The impedance of the coax used to make the stub won't affect the results as
long as the stub is cut to the correct length for the velocity factor --
i.e., the correct electrical length. Since the velocity factor can vary from
batch to batch, or even within the same spool of coax, you can't just
measure to the theoretical length and cut. You need to use a noise bridge,
SWR analyzer, or TDR (Time Domain Reflectometer) to measure the frequency
response of the stub. Stub response is relatively narrow, so I don't think
you will get very good results cutting stubs for a compromise between the CW
and SSB sub bands.
My stubs affect the SWR slightly on some bands. Sometimes it raises the SWR,
and sometimes it lowers the SWR!
You're right that the best medicine is separation between antennas.
73, Dick WC1M
> -----Original Message-----
> From: Robert Shohet [mailto:firstname.lastname@example.org]
> Sent: Monday, August 04, 2003 12:23 PM
> To: email@example.com
> Cc: firstname.lastname@example.org
> Subject: Re: [TowerTalk] RE: [CQ-Contest] Testing Stubs
> Yeah, but what happens when the stubs get "testy"? :-)
> I made mine out of Fused Disc 75 ohm hardline cut for a
> compromise of SSB/CW and I can tell you that even though I
> measured and cut them VERY carefully, they do not work very
> well. They do however raise the swr and reflected power on a
> few bands and make their use on 40 meters (with a 40-2cd) out
> of the question.
> While it is possible that cutting them for a compromise freq.
> was not a good idea, I just don't think that the stubs work
> very well at all. I had wondered if this had something to do
> with my using 50 ohm coax (RG213) with stubs cut from 75 ohm
> Fused Disc hardline. Although, when cut to the appropriate
> electrical lengths, I don't think that this should not really matter.
> The manual Dunestar 500's work pretty well and help reduce
> the crud, but the absolute BEST way to reduce interstation
> interference is to separate your towers as much as humanly possible!
> Bob KQ2M
> > Mike,
> > I agree that double-shielded coax might help (in fact, I should do
> > that in my shack!) I've found that stubs and filters don't
> always make
> > a big difference in the S-meter reading at the harmonic
> > but can nevertheless be very effective. What I mean is, I
> don't think
> > anything
> > reduce a 2nd harmonic to the point where you can receive within 1-2
> > Khz of it. However, effective stubs or filters can make it
> possible to
> > receive
> > *more* than 1-2 KHz away.
> > The first thing I look for is interference away from the harmonic
> > up and down the band. For example, let's say you are
> transmitting on
> > 7010 and listening on 14020. The signal strength might be
> S9+20 at the
> > 2nd harmonic, but when you tune more than 1-2 KHz away from it, the
> > noise
> > go away and there should be no S-meter reading. That's good. What's
> > bad is when you can hear that 2nd harmonic signal all over the 20m
> > band!
> > on the radios and strength of the signal, it can be modulated tones,
> > static, hash, rushing, etc. I believe phase noise plays a
> part in this
> > equation as well. Broadband interference makes it very hard to
> > operate, especially if it moves the S-meter. I've found
> that stubs and
> > filters can
> > quite effective at getting rid of that kind of
> interference. But even
> > when they do, you'll hear the 2nd harmonic loud and clear
> within 1-2
> > KHz.
> > I use a combination of stubs and filters. My stub box is
> setup pretty
> > much like the switched stub box described on K1TTT's site and comes
> > after the amps. I also use switchabale bandpass filters between the
> > rigs and amps (that's where I need to install double
> shielded coax.) I
> > have the stubs
> > filters configured so I can switch them off individually
> for testing
> > (TX stubs, RX stubs, TX filters, RX filter.) Sometimes the
> stubs work
> > and sometimes the filters work. Sometimes nothing works. I
> don't think
> > I have
> > case where both a stub and filter are required.
> > As it turns out, I have a nice pair of Dunestar 600
> bandpass filters
> > for sale... :-)
> > 73, Dick WC1M