[AMRadio] 1930's Old Buzzard Open Rack Transmitter Finally

Donald Chester k4kyv at hotmail.com
Tue Feb 21 18:56:28 EST 2006

>The amount of turns on the link coil impacts what you can load up to a LOT.
>I made my own link for the 812a rig, and had to try various link turns
>it loaded up right.  I have the series cap also, and that does change how
>rig loads up, but does not seem to change the efficiency or anything else.

I use a series cap on both my homebrew rigs for 40m and 80m.  However, even 
with higher capacitance, it wouldn't work on 160.  I could hit resonance and 
get some loading, but even with the link shoved in all the way, it would 
load up maybe 25% of full loading.

Using trial-and-error I eventually came up with a solution that works with 
both transmitters.  I put two fixed mica caps in series - somewhere about 
.0015 mfd each (not out in the shack where I can look at them) - whatever 
value it took to resonate the link with the 2 caps in series.  I have a 1000 
pf variable in parallel with one of the fixed caps for fine tuning.  The 
coax cable goes across the other capacitor.  The link operates as a parallel 
tuned circuit, with the load tapped down using a capacitive voltage divider. 
  With that lashup I can fully load both transmitters on 160 with the link 
about 65% into the tank coil.

The series capacitor would probably work on 160 if I had more turns on the 
link.  Both my links use 4 turns, and I use the same link to load up on 40, 
80 and 160.  I suspect 8 or 10 turns would do the trick, but then it 
probably wouldn't work right on 40.  I wired up a switching arrangement 
using a 4-pole DT switch to go from the 80/40m circuit to the 160m circuit.

Don k4kyv


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