The antenna consists of a large, outer resonant loop for picking up the RF signals and a smaller, magnetically coupled, inner loop that connects to the radio. See diagram below left:
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| (Source: http://www.aa5tb.com/loop.html) |
A 2' length of copper tubing was then formed into a smaller loop with one end connected to the center conductor of a length of RG-58 coax while the other end was connected to the outer braid. The other end of the coax was mated with a BNC connector to hook up with the receiver. This inner loop was then positioned in close proximity to the outer loop and secured in place with zip ties.
Some close ups of the various construction details:
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| Copper tubing inside PEX (large loop) with a soldered pigtail on one end |
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| Coax connected to ends of small copper (inner) loop |
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| Inner loop positioned closely to outer tube |
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| Vertical support strut added |
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| Tee connector at bottom of large loop |
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| Completed antenna minus tuning capacitor |
After assembly, I hooked it up to the radio to see if reception had improved. Definitely!! I also took measurements to determine the actual tuneable bandwidth. It seemed to show signal peaks at ~6MHz on the low end and ~18.5MHz on the high end. This corresponds roughly to the 40-17m amateur bands. I can only attribute the difference in high end response (27.9MHz calculated vs 18.5MHz actual) to additional parasitic capacitance of ~13pF.
I'm not worried about weather proofing the variable cap since the entire antenna unit can easily be moved indoors at a moments notice in case of rain or snow. The next task at hand will be to make additional antennas to cover different bands.
