Amp design question
 

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

Amp design question
 

Amp design question
 

"Paul E. Cater" wrote in message
...
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


I was tired when I made the post and for got to say that this would be a
mbile unit. antenna mounted on my tool box in the bed of my pickup. It was
really just a thought about mounting the amp in the tool box since room in
my little Nissan is scarce. From there I made the leap to thinking since the
feed point impdance of the antenna is already going to fairly low and feed
line length practically 0 why step it up to 50 ohms just to drop it back
down again. More realistically I am thinking a short antena on 20M may be
more like 10 ohms wich would be a 5:1 SWR and I dont think my amp would like
that much. At best I am thinking the 10M antena I am planning on using will
have the 25 ohm impedance you metioned, 15 and 20 m antennas would have even
a lower impedance.so an output impedance of say 15 ohms might be a good
thing.

Jimmie

Amp design question
 

Jimmie D wrote:


All OK Jimmie, now I get it. Mobile never entered my little mind.

If you were only going to use one band I can see how the idea might
work. But, looking at all the different impedances the output will see
on the different bands will be a problem.

I just tore apart an old plasma amplifier that utilized a solid-state
driver stage. It’s loosely based on the Motorola design. Anyway, the
output transformer uses three beads on each side. You are welcome to it
if it will help you out with your project. Its just collecting dust
here and I have no further use for it.

73,
Paul

Amp design question
 

wrote in message
...
On Sat, 24 Feb 2007 14:20:12 -0500, "Jimmie D"
wrote:


I was tired when I made the post and for got to say that this would be a
mbile unit. antenna mounted on my tool box in the bed of my pickup. It was
really just a thought about mounting the amp in the tool box since room in
my little Nissan is scarce. From there I made the leap to thinking since
the
feed point impdance of the antenna is already going to fairly low and feed
line length practically 0 why step it up to 50 ohms just to drop it back
down again. More realistically I am thinking a short antena on 20M may be
more like 10 ohms wich would be a 5:1 SWR and I dont think my amp would
like
that much. At best I am thinking the 10M antena I am planning on using
will
have the 25 ohm impedance you metioned, 15 and 20 m antennas would have
even
a lower impedance.so an output impedance of say 15 ohms might be a good
thing.

Jimmie

One reason to go from whatever andtenna feed impedence to 50 ohms is
on receive you still need a 50 ohm match so using standard amp design
with 50ohm output makes life easier for that. Also for testing life
is easier (got a 10 ohm load?) as then everything is standardized
at 50 ohms. Also at low impedence the I^R losses go up as the
magnitude of I gets very large

Allison.




Found an old CB amp in my junk box today. Spec plate on the amp says it will
cover 80 thru 10. I plan on doing some mods improving the bias.and add some
filtering I think this may give me what I want. I forgot I bought a 5 galon
bucket of these amps at a hamfest years ago and still have a few around.