>Measured using a Marconi Q meter I found that a Q of 600 and
>above is practical with normal aspect coils.
I'd be a bit suspicious of these high values of Q off a Marconi Q meter. I
have found incredible differences between TF1245 Q meters with the same
inductor when you get above Qs of around 200.
If you take a 2 microhenry coil, that's an XL of 176 ohms at 14MHz. A Q of
600 means an equivalent parallel resistance of 105kohms. In the TF1245,
that's of the order of the input resistance offered by the diode detector.
Incidentally, if anyone has any of the EA52 diode (6929 was an alternative
number, I think) as used in the Marconi Q meter, I'm interested at a
reasonable price.
I haven't gone through the sums before, so this has been interesting. I
suspect that the loss in efficiency on the high bands of many amps is
because the switch resistance becomes a greater proportion of the series
resistance as the inductor gets less, while the short circuited part of the
tank coil couples its resistance by transformer action back to the used part
of the coil.
If you don't short the unused part of the coil, what are the chances by
Murphy's Law that the unused part resonates in at least one of the bands,
leading to enormous voltages and problems?
73
Peter G3RZP
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