Optimising transmitter radiated power
 

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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Make c3 (1n) a variable cap too.

Clive

Make c3 (1n) a variable cap too.

Clive

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?

Paul-

You could use a tap, or add a winding to the coil. If it could be assumed that
the antenna was 50 Ohms, the impedance ratio needed would appear to be 140/50,
or 2.8. Therefore turns ratio would be 2.8 * 2.8, or almost 8:1. In other
words, if the coil had 8 turns, the 50 Ohm tap would be at one turn.

With a second winding, the capacitor would not be needed to block DC. However,
it would be a good idea to include a variable capacitor as Clive suggested, to
tune out any series reactance.

Remember that the telescopic whip is only one half of the antenna system. You
need some kind of counterpoise (ground) to balance it.

73, Fred, K4DII

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?

Paul-

You could use a tap, or add a winding to the coil. If it could be assumed that
the antenna was 50 Ohms, the impedance ratio needed would appear to be 140/50,
or 2.8. Therefore turns ratio would be 2.8 * 2.8, or almost 8:1. In other
words, if the coil had 8 turns, the 50 Ohm tap would be at one turn.

With a second winding, the capacitor would not be needed to block DC. However,
it would be a good idea to include a variable capacitor as Clive suggested, to
tune out any series reactance.

Remember that the telescopic whip is only one half of the antenna system. You
need some kind of counterpoise (ground) to balance it.

73, Fred, K4DII