Mal,
The only accurate way to measure loop inductance for an installation is
to use an inductance meter. Calculations do not take into account the
loop environment like trees and other objects in the vicinity.
100s of hours of measurements on my part indicate that trees have next to no
effect on TX loop performance, including inductance values. All my loops
amount to a conductor placed over the top of, and resting on at least 6
trees. The ground has some effect on the inductance, but less than 10%.
The same applies to natural resonant frequency, the loop or any other
antenna
needs to be measured in situ, calculations for real situations can be
totally different from theoritical values.
I find the resonant frequency also comes out very close to the theoretical
value once the inductance is calculated and/or measured.
This is where a lot are going wrong on LF.
I believe what is going wrong is that many guys fail to start out with good
reference material and not 'going through the numbers' on paper. Secondly
they don't reach a basic understanding or 'feel' of how a loop performs by
getting the Rac down to an acceptable value consistent with the loop
dimensions and conductor type. They will be putting 99%+ of the power into
that Rac and it is all wasted in heat - so it better be as low as possible!
A low power level must be used for this experimentation because only a tiny
breakdown of a component will totally mess up results/conclusions/opinions.
Then comes the problem of selecting a capacitor that can handle the voltage
and current in the loop. At 1w my loops run up to 1.5A at 150v RMS... no
problem for plain old 500v micas. At 100w the current and voltage is 15A and
1500v RMS. Getting tough? Darn right! - but doable. At 1000w you are at 47A
and 4700v. For those that are this crazy, the loop conductor will have to
use #6 or multiple runs of smaller size wire and the capacitor will have to
be...... well, something exotic! My gestimation is that at 1000w you will be
exceeding 1w ERP, anyway, so this likely a theoretical design.
Bill A
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