Making a Magnetic Loop Antenna

I've been using my recently acquired SDR for a while now hooked to a simple wire antenna, but as you might expect the results were far from ideal.  So, I made the decision to build (not buy) my own magnetic loop antenna.  The big advantages of this type of antenna over others are their small size, portability and decent bandwidth.

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:

(Source: http://www.aa5tb.com/loop.html)

For the large loop I used 10' of 1/4" copper tubing inserted inside an identical length of 1/2" PEX tubing to aid in stiffness and durability.  Pigtails were soldered to either end of the copper and then connected to an air variable capacitor I had in my junk box (thanks Bob, WA1EDJ), creating a parallel resonant circuit. The measured inductance of the copper loop was ~3.25uH and the variable cap I had selected had a range of ~10-200pF.  This gave me a calculated tuneable bandwidth of ~6.2MHz to 27.9MHz.

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:

Copper tubing inside PEX (large loop)
with a soldered pigtail on one end

Coax connected to ends of small
copper (inner) loop

Inner loop positioned closely to outer tube

Vertical support strut added

Tee connector at bottom of large loop

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.