160m: [Collins] Crystals

[Dr. Gerald N. Johnson, electrical engineer]

The KWM-2A won't tune below 3 MHz. It could be made to receive without
unreasonable changes with decent sensitivity. Transmitting isn't
practical.

Look at what is needed. To tune the slug rack down, you could unhook
jumpers on the first bandswitch position so its separated from the other
80 meter positions, then add enough tuning C to reach 1.8 MHz. You need
4 times the tuning capacitance to cut the tuned frequency in half. I've
done that to a 75S2 (has the extra crystal board like the KWM-2A)
successfully. The bandwidth is narrow, but that's a benefit when you are
tuning that close to the AM broadcast band.

Transmitting is another problem. You can't just quadruple the
capacitance on a Pi network. If you do that you double the loaded Q and
double the circulating current in the tank coil and capacitors and that
quadruples their power dissipation, to their destruction. The PA tank
coil in the KWM-2A and 30L-1 are not oversized to handle the extra power
dissipation and there's not room in the PA compartments to hold another
coil to double the available inductance. And no position on the band
switch to connect it. At least not in the 30L-1. 

To keep the Pi loaded Q reasonable you have to double the tuning and
loading capacitors and the plate coil. And you probably need to double
the plate feed RF choke inductance if its small. Then you get the strong
possibility of there being stray series resonances in some higher band,
most likely 17 meters. Its really hard to win the redesign in such a
compact package as the KWS-2A or 30L-1. 160 wasn't put in at the factory
because there wasn't room. Adding 160 later is unreasonably difficult.

Once upon a time, I thought of "building out" the existing Pi with an
external impedance matching circuit. My dad built my design in a home
brew PA. Don't try it. In essence, I left the first Pi network tuned to
10m, then band switched a second Pi to match impedances on the lower
bands. It was a great 10m amplifier, 3 813s with separate Pis from each
to a common point, then a reasonable sized tuning capacitor (impedance
at that point designed to be 600 ohms). But on 20 meters, the tubes
arced. Why? The impedance at 20m was matched. Trouble was that those 10
meter Pis were still resonant at 10m giving a high impedance at the tube
plates at 28 MHz but were unloaded which made that impedance really high
and the second harmonic plate currents resonated and developed arcing
high voltages. It was a GREAT idea so long as I only looked at the
impedance match at the operating frequency. When I looked at the plate
tank input impedance at 28 MHz while tuned to 14 MHz it was a really BAD
idea.

73, Jerry, K0CQ, Technical Advisor to the CRA
-- 
Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.