On Wed, 28 May 1997 21:49:31 EDT k1vr@juno.com (Fred Hopengarten) writes:
>Chosing a Stub
>
> Should the box for Station A, which is (let us posit) on 20,
>be
>switching IN a 20 meter bandpass stub? In this case, the bandpass
>stub
>would be selected by Station A's selector. Or should the box for
>Station
>A be switching IN 10 and 15 meter band reject stubs (the
>"cross-control"
>strategy)? If the latter, then wouldn't Station A's switch be
>controlled
>by Station B's controller?
Hi Fred,
Nice to see you at Dayton. The best article I have seen on stubs
was published in the NCJ, Volume 12, Number 3, Page 18. If you
need a copy, let me know. For maximum flexibility, the station A
controller should switch in stubs to reject ALL other bands, so
there is no need to know what band station B is on.
>
>Grounding
>
> With several stubs (at laest one per band, but we'll get to
>the
>number in a minute) hanging off the same switchbox, should the grounds
>all be tied together or should they float when not in the circuit?
The grounds can all be tied together.
>Stub Creation
>
> Would you cut one or two stubs per band? Why?
I would start with one set, and see if it does the job to your
satisfaction. I presume you are already using bandpass filters,
which are more effective than stubs. Sometimes stubs are not
required, except on the 40, 80, and 160 antennas to attenuate
2nd harmonic energy coming out of the amplifiers. Dedicated stubs
can be used for these antennas, so no switching is required.
>
> Would you make them out of hardline? Or what would you make
>them
>out of?
RG 213 is a reasonable thing to make them from. For example, a
shorted halfwave stub on 20 made from RG-213 (according to TL.EXE)
has an impedance of 1 ohm. If you use 3/4 inch 50 ohm hardline, the
impedance will be 0.3 ohm. If you pump one watt of harmonic
energy into those stubs, the voltage difference will be about 5 dB.
I have seen a typical attenuation for RG-213 stubs of 24 ~dB. RG-213
coils up neatly, has a predictable velocity factor, and is easy to
cut. (You can't make a proper stub until you have about 20 little
pieces of coax lying on the floor!) If you're a purist, make them
out of the best coax you can find. 75 ohm coax is ok.
>
> If making two stubs per band, for higher Q, what coax would
>you
>use for the quarter-wave link between the two stubs? More hardline,
>or a
>coil of RG-213?
RG-213 is ok. You are just rotating around the Smith Chart on
this section, just in case there is a big swr on the line, and
you happened to pick a low impedance point with your first stub.
>
> Would you make the shorted end or open end versions? (I would
>think that open end versions are faster to cut and try.)
To minimize losses, make the shortest version that will do the job.
To make the cutting go faster on shorted stubs, leave them unshorted
during the cutting, and put your measuring instrument on a frequency
that will show a low impedance. For example, if you are trying to make
a ~23 foot shorted stub to reject 14.2, set your meter at 7.1 and cut
for min Z with the stub unshorted.
>
> Would you use the AEA, Autek or MFJ box (which model?) for
>stub
>cutting?
I use a grid dip oscillator. I have an Autek analyzer, but
the instructions say the unit is subject to "diode suckout"
at very low impedances. There's a way to use it, using some
Mickey Mouse external resistors, but I have the GDO, so I
just use that.
Good luck, and wear gloves while you're doing all that cutting,
so you don't tear up your hands. 73.
Dave Hachadorian, K6LL
k6ll@juno.com
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