[TowerTalk] Re: Making and spacing stubs - questions

[Dick Green]

> Have you tested your system to see if you need all those stubs?
> All the stub does is prevent overload by harmonic energy. Most
> amplifiers are over 50 dB down. That's only about a dozen
> milliwatts or so of harmonic energy!

> I would expect your receivers to be the real problem.

I think Tom is right about the receivers being the real problem. Running
SO2R, I've found that most of the interference is caused by fundamental
overload.

For example, back when I had a TS950SDX and an FT-990, I found that when I
transmitted on 80M from the 950, the 990 would get wiped out on 40M, 20M,
15M and 10M. In addition to the usual harmonic interference at exact
frequency multiples on 40M, 20M, and 10M, there was also very strong (S9+)
hash across the entire width of all four bands (even 15M.) I've had similar
experiences working at a multi-multi where the 15M and 20M stations
regularly wiped each other out. Ditto on Field Day operations with stations
on bands that are not harmonically related.

This doesn't mean that stubs can't help. By adding a 1/4-wave shorted 80M
stub after the 950 and a 1/4-wave open 80M stub after the 990, the
interference pretty much disappeared (kinda neat -- the length of those
stubs just happens to work out to 1/2-wave multiples on 40M, 20M, 15M and
10M!) But I found that the shorted stub on the 950 didn't affect the
interference *to* the 990 (although it helped attenuate interference *from*
the 990.) It was very clear that the stub on the 990 was attenuating
virtually all of the 80M interference. In other words, filtering at the
receiver was most effective, which would indicate that it's not a matter of
attenuating harmonic energy from the transmitter. I think the bandpass
filters in the 950 weren't doing a very good job. In contrast, when I
swapped the 950 for a 1000mp, most of the 80M interference went away. As you
might expect, there's still a tone at the harmonics, but signal strength
drops off rapidly -- usually I can work comfortably within 1 KHz or so on
either side. There are still some band combinations where stubs help, but
fewer than with the 950/990 combination.

My SO2R setup automatically switches in various parallel stub combinations
for each band. For example, on 20M I switch in the 1/4-wave open 80M stub, a
1/4-wave open 40M stub, and a 1/4-wave shorted 20M stub. This combination
passes 20M while  attenuating 80M, 40M, 15M and 10M. This does not provide
the deepest attenuation, but it is quite adequate. The stubs are cut for the
CW portions of the band, where they provide the most attenuation. They work
reasonably well on SSB, but are definitely less effective there (i.e., the
bandwidth is pretty narrow.) If I was a fanatic about it, I'd cut a second
set of stubs for SSB contests. So far, I haven't found that to be necessary
(then again, I'm not a big fan of phone contests...)

Tom is right that LC filters would be more effective -- the nulls would
probably be deeper and the bandwidth would be wider. But I prefer the stubs
because they can tolerate high power, mismatches and switching accidents
better than outboard LC filters between the transceiver and amp (one mistake
with those babies and it's off to the repair shop!) In theory, stubs also
help attenuate harmonic energy from the amp, but that's probably not a big
factor with most well-designed amps.

I should point out, however, that the single most effective method for
reducing interference is antenna separation. The farther you get the
antennas apart, the less interference problems you will have. For example,
my tribander that is about 20 feet from the inverted vee picks up 80M
interference, while the tribander that is 200 feet from the vee can't hear
it at all. It's also very helpful if the antennas don't point at each other.
We eliminated virtually all of our Filed Day interference by moving the
beams far apart and lining them up so that the elements were parallel
("tip-to-tip".) Interference came back if we turned one of the beams just
20-30 degrees from the others.

I have explained to my wife that antenna separation is why I need four
separate towers for 40M, 20M, 15M and 10M stacks, but for some reason she
just isn't buying it...

73, Dick WC1M


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