[TMC] PMO/VOX etc

Roy Morgan k1lky at earthlink.net
Fri Oct 9 12:37:18 EDT 2009


On Oct 9, 2009, at 10:44 AM, Telegrapher at att.net wrote:

> ... All that is required is an RF source of modulated power, AM,  
> CW,SSB etc at around 2W or less to drive the Amps you have been  
> talking about...
>
> I recently picked up a radio that will allow me to run anything  
> from .5 watts upward to 100 and will be using that to run my PAL-500

Larry and others,

We did not hear what radio Larry has, but with the old tube type  
radios that can be throttled way back, there is something to look out  
for.  The problem is that when the transmitter or transceiver is set  
at a very low power, the spurious signals become larger with respect  
to the desired signal.  They produce much cleaner signals at moderate  
(or perhaps higher) output levels.

So you need an attenuator that will work at 100 watts or so and puts  
out 100 to 200 mw.  Bird makes/made these but they are few and far  
between, and they cost a bunch even at hamfests.  The Vishay company,  
I believe, made small solid state type attenuators that can handle 50  
or a hundred watts when mounted on a heatsink.  I think they are still  
available (put it on a heat sink with two connectors and it could not  
be simpler), but I don't know the source(s).

As I understand it, most TMC transmitters and linears will run to full  
power with a hundred milliwatts of signal or thereabouts.  (Do not be  
fooled when you find a 6146 or 2E26 as the "final" in their exciters  
and oscillators - they were chosen for high linearity in class A  
service at very modest power levels.)

One more point on the TMC SBE:  If I remember correctly, and without  
checking the manual, I seem to remember that the SBE needs an input  
frequency (or internal oven crystal) that is offset from the desired  
output frequency by something like 200 kc.  This is because the mixing  
scheme involves generating the SSB or other sideband signals at 200 kc  
or so and mixing that with the internally generated carrier.  (It will  
do two separate sidebands with independent information by being fed  
audio signals.)

I found that "getting my head around" the functions of the SBE was an  
activity that needed concentration and some study of the manual.

Roy

Roy Morgan
k1lky at earthlink.net
529 Cobb St.
Groton NY, 13073
Home: 607-898-3607
Cell: 301-928-7794





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