Sunday, December 23, 2012

PA-47 At Only 1 Watt

Original PA-47
with Large Heat Sink, 
Matching Transformer
and FET IRF510
In keeping with my goal of making my Homebrew projects as small as my abilities allow and using the same previously created PCB board (see previous post on my PA-47 Power Amplifier)

I have reduced the size of the output matching transformer and changed the output FET from a IRF510 (TO-220) to a 2N7000 (TO-92).

New PA-47  Running at
1 Watt Output

Knowing that the 2N7000 would not produce the same 4.7 Watts of the replaced IRF510 with its large heat sink, I planned to reduce the supply voltage; from 12.6 Volt down to 9 Volts, and reduce the size and turns ratio of the matching transformer form 1:3 to 1:2.

New Replacement Parts
of the Original PA-47
are the 2N7000 and 

Smaller Matching Transformer

Initial test produced 1 Watt RF output, and a Very Hot 2N7000 FET. A small 3/4 inch sq piece of aluminum flashing was super-glued to the FET which solved the heat build-up problem. Just in case, I also installed a pin socket for each leg of the FET (which means I can easily replace the FET, if I inadvertently cause it to release its Magic Blue Smoke :-).

For this first test, the DC input was 8.5 Volts (not a very fresh battery set) at 360 mAmps: or 3.06 Watts DC Input, which suggests the PA at only about 32% efficient (more work is maybe needed to increase this). The standby current (no RF input) is about 13 mAmps, and most of that is used to drive the green Power-On LED.

30 Meter Low Pass Filter

Without the FAN and Large Heatsink that was used in the initial PA-47, this PA is silent.

Like the original PA-47 an outboard Low Pass Filter is necessary and was used for this experiment.

TR Relay Mounted Under the PCB

The yellow-tag jumper on the header turns on the TR Relay, which is mounted on the under side of the PCB. Normally transmitter control from the exciter would be connected to this header.

The RF voltage measured at the 50 Ohm Load via the scope: 20 Volts PP, (20/2*.707)^2/50 => 1 Watt, or as per the web  RF calculator.

This smaller and lower power PA will be perfect for my ongoing Propeller 30 Meter Beacon efforts.

After I correct the PCB layout (see previous discovered errors),  and if there is interest, I may make the PA-47  board available, or as a complete Kit (but, as a board or kit, this would not be suited for first time builders, nor a builder without a Microscope).

The Exciter (Driver)
The Exciter
Outputs 12 mWatts

For this experiments and as explained previously, I used my Propeller Transmitter as the exciter which provides 12 mWatts of drive. The Propeller is the second board in the stack.


1 comment:

  1. Great article. When you get to the point of testing it on the air will you be posting that as well ? I live in Port Angeles and would like to see if I can hear it from here.