10 GHz signals
between WA7GIE (near Nephi) and
KA7OEI (near Fairview)
On June 11, 2005 the June VHF/UHF contest was in full swing and a number of us went from our comfortable shacks to places afield. I was at the cabin of WA7X, about 4 miles east of Fairview (at an altitude of about 8500 feet on a west-facing slope) and Dave, WA7GIE was near Nephi, about 27 miles to the west over several minor mountain ranges.
The cabin has, conveniently, a deck that faces west and its
roof provides protection from direct rain, and that was a good place to
set up. Using a lead-acid battery, an FT-817, and my homebrew
transverter I set up on the deck.
Giving Dave a call on 2 meter SSB, we arranged that he'd turn on his transverter and give time for it to warm up and stabilize in frequency. After this, I went out onto the deck to try to listen for the WA7GIE 10 GHz beacon. This beacon, if heard, would verify not only that the receive equipment was operational, but provide a common frequency reference for both of us to use: We could simply agree to operate, say, 2 KHz below the beacon frequency. After several attempts trying to detect the beacon, I concluded that it was likely that there were just too many mountain ranges in the way. Not only that, weather was moving in and the path that the signal would have to take had a lot of moisture (clouds and rain) along it. (Dave was unable to hear the beacon from his location, either.) I was fairly confident that my receive system was working, however, as I could see an S-0 meter reading with the antenna pointed skywards and an S-2 reading when pointing it at the ground: This meant that I was easily able to hear the thermal noise of the earth itself and that the receive system was working as well as it could possibly need to (except for EME, of course...) Another reason that I may not have heard the beacon was that I have not yet put together any antenna system better than a 17 dBi gain horn for use with the transverter and may simply not have had enough receive system gain.
After 10-15 minutes Dave and I communicated again (on 2 meter FM, this time) and I requested that he key down solid for several minutes. Immediately, I heard a signal - but it was not at all what I expected to hear: Instead of a clean CW note, I heard a fairly broad hiss with only a slight hint of a tone. To verify that I was, in fact hearing Dave, he sent his call in CW and sure enough, the hiss keyed. Despite its spectral spread (between 1 to 2 KHz wide) it was still 6 S-units (about 10-12 dB on the FT-817) above the noise using just the 17 dBi gain horn.
At this point, Dave moved his dish around while I gave him feedback as to what provided the best signal. As it turned out, I could hear his signal over a very wide swing of the dish and the dish heading that provided strongest signal, he reported, was with the antenna pointed directly into a mountain. Another interested effect of Dave moving his dish was the change in the way the solid carrier sounded: While it never became a "pure" CW note, it could vary from a broad hiss to an airy note. Even more interesting was that on two different bearings, the predominant note heard was quite different in pitch, different by, perhaps, 700 Hz - and this seemed to be stable in pitch. Because a change in pitch implies a doppler shift - and therefore movement - I have no good explanation why this frequency change was so constant. (A 700 Hz shift implies a net velocity of about 11 miles per hour in the length of the path: Neither of us were moving.) It is likely that the rain and clouds between us had something to do with it, but the weather and our schedules did not coincide with a clear-weather path to try this again to see what the effects might be.
After optimizing the antenna pointing, Dave sent more CW in my direction and then, SSB. While badly distorted, the audio was copiable: A recording of one of Dave's transmissions may be heard here. The signal, had the path been free of multipath, would have been pretty easy copy - but even as it was, almost every word during the SSB portion is copiable (if you are used to copying signals like this, anyway...) and the CW portion is easily 100% copiable. (Note that I retuned slightly during the CW portion, and yes, I know that the SSB portion is off frequency a bit, but I chose not to retune during that part.)
From the beginning it was not expected that Dave and I would be able to complete a 2-way 10 GHz QSO: Dave's transverter consists of a fairly quiet preamplifer, a 2-watt power amplifier, and a small dish antenna. My transverter, in contrast, while it has about the same receive system sensitivity, has no output amplifier and can produce only about 30 microwatts of power directly from the mixer (about 48 dB difference) not to mention the fact that I had only my 17 dBi gain horn to use as compared to his dish, which probably provides well over 30 dB of gain. This million-to-one (literally!) power difference in ERP made it unlikely, therefore, that he would be able hear a hint of my signal. Nevertheless, I had to try something... (Why not?)
In the event that I'd been able to try some wideband FM contacts (and I might have, had the weather been more cooperative) I brought along a 2-foot prime-focus antenna. The problem was that this antenna had been configured to use only mye Gunn transceiver, mounted in its entirety at the focal point. While I have most of the parts to do so, I have yet to devise an appropriate and convenient feed to use my transverter with this (or any) dish antenna.
Nevertheless, it was possible to use most of the gain of the dish to narrow the margin: I carefully placed the dish near the deck's railing, and on the railing I placed the transverter and it's feedhorn (tethered by a rather short coaxial cable) and positioned the two such that the feedhorn was in the focus of the antenna. In this case, I exchanged the 17 dBi gain horn for a much smaller feedhorn: The use of the larger feedhorn would not fully illuminate the dish and would actually provide a lower overall gain. Even though about half the dish was blocked by the railing and transverter, this would cause, at most, a 3-4 dB loss in gain. Because my antenna pattern included the ground anyway, the increased thermal noise of the receive system was inconsequential.
With very careful maneuvering it was possible to get a fairly high
reading. I then removed my GaAsFET preamp, decreasing system gain
by about 30 dB and increasing the noise figure from 2-3 dB to somewhere
around 12-13 dB. Further placement allowed me to hear the Dave's
signal with an S-3 to S-5 meter reading. With an estimated ERP of
5-10 milliwatts, I began transmitting and Dave tried to hear the
but with no luck.
The only other thing to try was to reconfigure and use the preamp as an amplifier to increase my transmit power from about 30 microwatts to as much as 30 milliwatts (if the preamplifier itself could produce that: I have yet to measure exactly how much power the preamplifier can produce when configured this way) and (maybe) reduce the power disparity to only 20 dB or so yeilding a couple of watts of ERP. Unfortunately, the only way to change the configuration was to physically move cables and thus it was not possible to determine if the position of the horn at the dish's feedpoint was disturbed by changing the cables. Again, Dave was unable to hear my signal - and because it was raining quite heavily where he was (and it was drizzling and in the clouds where I was) we decided to wait until better weather to try further tests. As you might have guessed, we never got around to trying later...
It should be noted that if this were a line-of-sight path, it would have been workable on SSB even with my microscopic TX power or on 10 GHz WFM using Gunn transceivers with no trouble at all.
Go to the KA7OEI 10 GHz page, or
Go to the KA7OEI main page.