Date:  Tue, Jul 12, 1994 3:07 PM CDT
From:  bill.lumnitzer@paonline.com
Subj:  RIG ISOLATION
To:    KA9FOX

Received 6 or 8 suggestions regarding ways to improve rig isolation
in a multi-setup including the following:

    1. Bandpass filters between rig and amp.

    2. Coaxial stubs at the amplifier outputs.

    3. Antenna/Rig selection.

    4. Separate towers for each band.

1. BANDPASS FILTERS: These come in XMIT or RCV-only types. Xmit filters are
available from ICE (and elsewhere) or can be home-brewed -see K4VX'articles in
NCJ and Sept '88 QST, and the article in May? '94 QST; these designs include
both 3 and 5 pole versions. Rcv-only filters can be built per the NCJ article
by W3LPL and are a 5-pole design I think; these must be installed in the 
"receive antenna" line of your xcvr since they won't handle any power. Some
stations use both types but it seems like the 5 pole Xmit filters would give
equal performance to the 'LPL ones (ICE filters are 3 pole and have 10-15 dB
LESS rejection than 5 pole types); filters can be cascaded but results unknown.

2. COAXIAL STUBS: Coax stub filters on "tees" (type N recommended) directly at
the amplifier output on each band; sometimes these work better 1/8 wave down
the line or a few feet after the power meter. Since my antennas are remotely
switched at the tower I'm thinking of trying stubs on each line BEFORE the
relay so they can be left PERMANENTLY mounted (isn't rf leakage from good coax
at HF pretty low?). You can use either 1/4 OPEN or 1/2 SHORTED stubs (I recall
seeing an old NCJ article on this by K2TR). Separate sets of stubs tuned for
CW and SSB should be used for best results, giving 25 dB attenuation or so.
Some stations use DOUBLE stubs at the second harmonic to increase the
attenuation while others recommend one stub at both HALF and TWICE frequency
(e.g. stubs for 40/15 and 10 on the 20m station). I tried coax stubs before I
got the ICE filters and they DID help but haven't tried them yet in combination
with the filters.

Another interesting suggestion was to measure the receive power on one band
while transmitting on another. Supposedly TENS or even HUNDREDS of watts
have been found in some situations! N4KG installs protective diode-resistor
networks at the receiver inputs to guard against burnout. I plan to use my
Bird to benchmark my station and then to track improvement? after each step. 

3.  ANTENNA/RIG SELECTION: Different combinations of antennas and rigs can have
a big effect on interference. Using multiple tribanders versus monobanders 
opens the door to more severe interference and should be avoided if possible. 
I use both tribanders and monobanders but plan to confine them to two separate
towers. I'm also thinking of trying some remotely located receiving antennas
(e.g. trap vert. and inv. vee or small tribander).

Some rigs "talk" to each other more than others (e.g. two TS-940s are
purportedly worse than two TS-930s) and combinations of "synthesized" vs.
"non-synthesized" (TS-830) can be tried (or try a Drake R4C for receive).

4.  IF ALL ELSE FAILS: Several reported of having operated up to six stations
pretty much interference-free (except for 2nd harmonic) using the above
techniques with a single tower or rooftop. But this degree of success has not
been universal. In these situations Separate Towers (widely spaced) for each
band were the way to go. I have the space but not the energy to put up and
maintain all this hardware (two towers are enough!). Nothing dramatically new
came out of this except to prove that it CAN be done and there's still room
for a lot of experimentation. Now where is that 50W slug for the Bird....
 
73' de Bill N6CQ/3      Lewisberry, PA
(n6cq@paonline.com)