A PIC-based "TDOA" DF antenna unit
(Now, with an audible field-strength meter!)

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.


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:

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

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:


Do people really use TDOA units in Foxhunting?

TDOA RDF units like this are still fairly popular, mainly due to their simplicity.  Presently, the most serious on-foot transmitter hunters use the combination of a Yagi and a receiver with a very wide-range signal level meter.  The advantage of this "Yagi-plus signal strength" scheme is that it gives two important pieces of information at the same time:
Having both of these pieces of information allows the user to determine something about how near or far he/she is from the source in addition to its possible location.  Also note that it is often easier to determine, with bearing and signal strength, whether or not the bearings are likely being affected by multipath or reflections.

Again, the TDOA system has the useful advantage that it can be fitted to nearly any narrowband FM receiver that the user may have onhand and that it is very simple and cheap to construct by comparison.  Additionally, version 2 of this unit has the ability to integrate a field-strength meter to help locate transmitters when they are very close-by.

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:

Recommended antennas:

The described circuit uses a standard 2-antenna diode-switched antenna array.  For details on construction of such an antenna array, see the FM Direction Finding Antenna page.  If you cannot find PIN diodes, 1N4007 diodes reportedly work, too - but only the 1N4007 type as the other types in the series - such as 1N4001, etc. - apparently do not work nearly as well!  In a pinch, 1N914/1N4148 types can work, but they tend to be more "intermoddy" and noisy.

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:

Note:  When the above modes are changed, the unit will restart.

Rectified voltage from diode D2 is input to pin 3, one channel of a 10-bit analog-to-digital converter, which provides for at least 5 millivolt sensitivity.  In reality, the sensitivity is slightly higher than this owing to extensive oversampling.  As it turns out, low-level remnants of the antenna switching pulses can make their way through C4 and C9 - unless the connected receiver happens to have a DC short across its antenna input.  This has the effect of weakly biasing the detector diode, often causing a slight indication of RF even if there really is no RF.  In reality, this actually improves the circuit's sensitivity slightly as it provides a "bias" signal on which an RF "rides", helping to overcome the diode's intrinsic voltage drop.  This fact, coupled with A/D oversampling techniques, effectively increases the A/D resolution to 11 or 12 bits, allowing for 1-2 millivolt sensitivity and resolution.  C10 and R12 filter the recovered DC and provide a reasonable time-constant:  It should be noted that C10 has the additional important property of providing a low AC impedance for the sample-and-hold input of the PIC's A/D input and is a required component for proper operation.

With version 2 firmware, there is also a "Version 1 compatibility mode":  If the version 2 chip is wired as a version 1 circuit (with pins 3 and 4 connected, as shown in the top diagram of figure 2) it will do ONLY TDOA functions.  Additionally, the OVERLOAD LED will function, as with version 1.  When wired this way, it is not possible to switch modes and use the field-strength meter function.
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.
                of the prototype PIC-based TDOA
                2 of the PIC TDOA, which includes provisions for a
                field-strength meter

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 in Figure 1, 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 (as shown in Figure 3) 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 in "Version 1" only.  This is primarily used to indicate an overload condition due to excessive audio input levels.  You may wish to choose to omit this LED.  This part was omitted in Version 2.

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 "Version 2" installed in an enclosure.
Bottom:  Inside the enclosure.
Click on image for a larger version
                of the PIC-based TDOA circuit in enclosure
Inside the enclosure with the prototype

Additional comments:

For the "Version 2" some changes were made and these include:

Note:  Those changes marked with an asterisk (*) may be applied to the Version 1 schematic as well.

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:



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.


Before anything else is done, the unit MUST be calibrated to the specific 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 (e.g. Version 2) 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.

The calibration information is stored in non-volatile memory in the processor and unless a different radio is used it need not be done again.  There is, however, no harm in recalibrating the unit every time you use it.

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 just high enough to get a reading.  In this way when the volume is increased, the LED will be correspondingly brighter to match the audio.

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.


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:

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-variable frequency ranges and two fixed-frequency modes available in the TDOA mode:  Approximately 400-525 Hz (the default) for the "low" variable-tone range, or about 640-770 Hz for the "upper" variable-tone range.  In the "Fixed Frequency" mode, the low range's tone is approximately 475 Hz while its high range is about 700 Hz.

