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[AMPS] Alpha question

To: <amps@contesting.com>
Subject: [AMPS] Alpha question
From: Mike" <W4EF@dellroy.com (Mike)
Date: Wed, 17 Oct 2001 20:59:02 -0700
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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