On 28 Nov 2002 at 13:16, Andy talbot wrote:
There is an awful lot of controversy over the G3LHZ theories on
loops......................
Yes, I accept this and you will see that I have included his formulae
on my website with the proviso that I do not necessarily believe
them. He derived them from real performance measurements of hf loops.
Certainly his formula for radiation resistance is many orders of
magnitude too optimistic for lf loops!
His formula for Q (and the one formula he is adament is accurate!!)
indicates the best achievable Q for any loop based on its diameter,
and is intended to show that even reducing resitive losses to zero
you will not get better than this figure. It is based on loop
bandwidth measurements. For my loop (not actively used at the moment)
the formula gives a Q of 35 and an effective bandwidth of 3.9kHz. The
fact that I achieve around 1kHz bandwidth (Q of 136) even with far
from minimised resistive losses indicates the formula is a bit
pessimistic!
There may be some truth in the mechanisms he is trying to demonstrate
- ie you can only increase Q up to a certain level and reducing
resistive losses below a certain level is a case of diminishing
returns - but I appreciate the values seem wrong.
Mike's theories may well be controversial but that doesn't mean they
should not be discussed on here. Rik, put up your loop and measure
what it does, then feed back the figures to G3LHZ. If enough do this
(as I have), he may rethink!
Incidentally Rik's figure of 30A loop current at 400W is pretty close
the the 26A I interpolated for my loop based on real measurements at
5W level (not many of us have 30A rf current meters!). Other
measurements I have made also tie in pretty well to the classical
loop theory values so we may well be coming to the conclusion that
Mike has got it wrong!
Well I guess controversy is good for us. Remember the CFA....?
73s Dave G3YMC
[email protected]
[email protected]
http://www.dsergeant.btinternet.co.uk
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