On 10/19/2015 2:28 PM, wrote:
Brian Howie wrote:
In message , bilou
writes

"Brian Howie" wrote in message
...

I've a 5 foot Octagonal loop for MF. The shield is copper water pipe, with
a gap , 7 turns inside plus a coupling winding. It does a good job
eliminating local noise (mostly ASDL hash from the phone lines) compared
with a vertical. However the capacitance between the shield and turns
seems to load it quite a bit meaning I can't get the tuning range I'd
like.

Brian GM4DIJ
--
Brian Howie
Hi
My own experience is that ,at least for receive, multi turn loops are
useless.
Instead you can use a single turn one with a good coil in serial.
The tuning range for a given variable capacitor is much greater
especially if ,at low frequency, the coil is using ferrite .
Switching the coil can increase the tuning range easily.
The coil, with a secondary winding,is also very useful to
adjust the coupling to the receiver.

I'd have thought I'd get a better signal from more turns, but maybe
better coupling and a higher Q from your suggestion would do the same.

Brian

To be a bit simplistic, the amount of signal captured is proportional
to the loop area; the number of turns has little to no effect on that.

I'm pretty sure that is not correct. The signal strength is
proportional to the number of turns *and* the loop area. I will have to
dig out my notes on this, but some factors (like Q) even out with
various changes in antenna parameters such as number of turns, loop
size, etc. But signal strength is proportional to the area of the loop
and the number of turns.

From
http://www.lz1aq.signacor.com/docs/f..._loop_engl.htm

E = 2pi w S µR e / λ
λ is the wavelength in meters
w - the number of ML turns;
S – is the area of the windings in m2;
μR is the effective magnetic permeability of the ferrite rod SML. μR is
always less than the permeability of the material used and depends from
the size, geometry and the way the windings are constructed. μR = 1 for
aerial loops.

The product:
А = w μR S (3)
is called effective area of the SML.

If you don't like this reference, I know I have seen this info in other
places too.


The number of turns greatly effects the inductance.

I believe it is N squared. Twice as many loops *and* twice as much
interaction between the loops. Picture a triangle formed by the sum of
the progression 1, 2, 3. That area is the inductance as you add loops.

1
1,2
1,2,3
1,2,3,4
1,2,3,4,5

I spent a great deal of time once trying to understand the formulas for
inductance. Seems the problem is the non-idealities of coils
significantly affect the results and vary a lot for different form
factors, etc. So it is *very* hard to produce an equation that is good
for all. The result is a number of different equations for different
shapes and many different equations over the years as better approaches
are found. I think the Lundin formula was the best one I found, even if
a bit complex. The Wheeler formula is not as general or accurate, but
simpler. Every formula I found used an N^2 term for the number of turns.

Wheeler formulae
http://home.earthlink.net/~jimlux/hv/wheeler.htm

I can't find a good reference for Lundin's formula, but if you want I
will copy my spread sheet data here or email a copy.

--

Rick