[Boatanchors] Link coupling?

[ianmwilson at earthlink.com]

I don't think there is any theoretical reason why the efficiency of link
coupling should be lower than that of an equivalent pi network.

However, there are practical difficulties. Paraphrasing from the 1963 ARRL
Handbook:

  For link coupling to work well, the reactance of the link winding should
  be about the same as the load (50 ohms, say), and the coefficient of
  coupling between tank and link coil should be as close to 1 as possible.

  The complete recipe involves choosing tank values that give a Q of 10 or
  more. This is a function of plate resistance (estimated from plate voltage
  and current, assuming Class C operation) and frequency. For example, at
  7MHz, plate voltage = 700, plate current = 100mA, the tank capacitance
  needs to be about 62pF (hence tank inductance =  8.3uH). Link winding
  inductance for a 50 ohm load would need to be at least 1.1uH (more if the
  coefficient of coupling is much less than 1).

These requirements being hard to meet, the Handbook goes on to recommend
tuned coupling (usually a large variable capacitor in series between the
link and the load).

Note that the original simple tank + link isn't so simple any more. You can
make an equivalent pi network with fewer components, and without the
mechanical complexity of the link).

Command Set note: if you are trying to use a 50ohm antenna, the 3.5MHz ARC-5
TX can be loaded up by using a large tuning capacitor in series with the
link. The 7MHz one needs the capacitor across the load (at least in my
experience).

Hope this helps.

73, ian K3IMW