On 31/03/2013 16:16, Sean Breheny wrote:
> As your message hints that you suspect, any structure that generates
> changing electric and magnetic fields IS an antenna to some extent.
>=20
> However, you can design structures which work better as an antenna and
> those which do not radiate as much power for a given input current.

Thanks to all who have offered help, I left it a while before replying
as I have learned lots from some of the interesting conversations.

Previous research had led me to Fair-Rite branded ferrite rods, made
from 61 material[1].  These have good permeability below 10Mhz and the
datasheet describes "a high frequency NiZn ferrite developed for a range
of inductive applications up to 25 Mhz".

Right now I have made a simple system around 4 transistors/FETs [2]
(crystal oscillator, buffer, modulation, power amp).  This was based on
a ham design and the basic carrier & modulation works, delivering ~25mW
into a resistor load from 3.3v PSU.

Coupling this to a transmit "antenna" made of the ferrite such that I
get good results from my passive pickup loop[3] is where I am now
encountering difficulty.  From a few inches I get some small mV at best
out of the coax connector.

How would you suggest driving a small coil round a ferrite core such
that it works well for magnetic induction?

David

1 - http://www.fair-rite.com/newfair/materials61.htm
2 - https://dl.dropbox.com/u/55193788/Temp/2013-04-02%2021.45.35.jpg
3 - https://dl.dropbox.com/u/55193788/Temp/2013-02-26%2021.40.56.jpg
--=20
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