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Re: [Amps] Alpha 87a

To: k7fm <>
Subject: Re: [Amps] Alpha 87a
From: Larry Benko <>
Date: Mon, 01 Jan 2007 17:40:02 -0700
List-post: <>
Just a small clarification:

A 3 dB pad will increase the return loss byt 2*3dB = 6dB.

A 2.0:1 SWR is equivalent to a reflection coefficient of .333 and a 
return loss of 9.542dB.  Therefore it takes a 4.77dB pad to guarantee a 
input SWR of <= 2.0:1.

However let's think of it like this: Assuming an original SWR of 3.0:1, 
then a 2 dB pad will produce a resulting SWR of 1.922:1.  A measly 6 ft. 
piece of RG-58 coax between the transmitter and the amp will have less 
than 0.2dB of loss at 28MHz which will not improve the input SWR much at 
all.  Even if the 6 foot piece of coax is made from RG-174 the loss at 
28MHz will only be 0.3dB which still won't do much.  Am I missing 
something here?

Larry, W0QE

k7fm wrote:

>One simple way to improve the input swr to any amplifier is to add a 1 or 2 
>db attenuator at the input.  The greater the attenuation, the greater the 
>improvement in swr.  It does not make sense to do this if you barely have 
>enough drive power, but the Alpha 87a drives easily.
>A 1 db attenuator only needs a 15 watt continuous rating for most 
>amplifiers.  Either a T or pi type will work.
>A 3 db pad would mean that you would never have greater than a 2:1 swr into 
>any amplifier, although the power dissipation would climb dramatically for 
>the attenuator.
>And, Happy New Year
>Colin  K7FM 
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