"Jim Lux" wrote in message
...
John KD5YI wrote:
"Jim Lux" wrote in message
...
John KD5YI wrote:

"Owen Duffy" wrote in message
...
Michael Coslo wrote in news:gvhhkj$do4m$1
@tr22n12.aset.psu.edu:

I'm starting up an old project that I never quite finished - my loop
antenna.

When I first put it together, I tried a simple loop of coax to couple
the radio to the loop. Was never really satisfied with that though.

I'm wanting to try a gamma match, possibly something a little like
what
Owen Duffy posted a nice version of.

The Gamma match is fairly understandable to me, and I expect it to
work
well.

What I am wondering about is in the coax loop coupling system, how is
the best match obtained? Loop size? orientation? Luck?

Mike, you haven't given much information about the loop (size,
frequency). Is loop balance / symmetry important?

If it was a small loop, I would not be thinking about a gamma match
because of the impedance ratios for just one reason.

Owen


Owen -

I've had this nagging idea for a couple of years: What if the
(single-turn?) loop passes through a toroid which already has, say, 10
or so turns on it? That should give an impedance step-up of 100 or so.
Would that not be a good way to at least get the matching closer? In
fact, the tuning capacitor could be connected to the high impedance
side to reduce the capacitor's value.


This has been done in a variety of ways (notably some papers from 20
years ago where they used this scheme to couple to the windshield window
frame on a jeep.

For Rx only, there's no real issues

For Tx, though, a small loop will tend to have high currents. There's a
LOT of energy stored in the magnetic field (and in the E field of the
capacitor that tunes the system). All that reactive energy will be
flowing back and forth through the transformer, so it will need a high
VA rating.

Consider this.. if the loop has an electrical Q of, say, 1000 (which
isn't unusual), and you're radiating 100W, then there's 100kW of
circulating power in the system.


Adding the transformer changes the circulating energy in the system how?
(Excluding losses)



It doesn't.. it's that the circulating energy in a high Q system is quite
large, so you need to size the transformer (core) appropriately.

The transformer size needed to handle a 100W Tx in a resonant dipole is
MUCH smaller than the transformer needed to handle a 100W Tx in a compact
loop with a Q of 1000. The transformer has to handle not only the
input(and radiated) power, but also the circulating power.

Actually, ALL the components have to deal with the circulating power, but
most builders of compact loops are aware of the need for low resistance in
the loop and high voltage ratings in the capacitor.


I must have missed something. I did not realize that Michael intended to use
the loop with a 100 watt transmitter. My assumption was that it was
primarily a receiving antenna.

However, it doesn't matter because your comments are equally applicable to a
short wire antenna. Therefore, I would urge readers not to consider the
small loop any worse than a short wire antenna in these regards.