[CQ-Contest] Coax Stubs for SO2R

[Leigh S. Jones, KR6X]

Having extensive experience operating from multioperator stations on the West Coast has given me some insight into interference issues.  I can confirm that quite often the source of trouble is not in the front end nor finals of the rigs.  It can indeed be elsewhere.  For instance, at W6HX we had a big problem on 1810 kHz.  The pull-down cable hardware on a large motorized crank-up tower mixed dissimilar metals.  In the wind it was intermittent but it was in effect a 70 foot wire with a diode at the bottom where it was grounded.  One AM broadcast station three miles distant at 1260 kHz was doubled and another at six miles distance on 710 kHz was subtracted, resulting in a meter-pegging signal modulated by both.  If you have anything in your yard that acts like a diode with wires connected there is a potential for harmonics to be generated where filters cannot possibly be of help.


Leigh S. Jones, KR6X

> On Sep 7, 2015, at 22:13, Dave Hachadorian <k6ll.dave at gmail.com> wrote:
> 
> Mike,
> 
> I guess the problem that you are seeing is that when radio A transmits on 40, you can hear interference on 20 and 15, around the harmonic frequencies?
> 
> I suspect that radio B is being overloaded by the 40 meter fundamental energy, and is generating the harmonics and possibly wideband interference internally, in the receiver.
> 
> In that case, you need stubs on the radio B antenna to pass 20 and reject 40, and to pass 15 and reject 40.  To test this hypothesis, take one of the 23' shorted stubs that you already made for 40,  convert it to a 23' open stub, and connect it to radio B on 20 meters.  See if the 20 meter interference is lessened when you transmit on 40.  A 23' open stub rejects 40 and 15 and passes 20 and 10.
> 
> Are you using bandpass filters?  A stub by itself may not provide sufficient isolation, especially since both of your antennas cover all bands.
> 
> Dave Hachadorian, K6LL
> Big Bear Lake, CA
> 
> 
> -----Original Message----- From: Mike Smith VE9AA
> Sent: Monday, September 7, 2015 6:07 PM
> To: cq-contest at contesting.com
> Subject: [CQ-Contest] Coax Stubs for SO2R
> 
> This is about coax stubs.
> 
> 
> 
> Sent this earlier to "Towertalk" then I thought perhaps it was better suited
> to contesting.
> 
> 
> 
> Reposting, with some edits:
> 
> 
> 
> I've seen the K2TR stubs, K1TTT page and have recently bought the FB book by
> W2VJN
> 
> Today, for a change, I had a little spare time so built 2 stubs, but lets
> just concentrate on one.
> 
> It was supposed to be for the 40m transmitter (Radio A).
> 
> I will add  Radio A (transmitter) is using a multiband vertical.
> 
> Radio B (receiver) is attached to a multiband horizontal dipole (ZS6BKW) in
> case it makes any difference in my question.
> 
> I see a signal of roughly S9 or S9+ 20/40 on the receiver (varies per band &
> harmonic)
> 
> 
> 
> I roughed out the length, attached a T and a 2nd antenna switch (a la VA2UP
> method) (which I'll only use for stubs), trimmed it with my AA-230pro
> 
> and when it's inline (parallel to my primary antenna switch) I see no
> difference at all on 20m or 15m on my 2nd receiver when the stub is in or
> out of line
> 
> NONE.  Like, not even a titch ESP hopeful. (hi)
> 
> 
> 
> I presume this is a type 1 stub (shorted 1/4WL type, x velocity factor)
> 
> 
> 
> Thinking I had done something wonky, I made another stub from different
> coax..same result.
> 
> Later I made one up for 20m - same result. No change whatsoever in the
> received signal harmonic(s) on the receiver.
> 
> 
> 
> Anyone been down this road?
> 
> 
> 
> Too close to rig?
> 
> 
> 
> Interaction from my T and 2nd antenna switch?  Gremlins? Wrong method using
> the AA-230 pro maybe?  I am close to that 23' mark on the 40m one.
> 
> 
> 
> de Mike (SO2R hopeful) VE9AA
> 
> 
> 
> 
> 
> Mike, Coreen & Corey
> 
> Keswick Ridge, NB
> 
> 
> 
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