[Elecraft] Receiver Damage from another Close Station?

[Jim Brown]

On 3/2/2011 5:38 PM, w0ih wrote:
> My next door neighbor is a ham and recently upgraded to a Icom 7700, 200
> watt.

This is the sort of operating conditions that we typically set up for 
California QSO Party county expeditions, and that also occur at some 
contesting stations (like PJ2T, PJ4A, N6RO, etc.). The CQP group that I 
go with uses K3s driving 500w amps, wire dipoles, and tri-banders on 
tower trailers roughly 130 ft apart. Stations like this will typically 
use very good bandpass filters between the rig and the power amp 
(usually W3NQN filters) and also stubs following the power amp. Antennas 
are usually carefully arranged to minimize the coupling between them, 
and most contests, stations are on different bands. In CQP though, we 
often had a CW and SSB station on the same band. Filters don't help with 
that -- it takes separation of antennas.

At CQP, we had one incident of actual damage to a K3. It happened 
because both CW and SSB were on 40M at the same time, using dipoles that 
were end to end with only a few feet between them. That's dumb, but we 
did it. :)  We had no problems with both stations being on 15M or on 
20M, and the year after blowing up the front end with the 40M dipoles 
end to end, we moved the dipoles much further apart.

Also, at CQP, our tri-banders were carefully arrayed so that they were 
essentially parallel to each other when pointed where we intended to 
operate them.  That greatly reduces the coupling between them as 
compared to pointing them at each other -- I'd guess a difference of 
roughly 30-50dB (30-300 times less voltage by having each in the null of 
the other's pattern, as opposed to 6dB of gain from each).

One of the best resources on this is W2VJN's book on Managing  
Interstation Interference. Buy it from Inrad (the folks who sell the 
crystal filters) for about $25. George talks about how to calculate and 
how to measure the coupling between antennas, how much RF voltage it 
takes to cause damage, how to reduce the coupling with filters and 
stubs. Very solid engineering, very practical, written at a very 
understandable level. George is a contributor to the ARRL Handbook.

73, Jim K9YC