24.3.1998

A discussion of  The Select-a-Tenna's design and whether or not it contained a toroid, prompted this from Bob Foxworth.

In a post to amfmtvdx@qth.net he writes,
 

I believe this circuit is simply a parallel-tuned L-C circuit, just like any simple air-core loop. There are two ways to get the signal out that is developed across the L-C circuit. You can inductively couple it to the ferrite core antenna of your receiver, by holding the receiver in the near-field of the loop. This applies to small portables that do NOT have an external input jack.

Or, you can introduce a coupling winding in the field of the L part of the circuit, i.e. the tank winding, and feed that to a transmission line which goes to the terminal lugs on the back of your HQ-180 or whatever receiver you have that has a antenna connector input.

The probable way the SAT would approach this problem  would be to pass one run of the tank winding through a winding of several turns on a toroid "doughnut" form and have another winding feeding the output jack, if one exists. I do not know if the SAT even HAS an output jack, but IF it does, I think it is likely that a small coupling transformer _has_ been included, a detail the casual user wouldn't think of too much.

The typical air core loops I used long ago consisted of 11 turns, spaced a half-inch per turn (1.2 cm) , of #14 stranded wire, on a cross frame such  that each run, from corner to corner, was 3 feet or about 0.95 meter. This then required approx 11 x 3 x 4 feet of wire, totalling actually about 135 ft. when allowing for adding spreaders, and lead length to the cap. This was tuned with a 365 pf. air cap. The link winding was a single turn wound along the same path as the tank winding, feeding a run of coax leading to the receiver. This fed the low-Z rcvr input reasonably well and did not degrade the loop Q too badly.

Bob