The Panorama Point repeater "technical" page

This page contains some of the more technical details of the temporary radio system used at Panorama Point during the Friendship Cruise.

Receiving signals at Panorama Point:
Fig. 1 - This is the receive site for the Panorama Point repeater system showing the three Yagis, weatherproof box with the equipment, and the solar panels.
Click on the picture for a larger version.
The Panorama Point receive site in action

When it comes right down to it, while Panorama Point is in a geographically favorable location to have a fairly straight shot up both the Green and Colorado rivers, there are several strikes against it when compared to the "ideal" radio site:
For these reasons, one can't simply throw a portable repeater on the air and expect it to work very well.  From the initial survey done in 1997, it was clear that any "upriver" signals that arrived at Panorama Point would likely be extremely weak and "multipathy" - not surprising considering that the signals will have had to find their way out of deep, narrow river gorges some distance from the receiver site at Panorama Point.  It was decided that, from the first moment, that extraordinary efforts should be implemented on the receiver to maximize performance.

Duking it out with Boltzmann:

For any receive system, there is one fundamental limiting factor that imposes an absolute limit on how sensitive it can be:  Thermal Noise.   This thermal noise comes from two places:  The equipment itself, and the environment in which the receive system is used.

The "Noise Environment":

As it turns out, everything gives off noise - assuming that it is warmer than absolute zero - and the warmer it gets, the more "noise" it puts out.  For an example, consider a piece of metal.  At room temperature, it does not "glow" visibly, but if you were to heat it, it would begin to glow - very dull red at first, but as it got hotter, it would become closer to being "white hot."  As it cools again, its glow disappears once more to our vision.  Just because it may have cooled off to room temperature, don't think that it isn't still "glowing" - because it is!  As it turns out, any object that is above absolute zero does glow - not only at infrared wavelengths (as you may have seen in footage of police finding criminals in the dark using "heat sensitive" cameras) but also at plain radio wavelengths where it manifests itself as noise.

Keeping in mind that all warm surfaces "glow" at radio wavelengths, one can complete the analogy by likening a radio signal to a light source on this glowing surface:  If the surrounding surface is "glowing" more brightly than the radio signal, that radio signal is simply lost!

The magnitude of this noise can be calculated using this equation:
N = k * T * B

N = noise power in watts
T = temperature in Kelvin
B= bandwidth in Hz
k = Boltzmann's constant - which is approximately 1.38x10-23 (expressed in Joules per kelvin, or J/K)
When numbers are crunched, assuming a 300K (about 80 degrees F) temperature and a 15 kHz receiver bandwidth, we can calculate that in a 50 ohm system, as is used on a typical receiver, this is equivalent to a noise voltage of 0.056 microvolts when using an isotropic antenna immersed in a 300k environment.  (Yes, that also means that there is an equivalent noise voltage of 0.056 uV coming out of a 50 ohm dummy load that is at 300K:  It is through the use of the intrinsic noise of dummy loads the EMEers can check the relative performance of the antenna/preamp system!)

This example assumes that we are receiving a signal using an isotropic antenna, surrounded on all sides by matter that is at 300K:  While this isn't exactly the case in a "real world" scenario, it is a reasonable approximation of actual operation of a receive system in which the signals are emanating from the surface of the Earth and the numbers calculated above illustrate that our receive system is, in fact, limited by more than the fact that the signal is just getting weak:  The signal is, in fact, being lost in thermal noise being radiated by the earth itself!
Fig. 2 - Top:  One of the preamps, in-use and connected directly to the receive antenna with a minimum of loss.  Center:  The inside of the preamp showing simple "dead bug" construction with the input on the left side.  Bottom:  The schematic of the preamp.
Click any picture for a larger version.
One of the GaAsFET preamps mounted directly onto the antenna connector
The 'guts' of the preamp
Schematic of the GaAsFET preamp


On a narrowband FM receiver, 12 dB SINAD correlates approximately with a noise voltage at 60% of the signal voltage for a typical receiver.  This means that in a 50 ohm receive system that is terrestrially based (that is, the antenna receives a signal originating from the Earth's surface) or in a test setup involving dummy loads/attenuators that are at room temperature the maximum sensitivity possible - no matter how good your receive system might be (using a standard 15 kHz FM voice channel) is approximately 0.09 microvolts for 12 dB SINAD!  Without cryogenic cooling of the entire planet, better sensitivity than this is impossible to obtain with the aforementioned receive system!  Figure 3 shows the approximate relationship between noise figure and the sensitivity of a typical narrowband FM communications receiver.

