On Thursday, October 22, 2015 at 1:20:31 PM UTC-5, Brian Howie wrote:


It's pretty complicated .There's distributed capacitance between the
windings and distributed capacitance between the windings and the
shield. I know the inductance of the loop and the resonant frequency
with the variable C set to minimum, so I can work out an effective
capacitance 100pf that causes the resonance.

There's a further complication in that the coupling loop is also
capacitively coupled to the main windings and the shield, so
connecting that up changes the resonance as well.

If adding the coupling loop changes the resonance, it's so small
as to be ignored in the real world.


I can't even begin to think how to model it.

Modeling it can be a pain I imagine, but it's all quite easy
to calculate using Reg Edwards program rjeloop3.exe.
Which is a DOS program, but I installed "DOSbox" on my Win 7 64 box,
and it runs just fine.

Lets take it for a quick test drive.
Lets make a square loop 1000mm per side, with seven turns of 1mm wire,
with a ratio/wire spacing of 5mm between the wires.
We want to tune it to 500khz for an example.
The program proclaims that the inductance of the loop is 155.6 mh.
The inductive reactance is 489 ohms.
The HF loss resistance of the wire is 2.32 ohms.
The self resonant frequency of the loop is 4.6 mhz.
Total cap value to tune 500 khz is 651 pf, - stray capacitance of 8 pf,
leaves a setting of the cap at 643 pf.
The appx Q of the coil is 210, and the receive sensitivity is 53 db
below a 1/4 wl vertical.
Total width of winding is 31mm, and the total length of wire is 28m.
Impedance seen across loop when tuned is 102.8 K-ohms.
Impedance seen by receiver is 2.1 K-ohms via a 1 turn coupling loop.

The program can be used to play "what if" until the cows come home,
and any single specification can be changed and tested to see the
difference.


The unshielded wide-spaced loop makes it easier to design, but you
get ( arguably) no electric field shielding, which is where we came
in.

You would get an argument from me, as the "electric field shielding"
is the part I consider total malarkey, and I believe W8JI was of
pretty much the same opinion when I took a quick glance of his article.

I've built nearly every type of small receiving loop there is,
and the tested results reinforced my feeling that the concept
of "electric field shielding" is malarkey. It's all about balance,
not electric field shielding as far as I'm concerned.
The gapped shield loops were no better at reducing noise than a properly
balanced solenoid or pancake loop.
Heck, I even tried using a single turn shielded loop as the coupling
loop. Worked fine, but no better than a plain wire coupling loop,
being as I had no balance issues.