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Date: Thu, 07 Mar 2002 10:04:11
To: rsgb_lf_group@blacksheep.org
From: "Rik Strobbe" <rik.strobbe@fys.kuleuven.ac.be>
Subject: Re: LF: Loop theory...
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At 18:07 6/03/02 -0600, WE0H wrote:
>Now a vertical has high voltage and lower current than a loop. A loop has
>high current but lower voltage than a vertical. Power = current x voltage.
P = U x I counts only if the load is purely resistive, the more general
formula for AC is : P = U x I x sin(a) where a = angle between u and I.
Example :
I'm running about 400W into my antenna, resulting in an antenna current of
1.8A and an antenna voltage of 5kV.
Taking P = U x I would result in a power of 9kW ... not bad for 400 W input
But in reality my antenna has a dominant capacitive component so phase
between voltage and current is close to 90 degrees.


>Verticals have their place on a treeless lot. Loops have their place on a
>tree filled lot.

I happen to live on a tree filled lot (several 100's of these lossy
capacitors within a range of 50m arround the antenna).
I tried a loop (12m high / 23m long) and a vertical (inv-L, 12m high / 23m
topload). The vertical was about 6-10dB better than the loop.

But maybe things would have been different for a larger loop :
Simplified one could state that the loss resistance of a vertical is rather
independent of the height (ignoring the 'footprint theory', in reality the
loss resistance will slightly decrease when increasing the heigh). On the
other hand the radiation resistance increases with the square of the
height, so the efficiency of a vertical antenna will (more or less)
increase with the square of its height. So doubling the height of a
vertical will result in a +/- 6dB improvement.
For a loop antenna the loss resistance (assuming that it is mainly
'copper-loss' in the wires) will double if you double the dimensions of a
loop (what will increase the loop area with a factor of 4) you will
increase the radiation resistance with a factor 16 (since it is
proportional to the square of the loop area). So the efficiency will
increase by a factor 8 (= 16/2) if you doulbe the loop dimensions (= 9 dB).
This means that by doubling dimensions a vertical will improve 'only' 6dB
will a loop will improve 9dB. So it is clear that by increasing the
dimensions a loop at a certain point will outperform a vertical, but I'm
afraid that these dimensions will be far too large for an average European
property (maybe different for the midwest US properties ...)

73, Rik  ON7YD