[RFI] Inter-station interference question.

Martin Sole hs0zed at gmail.com
Wed Nov 20 23:22:01 EST 2019


This is about a two radio inter-station interference problem. A bit 
different perhaps but still RFI of a sort
Looking for some input to help me understand a bit better what is 
happening and maybe correct some errors in my thinking.

I will describe the setup:

Two radios are FT2000, two amplifiers are OM2500, output filters are 
4O3A high power band-pass filters followed by dual, suitably spaced, 
stubs. Antenna is an Optibeam, 2 element short 80, 3 element short 40, 
and 5 element full size 20 interlaced on a common boom. All feed points 
have large #31 chokes installed at the feed point. Antenna is at 
approximately 50m and feeder length to the station is approximately 49 
metres of heliax type half inch followed by 36m of RG214, 85m in total.

The 403A 80m filter swept with a VNWA3E shows -55dB at the second 
harmonic (40m). The dual stubs show approx -55db at 40m, essentially no 
attenuation in the band at 80m of course. Total attenuation around 90dB 
as far as can be figured at the 7MHz second harmonic

With no filters or stubs fitted and the antennas connected to the radios 
directly a 5 watt signal on 80m shows a harmonic measuring about S8 on 
the 40m radio representing about 116dB being the combination of the 
harmonic attenuation in the radio/amplifier and the isolation between 
the antennas.

Inserting the 4O3A filter in the 80m feed line after the radio shows the 
harmonic is reduced from the previous S8 to around S4 (circa 20dB?). 
Addition of the stubs after the filter shows the harmonic to be 
unchanged from the S4 with the 4O3A filter alone. Testing in isolation 
with a Rig expert AA230 to an SDR running on a battery laptop showed an 
attenuation of the second harmonic of approximately 17dB this seems very 
much in line with that seen on the radios.

The 2 element Optibeam 80m antenna when swept at the second harmonic of 
40m and at the end of the feed line shows 22-j65. The output of the 
OM2500 with the tube replaced with a 2k2 load resistor and tuned to 80m 
shows approximately 26+j413. Sim-Smith simulates the stub performance 
identically as measured with the VNWA3E. Changing the input and output 
impedance to match the values measured as above shows even more 
attenuation at the second harmonic (40m).

Irrespective of identifying an ideal location for insertion of the 
stubs, fully appreciated from reading the various W2VJN and K9YC 
articles on stub location, I would expect to see even in the worst case 
a marked reduction in the harmonic signal level over and above that we 
are experiencing. Having measured the stubs and shown them to work as 
predicted and having tried a couple of locations for the stub a metre or 
two either way with no discernible change coupled with the poor apparent 
filter attenuation suggests something else is the prime culprit.

Thoughts and suggestions very much appreciated.

Martin, HS0ZED

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