Minimizing receiver system noise figure:

Now that we know the magnitude of the noise emanating from the receive system's environment, we should consider the other major source of thermal noise:  The receive equipment itself.  While we can't really control the noise from the environment, we can design a receive system that does not add to it and unnecessarily degrade the system.

When determining "receiver performance" it is worth little to consider just one or two individual components, rather it is absolutely necessary to take into account the entire signal path and hence, we calculate it all as a receiver system.  There are several things that degrade the performance of any receiver system from one that is "perfect" and these include:
Typically, a high-performance receive system will have a low-noise preamplifier in front of the receiver and for the most part, the performance of the receive system is set by the performance of this preamplifier and the losses that are between the preamplifier and the antenna.

One common misconception is that a low-noise preamplifier will cure all woes if it is installed in the system.  What is often not realized is that with modern preamplifiers (those that have noise figures of under 1 DB) it is not necessarily the performance of the preamplifier that limits receiver system performance, but the performance of the system is limited by those losses in front of the preamplifier. 

Take, for example, a hypothetical preamp that has a 0 dB noise figure.  If that preamplifier is connected to the antenna (assuming a "perfect" antenna) via a piece of coax with 0.5 dB of loss through a bandpass cavity that has 1.0 dB of loss (for a total loss of 1.5 dB) then we have just created a system that has an equivalent noise figure of 1.5 dB:  That is, every dB of loss preceding the preamp contributes to a noise figure increase of a dB!

What's worse is that once this "1.5 dB" is lost forever, the increase in noise cannot be recovered - no matter how good the preamp may be or how high gain it is!  Why is this?  All of these losses are subject to the laws of thermodynamics and are resistive and/or dielectric and therefore "generating" noise energy.

The GaAsFET preamplifiers:

The preamplifiers used on the Panorama Point receive system are homebrew, built in 1997, using the Mitsubishi MGF1302 GaAsFET - a now-obsolete device capable of a typical noise figure of 0.4-0.7 dB on 2 meters.  These preamplifiers are designed to have about 18 dB of gain (an "amplification factor" of 63) so we can tolerate the inevitable intrinsic noise of our receiver as well as additional losses after the preamplifier and it is assumed for the purposes of these discussions that this preamp has a conservative noise figure of 0.8 dB.  For the Panorama Point receive system, the losses in the coax and connectors following the preamp is about 1 dB, yielding an equivalent total system gain of 17 dB (an amplification factor of 50) prior to the receivers.

These preamplifiers are built into small die-cast aluminum cases with BNC connectors and a feedthrough capacitor for power for a cost of $20-$25 each in materials.  The original circuit found in a late-80's ARRL Radio Amateur Handbook was modified to reduce gain (from the original design's 24 dB) and to improve stability by using a simple 100 ohm resistor (R2) to decouple the drain circuit from the power supply rather than the traditional tuned circuit or 4:1 balun.  This gain reduction was done primarily to prevent overload of the receiver as the gain exhibited by this amplifier (18 dB) is more than adequate to override the intrinsic noise of the receivers enough to assure that the received antenna noise and preamp performance was the limiting factor in system performance.  In later testing, it was observed that it takes about 6 dB of additional attenuation after the preamp before the system sensitivity begins to measurably degrade.

Performance of the Panorama Point receive system:

For the reasons mentioned above, the preamplifiers on the Panorama Point receive system are mounted directly to the antennas.  It is expected that the losses incurred by this configuration are below 0.25 dB (a typical antenna value) taking into account losses in the matching network of the antenna itself plus those of the adapters used.

Comment:  We know that the losses of these antenna/matching networks are quite low owing to the fact that one of these same type of antennas is used for transmitting and when we dump 300 watts into them, they don't burst into flame or otherwise get hot!

The receiver itself has its own noise figure, but the sensitivity of FM communications receivers is typically measured in terms of 12 dB SINAD for a given number of microvolts.  While it is not particularly easy to quantify the performance of an FM system in terms of noise figure, typical values can be determined and it turns out that a modern receiver with a 15 kHz IF bandwidth and a sensitivity of  0.15 uV for 12 dB SINAD (3 kHz deviation of a 1 kHz tone) has an equivalent noise figure of  approximately 5.7 dB - and this noise figure adds slightly to the overall noise figure of the system.  See figure 3 for a chart that approximately equates receiver sensitivity with noise figure.

