Monday, February 17, 2014

Analysis of a repeater's antenna pattern

Back in 1997 the antennas on the Utah Amateur Radio Club's 146.760 repeater were relocated and replaced - this, because the original, guyed tower on which the antennas were located was being replaced by a free-standing 120' tower.

Because the (separate) transmit and receive antennas were, at that time, over 20 years old (but still in perfect condition owing to radome placed over them when they were originally installed) we decided to start anew with the 2 meter antennas, putting the new antennas at the locations prescribed by the owner:  The receive antenna on top at the 120 foot level and the transmit antenna at the 60 foot level.  Upon installing the new antennas and running the new Heliax (tm) in a cable tray with almost nothing else in it (yet) we noted that we were the first to attach anything to the (also) brand-new ground system.  (We also noted that some hardware for part of the ground system had been installed incorrectly - which we fixed!)

While the receive antenna - being the tallest thing on the tower - worked quite well we could tell that something was amiss with the transmit antenna.  From the time that it had been installed we got reports that the signals to the north were noticeably weaker than they had been on the old tower/antenna and anecdotally, they seemed to get worse as the tower was finally built-up and more antennas, dishes and cables were gradually installed over the years.

signal strength
signal strength
0 <-108 <-114.5
1 >-108.3 >-113.8
2 >-107.3 >-113.0
3 >-106.7 >-111.6
4 >-106.0 >-110.2
5 >-105.1 >-108.8
6 >-104.2 >-106.4
7 >-103.0 >-104.7
8 >-100.4 >-102.8
9 >-84 >-101.0
A >-74.5 >-99.5
B >-70.1 >-97.8
C >-58.9 >-96.8
D >-50.8 >-95.8
E >-40.8 >-94.6
F >-30.1 >-93.5
Table 1
Serial-port S-Meter readings versus signal input (as read via the serial port) on 2 meters for my FT-817 as shipped from the factory.
Not wanting to rush into these things, it wasn't until 2001 that we decided to make some scientific measurements.  One option was to drag along a signal level meter or spectrum analyzer and, every so-often, stop and make signal level measurements.  Since this method was likely to be very tedious and, in some areas may not even be very practical, I decided that there had to be a better way!

The FT-817 as a test instrument:

Not too long before this I'd bought a Yaesu FT-817 and noticed that it had the capability of reading the S-Meter via the serial port, but it had a rather useless signal strength span when it came to making meaningful measurements of real-world repeaters.

As can be seen from TABLE 1 the readings aren't entirely useful.  While each step is approximately 1 dB (more or less) the useful range goes from -114.5 to about -93.5 dBm - this entire range being generally weaker than what one might see from a local repeater.  At the same time I also made measurements of the S-meter reading when in SSB/CW/AM mode to see if that would be useful and while it covered far more range, the steps were uselessly small at the weak signal end (e.g. <1dB) but uselessly large at the high-signal end! (This indicates another, well-known problem with the FT-817's AGC, but that's another story...)

At about this same time I'd become interested in another aspect of the FT-817:  It's "soft" calibration settings.  I believed that these settings, in a special "calibration" menu, were too numerous and tedious to have someone on an assembly line adjust so I figured that there MUST be a way in which a radio was semi-automatically calibrated at the factory - and I was right!

What I found were some "undocumented" commands via the serial port - some of which obviously read from and wrote to the EEPROM - and I quickly wrote a program that would allow me to determine what memory locations were used for what:  The program would download the current EEPROM content, I would change a setting, and then the program would tell me what had changed after downloading it again.  I'd documented my findings on a web page and in the years that followed, all sorts of things followed-on from this information (e.g. "FT-817 Commander", the "SoftJump" program, various remote meters for FT-817 signal strength, ALC, SWR and transmit power - just to name a few).
0 <-110.7 8 -94.2
1 -108.9 9 -91.5
2 -106.2 A -89.2
3 -104.2 B -87.2
4 -102.3 C -85.2
5 -100.6 D -82.1
6 -98.9 E -78.1
7 -96.7 F >-75.7
Table 2
Serial-port S-Meter readings versus signal input using FM mode (as read via the serial port) after the described recalibration of the FM-S1 and FM-FS parameters. 
In  this early stage there were two "Soft Calibrate" (and now, EEPROM) settings that most interested me:  The ones that corresponded with S-Meter calibration, namely #9 - "FM-S1" and #10 - "FM-FS" which, I correctly surmised, related to the settings for the S1 and Full-Scale readings.  Through experimentation by using a calibrated signal generator and observing the readings on the serial port I determined that the original settings badly shortchanged the dynamic range of the FM S-meter and simply by readjusting these two settings could provide a wider and more useful FM S-Meter range as TABLE 2 demonstrates.

Now the meter was useful over a range of more than 30 dB and it still had reasonable resolution - between 2-3dB per step, but I still had a problem:  The usable range - from about -108 dBm to about -80dBm was still too low for the expected signal strength of typical, local repeaters which could vary from about -50 to -80 dBm at the receiver's input terminal.

Fortunately, I knew of another setting or two within the radio that proved to be useful - Calibration menu # 5 "VHFRXG".  This setting adjusted the bias of a PIN diode in the FT-817's IF and I found that it could usefully add at least 30 dB of attenuation, extending the S-meter to signals stronger that -50dBm!

