Efficiency of MFJ remotely-tuned loop antennas

[ANDY DURBIN]

"I wonder if the SWR runaway at 150W could be due to the PVC pipe used as
the insulator for the variable capacitor?  PVC is quite lossy at RF.
Perhaps when it gets hot it detunes the capacitor.  It would be easy to
check by simply feeling the cap to see if it gets warm."

The insulator/dielectric is PEX not PVC.   I have studied the impact of temperature on the antenna and it exhibits some interesting and sometimes non-intuitive effects.  The biggest change is seen when the sun hits the antenna soon after run rise.   Study of heating during TX would require sensitive thermal imaging.  There is no change detectable by touch after a TX.  Touching it while transmitting would be counter productive and perhaps painful.

I have lots of notes and some plots of the thermal characteristics.  Maybe I'll get back to working on it one day.

73,
Andy, k3wyc

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[Augie "Gus" Hansen]

On 1/19/2021 8:20 PM, Andy Durbin wrote:
> A couple of people expressed an interest so here is a link to a presentation on my mag loop.   It's a while since the presentation was given but the loop is still in service in the gamma match configuration.
>
> https://www.dropbox.com/s/5va4ygzbd410le4/Prototype%20Magnetic%20Loop%20rev%20-.pdf?dl=0

Nice job Andy. Looks like we've both had some fun with these antennas
that have become quite popular in recent years.

My interest in Mag Loop antennas started back in the early 1980s, and I
have designed and built a couple of dozen over the years since, mostly
as experiments to gain knowledge and experience, and several to help
friends who lived under extreme antenna restrictions.

The two pictures in this Dropbox folder show one incarnation of a mag
loop that uses flat mill stock for the main loop, and a trombone-style
capacitor for tuning:

https://www.dropbox.com/sh/gqbzz60bcwkc54h/AACAjWK6JFfyLN9qcvq9oSrqa?dl=0

My capacitor design uses PEXX tubing for the dielectric layer (must be
protected from UV), and has fiberglass stiffeners inside the sliding
part. The PEXX allowed me to run up to about 300 watts. And the
stiffeners hold the tubes in line at the low capacity end of travel to
prevent misalignment (sloppy tuning).

I long ago moved to all copper loops and vacuum variables to handle high
power and obtain better efficiency. An example of a two-turn version of
a 40-80m mag loop is on my QRZ.com page:

     https://www.qrz.com/db/KB0YH

It uses 7/8" Heliax (the shield) as the main loop conductor. Kinda
heavy, so some of my newer mag loops use 1/2" Heliax or soft copper
tubing instead. Performance suffers a little, but no one notices the
difference.

Have fun and keep safe.

Gus Hansen
KB0YH





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