So, taking into account these numbers:
Now, to crunch numbers:
For details of how noise temperatures, amplifier gains, and system losses add up, or for a link to an online calculator that converts to/from noise figure and noise temperature, see the links at the bottom of the page.
Fig. 3 - This table shows approximate equivalents of receiver sensitivity (12 dB SINAD, 1 kHz modulation, 3 kHz deviation, 15 kHz bandwidth) with receive system noise figure.
(This information derived from data published by E.F. Johnson Corp. as well as my own empirical testing of radio gear.)

12 dB

Noise Figure (dB)

12 dB
SINAD Sensitivity

Figure (dB
0.1 uV

0.3 uV
0.125 uV

0.4 uV
0.15 uV

0.5 uV
0.2 uV

0.75 uV
0.25 uV

1.0 uV

As can be seen, the fact that the preamp (plus coax losses) has significant amplification (50x amplification of the signal) means that the intrinsic noise of the receiver itself contributes only about 10% to the overall system noise and therefore it is the preamplifier's noise figure that plays the major part in setting overall system performance.

It should also be noted that if our preamplifier had even more gain  (let's assume that it had an infinite amount of gain) OR if our receiver had a zero noise figure of its own instead of the 4dB mentioned, we would only lower the "noise temperature" of our receive system by only 15.76K - or about 0.23 dB!

If our system noise temperature is, in fact, 95K (a noise figure of about 1.23 dB) that would equate to a thermal noise voltage of about 0.031 uV.  Because we already know that the intrinsic sensitivity of any terrestrially-based system can be, at best, 0.09 microvolts (because of the fact that the Earth is, in our example, at 300K) we now know that our receive system's sensitivity exceeds this by about 9.3 dB and this means that the 0.031 uV noise will have relatively little effect on a 0.09 microvolt signal.  Bench testing with signal generators and attenuators (which are at about 300K and are contributing their own thermal noise) shows that the actual sensitivity of the receive system is, in fact, 0.09 uV for a 12 dB SINAD.

Comparison of other hypothetical receive systems:

Case #1 - Using a typical receiver with coax and cavity losses but no low-noise preamplifier:

Let us compare the above system with what one might see if one used the same receiver, but added 2 dB of cavity losses and didn't use the preamplifier:
To crunch numbers:
From the above noise figures alone, one would have a system noise figure of 8.7 dB (about 1860K) - a noise voltage of equivalent of 0.14 uV - more than enough to completely bury our "Earth limit" weak signal of 0.09 uV and would effectively limit the receive sensitivity to signals above 0.23 uV - more than 8 dB worse than the Panorama system.

Case #2  - Case #1 with a low-noise preamp:

Supposing that one were to "improve" the system by placing a low-noise preamp right at the receiver - but after the coax and cavity losses:
To crunch numbers:
From these numbers one would expect an equivalent thermal noise voltage of about 0.07 microvolts - about 2 dB above the 0.056 uV intrinsic noise voltage that one would experience on a terrestrially-based system, implying a maximum receive sensitivity of approximately 0.12 microvolts at 12 dB SINAD.  This figure is about 2.5 dB worse than the Panorama system - but is 4.7 dB better than case #1 - a significant improvement!

It should be pointed out that with FM (unlike amplitude-modulated modes such as AM or SSB) is affected by noise in a nonlinear way once the signal drops below what is called the "limiting" threshold:  On AM or SSB, the addition of 3 dB of noise into the receive system will, in fact, degrade the signal to noise of the received audio by 3 dB no matter how strong or weak the signal is.  FM, on the other hand, is not affected in this way once signals fall below this "limiting" threshold.

When signals fall below the "limiting" threshold of the FM receiver being used, the noise abruptly gets worse:  Depending on the receiver's design characteristics and the exactly where on the "curve" we happen to be, a 1 dB decrease of the signal to noise of  signal that is already at or just below the limiting threshold can cause a 3-10 dB increase in the amount of noise in the recovered audio and the received signal rapidly falls into the noise.  In light of this effect, "2.5 dB" deficit in Case #2 above can amount to a significant degradation in the quality of a received signal:  When this 2.5 dB difference straddles the limiting threshold of an FM receiver, it can easily turn a somewhat noisy but perfectly copyable signal into one that is completely lost in the noise.

Comments on preamps, mismatch and coax losses:

One factor that is not often realized is that high-performance preamplifiers are tuned at the factory for best NOISE FIGURE - something that does not usually coincide precisely with maximum gain and is seldom associated with anything resembling a good 50 ohm match.  What this means is that many preamps will terminate a piece of coaxial cable with a rather poor SWR - something that will exacerbate losses of the interconnecting coaxial cables, degrading system more than just the coaxial cable loss alone.

