Hi gang,

I've just developed a three stage RF amplifier for 40Mhz and the final
PA stage is shown below. The maximum power transfer occurs when the
load resistor has a value of 140 ohms whereupon it dissipates a
thumping 475mW of RF power! All well and good, but I need to radiate
as much of that power as I can - with only a telescopic whip antenna.
So I snip the load resistor out of circuit and the following questions
arise:

Where do I take the feed point to the antenna from? I realise I don't
have a hope in hell of matching 140 ohms without an "L network" or
whatever, but is that really necessary? Could I get away with taking
the antenna feed direct from a tap off the tank coil?

I only *need* 50mW ERP so am not *too* bothered about losing most of
the useful power.




+--------------|--------+---------------------------+12V
| | |
| | |
| C| | VC1 (5-60pF)
| L1 C| ---
.-. C| --- C3
820R| | | | 1n
| | | | ||
'-' +--------+---------------+||-------+
| | || |
| | |
|| | |/ |
-----||-----+------------| 2N3904 |
|| | | |
| | |
C1 | +--------+-------+ |
1n .-. | .-. .-.
| | --- | | Rload| |
| | --- | | | |
820R'-' C2 | '-' '-'
| 1n | | 160R |
| | | |
| | | |
| | | |
| | | |
| | | +

+-----------------------+-------+-----------------+----
gnd


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