Hi all,
We had an interesting interference problem crop up this last weekend at NA6Q,
and I'd be interested to hear ideas from anyone else on a possible source.
Here's the background:
For WPXCW at the last minute (about 2 weeks notice) we decided to put
together a multi-2 setup (which turned into a 3 station M/M). The station was
provided by W6EMC and has never before been used for an M2 or MM operation. The
three
radios were: FTdx9000D used for 160, 80 and 20 driving an Alpha 87A, an Icom
756Pro2 driving a Henry 3KA used for 10 and an Icom 7800 driving an Icom PW1
used for 40 and 15. Antennas were an inverted vee for 160 (center at 70 feet
just below the 20 antenna), an inverted vee for 80 (center at 70 feet just
below
the 15 antenna), a roof mounted full size 40M 1/4 wave vertical, a 6 el
Telrex 20M monobander, an 8 el Telrex 15M monobander, and a 10 el Telrex
monobander. Each of the monobanders is on a separate tower, all about 70
footers. There
are no filters or stubs on any of the transmitters
Interference between stations was almost nil ... due to some superb
installation and engineering by the station owner for all the stations and
antennas ...
except for one particular pair, 10M and 15M. When the 15M station was
transmitting at roughly 1KW, anywhere between .000 and .075, and the 10M
station was
listening anywhere between 28.0 and 28.1, there was a strong signal from the
15M station. Now here's the strange part ... the interference sounded like a
pure CW note, EXCEPT that changing frequency on the 10M radio, or for that
matter on the 15M radio, made no difference to the note or apparent signal
strength. In other words, it sounded just like a sidetone signal. We tried
changing
the relative pointing angles of the two antennas, and as one would expect, the
closer they were to pointing at each other, the stronger the interfering signal
was. BTW, the stations are physcally about 150 yards apart in different parts
of the same building and the antennas are about 200 yards apart.
So as an experiment, we temporarily replaced the 756Pro2 with a Kenwood
TS2000, and the interference went away, or at least reduced itself to just the
kind
of phase noise that one would expect in such a situation, and could be
eliminated with the appropriate use of stubs or filters.
Discussions between us came up with a couple of different ideas as to causes,
but given that this was in the middle of the contest on Sunday AM when 10 was
open, we didn't take the time to conduct real experiments. One thought was
that since the 1st IF of the 756Pro3 (and we assume also the 756Pro2) is about
64 MHz and the 3rd harmonic of the 15M signal from the IC7800 was about 63 to
63.15 MHz, there might be overload of the IF in the 756Pro2. Another idea was
that if the "monitor" circuit in the 756Pro2 was on it might have been being
overloaded by the 15M signal.
So here are the questions: Has anyone had a similar experience using the
756Pro2? Does anyone else have any thoughts as to the causes of such a
situation.
Please note again, that in general, we had essentially no interstation
interference between the other stations, and consistently, without any special
filtering or stubs, there was no problem in operating simultaneously on other
band
pairs.
Any thoughts or ideas will be appreciated ...
73 de Bill, W1HIJ/6, aka FOØSCH (2000, 2001, 2002), FO8DX (WPXCW 2001)
Pina Colada Contest Club (KP2AA)
NA6Q Team, WPXCW05
Field Day Teams: NP4A (02), COØUS (03), KP2AA (04), KP2AA (05)
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