 2-antenna TDOA units, proper LEFT or RIGHT indication depends completely on two factors:

• Whether the antenna array is upside-down.  If the antenna is upside-down, the orientation of the system will be reversed.
• Whether the signal is in front of or behind the user.  If the signal source is BEHIND the user, the left/right indication will also be reversed!
  

Users of this (or ANY) TDOA or other direction-finding system should PRACTICE so that its operation becomes second nature.  ONLY if this is done will the user be able to get the "feel" of how the unit responds to signals of various qualities.

Determining the signal bearing:

My personal preference, when looking for transmitters, is to constantly sweep the antenna array back-and-forth, as one might using a metal detector, while one is walking or driving.  When doing this, you will note several things:

• As you sweep past the "center" (that is, the direction of the transmitter) you'll note that the tone will go up or down:  The point at which it changes direction is the apparent bearing of the signal.
• As the tone changes, the unit's LEDs will also change, indicating that the signal is to the right or left.  This is also true of the meter, if you have used it.
  
• As the antenna is swept through the "center", the tone will disappear (that is, "Null" out.)  This is another indication that the transmitter's signal is coming from that direction.

• As you sweep the antenna back-and-forth, you get accustomed to the tone going up and down as you move the antenna:  If the pitch of the tone suddenly does something different - or starts to go backwards (that is, it goes up when it should go down) then you may have walked past the transmitter or entered an area with strong reflections.
  
• When hunting for transmitters - particularly in cluttered areas - there are likely to be many reflections and multipath effects.
• These reflections can confuse the readings, making it difficult to get a definite "Left/Right/Null" indication.
• If one is constantly walking, one will go in and out of these random reflections:  It is most likely that the "general" trend of the bearing is more indicative of the transmitter's bearing!  That is, if you are in an area with a lot of reflections, keep walking in a straight line, keep a mental note of where the signal usually seems to be coming from, ignoring those brief, individual "stray" readings that seem to randomly vary.
• As you walk about, avoid stopping to investigate every seemingly good bearing.  If you seem to find a location that provides a "solid" bearing toward a signal, make a note of where that bearing occurred- but keep walking in a straight line for a little while:  If you consistently find that future bearings along that track indicate the same general direction, then you can go back and investigate.  In addition, you might observe that the bearing has changed as you've continued walking, possibly telling you how close the transmitter might be!
• Again, there is no substitute for practice!

When using a system such as this, I rely almost entirely on the audible indications rather than looking at the unit's LEDs or front-panel meter:  This is highly recommended, as it is important that you watch where you are going, keeping an eye out for obstacles, traffic, or even the transmitter itself!  Above all, be safe!

If you have immediate interest in this, you may send email - follow this link.

Pre-Programmed PIC for this project:

If you are interested in building one of these units, go here for more information on pre-programmed chips - keeping in mind that this is a prototype!

Related Links:

• UARC RDF page.
• KA7OEI RDF page.
  

This page updated on 20110531