[Lowfer] Re: LF PAs

[David Willmore]

> 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.

*sigh* *drool*  *wipe*  Uhh, sorry.

> >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...

This confuses me.  If the Q is so high as to make the reactance change a lot
over the width of the band, won't that same Q be good for filtering out
harmonics?  I feel I'm missing something.  

I take your point about 50 ohm being good for being able to use common tools,
but won't the values of other types of feeders be useable as well?  So, 75,
90, 300, 450, etc. would all be fair game?  Of course, they're even less ideal
for solid state design than 50...

> >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.

Hey, I've got a tube of MTP3055Es and I'm not afraid to sacrifice them all. :)
I can always get more.  It's not like parts for this band are going to get
more expensive--like working microwave.  How much does a 1W SS PA for 147GHz
go for? ;)

Most design of circuits in this range is for audio applications--amps which are
driving much less reactive loads.  I imagine people just aren't using the
right mind set in their designs. :)  Think of it more like driving a motor
with an off balanced load. 

Cheers,
David N0YMV