For a system in which the preamplifier is wholly integrated with cable and cavity assemblies, the preamplifier may be tuned in situ to mitigate some of these problems - but it is also true in many of these cases that with the additional losses of components in "front" of the preamplifier, the ultimate system sensitivity is more limited by the losses of these additional components than the performance of the preamplifier itself.  Knowing this, one can further understand why it is that preamplifiers mounted at the antenna are the preferred configurations for those systems where preamplifier noise is the limiting factor - most notably in Satellite and EME ("Moonbounce") operations.
Fig. 4 - This shows the voting controller (in the blue box) with the two modified Tactec receivers and the UHF link transmitter (a Yaesu FT-470 HT).  Click on the picture for a larger version.
The voting controller, receivers, and link transmitter.

How to beat Boltzmann:

With the existing system, we now know that we are nearly (within less than 0.25 dB) of the theoretical maximum sensitivity possible for an Earth-based receive system:  Any further improvements in the performance of the receiver itself and the preamplifier would NOT offer any more than a very minor (and operationally insignificant) improvement that could be measured only with sensitive instruments.  How is it possible to obtain further improvements in system performance?

One obvious answer is with the use of directional antenna rather than omnidirectional antennas.

Using directional antennas:

Because our major limiting factor is thermal noise from the Earth itself it would, therefore, make sense to limit the degree that the Earth's thermal noise "dilutes" the signal that we are trying to receive.  Consider, for example, a system with an omnidirectional vertical antenna that has gain over 360 degrees of azimuth and compare that with, say, a 12 dBi Yagi that typically has a 3 dB beamwidth of about 60 degrees.  Assuming that the vertical beamwidths of these two antennas are comparable, that implies that the Yagi is receiving a noise contribution of less than 1/6th (360/60) of the Earth as compared to the vertical.  This (theoretically) corresponds to at least a 7.8 dB of improvement.  Note:  The improvement is actually greater than this owing to the fact that the beamwidth of the Yagi is roughly circular whereas our 1/6th "section" of the vertical's beam pattern is a rectangular piece of the cylindrical shape.

At first guess, it might be assumed that the fact that our antenna has a gain of 12 dB (an "amplification" factor of about 16) that the increased signal voltage is the source of improved performance in our system - but it is not:  In the case above where our receive system's noise figure is already about 1.2 dB, the effect of the "gain" of the antenna on increasing the signal's voltage doesn't help us as much as our system's sensitivity already allows us to easily "hear" the Earth's thermal noise - and the antenna's gain effectively increases amplitude of that thermal noise.  What does affect performance is the fact that the "spotlight effect" of the antenna's beam pattern is rejecting the thermal noise from directions other than that of the source of the signal that would otherwise "dilute" it.

If, however, our sensitivity was limited by the thermal noise of our receive system rather than that of the Earth itself (as in "Case #1" and "Case #2" above) then the "gain effect" would improve the system performance as it would help overcome the intrinsic receive system noise.  In "Case #2" where we already have a preamp inline, the use of the Yagi would overcome much of the thermal noise generated by our "3 dB loss in front of our preamp" losses caused by the cavity and coax:  In "Case #1" the improvement would be even more dramatic.

Voting receiver system using Space Diversity:
Fig. 5 - These pictures showed the pair of "married" Mocom-70 amplifiers that were used in 1998.  Using a Wilkinson power divider on the input to split drive power - and another on the output, the 110 watt output of each amplifier was combined to provide over 200 watts of RF output with only a watt or so of RF drive.
Click on either picture for a larger version.

The "circuit" side of the paired Mocom 70 amplifiers
The 'heat sink' side of the paired Mocom 70 amplifiers

There is yet another technique that can be exploited to further improve performance of the antenna system:  A voting receiver system using space diversity - that is, having independent receiver/antenna systems spaced a short distance apart from each other.  Under multipath conditions and at any given instant, one antenna is likely to have a better signal than the "other" and furthermore, the likelihood of both antennas experiencing weak signal conditions at exactly the same time is reduced.  Although the amount of improvement is difficult to quantify, published literature indicates that a typical 2-receiver voting system with fairly closely-spaced (1/2-1 wavelength separation) antennas improves the probability of signal intercept such that the improvement of the receive system is between 6 and 10 dB when the major cause of degradation has to do with multipath.

