[Elecraft] OT: SGC Coupler / Small Wire Loops

Tony Wells [email protected]
Fri Mar 28 05:11:01 2003


Ron in his previous posting, kindly wrote a lot of stuff about Mag Loops and
the voltage and current phase relationships, and the summary was:

>So, please let me amend my statement above and say that the "phase
>relationship of the voltage and current are absolutely critical in order
for
>the energy to flow efficiently..."

Hi Ron,

Well, I ain't JC Maxwell, my education was in quantum mathematics, but I
have been a mag loop obsessive for a long time, and I would endorse your
caution about stating the exact phase relationship.

There is some evidence that the phase relationship varies around the
curcumference of a mag loop circle. Stuart K5KVH alluded to this I think in
an earlier posting on this off-topic topic. There are certain areas around
the circumference of a mag loop where low resistance does not seem important
from the point of view of radiating into free space. I suspect, but do not
have the tools to prove, this is where voltage and current are more in phase
than elsewhere in the circle diameter, and the current is relatively low.

Typically the area concerned is approximately 1/8 of the circumference
spanning the location of the capacitor. Also, depending on the type of
matching network (eg small loop transformer vs gamma match) there is a
similar area which seems to vary in size between 1/16 and 1/8 around the
location of the matching section.

I've also done experiments to extend the unimportant areas of radiation
resistance by putting multiple capacitors around the circumference of a
loop. I used 5 high-voltage caps and one variable tuning cap and ordinary
mains flex connecting the lot. However the results were inconclusive because
when I was checking the far field strength I blew all 5 caps on 5W !!!! Lots
of smoke and sparks! When I get some more caps I'll report back!

There is one other point. Many mag loop users want to get more than one band
out of their loop and can get a reasonably good SWR across 2 or more bands
when they adjust size and capacitance. However the radiating efficency in
the far field is considerably effected by trying to achieve this. This is
caused by the "traditional" approach of using fixed ratios in the matching
network - eg ratio of diamete of matching loop/gamma match vs main loop. I
*think* this is heavily related to the phasing issue, but will probably
never prove it.

When using an antenna analyser in conjunction with field strength tests, and
drawing lots of graphs, it is possible to see that there is only one optimum
transformer ratio for a particular frequency for a particular loop diameter.
In practice, for frequency, read "band". The way to handle the ratio problem
is to use multiple matching ratios - gamma matches are best for this - using
relays for example to swich in different gamma match lengths.

Anyway....

Regards,

Tony
M3CJF
G7IGG

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