What was more, I found that this setting - because it was done in the IF - was the same for every band (using the corresponding calibration points for HF, 6 meters and UHF) and it turned out to be consistent (within a 2-3dB) over a very wide temperature range (e.g. "Hot Car" to "Deep Freeze").  I found three more values for the "xxxRXG" parameter that adjusted the gain by approximately 10 dB (and precisely measured that amount of attenuation) and was ready to go!
0 <-110.7 <-98.5 <-88.1 <-78.7
1 -108.9 -96.8 -86.7 -77.7
9 -91.5 -79.5 -69.6 -60.4
D -82.1 -70.5 -60.2 -50.9
E -78.1 -66.3 -56.5 -46.9
F >-75.7 >-63.8 >-53.6 >-44.5
 - 12.0 22.0 31.1
Table 3
Sample values of the VHFRXG parameter (soft calibration menu item #5) versus the signal input level.  The bottom row shows the average difference between the "unattenuated" reading (VHFRXG = 99) versus the reading obtained with differing amounts of "attenuation".
Note:  The above values are for my FT-817.  Every '817 will be different, requiring individual calibration to assure accuracy.

Putting it all together:

I could now get down to the business of writing a program that would take all of this data and make sense out of it.

What I had now were lots of bits of information that I could use to analyze the problem related to the repeater's transmit coverage:
  • Using the FT-817, I could now read the signal level arriving at its antenna terminal.
  • Knowing the type of antenna and amount of coax, I could make an estimate of antenna gain and other losses to correct the signal level reading.
  • The GPS location of the repeater was known from previous on-site measurements.
  • The repeater's transmit antenna gain and losses (coax, cavity, etc.) were known.
  • Using a portable GPS receiver connected to the computer, I knew MY location via the NMEA strings emitted by a GPS receiver and fed to the computer.  The laptop that I was using had only one serial port so I used a relay controlled by the handshake line two switch between the FT-817 and the GPS receiver every 30 seconds or so to record the location.
  • Knowing my location with respect to that of the repeater, I could calculate the distance between my antenna and the repeater's antenna as well as the bearings to/from the two antennas.
  • Using fairly simple formulas, I could calculate the free-space path loss between my current location and the repeater antenna.
  • Knowing the transmit antenna gain and loss parameters, my own receiver's antenna gain and loss parameters and the amount of expected path loss, I could could calculate how much signal I should (theoretically!) expect from the repeater.
  • Since I was able to directly measure my received signal strength, I could calculate the "Excess Path Loss" - that is, the difference between the predicted signal level and the actual signal level.  This value could vary from being negative, indicating a higher signal level than expected, to positive, indicating greater path loss than expected.  Both a "real-time" and a "sliding average" reading were made available, the latter smoothing out short-term variations in signal level due to Fresnel effects, uncertainty in measurements and the effects of nearby obstructions such as buildings and vehicles.
  • Since it was a computer, this was done automatically and the results saved to a text file for later analysis.  This included time stamps and all of the raw data as well as the "cooked" data such as excess path loss, bearing to/from the site, etc.
  • The program also allowed brief text notes to be inserted in the file permitting one to take notes about local obstacles that might skew readings, etc.
What this meant was that while I drove a path that circumnavigated the 146.760 repeater, my passenger could look at the computer's screen which was providing a real-time display of the calculated parameters.  The biggest advantage was that we could be zooming down the highway, taking readings very frequently.  With the real-time display we could also take a different route if we suspected that some local obstructions excessively skewed the readings.

So, during April 2001 - after testing the program on a few other local repeaters and finding that the readings agreed within a few dB of theoretical - Gordon, K7HFV and myself took a day-long drive, circumnavigating the 146.76 repeater.  While much of this was via paved roads there was a significant segment consisting of high-clearance four-wheel drive dirt and gravel roads that took more time to traverse than the rest of the trip put together!

Having made the trip "behind" Lake Mountain to the west we were coming close to closing the circle when, while driving along the highway, Gordon started reading out "additional path loss" numbers like "-10... -15... -25... -35... -25... -15... -10..."  While in full, line-of-sight view of the transmit antenna we had passed through a 30+ dB deep null in the transmit pattern while traveling a fairly short distance!  Not sure of what we just saw, I did a legal U-turn and re-traced the path going the other way - and then back again, each time seeing the same numbers go by on the display on three occasions!

Figure 1: 
The measured antenna pattern (the shaded circle near the center) and the calculated coverage of the 146.760 repeater based on this pattern and actual terrain data.
Click on the image for a larger version.
We now had our answer as to how severe the null was - and the results may be seen in Figure 1.  After analyzing the logged data I was able to determine the approximate antenna pattern and input this data into the "RadioMobile" program by VE2DBE.  As expected, it showed a rather deep null almost exactly straight north, encompassing a significant portion of the Salt Lake valley and communities to the north.

What to do about the null?

Even though we've known about this problem for some time now, the big question is "What to do about it?"  On this site, the receive antenna is just that:  A receive-only antenna, and we cannot transmit from that location - which, being on the top of the tower, is free of this null.  At the level of the transmit antenna we have the problem of there being very limited options as to where and how we may mount our antenna to avoid the mechanical obstacles.  We have some ideas in mind, but we are still considering the options!

A slightly more in-depth version of this article may be found here (link).

For more information about the FT-817's inner workings, visit the KA7OEI FT-817 pages (link)


In the fall of 2014 a "fill" antenna was added to (hopefully) minimize the null caused by the "tower clutter".  While anecdotal evidence indicates that this has improved coverage in the "null zone", at the time of this update (3/15) we have yet to re-do at least part of the circumnavigation to quantify the effect of this change.


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