Using both directional (gain) antennas and voting receivers, it is reasonable to expect an apparent receive system improvement of well over 10 dB as compared to just a single omnidirectional antenna under the majority of conditions encountered.   It is likely that further improvement could be obtained by the addition of more voting receivers - possibly placed further away from the existing receiver site - but that still doesn't address the problem that the user must still be able to hear the repeater that he/she is using, but in this case the user has the advantage of being able to move location to find a usable signal.

The receivers:

The receivers used for the voting receiver system are a pair of identical Tactec transceivers that have their transmitters disabled.  A number of modifications have also been done to these receivers - mostly to minimize current consumption (such as replacement of the programming EEPROM with a CMOS version which alone saved about 100 mA) as well as modifications to allow the disabling of the audio amplifier and front-panel LED displays.  With all of these modifications, the receivers consume only about 180 mA each, compared to about 360 mA in their original, unmodified state.

One important feature of the Tactec receiver that makes its rather high power consumption worth the trouble is that it has an excellent receiver on at least two fronts:  The receiver's synthesizer is an unusual design that has very low phase noise - something important in a high-dynamic range receive system.  Secondly - and more important - is that the receiver unconventionally (for a "mobile" radio) uses a passive diode-ring doubly-balanced mixer (DBM.)  The use of the DBM is important in that it gives the receiver a much higher dynamic range than more conventional receivers with active mixers, allowing the receiver to function in the presence of other strong signals.  Both the low phase noise and the high dynamic range allow the receiver to function well in the presence of the very strong signal from the transmitter itself.  ("Barefoot" these receivers have an average 12dB SINAD sensitivity of about 0.15 uV - equivalent to a noise figure of approximately 5.7dB.)

The Voting controller:

The voting controller originally used a PIC16C84, but because this processor is now obsolete - and I needed to add more functions - it now uses the pin-compatible PIC16F819.  This newer processor not only cheaper, but it has a lot of on-chip peripherals (such as A/D converters and comparators) that would have made the original design simpler.  With the original source code written in C, it was a fairly simple matter to "port" it over to the newer processor.  Wanting to retain compatibility with the original firmware and being somewhat reluctant to simply toss already-proven hardware and software, I chose to retain the use of an LM339 outboard comparator rather than use the chip's own comparators and A/D converters for signal-quality voting.

Another of the modifications to the Tactec receivers is to bring out discriminator audio and the "noise voltage", a signal generated inside the radio used to determine now noisy the signal is and thus determine  whether or not what is received should be squelched.  The voting controller looks at this squelch voltage to see which receiver (or both) is receiving a signal.  If both receivers "see" a signal, another comparator is used to determine which one has the lowest "noise voltage" and selects that receiver so that, at that instant, it is used to provide audio from the receiver with the best signal.

When a signal is received from either receiver, the processor keys the link transmitter (a modified Yaesu FT-470 HT) and routes audio from the receiver with the best signal to the transmitter.  This FT-470 has been modified such that an audio path is available on a jack that directly feeds the modulator.  In this controller, "discriminator" audio is used throughout and passed through to the transmitter without any de-emphasis or pre-emphasis:  The only processing done is a 3 kHz lowpass filter to prevent high-frequency noise inherent in weak FM signals (the same noise that is used for squelch detection) from being transmitted and causing the UHF link receivers to experience "squelch clamping" when the 2-meter input signals are weak.  In doing this, the audio response from the 2 meter receiver and through UHF transmitter is flat to +/-1dB from 300 Hz to 2.5 kHz and flat to +/-3dB from 75 Hz to 2.75 kHz, thus avoiding any discernible "coloration" of the link audio.  The controller also monitors the UHF link activity by "looking" at the transmit/receive LED on the FT-470 using a CdS photocell to reduce the chance of the controller IDing atop a transmission originating from another repeater in the system.
Fig. 6 - Front and rear views of the Vocom 300 watt RF power amplifier.  This amplifier requires about 50 watts of drive to produce full output.  At 13.8 volts - the designed voltage - it will produce about 350 watts, but when powered from a partially-discharged battery bank under full load the voltage drops to the 12.2 volt neighborhood and the output power is in the 200-225 watt range.
Click on either picture for a larger version.
Front view of the 300 watt Vocom power amplifer
Rear (heat sink) view of the Vocom 300 watt amplifier

A more-recent addition (which required the use of the newer processor) is a power management function.  When the receivers have been idle for a certain period, they are power-cycled to reduce the average current consumption and extend the charge life of the battery.  Under normal conditions, a "fast" power save function starts one minute after the last transmission in which the two receivers are alternated between each other at of about 3 times each second.  The instant either receiver detects a signal, both receivers are powered up and the "power save" cycle timer is reset.  After 5 minutes of "fast" power save, a "slow" power save kicks in where each receiver is alternately powered up for 1/5th of a second, each second, further reducing power consumption.  As in the "fast" power save function, both receivers are instantly awakened should a signal appear, restarting the "power save" cycle.

