[RFI] Inter-station interference question.
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
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.
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