[Lowfer] Re: LF PAs

[James Moritz]

Dear David, Lowfers

At 04:01 12/07/2002 -0400, you wrote:
>A pair of MOSFETs will be a full legal limit (US/Canada) amp with plenty
>of head room--until/unless they up the power limit from the proposed
>100W PEP.  Even if it goes up to the 1.5KW allowed elsewhere, that's
>still solidly within the range of common parts.

This is certainly a nice thing about LF - I have built a 1kW class D PA 
using 4 MOSFETS costing 3 pounds each (about $18 in total). The active 
devices are negligibly cheap compared to the rest of the circuit.


>I'd actually like to see some creativity shown in pairing amp designs with
>antenna designs.  For example, semiconductors tend to work better at lower
>voltages and higher currents as compared to tubes.  Loop antennas are high
>current, low voltage devices.  Sounds like a good pairing?  Pair a tube
>amp with a vertical.  Skip a lot of impedence transformation.

Hmm - Not really - if you put the capacitor in series with the loop, you 
get a low resistance, put it in parallel and you get a high resistance. And 
much the same applies to a vertical with a loading inductor. The resistive 
component is probably never quite what you would like it to be for a 
feasible PA design, plus it is rather unpredictable and varies a lot with 
design details, location, and even the weather, so you will end up doing 
impedance matching anyway. Designing the PA for 50ohm Rout is very handy 
for things like power measurement, dummy loads, feeders, output low-pass 
filters and so on. Having said that, at least one UK amateur used a loop 
antenna as the tank inductor in an LF valve/tube PA - can't have been great 
for harmonic suppression though...

>Heck, with the simplicity and low cost that such an amp can have, just
>build a few and replace them as they blow. ;)  Might be cheaper than
>worrying about protecting them.
Some transmitters that have been used on LF are rather like that! Their 
owners have seemed intent in qualifying for bulk discounts from 
semiconductor suppliers:-) But most cases of blown MOSFETs seem to be due 
to "self destruct" circuit designs, or antennas breaking down rather than 
lightning or other external causes. With a good PA circuit, MOSFETs seem to 
be highly reliable.

Cheers, Jim Moritz
73 de M0BMU