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Magnetic Loop Antenna

My primary QTH is an apartment with very little possibilities to install an antenna so it was a difficult decision finding the right setup to be able to be in the air with the less impact as possible. One of the selected solutions has been a hombrew small loop antenna. This antenna fits in my balcony and offers good enough performance for this compromise installation. Since the easy accesibility to the tuning capacitor I don't need remote tuning control so simplifying the implementation. The antenna is able to cover the 30M to the 15M bands and tested up to 100W. The performances are quite satisfying considering the loop installation conditions with the balcony metal rails close to the loop.

 
 
All materials for the construction of this antenna can be easily find in the local hardware store. The loop has a diameter of 1.20m and it has been built from 15mm flexible copper tube. This type of copper can be easily bend to form the circular shape using a proper support structure. In my case I used a round table with exactily the same requested diameter. The loop support is made out of PVC tubing and two clamps for supporting to the balcony.


The loop capacitor is a butterfly type which has been completely handcrafted. My capacitor has 15 sections. The plates have been built using 0.8mm aluminium sheet but I recommend 1mm or thicker material. For building the plates I created the shape patterns and printed to a self-adhesive printer paper that is later applied to the aliminium sheet. Then the aluminium is easily cut with a fretsaw following the shape patterns. For the rotor and stator I used M6 size brass hardware, including 5 threaded rods, nuts, and washers. The plate separators are M6 brass nuts providing enough separation for the required isolation voltage. The rotor knob control is just a PVC cap.


The capacitor support is made out metacrilate and it has two clamps to fix to the loop support. These clamps are the standard used for PVC tubing. For the connection to the loop I used two layers of copper sheet.


The primary loop is a faraday loop with dimensions 1/5 of main loop made out of RG213 coaxial cable and a SO239 T adapter. I tested with RG58 but the performances were not as good.


 
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Melchor Varela,
Jan 14, 2012, 3:49 AM