[TMC] Putting PAL 350/500 on 160 meters?

[Fred Telewski]

Hi Folks,

I am looking at what it would take to put the PAL350/500 on 160 meters
without really tearing up the unit. I would also like to keep the unit
functional on the rest of the bands. The 200 kHz required to cover 160
meters is a fair stretch as the low frequency position on the PI section
bandswitch covers 2 to 2.5 MHz.

The conventional wisdom suggests that the PI inductor is not large enough,
but this is really not the case. I simulated several PI networks of varying
Q for use with a 2000-ohm plate impedance (2 times 4CX350 in parallel). The
values in the unit looked OK with the exception of the loading capacitor
being too small. My simulations indicated that the unit would require about
1000 pF of additional loading capacitance.

I then placed a 2000-ohm resistor from the plates to ground (with the unit
unpowered of course) and looked backwards into the PI network with a network
analyzer and began to follow the match point down below 2 MHz. I ran out of
range at 1.95 MHz. I also noticed that the tuning did not behave the way a
normal PI net might be expected to respond. I became suspicious when I
noticed that the plate choke inductance was only 50 uH. 50 uH computes to
around 500 ohms at 1.8 MHz; which is too low for normal PI network
operation. Additional simulation shows that the effect of the low plate
choke inductance is to require a lot more plate tuning capacitance to force
the choke towards parallel resonance driving its effective impedance
upwards.

The solution is to remove the 50 uH choke and substitute a step wound choke
in its place. This is an easy nondestructive modification. With a new plate
choke and the 1000 pF loading padder in place, the PI network tunes the
160-meter band in a normal manner.

The only remaining question is how to switch the padder in and out using the
bandswitch?

I noticed that the bandswitch is used to switch a 100 pF capacitor across
most of the tank coil on the lowest (2-2.5) range. This action parallel
resonates the tank coil at roughly 4 MHz. Was this done to reduce the second
harmonic? Is anyone familiar enough with the design to know why this was
done?

I would like to ditch the 100 pF cap and use this switch position to
automatically engage the 1000 pF loading padder to ground.

If these two simple modifications are possible, the output PI network can be
made to cover 160 meters nicely without any sawing or drilling.

It appears that the driver tuning can be shifted low enough to get good
coverage too.

Any comments would be appreciated.

Fred WA7TZY