From: "Dave Sergeant" <dsergeant@connectfree.co.uk>
To: rsgb_lf_group@blacksheep.org
Date: Thu, 28 Nov 2002 06:36:08 -0000
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Subject: Re: LF: loop antenna
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On 25 Nov 2002 at 17:58, Rik Strobbe wrote:

> I am about to put up a transmitting loop antenna. Dimensions will be 23m
> long and 15m high. Thanks to Dick (PA0SE) I could achieve a wheel of 7mm
> litz wire, so wire losses should be minimized (about 0.04 Ohm).

I have just done a couple of calculations on Rik's loop using 
formulae from Mike Underhill G3LHZ (which are quoted on my web site). 
Mike says that the maximum Q is in fact limited by the size of the 
loop and has a limiting value of Q=500/D, where D is the diameter of 
the loop (or for a rectangular loop the circumference divided by pi). 
Feeding Rik's dimensions into this gives a maximum Q of 20, and it 
will in practice be lower than this due to resistive losses.  This 
formula has been derived by Mike from actual measurements on loops 
and while I would vouch nothing for its validity myself, measurements 
I did on my loop some time ago did indicate it was in the right ball 
park figure. Reducing dc resistance will not increase the Q above 
this figure and the astronomic voltages and currents rumoured will 
not arise.

The second formula from Mike is his for loop inductance - it gives a 
value of 123uH which sounds about right.

The third of Mike's formulae on my site is his for radiation 
resistance. As I have previously found it produces silly values of 
radiation resistance for 136 loops - in Rik's case of about 2.5 ohms, 
clearly nonsense.

Regarding currents flowing in the loop (and assuming the Q stays 
within the sensible limits above) measurements on my loop done at the 
5W level and scaled indicate a loop current of 26A at 400W. This is 
probably well in the handling range of Philips 376 polycarbonate 
capacitors - GW4ALG used this power with his loop with no problems. 
These capacitors are cheap and worth experimenting with.

73s Dave G3YMC

dsergeant@iee.org
dsergeant@btinternet.com
http://www.dsergeant.btinternet.co.uk