Stefan schrieb:
> 1 Ohm is to less i mean,

I was thinking in terms of, say 100W from a 12V supply,
so you'd want to offer a load of less than 1 ohm to the
PA.

> swr meters.

I'm not sure what use an SWR meter would be.  I think the
only measurement you'd really need is an ammeter in the
DC supply to the PA final stage.   A kV meter on the top of
the loading coil would be nice too.  You would tune for max
ammeter reading, and adjust taps so that the current drawn
does not exceed some limit.

> The region of 50 to 100Ohm is a good choice.

Yes, if you are separating the PA some distance from the
loading coil, then an intermediate impedance might be desirable,
on cost alone if  nothing else - 50m of speaker cable is a lot
cheaper than 50m of car jump lead cable.  It doesn't have to
match the cable impedance.   And as you say, if you have some
50 ohm filter or something then you will have to get near to
that to get the right filter response.

Build in plenty of range of impedance transformation.  One thing
I've always found with driving LF into coils is the impedance is
always higher than you hope it will be!  You always seem to need
more voltage from the driver.  Predicted Q always seems to be on
the high side of reality.   Build in lots of step-up!

The system Q will vary from day to day, hour to hour, even minute
to minute in some cases.  I have some interesting experimental
results from long period automatic Q measurements which show all
kinds of variation not accounted for by temperature coefficient
of copper, especially when set up over grass/soil in temperatures
around freezing.

I will post about the 13th harmonic over the weekend but just now
am busy here making a new calibration source for the loop receiver.
--
Paul Nicholson
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