A while ago I was looking for info on making/using coax stubs for
filtering between two stations. Thanks to W1PH, VS6WO, KY1H,
K3NA, K1VR, W0CP, K3LR, KR0Y, WA6OTU, WM2C, ON6TT, N4OGW,
(and maybe a few others who got lost while cutting and pasting
this together) for the hints and comments. Several cited the K2TR
article in the May - June 1984 NCJ. Following is a collection of
the various comments received (in no particular order) beginning
with the original post.
Mike N0BSH
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I'm looking for information on using coax stubs for filtering
between the two stations. Some questions that come to mind
include:
1) Lengths required for nulling various bands - open/shorted?
2) Can stuff like RG58 be used or should we stick with RG8 (will
be running about 1KW on each station?)
3) Do they need to be made with 50 ohm coax or can 75 ohm be
used (have some of both laying around?)
4) Can we put these on a coax switch and switch in the ones we
need depending on band?
5) Is it worth all this or should we just get out the MC/Visa and
buy the Dunestar filters?
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1. Lengths required: This depends on the velocity factor of the
coax being used to make the stub. In addition, you will have to
have an appropriate serial coax impedance transformer between
the point where the stub and antenna feed attach and the radio.
Otherwise the radio will see a wierd impedance and potentially
high SWR. The lengths of the serial matching coax sections will
be different for each combination of antenna and band-stub. (If
you are getting the impression that this is complicated, you are
correct!) Either shorted or open-ended stubs can be used.
However, shorted stubs are easier to build properly. Open stubs
tend to leak at high powers (remember, that will be a very high
voltage point at the end of the open stub!)
2. The quality of the stub as a filter depends on the loss of the
coax. RG58 coax has high losses and you will lose many 10s of dBs
of filtering. Even RG-213 is pretty marginal... (This is another
problem for the use of stubs in an expedition...)
3. Effective stubs can be designed using any impedance of cable,
although the lengths of the matching sections will change.
4. Coax switching of stubs is very complicated because the
matching sections must also be switched.
5. It is simpler to buy the filters. You will get similar
performance to reasonable-quality stubs (altho not as good as
excellent stubs, and not at 1500 W power levels). Switching is a
breeze. And Dunestar or ICE filters are smaller and lighter to
carry with you to the site.
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The stubs are easy and cheap. I use RG62A/U that I bought by the
pound from a metal salvage yard. Impedance of the stubs does not
have to match the transmission line. RG62A/U is 93 ohm coaxial
cable. Small cable is fine as it only has to carry the spurious
signal. I use 6 dedicated feed lines for the 6 bands and have stubs
permanently attached to each line. There is no reason not to use a
switch to select the right stub. But if you forget to switch, the
amplifier may be looking at a short circuit with the wrong stub.
Stubs are not real high Q devices and seem to cover an entire band
easily.
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An effective combo of stubs and filters allows almost any
band/antenna combo to be on the air at the same time.
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Yes, stubs are worth it even if you use ICE or other tx filters I
think. Most of the other filters are only exciter level so you put
them between the exciter and amp, but they don't do anything for
amp produced harmonics. In general the stubs are 1/4 wave
shorted pieces of coax, except on 40m to null out 15m harmonics.
If you are running kw's you want to use at least rg8x, preferably
rg8 or rg-213. We melted down a stub using rg-59 here one
contest. No, the impedance of the coax doesn't matter, but the
lossiness of it does. Going from rg-58 to 3/4" hardline increases
nulls from about 15 to about 40db. Using decent quality rg-8 gives
good results. Yes, you can switch stubs, just remember that the
size of the switch and any connecting coax pieces add to the
length of the stub so shorten up the stub to compensate for it.
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As most expeditions involve dragging too much stuff through
reluctant airline channels, I'd recommend buying the DuneStars.
Yes, you may bandswitch stubs in and out. Yes, you may make them
out of 75 ohm cable. But to make stubs that work, you MUST have
a testing machine that will read out frequency (AEA, MFJ, network
analyzer, etc.) RG-58 vs. RG-213: The less loss, the better the
stub performance. Remember, DuneStars go between xcvr and amp.
Stubs of RG-213 can go after amp. I use ICE bn xcvr and amp,
stubs of .750 inch diameter between amp and antenna.
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Coax stubs are best at reducing broad band phase noise from
nearby transmitters. They will provide about 20-25db attenuation
on the next nearest band. The lower the loss of the coax used on
the stub, the deeper and narrower the notch will be - take your
pick from any coax that will handle the power. Note that stations
have had stubs melt in the 'heat' of battle!. Better stick with 50
ohms... Many stations don't bother with stubs in multi-single -
just good rx filters.
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There is a middle way, which is to use an ant tuner as a "tunable
stub". This doesn't work in a m/m operation, but for what you guys
are doing, it might be the best approach. I have used MFJ 16010
tuners for this, which cost about $30. I can get about 30 db of
rejection on the offending band. Operationally, you simply switch
to the offending band and tune the device for minimum band noise,
and put little notation marks on it for reference during the
contest.
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