Hi Jack,
A single 1/4 wave stub made from low-loss coax will give you between 20
and 25 dB of attenuation when properly adjusted. If the second station doesn't
shut down the amp on the first station when the second station is only running
barefoot, then a single stub should be more than good enough when running
the amp on the second station.
The filtering scenario you describe needs to be modified slightly to work
properly. A open 1/4 wave stub cut for ten meters will look like a 3/16 wave
open
stub 15 meters. This will look like a shunt capacitor and will wreak havoc
with your 15 meter SWR. Use a 3/4 wavelength shorted stub across the amplifier
output when you are operating 15 meters (this will look like a 1/2 wave and a
one
wavelength shorted stub on 20 and 10 meters respectively). When operating
twenty meters, use a open 1/2 wavelength stub on the amplifier
output to kill 15 and a shorted 1/4 wavelength stub to kill 10 (you can put them
in parallel to kill both 10 and 15 simultaneously). Finally when running ten
meters,
an open 1 wavelength stub will kill 15 and a shorted 1/4 wave stub will kill
20.
Since the 3/4 wavelength 15 meter stub will be the same length as the 1
wavelength
10 meter stub, I have summarized everything below in terms of wavelenghts
measured at 10 meters to give you an idea of how one stub can be reused for
several
different combination of bands (remember I am referring to electrical length so
you
need to account for the velocity factor of the coax you are using when sizing
the
stubs):
XMT
BAND
10 1 WL OPEN (Kills 15, Passes 10/20) 1/2 WL OPEN (Kills
20, Passes 10)
15 1 WL SHORTED (Kills 10/20, Passes 15)
20 1 WL OPEN (Kills 15, Passes 10/20) 1/2 WL SHORTED
(Kills 10, Passes 20)
Note: stub lengths are expressed in terms of wavelengths on 10 meters.
As you can see, if you have a switch that allows you to change from an open to
a short on the
stub termination, then you really only need a single pair of stubs one each
station to handle
all of the possible combinations. BTW, I have had excellent results using
LMR-400 for stubs.
Good Luck,
Mike, W4EF..........
----- Original Message -----
From: "jsschuster" <jsschuster@snet.net>
To: <amps@contesting.com>
Sent: Wednesday, October 17, 2001 9:16 PM
Subject: [AMPS] Alpha question
>
> I will be operating at a station with two Alpha 87A's , and the owner
> says there is a problem if both amps are on 10, 15, or 20 (not on the
> same band). The reflected power goes up and the amp shuts down when the
> second amp is transmitting. I am thinking of using a single stub on
> each. If "A" is on 15, and "B" is on 10, Ill put a qurter wave open stub
> for 10 on "A", and a similar one for 15 on the output of "B".
> Has anyone experienced this with 87A's, and do you think a single stub
> will do it as described? Sorry if this has been discussed before. Tnx
> JACK W1WEF
> (Antennas are three F12 tribanders on one tower spaced at least 30
> ft...each rig has a Dunestar 600)
>
>
> --
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>
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