[CQ-Contest] Re: 2-Radio Interference

Marty Tippin martyt at pobox.com
Thu Dec 21 18:42:02 EST 2000

(Let's try this again. Was using a new e-mail program and it really munched 
on my carefully crafted words...

I'm trying to get my station set up for SO2R low power RTTY operation 
(Hopefully in time for RTTY Roundup) and have some questions - have been 
through the CQ-Contest archives and found a lot of useful information but 
still need more...

On most bands, I have S-9 or higher broadband hash on the second radio when 
transmitting on the first (there are a few band combinations where there's 
almost no noise at all, and a couple where there is full-on rectification 
of the transmitted signal). Running a FT-920 and FT-100, with a tribander 
on the roof and a vertical in the back yard, probably 25 ft from the 
tribander (not much of a contest station, I know - but it's what I have...)

I presently have no filtering in place at all and am considering either 
coax stubs or a pair of Dunestar filters. (Yes I've been taking a chance on 
blowing out the front end of one or both radios during my testing...)

The question I have is, how can I tell whether I'm getting fundamental 
overload vs. harmonics on the receiving rig?

If I understand things correctly, a bandpass filter won't really do much 
for the fundamental overload problem but will alleviate the harmonics 
(correct me if I'm wrong). And I don't want to go drop $500 on a pair of 
Dunestar filters if they're not going to solve my problem! I gather there 
are others with more closely spaced antennas, running much higher power, 
who have solved the interference problems (albeit with high-end rigs like 
the 'MP) - so this should be something I can do. Just a question of whether 
I can do it with the rigs I have.

Also, from W6XX's page I see the out-of-band rejection of the Dunestar 
filters is on the order of 40 to 50 dB for most band combinations - if I'm 
getting S-9 interference noise, a 40dB drop really isn't going to help 
much, is it?

I don't have much in the way of test equipment - MFJ-259B and a DVM (and 
lots of duct tape!). So properly cutting a bunch of coax stubs is going to 
be challenging to say the least. I'd prefer to build something myself over 
buying it, but lack of tools makes that a sub-optimal idea.

Any advice, suggstions, web links, etc. would be appreciated. Have seen the 
K1TTT references on stubs and K6XX's analysis of the Dunestar filters.

At 01:54 PM 12/21/00 -0600, Marty Tippin wrote:
>(a lot of gibberish...)

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