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Interference between stations at Field Day
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by W7BUN on November 3, 2000
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Our local radio club has a problem every Field Day. The
20 meter phone and CW stations wipe each other out. We
use the maximum separation allowed by the 1000' diameter specified in the FD rules. The rigs this year
were a KW TS-440S on CW and a KW TS-530S on phone. Is it the specific radios, or is there a simple solution to this? A cavity to isolate the radios would be as large as an automobile and definitely not practical! HELP!!
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RE: Interference between stations at Field Day
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by ve3iay on November 3, 2000
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One thing you could try is to make use of the antenna patterns to put each antenna in a null of the other's pattern. Another is to cross-polarize them (vertical vs. horizontal). For example, you might try using a vertical antenna for one radio, placing it in a null of the horizontal antenna's pattern and putting them as far apart as you can manage.
At VA3RAC we had four 40m dipoles (CW, phone, digital and novice) oriented end-on along a single line, with a couple of 15m beams on towers in between pairs of the 40m dipoles. On 80m we had two dipoles in a line plus a vertical. We oriented all of our beams on each band so they would not be pointing at or near one another. You get the idea. Our only problem of the kind you describe was some minor pickup of the 10m CW station in the 10m digital receiver (see next paragraph for my explanation of this). None of our other band captains reported this kind of problem, although we did have an unusual case of receiver-to-receiver interference caused (I believe) by identical IF frequencies in receivers on 20m and 15m digital using a "shared" antenna (actually separate two-element monobanders on a common boom). By the way, the credit for our site layout planning should go to Brice Wightman, VE3EDR, whose careful planning made our 35A operation possible.
I believe it also helps to choose receivers with good immunity to strong signals, i.e. high dynamic range. I will quote some blocking dynamic ranges from QST reviews, but IMD DR may be just as important. Anyway, according to the QST reviews quoted in my 1993 "ARRL Radio Buyer's Sourcebook", the blocking DR of the TS-440S on 20m is a relatively low 111 dB, and that of the TS-530S is 120 dB. Rightly or wrongly, I attribute the pickup of the 10m CW transmitter in the 10m digital receiver at VA3RAC to the receiver on 10m digital, which was my old TS-180S. ARRL measured the TS-180S's 20m blocking DR at 114 dB. To show what is possible, the best three 20m blocking DRs measured in the list quoted above were the Yaesu FT-1000D at 154 dB, the Kenwood TS-850S at 148 dB, and the ICOM IC-765 at 146 dB. Some newer radios may or may not do equally well or better; I don't have recent data.
As further anecdotal evidence for the effect of dynamic range on receiver performance in strong signal environments, I can tell you that with the TS-180S I used to be almost unable to operate during contests when VE3HX, about 1500 feet from my QTH, was on the same band. With my TS-850S I have almost no such problems. Alan and I can coexist on the same band as long as we stay a few kHz apart. 34 dB of additional blocking dynamic range makes a big difference to my enjoyment of ham radio. Maybe someone else has evidence to confirm or refute my belief in the importance of dynamic range.
73, Rich
VE3IAY
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RE: Interference between stations at Field Day
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by kr6x on February 13, 2001
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During the decade 1970-1980, the West Valley Amateur Radio Club, WA6LXN, of which I was a member, operated numerous Field Day contests in the 4A class, and we were highly successful. That's enough simultaneous transmitters to force frequent dual mode band occupancy -- 1 phone and 1 CW rig on each band in use -- in fact, nearly continuous interference throughout the entire period of the contest. We did find ways to allow some additional movement -- eventually daytime 40 M SSB activity in California was fruitful enough to allow us to move one of the 15 or 20 meter rigs out of the high interference zone (10 meters was rarely a factor for that sunspot cycle).
However, this was not the source of our success. The following techniques were put to use, although occasionally we forgot an element or two that was required to obtain the necessary intra-band interference rejection:
1) Separate generators -- we used two generators to allow us to power stations with a high potential for intra-band interference off of completely different power sources. The two generators were placed in a central location, about 30 feet from each other. In a pinch we were able to power the entire 4 transmitter station from a single generator by running an extension cord from one generator to the other and interconnecting. And the pinch came every time one of the generators had to be turned off for refueling and lubrication, in addition to every time that one of the generators quit unexpectedly.
2) Antennas off-end to each other -- when there was enough flexibility in the station layout, the antennas would be located with all of the elements in line with each other. We never bothered with rotators, as the majority of our contacts were all in the same beam heading -- about 70 degrees from Southern California.
3) R4B Receivers -- the Drake R4B/T4XB was the rig of choice for a WA6LXN/6 Field Day. After comparing the Drake B-line with the Heathkit SB line, the KWM2A, the Swan 350, and the Drake TR4 rigs, we concluded that the multi-section filters in the Drake R4B (and R4A) provided a unique performance advantage. This front end used about 6 tuned circuits with permeability tuned coils in the RF gain stages and more tuned circuits peaking the local oscillator -- and all 6 tuned circuits were simultaneously adjusted with one knob. They provided highly selective (~25 KHz bandwidth) front end filters peaking right on the frequency of operation, and rejecting signals from the other rig on the same band to a significant degree. The Drake rigs also maintained stability when the power line voltages dipped a little bit, which was not at all rare. The Heathkit SB line, the KWM2A, and the Swan 350 all showed problems of one sort or another under the influence of power flutuations.
4) Sola constant voltage transformers and brute force power line filters -- Several club members found 750 VA Sola constant voltage transformers on sale at a local electronic surplus house. They masterfully held the power constant, protecting the rigs that were connected to them against power fluctuations through several of our most successful Field Day efforts. In addition, they provided a couple of additional dB's of rejection of RF on the power lines. Several of the stations were also powered through RF chokes identical to the ones that you would find on the filaments of grounded grid power amplifiers. The most critical place for this choke was on the power cable that ran between the two generators.
5) Separate station grounds -- the first club Field Day, someone tried grounding all of the rigs together (daisy chain style) with a long piece of #14 stranded copper wire. It made the interference worse, so we left the wires connected to the rigs, but cut each of them at the midpoint and separated the ends. We were grounding the rigs with 2 random length radials, and it seemed to work OK under some soil conditions.
A more modern approach would probably depend on highly selective receiver front-end filters in lieu of the built-in front-end filters of the Drake Lines. Not that filtering was unheard of in those days.
I also built tunable rejection filters for the rig that I was going to operate on the night before Field Day one year. It did prove useful.
We were forced, on occasion, to endure objectionable levels of intra band interference -- usually 40 meter SSB blocked 40 meter CW reception to a degree when they shared the same time slot. We were often running a beam of some sort on each mode on 40 meters, and that did help with the rejection. Often times the interference would not be present early in the contest, then inexplicably it would appear and continue to trouble us until the end of the contest. There was also some direct radiation and pick-up of the IF signals providing some low level interference.
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