[Elecraft] Nearby Ham Interference

[Jim Brown]

On Tue,1/19/2016 5:17 AM, k8kzb at charter.net wrote:
> Our tx antennas are roughly 300 feet apart, he has a vertical right now and i have both a vertical and a beam to switch between. He plans on a beam this summer and it will be the same distance apart.
>
>
> Thoughts...

For operation so close together, you really must view this as a multi-op 
station. :) Rules -- very practical ones.

1) Both stations must use rigs with excellent strong signal performance.

2) Both stations must use rigs with very clean TX performance, and 
operate them from supplies of at least 13.8V under load.

3) If using power amps, both stations must tune their amp very carefully.

4) Yes, antenna directivity matters.

At county expeditions for the California QSO Party, we set up three 
stations -- two CW, one SSB, with antennas that are as widely separated 
as possible. Each station is a K3 with new synth board, a KPA500, and a 
KAT500. With 2-el Yagis spaced about 250 ft apart, we can operate CW and 
SSB on the same band with no QRM. The Yagis are carefully located and 
aimed so they are colinear when pointed 70 degrees azimuth (what works 
in W6 for US contests). Our wire antennas for 80 and 40 are about 300 ft 
apart, and we can operate CW and SSB on those bands too.

5) Bandpass filtering helps when you're NOT on the same band. We use 
TXBPF filter sets between the K3s and the KPA500s, and we have 
double-stub notch filters on the 40M and 80M CW feedlines to kill the 
second harmonic of the power amp.

6) Cross polarization can help.

7) Both stations need very good coax with well installed connectors. 
Coax shielding effectiveness depends on a shield with very low 
resistance and very good uniformity. THAT'S why there's no RG8X or RG58 
in my station -- shield resistance is directly related to skin effect, 
and a larger diameter shield has less resistance than a small one.

8) Both stations need very good common mode chokes at the feedpoint of 
their antennas. This prevents radiation and reception by the feedlines.

9) Every little thing matters -- little things add up to a lot. I wasn't 
satisfied with the effectiveness of a 2-stub filter on my 40M antenna -- 
there was still too much second harmonic in my RX on 20M. Last fall, I 
bought some very good coax (Buryflex) and some Amphenol 83-1SP 
connectors and replaced every piece of coax inside my station. I have a 
lot of antenna switching, so that was nearly 100 ft of coax and 50 
connectors. The second harmonic dropped by about 10 dB!

Observations -- the only problem we've ever had with RX burnout was the 
first year we were there -- we strung two dipoles end to end with only a 
few feet between the ends and connected them to different K3s.

73, Jim K9YC