On Mon, 1 Mar 2010, James Moritz wrote:
>
> Well, scaling the 12 inch diameter coil on 160m would require about a 60
> metre diameter coil at 9kHz, so still quite "challenging". It would need an
> impressive no-entry sign too ;-)
Dear Jim,
to get constant Q one should scale coil dimension as sqare root of
wavelength (if neglect dielectric losses). May be it sounds strange but it
is so, sure. Conventional scaling wellknown in antenna design is wrong
here because there is yet another scale exept wavelenth, pariculary skin
depth which is critically worth here. 33000/160=200 then dimensions should
be scaled by factor SQRT(200)=14. This yelds scaling to 4 m coil diameter.
One should remember welldesigned 300mm coil has Q about 800 on 160 m. If
decrease Q to say 200 then diameter should be about 1 m, may be
little more but not 60 m:-)
There is another problem to get high Q on 9 kHz. C of ant is too small.
Then reactance (omega*L) of a coil should be huge. Dielectric losses is
worth then. One shoud use very perfect dielectric in such a coil.
Regards,
Alexander/RA9MB
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