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Re: LF: LF Antennas

To: [email protected]
Subject: Re: LF: LF Antennas
From: "Mike Dennison" <[email protected]>
Date: Mon, 9 Apr 2001 15:03:12 +0100
In-reply-to: <[email protected]>
References: <[email protected]>
Reply-to: [email protected]
Sender: <[email protected]>
G2AJV 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).
I replied:
>If it is less efficient, where does the lost power go? I am puzzled.
ON7YD wrote:
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).
I have done a lot of work with ferrites in order to get the 12mH or so that I
need to get me onto the 73kHz band. I have found, as you and others have,
that broadcast type ferrite gets very hot. Plainly power is lost here.
I have at present some bits of ferrite from broken up audio pot-cored coils,
and these seem to work fine, getting only slightly warm after long periods of
QRSS. I presume that if the ferrite stays cool, the lost power is insignificant,
compared to the additional turns required to achieve this inductance without
ferrite. My 4mH coil goes to well over 7mH when only a small amount of ferrite
is inside, so I do not expect to fill the former completely with ferrite - perhaps
10% will be enough.
Even if there is a loss of efficiency, since I am replacing a 7mH ordinary wire
coil with a 3mH Litz-wound coil, my resistive losses before ferrite loading must
be much lower. So long as the ferrite loading (to get the 3mH up to 7mH) does
not increase the losses too much, I am still gaining.
If the losses are not heat, then where are they? The R has not changed, and it
is my understanding that a ferrite-cored inductor has a higher Q than an air
cored one.

Certainly any radiation from the coil will be altered, but the coil is only 300mm
long in an antenna 32m long. This radiation will be very small indeed.
I am still puzzled at Roger's remarks. I hesitate to argue with someone so
clever and experienced, so have I missed something?
Just to clarify, the problem I am trying to fix is to get the lowest loss 7mH coil
on top of the mast with minimum visual impact. I am already doing this with a
lot of wire on a drain pipe, but at the expense of resistive loss. My Litz coil
(with thicker wire) is the same physical size, much lower loss but half the
inductance.



Mike, G3XDV (IO91VT)
http://www.lf.thersgb.net



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