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Date: Mon, 09 Apr 2001 10:52:59
To: rsgb_lf_group@blacksheep.org
From: "Rik Strobbe" <rik.strobbe@fys.kuleuven.ac.be>
Subject: Re: LF: LF Antennas
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At 09:46 9/04/01 +0100, you wrote:
>
>> If you load it with ferrite you will certainly increase its inductance and
>> also its efficacy as a receiving antenna but the  radiation efficiency on
>> tx will decrease accordingly  (the difference between B and H).
>> 73,
>
>Roger,
>
>If it is less efficient, where does the lost power go? I am puzzled.

Hello Mike,

I'm using a loading coil with ferrite rod for 'fine tuning' the antenna (I
can slide the rod in/out the coil). Without rod the coil is 700uH, with the
rod I can tune it from 800-1500uH. The Q is 400 without the rod and sinks
to 300 (at 800uh) - 250 (at 1500uH).
So the rod introduces an extra loss of 0.5 to 2 Ohms, looks quite
acceptable to me (compared to the +/- 120 Ohm overall loss it is less than
0.1dB).
But the choise of rod is very important, I tried all (+/- 20) rods I had in
my possesion and found only one that was good. With most other rods the Q
of the coils 'crashed' to below 150 and I even managed to break a rod (by
heating) with 200W RF power.

With the right rod it works fine, at least for a rather small coil. But I
believe that you want to make a +/- 4mH coil. So assuming you can make a
2mH coil with a Q of 400 and you find the right rod you can make a 4mH coil
with a Q of 250. This would respresent a loss resistance of 13 Ohm (versus
7 Ohm for a similar coil with a Q of 400).

Regarding 'where goes the power' : a bit will be dissipated by the rod (it
will heat).

73, Rik  ON7YD