Yes--the coax length does have an effect, and though my
Alphas are from 1993 and earlier the manuals (which
were all "preliminary") did suggest the use of a
connecting coax of about 5 or 6 feet. Without getting
into boring detail, at 10m and thereabout this length
of line is enough to have an impedance transforming
effect but roughly speaking the same length would not
have such an effect at 20 meters and below. Usually a
length could be chosen so that the SWR reported by a
transceiver would be under 2:1 on all bands.
Unfortunately some transceivers begin to show a
significant foldback before 2:1 VSWR in order to
protect output transistors. Sometimes--but not
always--the transceiver's ATU can be used to help the
match, but the amp input "impedance" is likely to be
complex and to vary with drive, which may confuse the
auto tuner. Usually though a coax length can be found
to make the transceiver happy without using the ATU.
I am sure all modern Alphas are designed with attention
paid to the matching issue.
73
Bob W2WG
-----Original Message-----
From: Amps [mailto:amps-bounces@contesting.com] On
Behalf Of KB8NTY
Sent: Saturday, January 10, 2015 4:01 PM
To: amps@contesting.com
Subject: Re: [Amps] Alpha Input SWR
My Amp Supply LK-500ZB GG. amp manual also states to
use a 6' length coax between your exciter's RF output
and the LK-500ZB RF input connector.
Possibly the feeder length does factor in!
-73-
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