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Paul E. Cater · February 24th 07, 02:40 AM posted to rec.radio.amateur.homebrew

wrote:
I am planning on building a solidstate amp of about 100watts to use
with a low power transceiver. My interestrare 20 thru 10 meters and I
thought I could save a little board space and maybe money if I
designed the output transformer to cover just these bands. I have half
the cores I need to build a transformer that covers 80 thru 10 I was
wondering if this would be good enough for a transformer for 20 thru
10. Also the amp will be mounted right at the base of the antenna I am
thinking of designing the amp with an output impedance considerably
less than the usual 50 ohms. I am thinking this would make fial
matching to the antenna easier. Any comments would be greatly
appreciated.

Jimmie
KD4RQE

Hi Jimmie,

As for the transformers needed for your project, thinking about it
quickly, the inductive reactance would decrease as you leave out cores
so at the higher end of the band I think you are going to have problems.
I’m sure there are others in the group that can guide you in this area.

Whatever you end up doing put an output filter on the thing as it will
amplify whatever you feed it just like the fundamental. Yea, I know,
all modern transceivers are clean. A little “over-kill” never hurts.

I'm very curious why you are interested in mounting the amp at the
antenna? Do you intend to use it as a matching device for the antenna
as well as amplification? I believe that is where you are coming from
with the idea.

I want you to think about a couple things. The solid-state amp is going
to be a DC powered device. How long will the power lines have to be to
reach the amp at the antenna? You would need to run two wires, of
course, that are going to have considerable DC losses especially with
the current the amplifier has to draw. The “I” squared “R” power losses
would be substantial even on a short run.

The second thought is about the actual coaxial loss that could take
place at 28 to 30 MHz. As long as you are not planning to use RG-58 or
cheap RG-8, or making a few hundred foot run, the line loss would be
negligible compared to what you might gain from the amp being at the
antenna. Also, it sure is much easier to work on it in the shack
instead of having to climb a tower to yank it down.

As a final thought, if the antenna is 25 ohms, do what you need to do to
make it OK. The worst case at a 2:1 SWR is around 10% power loss. I am
a bit of a perfectionist with the impedance thing. So, I recommend
choosing whatever matching system hits your fancy. If it is a balanced
antenna by all means use a BALUN!

If we were talking about 2 meters or higher I might be a bit more concerned.

I do hope this helps with your ideas and project. I would be interested
in knowing what you decide.

73,
Paul