[ARC5] ARC-5 TX output circuit modeling [long]

[Ian Wilson]

I’m here.

 

The large capacitor goes across the 50 ohm load. The parallel combination of
50 ohms and 1000pF is equivalent to about 9 ohms in series with 1200pF.

 

I should perhaps clarify something here. I was not trying to analyze how the
BC-459 was used in its intended environment. I was trying to understand the
results that I got trying to use it in conjunction with a 50 ohm load. I
could not get maximum output using a 50 ohm load without adding parallel
capacitance. Adding a large series capacitance did not work for me. With no
series 50pF or 100pF capacitor, output was maximum with the roller inductor
set to zero; but adding 100pF and using the roller to resonate out it – and
the link winding inductance – produced more output than without the 100pF
capacitor. The model that I described agrees fairly well with what I measure
here on the bench.

 

(Incidentally, the very poor efficiency in my unit was caused by a broken
padded capacitor and some other problems, efficiency now is moderate. I am
getting about 50w out for 93w in. This is with 1000pF loading and 200pF
series capacitance, at 7MHz).

 

I did not mean to claim that all these transmitters were designed to operate
into loads with a 4 ohm resistive part. I would think, however, that most HF
aircraft transmitters had to operate into antennas that were electrically
short (compared to ¼ wavelength), and had a radiation resistance
significantly less than 50 ohms. The capacitive reactance is irrelevant, of
course, since it plays no part in radiating energy.

 

73, ian K3IMW