Additional code was written to protect/prolong battery operation:  If the battery voltage exceeds 14.5 volts - possibly due to charging by the solar panels - the power save functions are inhibited (to increase current drain) until the voltage drops below 13.9 volts in order to prevent battery damage due to overcharging.  If the battery voltage drops below 11.7 volts, the "fast" power save mode begins immediately after a received transmission ends and the "slow" power save mode begins 1 minute after the last transmission:  This more-aggressive power saving remains in effect until the voltage rises above 12.5 volts.  In testing, the "fast" power save function causes a negligible "wake up" delay while the "slow" power save function might, at worst, cause the loss of the first word in a transmission.  For continuous monitoring of the "health" of the power supply,  the controller sends the battery voltage just after each ID.

In addition to canceling the power save features at a high battery voltage, an N-channel MOSFET is used to disconnect the negative lead from the solar array:  If the voltage exceeds 14.5 volts, the solar panels are disconnected until the voltage drops below 13.5 volts.  In actual practice, the results in the battery voltage averaging about 13.8 volts under full sun.

In testing, it was noted that this receive system will function without degradation down to 10.5 volts and continue to operate with sensitivity degradation down to about 9 volts (at which point the receivers start to fail and begin to show serious signs of performance degradation) allowing for a significant margin in battery voltage.  Originally, the receivers and controller consumed nearly 700 mA when idle (about 350 mA more when the transmitter was keyed) making power management of the receive site somewhat of a challenge - particularly overnight following cloudy days.  Now, with the "fast" power save mode, the average current is about 300 mA and it drops to less than 120 mA when in the "slow" power save mode.


The controller also provides a bit of telemetry as well. After each transmission, there is a "courtesy beep" that indicates which receiver(s) were used and to what extent with the "A" receiver, having a low-pitched tone and the "B" receiver having a high pitched one.  The beep - which is always the same duration - will sound high or low pitch in proportion to how much one receiver or the other was used:  In other words, if both "A" and "B" were used equally, half the beep would be high-pitched and the other half would be low-pitched while if only one receiver were used, the beep would be a single pitch.  When it is time to ID, the pitch of the ID will reflect that of the last-used receiver.

Following the ID is a readout of the battery voltage in Morse using the letter "R" as a decimal point as has been long tradition.  This "R" also has significance:  If the "R" in the battery voltage is of a different pitch than the rest of the ID/battery voltage, that indicates that since the last ID, the battery bank has achieved full charge and that the solar array has been disconnected at least once.

These bits of telemetry have proven to be invaluable as the receiver is located some distance away from camp and keeping tabs of the battery voltage provides a good indication that the solar charging system is working.  In at least one instance, high (>50 MPH, or 80kPh) winds have flipped over the solar panels and inhibited charging:  It was by monitoring the telemetry - and noting that the voltage had not been increasing under full sun as expected - that we knew to make the 0.83 mile (1.3km) walk to the receive site and investigate!

Receiver placement:

You may have already noticed that this receiver system does not have any bandpass or band-reject cavities in it:  Having them would most certainly degrade system performance.  Typically, a cavity filter is required on a repeater system owing to the fact that the receiver will be overloaded by the transmitter's signals and/or that the transmitter itself can radiate a low-level masking noise that will effectively degrade the receiver's performance.

At Panorama Point geography is used to allow such a system to be implemented without the use of any additional filtering as the transmit and receive sites are about 2900 feet approx. 900 meters) apart (yes, we have to carry armloads of gear to/from the receive site - but with three people, we can make it in just one trip...) from each other with the receive site being "downrange" of the transmitter's antenna pattern.  This spacing alone provides significant transmit/receive isolation (about 75 dB) but there is also some terrain blocking the direct view between the two sites as well as the fact that the receive antennas are directional, having side and rear pattern rejection.  See Figure 8 at the bottom of the page for a map and picture showing the relative locations of the sites.


