[Fwd: LF: Yet more loops]
Mon, 15 Jun 1998 10:17:28 -0400
Peter Martinez wrote:
> >From Peter Martinez G3PLX Kendal Cumbria.
> After my last about multi-turn versus single-turn loops, I realised that
> there is another more interesting question worth answering: Given
> that I have a fixed amount of copper, is it better to make a large loop
> of thin wire or a small loop of thick/parallel wires?
> Since we now know that it doesn't matter whether we connect wires
> in series or parallel in a loop of a given size, I think this question can
> still be answered with thought alone. Suppose I have paid a pound at
> a boot sale in Canterbury and bought 100m of wire. I feed 1 amp of
> RF into it and keep this constant throughout the next part of the
> experiment. The power loss heating the wire (assuming it is much
> greater than the radiated power) will be constant during the next
> I now coil it, first as a square loop of one turn, which will have a side
> of length 25m and hence an area of 25^2, and hence a radiation
> resistance of K*25^4, where K=320*pi^4/L^4. The radiated power will
> be I squared times this, that is K*25^4 since I=1.
> Next I coil it as a two-turn loop, which will have a side of 12.5m and
> hence an area of 12.5^2 and an Rrad of K*12.5^4. Remembering that
> 2 turns of 1 amp is the same as 1 turn of 2 amps, the radiated
> power is 2-squared times K times 12.5^4. (You get the same result
> if you do it Andy's way)
> The radiated power of the single-turn loop is thus 4 times higher than
> that of the 2-turn loop WITH THE SAME WIRE AND THE SAME
> TRANSMITTER POWER. It therefore seems to be best to build the
> quantity of wire that you have into the largest single-turn loop that
> you can get into your space.