[ARC5] Modeling the PA stage

[Mike Hanz]

Ian Wilson wrote:

>I'd like to analyze the output side of the BC-459/T-22 in order to
>understand how to load it properly, etc.
>I can estimate the source resistance of the 1625's from the HT voltage &
>current drain using the ARRL formula 0.5 * Vp * Ip. The inductance of the
>tank coil can be calculated.
>

I think most of us have gone through this exercise at one time or 
another, Ian - it's a fun thing to do.  Unfortunately, the textbook 
values you get aren't particularly accurate and are somewhat secondary 
to the design parameters for the antenna load given in the manuals, 
which suggest a value of 5 ohms and 100pF as the design center.  Looking 
back into the transmitter antenna terminal you should see the conjugate 
of that (with the settings described for the T-22 in the test portion of 
the manual, for example) so you can actually calculate the coupling coil 
contribution and tank inductance from that data if it seems important to 
you (for modification of a hacked up set or whatever.) 

For an unmodified set, the article at 
http://aafradio.org/docs/Aircraft_Antenna_Design.html might be more 
relevant for you.  Relatively high capacitance and low resistance is 
normal for these antennas, and typical figures are discussed.  If you 
need a long feeder between your ham antenna and the command transmitter, 
you might want to think about impedance matching at *both* ends of the 
feeder to minimize losses.  Dave Stinson has suggested using a series 
capacitor at the transmitter antenna terminal, and that gets you 90% of 
what you need for most situations.  If you have a long coax feeder and 
want to be a purist about it, you'll want to go a bit further.

73,
Mike