Hello Dave,
A Q of 20 seems very low, it would mean that losses are as high as 5 Ohm (Q
= XL/R).
But maybe Q is calculated based on the Rrad of 2.5 Ohm, that would explain
the low value.
73, Rik
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
[email protected]
[email protected]
http://www.dsergeant.btinternet.co.uk
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