On Nov 21, 5:18*pm, Wimpie wrote:
On 21 nov, 16:44, Art Unwin wrote:



On Nov 21, 5:38*am, Wimpie wrote:

On 21 nov, 04:47, Steve wrote:

I've seen several programs that will help you calculate the precise
dimensions of a single-turn loop, given the composition of the
radiating element, its thickness, and so on. However, none of these
programs are written to cover the case of a two or more-turn loop.

Does anyone know of a program that will offer guidance in the
construction of a two or more-turn loop?

Thanks,

Steve

Hello Steve,

You probably did some loop calculations and found that in a transmit
case the voltage across the tuning capacitor is very high (and
bandwidth is limited). Also for small loops, most input power is lost
as heat due to copper resistance.

When you make a two turn loop, the radiation resistance will increase
with factor 4. So with half the current through the loop, the radiated
power is same (as for a single turn loop). *When the 2 turns of the
loop are relative close together, the inductance increases with factor
4, hence the reactance.

The current has been halved, but because of the reactance, the voltage
across the tuning capacitance will be 2 times the value for the single
turn loop with higher probability on corona effects. *An advantage can
be an almost 4 times smaller tuning capacitor.

One may expect that the loss resistance due to heat of a two-turn
inductor will be twice as high (w.r.t. single turn case). This is not
true; the loss resistance will be more then twice as high because of
proximity effect. The current will not equally distribute along the
circumference of the tube/wire. *So the efficiency of the loop will be
less then twice as high (w.r.t. single turn case).

When the turns are far apart (with respect to wire/tube diameter),
inductance will not be 4 times higher and proximity effect will be
less. You will get better performance than the single turn loop made
of same diameter tube/wire. The result will be the same as when you
place the two turns in parallel. Inductance will decrease somewhat
(hence lower voltage across capacitor), AC resistance also, hence
radiation efficiency).

There is an "however". When you make a single turn loop from flat
strip that has the same width as the length of your two-turn loop, you
will notice: *1. reduced AC resistance (because of the significantly
larger circumference of the flat strip with respect to a thin round
tube, 2. inductance will decrease (H field lines have to take a longer
path around the wide strip), 3. radiation resistance will not change
with respect to a single turn loop from wire/tube.
This results in higher efficiency and increased bandwidth. * The
overall result will be better then for your two-turn loop. I think
that is the reason why most programs are for single turn loops.

So for the transmit case, given fixed diameter of your loop, the
larger the copper surface (=length*circumference), the better the
efficiency. *Best thing to enhance conductor surface is to use very
wide flat strip (high wind load), or multiple wires (with some spacing
in between) in parallel (limited wind load).

Off course for the receive-only case, a multi turn loop can be helpful
as you can use a smaller tuning capacitor.

Best regards,

Wim
PA3DJSwww.tetech.nl
In case of PM, don't forget to remove abc.

Seems to me you are recommending the "?slinky" !
Is that correct?
Art

Sorry Art, I am not talking about a slinky.

I am just talking about a multi turn (2 turns) loop where overall wire
length is 0.25 lambda so you can assume that current in wire is
constant along the length. It must be tuned by external capacitance.

Regarding the strip. When you take a 3.14m long 20cm wide thin copper
strip and make a loop of it (1m diameter), it will have a better
efficiency then when you take 6.28m *copper tubing with Dtube=2cm and
make a two-turn loop (Dloop=1m, turns 18 cm apart).

In the strip case, the current has more circumference to flow (40cm)
instead of 6.28cm for the copper tubing. *AC resistance of copper
tubing will be about 10 times higher. Off course, current in two-turn
loop will be half (for same radiated power), but still heat losses
will be 10*0.5^2=2.5 times higher (for the two-turn loop).

When both loops have good efficiency (so radiation resistance
dominates), the strip loop will have better bandwidth as flux path is
longer and therefore results in less inductance.

I hope this clarifies my posting.

Best regards,

Wim
PA3DJS
Please remove abc in case of PM.

I think I am missing something Wim. A slinky has a strip winding that
is edge wound which provides the largest disparity
between the inside radius and the outside radius. On one of the top
transmitters the inductance winding is such that the inner radius
is close to the outside radius. Naturally the different pitch of the
windings is very different as is the inter coil capacitance.
As Roy stated charges accumulate on sharp edges which I see as correct
but I cannot see how that alteres the diference all that much as the
same clearance is required So in the final analysis for less
inductance which form is which., the longer inductance or the shorter
inductance on the assumption that the number of turns are similara nd
I can acceptt your word for it? I referred to a slinky purely to
emphasize the importance of reverse windings so that lumped loadings
applied are cancelled. Actually the modern slinky is not contra wound
for some reason but I assume that is for the novelty movement reasons
for children and not because of radiation reasons. The slinky patent
is now defunct if that matters and iI am assuming that the fed would
be centre fed.
Thank you so much for responding
Best regards
Art