After the May 3rd meeting, Fred KC2QFR, JB WA2WMJ, and Stan WB2LQF sauntered out into the parking lot to see how well Stan’s newly revised 20 meter mag loop would work in front of a couple impartial witnesses.
The mag loop has a circumference of approx 11.5 feet and is made of 1.5″ copper pipe. It has a 10-100pf vacuum variable tuned by a 1 rpm motor drive. When designing a mag loop, one can have any two of the following: High Q, broad bandwidth, or high efficiency. Stan chose High Q and high efficiency. This resulted in a very narrow bandwidth. Hence the one rpm motor. Tuning is very sharp. The original Farady loop coupling was replaced with a cut-and-try Gamma match that resulted in 1.1:1 SWR . JB made a CW contact with N0ZH in Missouri. Mike was running 5 Watts on 14.060. JB was putting out around 50 watts. Mike gave JB a 559. JB gave Mike a 229 (not too bad for a QRP signal into our loop in the parking lot behind a big steel beambuilding with a high noise level from the parking lot lamps.
A mag loop operated with vertical polarization can be used right at ground level. Thus, a mag loop is quite easy to set up – there simply IS no setup. Just put it down and hook it to the radio! The radiation pattern is a figure eight with deep nulls broadside so the loop is quite directional. The front to back ratio is, of course, 1:1.
More testing will be done but initial results suggest that having moved the capacitor and motor from their original location in the center of the loop to the point outside and above the loop improved transmitting performance significantly. This loop – in both its original configuration and this new revision – was always a fantastic receiving antenna. Loops are known for that! 73, Stan