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" quickly 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" again, but more slowly.  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.

The various tone modes are accessed sequentially, as in:

Low Variable (400-515 Hz) -> High Variable (640-770 Hz) -> Low Fixed (475 Hz) -> High Fixed (700 Hz) -> and back to Low Variable

The "Fixed Frequency" modes, as you may have already guessed, are those in which the tone does not change frequency with respect to relative bearing so this means that if you are using this mode, you will have to depend on audibly detecting the null of the tone as well as looking at the indicator LEDs (or meter, if you've included it) for a "left/right" indication.  The fixed frequency modes were added to help deal with those situations where the signal being receive is being very strongly modulated - particularly with a tone - that might tend to confuse the circuit and cause false left/right indications or, at the very least, "dilute" them with noise.  While it is usually possible to deal with this situation by appropriately selecting the low or high variable tone range, having a "fixed" tone range with the frequency selected to be as "far as possible" from that which is being modulated on the signal may be of help.

Once again, if you are having trouble getting sensible left/right indications and you know that your antenna is working properly (e.g. both diodes are fine) and local multipath isn't the issue, try recalibrating!  You shouldn't need to recalibrate unless you change radios, but it may be possible that somewhere along the line it wasn't calibrated properly!

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.

Using the field strength meter:

If so-configured, Version 2 firmware offers a "Field Strength" meter capability.  There are two modes of operation that provide this:
A few important comments about using the field-strength meter:
Troubleshooting during operation:

Finding transmitters:

Before you start:


If you are using a transceiver - such as a handie-talkie, 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:

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:

Constantly sweeping the antenna back-and-forth while walking has another important function.  In addition to determining the direction, it can tell you something about the signal itself:
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!

Using the field-strength meter in Version 2:

With the simple field strength meter shown in Figure 2, in the absence of other nearby transmitters, you would get a field-strength reading only if you were fairly close to the the hidden transmitter!  In testing using a signal generator applied to either J2 or J3, signals of -30dBm could just be detected when a 1N34 was used for D2:  And the sensitivity was fairly flat (within 2dB) from 500 kHz to 1 GHz.  Of course, sensitivity will vary with the diode used for D2 and the layout of its related circuitry.

What is the range of the field-strength meter?  Using the 2-antenna switching array and a standard 1N34 germanium diode for D2, a 10 milliwatt transmitter was detectable from a couple of feet (about a meter) or so away, while a 1.5 watt handie-talkie could readily be detected at a distance of at least 50 feet (15 meters.)

It was noted that the 2-antenna switching array provided no directionality at all in Field-Strength mode!  This should not be too surprising, as it consists of two switched dipoles, only one of which is "on" at any given instant, so they do not interact with each other at all.  As you might expect, it does respond to vertical/horizontal polarity and it exhibited the expected "edge-on" null that is characteristic of dipoles:  Both of these properties could be potentially useful when attempting to locate hidden transmitters!

What about using a directional/gain antenna instead of the switching array?  With a small, 3-element ("Tape Measure") Yagi, this detection range increased to over 100 feet (about 30 meters) when used with a 1.5 watt handie-talkie.

How about strong signals?  With the 1.5 watt test transmitter, I was unable to completely "peg" the audible field-strength meter even by placing the antenna (either the switched dipole or Yagi) against the HT's antenna - but it was fairly close to being "full scale."  What this indicates is that one should be able to approach even a fairly high-power transmitter to within several feet.  Be aware that placing your antenna too near a high-power transmitter could damage the antenna's switching diodes in the TDOA unit and/or your receiver!

Remember:  If you use an antenna other than the TDOA switched array (e.g. one without switching diodes, such as a Yagi) you will need to listen to the switching tone through the headphone jack, as it will not be audible through the receiver!

In other words:  If you detect anything on the field strength meter - and you are sure that it isn't another transmitter - you are very close to the transmitter for which you are looking!

Contact info:

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

Pre-Programmed PIC for this project:

Related Links:

This page updated on 20130304

Since 12/2010: