OBSERVATIONS FROM SCOTLAND                            28 May 2008                                GM1SXX

Mag-Loops for tightwads.

Following yesterdays modest success with the DIY  Mag-Loop, it was obvious that a portable version of the loop for outdoor or temporary operation was a possibility.

This is presented simply as an idea, for your perusal.  Hopefully I'll get time to actually make one of these loops because it looks very useful indeed.

The concept is simple, replace the solid copper loop with one made from the braid of some decent co-ax such as RG213 and support it on a temporary structure (AKA a fishing pole, with a horizontal boom.  You need a 4M length of braid.

The boom  can be just a piece of dry wood with a hole to suit the diameter of the fishing pole used and a notch or holes on each end to hold out the wire into a square. I'd suggest using a miniature three way switch and a set of three small trimmers to tune the loop, one for each band.  Do remember that even at 5W PEP, voltages of 1-2Kilovolts will be present across the capacitor on transmit.

To resonate a loop of 4M total length arranged in a square, you will need the following approximate values of capacitance.

3.78Mhz  485PF

7Mhz      140PF 

14Mhz  31pf

The capacitors should really have a rating of 2KV or better.  Old mica 'postage stamp' types would be perfect for the job.

Such a loop should allow operation on 80, 40 & 20M. The efficiency on each band will be around 1%, 9% and 54% respectively  Although this looks very poor, especially on 80&40 its worth remembering that a 1/4 wave vertical with radials on 80M typically manages under ten percent efficiency.  On 20M, it looks not at all bad.

The feed system could be copied from my 'El-Cheapo magloop, namely the use of an FT200 core as a transformer with taps to suit the particular band(s). A 4 way rotary switch to connect to taps at 5,10,15 & 20T should do the job. Alternatively, you may wish to try 4,8,12 & 16 turns.  Experiment!

Remember to place something soft, like tape, around the toroid. They are brittle and easily shattered as I know to my cost.

A couple of ferrite sleeves over the co-ax where it connects to the matching switch wouldn't go wrong. Wrap several layers of PVC tape around them to avoid breaking them against hard surfaces.

 This could make a very nice aerial for field use, especially for 20M.

73 Al.

GM1SXX

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