[Lf] Ferrite Rods in LF Antennas

[Andre' Kesteloot]

Rik Strobbe wrote:

> 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