[VHFcontesting] proximity of my various V/U antennas

[MICHAEL SAPP]

>Proximity of my various V/U antennas

Hi all: This became an issue at w3kwh a year or so ago after having repeated failures of the front end HEMT transistor in a TS-2000X on 23cm. In our case, the culprit was the local ATC radar at 1310/1330 Mhz, which was about 1.5 miles away. With the 23cm loop yagi pointed at the radar site, a whopping 10mw of radar signal was present on shack end of the coax cable as measured with an HP141T/8554/8552. (We also have a cell phone tower on the property, that's another issue).

At the time, Dave, K1WHS had suggested to me that even -10dbm was danger zone levels for a HEMT transistor, so we were 100 times over that. To make matters worse, the club site in an an approach flight path and random reflections from aircraft would cause 10mw spikes even if the loop yagi was not pointed at the radar site. This was discovered by watching the spectrum analyzer display on 1310/1330 MHz for a few hours one evening.

The solution was an interdigital filter followed by a "pipe cap wa5vjb" type filter I made by cutting down an older F195/U twin cavity filter (cavity height 2.0 inches, probe length 1.375 inches. The combined filter bandpass lowers the 1310/1330 response to around -20 dbm while having around 1 to 1.5 db throughput loss. No further HEMP losses have been encountered with the TS-2000X using this approach.

As an observation, I ran an LT23S ssb electronics transverter and and outboard preamps of the MGF-1302 and 1402 variety in years prior at w3kwh and never had a 23cm preamp device failure from the nearby radar's 10mw poundings.  I did lose an ATF-10135 on 2.3 ghz from the cell tower overload once at w3kwh some 15 years ago.

A preferred solution for a more typical home or rover station is an appropriate harmonic trap filter on the amplifier outputs versus having to eat a 1db or more NF deterioration with a reciever bandpass filter. For example, run separate radios and listen to your second or third harmonic with the preamp off. If the received harmonic pins your s-meter does the radio's 10db attenuator bring the signal down off the peg? If not, how many 10,6, or 3 db pads does it take to get the S meter below S9+60 or whatever the top of the scale is?? If you assume a pegged S-meter as being close to the danger zone then the added attenuator pads will give you a clue as to how much harmonic notch filtering you need as a minimum for rx front end safety.

At w3kwh I decided to put double coax stub 1/4wave harmonic filters (and 1/4 wave spacng) on all the V/U amplifiers just for safety sake. As an example of the results the second harmonic from our AM-6155 amp on 222.1 was pinning the S meter on the TS-2000X on 444.2. After installing the double stub filter on the output of the AM-6155 amplifier the 444.2 signal was down to S5. The two antennas involved were on the same mast about 4 feet apart and in the clear at 55ft up. Throughput loss was negligible on 222, perhaps a slight tuning change was noticed. You might have to try different jumper cables on the output of a solid state amp to a notch filter  to find a cable length that keeps the amplifier happy (i.e. Mirage Power Oscillators, sri I digressed :)).

What I found was easist to fabricate for the double stub fllters was to use a pair of N-tee female adaptors linked together with an electrical quarter wave of flexible coax of the desired harmonic frequecy (get as reasonably close as you can, use an antenna analyzer to estimate desired resonance). For the quarter wave stubs I used copper water pipe and brass hobby tubing to make rigid 50 ohm lines with N connectors. A little epoxy will hold the center pin on place inside the N-connector shell. A (home fabricated) teflon standoff washer aligns the center conductor near the open end---these rigid lines are easier to cut precisely with a bandsaw or miter box---rigid 50 ohm cable may work just as well.  One thing I found with the copper pipe coax stubs is that I could put a pipe cap on the end of the stub and tune it with a screw---which is faster than several cut and measure iterations to get a deep harmonic notch effect for the double stub pair. You can (must) also tune each stub individually and then re-tweak for the best notch performance with the assembled pair. I've consistently achieved 70 db nulls with this approach.

With two 10W erp beacons running at home continously on 2M and 432MHz, I've measured a few -10 and 0 dbm in-band reflection points on the corresponding chimney yagis with the omni loop stacks about 15 feet away--most likely tree clutter reflections.  I imagine that a wood roof and shingles in near proximity to attic-installed antennas would have some similar reflection effects.

 At this point I should thank Dave, K1WHS once again for his insight and give a plug to W1GHZ for his low-power RF meter, as it costs less than the two-way shipping expense for a radio front-end repair these days.  73 Mike wa3tts