The first year that the Panorama Point receive system was deployed (in 1997) it used the receive system described above, but its transmitter produced 50 watts into vertical antenna, yielding an EIRP of, perhaps, 150 watts.  It soon became apparent that while we could hear the fixed stations and th boats on the river quite well, those in farther-flung locations (e.g. near Moab and at the northern end of the Green River) were having trouble hearing the Panorama Point repeater in some locations - that is, our repeater was an "Elephant" (e.g. big ears, small mouth...)

The next year (1998) I decided to improve things a bit.  Because the river course is located in mostly one general direction from the Panorama Point, there was absolutely no need to use an omnidirectional antenna:  One could use a modest-gain Yagi to beam the signal in the direction of the river course, "tweaking" the pointing of transmit antenna as necessary to accommodate the crowd of boats as it moved along, or if a particular station had difficulty in hearing the signal.

In addition to the Yagi, more transmitter power was used.  Having a bunch of old Motorola Mocom 70 radios kicking around, I "married" a pair of 110 watt power amplifier modules together for a combined total power of over 200 watts of RF.  In conjunction with the 5-element Yagi, this produced well over 2500 watts of effective radiated power - more than a 12 dB improvement in signal from the previous year.  Unfortunately, after a day or two of operation, one of the Mocom 70 amplifiers failed - but a simple jumpering allowed us to continue with just one of the amplifiers and over 1200 watts of EIRP:  No-one really noticed...

Was the extra power worth it?  Reports from those on the river indicate that it was:  At this power level it is possible to hear the Panorama Point repeater nearly everywhere along the rivercourse - although, as before, it is sometimes a bit spotty near the beginning (just south of Green River) and nearer the end (near Moab.)

Fig. 7 - Top:  A view of the transmit site showing the old 2kW generator and the two antenna masts.  Bottom:  A view of the equipment used for the transmitter.
Click on either picture for a larger version.
View of the transmit site showing antenna masts and the generator.
The transmit site gear and batteries.
A few months after the 1998 Friendship Cruise I was given a broken Vocom 300 watt amplifier.  As it turned out, only "half" of the amplifier (which consists of a pair of 150 watt amplifiers operating in tandem) was bad so, in 1999, I operated this "kludged" amplifier at 150 watts.  By the time the 2000 Friendship Cruise rolled around I had completely repaired the amplifier and operated it at its full 300 watt output power (when the battery bank was at full charge) yielding a transmitted EIRP of over 4500 watts.

Even with this much transmitted power, users still report that the repeater is still somewhat of an "Elephant" - but it is unlikely that either logistical practicalities would allow us to provide a dramatic increase in effective transmitter power:  Going from 300 watts to full-legal power of 1500 watts would be in increase of 7 dB - just over 1 S-unit!  On one occasion, I used a 7-element Yagi instead of the usual 5-element Yagi, but it soon became obvious that while the 7-element had higher gain, it had a noticeably narrower beam pattern and did a poorer job of covering the entire breadth of the rivercourse at once.

The radio used for transmitting is a Kenwood TM-733A dual-band mobile radio.  This radio can be configured to provide cross-band repeating and it is this mode that is used to receive the UHF signal from the Panorama Point receive site - or from other repeaters in the system.

Because repeater operations often involve long periods of transmitting, the TM-733A itself cannot be used to directly drive the Vocom power amplifier - which requires 50 watts of drive:  While the '733 can output 50 watts, it cannot do so for extended periods without overheating.  Instead, the '733 is set to a 5 watt output (a power level that it can handle easily) and an outboard 50 watt amplifier (a VHF Engineering unit that is, fortunately, an exact copy of the 75 watt amplifier in Motorola App. note AN-791) is used to boost the power.  This outboard amplifier, with its larger heatsink, is placed in the airflow of the 300 watt amplifier's fans and can sustain continuous-duty operation.

A look at the pictures reveals two antenna masts.  The shorter mast is one that is erected immediately upon arrival and is used to hold not only the UHF link Yagi antenna, but a "backup" VHF transmit antenna and until the main mast is set up, this "backup" antenna is used for transmitting (at 50 watts) - or if some maintenance/adjustments are required on the taller mast during normal operations.  The taller mast is a military-surplus sectional guyed mast with an added pipe section on top for total height of about 27 feet and it allows adjustment of the beam heading from the base.  With 300 watts of RF being used, prudence dictates that such a strong RF field be moved up in elevation atop the mast to minimize any RF exposure hazards.

