This aerial covers the 20, 17, 15, 12 and 10 metre bands and is made from a single 3 metre length of 22 mm copper tube cut into 4 pieces of equal length and joined in the corners with 90 degree elbows. Some plumbing skills are required here as you will need to use a blow torch with plumbers solder paste and solder so do not attempt this part if you do not have the required confidence and equipment. The pictures below show the full story at each stage of construction.
The wide spaced tuning capacitor being used is a very old 200pF + 200pF type but the fixed vanes are connected in series, the moving vanes will not be connected at all therefore the capacitance can can be varied from about 7pF min to 100pF max. The minimum capacitance is important as it is this that determines the highest frequency that the loop can be resonated at. The tube at the opposite side to the gamma match is marked in the middle and then the tuning capacitor positioned and an equal amount cut off to exactly accomodate the tuning capacitor used so that direct connections can be made. The dimensions of the gamma match are quite critical in order to achieve a good match on each of the bands covered, it is made from 8mm micro bore heating pipe.
The tuning capacitor is driven by a geared down motor which produces 2 rpm at 12 volts. There is a wire wound 10k ohm pot employed which is inserted between the motor and the tuning capacitor isolating coupler which sends the required information back to the shack to drive a moving coil meter which can be calibrated in frequency. The loop covers from 14 to 29.7Mc/s and presents a one to one swr throughout.
I believe MFJ are the only commercial manufacturers of these but they are VERY expensive!
The main problem with mag loops is that they are very high Q and need to be re tuned even for a relatively small change in frequency so a motorised tuning capacitor arrangement is most definately required.
The big advantages are that they can be used indoors, are small in size and it is possible to cover quite a few bands with the same loop, they also tend to pick up less locally generated noise. The loop does have directional properties and can be used in the vertical or horizontal mode.
This mag loop will of course work well outside but quite a bit of thought will need to be put into its weather proofing……READ FULL ARTICLE
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