While doing a bit of clean up in the garage, I came across the CML-47205 antenna loading coil units. They came as part of a couple of US Navy TCS-13 transmitter/receivers that ended up in my collection a few years ago.
They’re simple black boxes, 6 x 8.5 x 5.75 inches with a 7 position dial switch numbered 0-6, 2 thumbscrew terminals and a ground terminal. Inside it’s pretty simple, a multiposition switch and a tapped inductor coil on a plastic form.
From the manual, it’s meant to be used with a 20 foot whip between 1.5 – 3 MHz. Dial setting 0 selects the highest inductance for the lower frequencies, while dial setting 6 selects the lowest inductance for the higher frequencies.
Using my AADE LC meter connected across the two terminals, I measured the inductance for each position.
| Dial setting | Inductance (μH) SN 1272 | Inductance (μH) SN 1793 |
| 0 | 98.32 | 97.76 |
| 1 | 76.49 | 75.81 |
| 2 | 59.08 | 58.57 |
| 3 | 41.78 | 41.51 |
| 4 | 25.83 | 25.53 |
| 5 | 12.98 | 12.72 |
| 6 | 0.46 | 0.38 |
Might be interesting to see if I can incorporate these into a vertical antenna for Field Day.
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