Linking the sites:

Because the transmit and receive sites are separated by some distance, it is necessary to link them together - and the most convenient way to do this is via a UHF link.  For the Panorama Point receive site, an old Yaesu FT-470 Handie-Talkie is used, operated by the voting receivers' controller.  Having a UHF link such as this conveniently supplied the means by which other sites - such as the Canyonlands Overlook site - could be linked together with this one.

When a signal is received on the Panorama Point receiver, it is transmitted via UHF.  A crossband repeater at the transmit site at Panorama Point (over 1/2 mile away) receives this and simply retransmits what it hears.  Simultaneously, a UHF receiver at one of the other sites (such as Canyonlands Overlook) receives this signal and retransmits it, on its output frequency and provides a simulcast of the transmissions received at Panorama.  Conversely, any transmission received at Canyonlands Overlook is also transmitted on UHF, received by the same receiver at Panorama that receives the signals from the Panorama receive site, simulcasting the signals received at Canyonlands Overlook.

Even though there is not line-of-sight between the two primary repeater sites, Panorama Point and Canyonlands Overlook, the path is quite reliable as it seems to "knife-edge" over the Island-in-the-Sky district of Canyonlands.  The UHF link transmitter (the FT-470) is set at just 1/2 watt output and uses an 8-element Yagi pointed in the direction of Canyonlands Overlook and provides a very strong signal.  It has been noticed that on those occasions where both Canyonlands Overlook and the "local" Panorama Point UHF link transmitters are active, it is not uncommon for the more-distant Canyonlands Overlook signal to override the "local" Panorama signal.

It is worth mentioning that each of the linked repeaters operate on their own 2-meter frequencies, requiring the users to select the one that covers best in their respective locations.  Having all of the repeater systems linked to each other allows the users to stay in contact no matter which system they might be on.

Fig. 8 - Top:  A topographical map showing the relative locations of the transmit and receive sites.
Bottom:  A composite "Google Earth" (tm) image of the terrain highlighted on the topographical map, showing the locations of the radio sites.  Note that the darker mass toward the top is actually at a lower elevation than the narrow isthmus on which the Panorama Point receive site sits:  It's the shadowing and coloration that makes it appear differently.
Click on either picture for a larger version.
Topo map of the area around the TX/RX sites.
Aerial view of the TX and RX sites.
Power supplies:

As mentioned before, the Panorama Point receive site is self-sustaining in its power requirements, using batteries and solar panels for charging.  The transmitter site, however, is a different story:  At the full 300 watt output, the Vocom amplifier, the 50 watt driver, and the TM-733A pull nearly 70 amps of current.  To supply this current, several large lead-acid rechargeable batteries are used.

Because of the high current consumption and frequently heavy repeater use, the batteries need to be charged - typically twice each day - to maintain the system.  To do this, a generator is used along with several high-current power supplies along with several high-current switching power supplies to fully-load the generator and effect as rapid charging as possible.  In the past, 3-4 hours of run time per day have been needed to maintain the batteries.  While the generator is being used to charge the transmitter's battery bank, other batteries - such as those used for running an HF rig or tabletop VHF rig or other gadgets that we might bring along - are also charged.

Earlier, I used an old 2 kW generator that had been refitted with a very large noise-suppressing muffler, taking the sound from a deafening din to more of a distant hum.  Typically, the generator was placed at the end of an extension cord, behind rocks from the campsite so its noise was just "there" and not at all offensive.  After the battery bank it was brought to full charge, it was shut off and re-fueled in preparation for the next time.

Several years ago, I bought a 2kW "inverter" type generator - a "Kipor" model (essentially a Chinese "knockoff" of the similar Honda and Kawasaki generators) that varies its engine speed depending on load and is thus more efficient.  With this generator there is more of a tendency start it up and walk away, allowing it simply to run through an entire tank of fuel at its own rate:  In the first two or three hours, it is fairly heavily loaded as the battery bank is quickly charged, but once the charging current drops off, it would simply idle along for another 4-6 hours - kicking its speed up only when the transmitter gets keyed up and the load increased momentarily.  As it turns out, keeping the battery bank fully-charged with the newer generator requires that we use 2.5-3 gallons (10-12 liters) of gasoline over the weekend while the older "conventional" generator required about 6 gallons (24 liters) for the same task and duration.

Additional Friendship Cruise-related pages:

Go to the KA7OEI main page.

If you have any questions about this event, or about any of the equipment or techniques used, you may contact Clint, KA7OEI via email.

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This page was last modified on 20110606.  Text and images are copyright 2001-2011 by Clint Turner and all rights are reserved.

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