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The Ugly Magnetic Loop

Well its about time I pulled my finger out so today I tried it and succeeded.                    

Its one of those things I suppose, I have tons of them, all whirling around up there, yes I'm talking about ideas for antennas.

Will it work if I scrimp on parts?
Will it work with just all this old tat in my draw?
Do I need specific value components?

The answer is "you'll never know if you don't attempt to find out".
You don't have to have a certain callsign or be a certain age to enjoy tinkering with antennas.

Today I built my first magnetic loop antenna.
Here are a few pictures while I get on with the serious business of editing and rendering.

Told you it was ugly..

The inner loop is made from 80cms of Westflex W103 Coax with the inner dielectric soldered to the outer braid, its a bit to long as its larger than 1/5th-1/6th of the outer loop so this might have to be adjusted.

The W103 loop terminated in a chock block ready for the W103 feeder.
Plenty of heat shrink to tidy up the job.

The loop complete but with manual tuning only, not for long I have a cunning plan.
A very simple 30amp terminal strip joining the inner loop to the feeder.

Things went amazingly well with an SWR of 1.1 was achievable on 40 meters though the bandwidth was narrow but then I had 20 meters in mind for the center of the possible tuning.
I ended up run out of time and battery power before I could check any other other other bands to any extent.

My first magloop attempt

According to online calculators a circumference of 3.6 meters is too small for 40 meters with an efficiency of 12% (-9.7 dB below 100%) this percentage increases to 57%  (-2.4 dB below 100%) efficiency on 20 meters and up to 76% (-1.2 dB below 100%) on 17 meters
I couldn't reach 15 meters because 27pf was needed and the lowest value I had was 40pf to work with.

I hope to continue testing in the next few days to see what available bandwidth is available on the other bands and perhaps I could tempt it onto 60 meters where the loop is far to small.

To be continued...

UPDATE 26/07/2013
I reduced the length of the inner loop to bring its circumference closer to 1/5th

With the antenna 1.2 meters above the ground a 1.1 SWR was obtained on 60 meters (5mhz), although only having a bandwidth of about 10khz between having to re-tune it, it covered all the band.  It also covered all of 40 meters with a 1.1 SWR achievable anywhere between 7-7.2mhz
30 meters is the same story, it covers the whole band with ease.

Now 20 meters is where my sums didn't add up, It will tune all the way up to 13.6mhz but no higher, the airspaced capacitor is 99% un-meshed and just doesn't have the range, so close...
I'm now torn between reducing the outer loop to make bring 20 meters in but then risking loosing 60 meters..

Its a hard choice.
I'll leave it lone for now.. Perhaps I can adjust the capacitor to reduce the value.

*update* 27/07/2013

Hey all
I've worked out how I'm going to turn the capacitor remotely, after more tests last night after I'd reduced circumference of the inner loop and found that its is actually going to work.
I think its now worth purchasing more parts to see this project to its end.

I'm going to use this 0.5rpm motor to turn the capacitor, It should be slow enough to able to fine tune the loop.

I thought at first I will just control this motor with a simple rocker switch to spin the motor clockwise and counter-clock wise, maybe I'll leave this particular loop manual electric control.

If I move the loop up on the roof or up in the loft its not going to be so easy perhaps to run the DC control cable.  This is where my next gizmo comes into play.

This circuit makes the whole thing wireless!
It will be as easy as pressing a button and listening for the sound peak.

The potentiometer is a speed adjustment just in case 0.5rpm is not slow enough for fine tuning on some of the bands.  So eventually I would like to build a magnetic loop which will handle the UK limit of 400 watts.   The air spaced capacitor I used in this loop can handle the 2,244 volts RMS that is generated from the 100 watts input on 7.130mhz but at 400 watts this voltage increases to a scary 4,488 volts RMS! Get ready for some spectacular flash overs!

Thanks for stopping by

Lets hope it all works when it all goes together!

73 de Mark

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Thanks for all your support.