Hi Steve,
Thanks for looking over the design. I didn't consider the impedance change of
the relay, maybe somehow tune it out?
>I've wondered before if you could use the two halves in parallel
>at lower frequencies, with the rf in and out from the centre tap
>of the transformer primaries.
I think what you describe is similar to this example, which is used in the THP
HL-2.5Kfx
http://www.arrl.org/qex/2006/09/qx9ohsawa.pdf
>Another whacky idea I wondered about was making the transformers
>using E/I cores chosen to give lf performance and then have a
>mechanical arrangement to remove some or all of the 'I' pieces as
>needed as the frequency goes up.
I'm currently using the semi-rigid coax transformers with the type 61 core
material. According to the app note, using type 43 material would pretty much
get me to where I need to be, but unfortunately, I have been searching the web
for type 43 material cores for a while now and haven't found any. Maybe moving
away from the coax transformers to a bionocular style of type 43 would do it.
Paul
Message: 5
Date: Thu, 13 Dec 2007 07:48:40 +0000
From: Steve Thompson <g8gsq@eltac.co.uk>
Subject: Re: [Amps] input/output transformer switching
To: amps@contesting.com
Message-ID: <4760E3D8.3010806@eltac.co.uk>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
> Hi everyone,
>
> Thanks for all the replies on the LK500, everyone seemed to converge on the
> same area to check out. I'm in the process of replacing the parasitic
> suppressor resistors.
>
> While I wait for parts....
>
> Background: I've mostly finished up my solid state amp based on the MRF141G
> using Motorola app note AR313. One of my original design goals was 160 - 2
> meter operation & 70cm, but that is another PA strip. As it seems to be
> turning out, it's basically impossible to make 9:1 and 1:9 input/output
> transformers which work over that broad of a range. The transformers I have
> work accoring to the app note, but that doesn't give any performance on 160,
> 80, and 40 meters.
>
> Solution: Remove the original matching transformers and replace them with a
> circuit board with two input transformers and two output transformers and
> some
> relays to switch between them.
I doubt it would work. The impedances at the transistor side of
the transformer are very low (few ohms) so any inductance from the
switching will totally mess up the higher frequency operation. I
suspect you would have stability problems too.
I've wondered before if you could use the two halves in parallel
at lower frequencies, with the rf in and out from the centre tap
of the transformer primaries.
Another whacky idea I wondered about was making the transformers
using E/I cores chosen to give lf performance and then have a
mechanical arrangement to remove some or all of the 'I' pieces as
needed as the frequency goes up.
Steve
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