7536Re: [loopantennas] Re: Feeding A Magnetic TX Loop
- Jun 3, 2010In a message dated 6/3/2010 1:31:57 P.M. Eastern Daylight Time,
> > Hi All,Hi Irv,
> > I am wondering if anyone has tried locating the feed loop
> > or Gamma match next to the tuning capacitor and if
> > they had success with this arrangement?
> > 73 Todd WD4NGG
>The portion of the loop opposite the capacitor is the only real
>place to get a low impedance feed point which the gamma match
>requires. You would have difficulty feeding the loop anywhere
>around the capacitor because of the very high voltages there.
It is not really difficult to couple to the loop at the tuning
capacitor. The circuitry is simple ... the problem is as you say ...
dealing with the high RF voltages. It also relates to the amount of power
you run. If you run QRP no problem.
>Believe me, it would greatly simplify the construction of theThe reason you don't see it done is that it requires non-traditional
>magloops if the feedpoint and capacitor could be colocated -- thats
>why you never see it in magloop articles.
MFJ made several versions of a Mag Loop Tuner that operated in this
fashion. Simply connect the loop to the 2 stand-off insulators. the tuner
acted as the base as well as the tuner and it could sit on the table next
to the rig. I didn't particular like the idea of the mag loop sitting a
couple of feet away from me ... even though I was only operating 10 watts
Thanks for the many ideas and suggestions on trying to feed a
magloop next to the tuning capacitor. I had forgotten about the MFJ
loop tuners that use a capacitive match right next to the tuning capacitor.
It looks like the Gamma Match is not really suited to feed a loop with
at a high-impedance point like right near the tuning cap, but works well at
low impedance point opposite the loop tuning cap. Plus by its nature one
of the Gamma Match is grounded. That would upset the balance of the loop
if placed right next to the loop tuning cap.
About 15 years ago I did successfully feed a balanced, ungrounded loop
next to the tuning capacitor once
with an unusual arrangement. I was using a QRP transmitter with a tuned
tank coil and inductively coupled the tank coil into a corner of a magloop
next to the tuning capacitor. I made the tank coil out of about 3 turns of
about 1 ft in diameter feeding a loop 10 ft square or 40 ft circumference.
was in the Medfer band about 1645KHz. The tank coil was resonated with its
own tuning capacitor. Then I varied the spacing to the loop for best
I was basically coupling one tuned coil into another larger tuned coil (the
tuned tank coil of the Mosfet final was not a real high impedance like you
would find in
a tube final but I would guess was somewhere in the range of 250-300 ohms.
This makes me think that perhaps stepping up the 50 ohm output of
common coax feedline with a 1:2 or 1:3 ratio transformer to start with and
that into either a larger-size single-turn feed loop or perhaps a
feedloop would give a better match at the higher-impedance feedpoint next
the loop tuning capacitor? Then keep experimenting with the step-up turns
size of the feedloop plus spacing to see if best match can be found?
I will report back on this if I can get any success with this method over
73 Todd WD4NGG
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