SSTRAN AMT3000 Output impedance.

[jpdesroc]

Hi list,
I'm about to build a small push-pull RF amplifier
to boost a little my AMT3000 transmitter output.
See both attached schematics. One is the Push-pull RF amp
I'm about to build and the other is the actual AMT3000 output stage..
I chose to use 1280khz as a working frequency in my neighbour.
Here in the province of Quebec there are about no AM stations
since the 80's so the choice is easy.
At that frequency, keeping only L4 (56uH) tuned with C5
I found that the output impedance was around 270 ohms.
(The RF voltage swing at connector J4/P4 goes down halfway with that resistive load).

And by the way all of these will be mounted in my basement.
No outside stuff.

Here are my questions:
- So to keep things nicely balanced my AMT3000 output
connected to my boost RF amplifier should 'see' an input transformer primary
that shows around 270 reactive ohms or little higher right ?

- And since the original AMT3000 has a 270 ohms output impedance
(L4 & C5 tuned at 1280khz) Should I design the boost amplifier
to have the same output impedance ?? I saw MANY RF amplifiers
use 50 ohms antennas, but these are way above my 1800khz range.

My antenna will be made of a 5/8" copper pipe 6 meters long
that will be bent in a 'Z' shape with 2 x 90degs knees in the middle
to accomodate the lack of height (8' ceilings) ,
connected in series with a loading coil. And the final amplifier
will be grounded using a copper rod inserted in my basement's
concrete floor close to the amplifier.
I know this is not at all a 'standard' way to do these things
but believe me this will be the best way for me to have an inside
setup that will generate a 'no static back ground noises' signal
for all my house vintage radios.

I need your help.
Thanks.

[G8HQP Dave]

There is no need to match impedances here. The output of the original transmitter is essentially a collector (so high impedance) driving a resistor (820 ohms) when then goes through a pi network to the output. I guess this was designed so no harm is done whatever impedance is attached to the output. 270 ohm load will maximise power from the transmitter, which may or may not be what you want.

Why do you want to boost the output power? Better antenna matching should give you a better signal.

[Paul Stenning]

Breach of rule F2, closed.