I Need Help - AI Part 2313

Prompt: Minor correction: the inside of the box is 7.5 inch, the pipe is 7 inch and the total wire winding is 6 inches. Yes, let us do the primary. I have many wire gauges available and I have sticky foam tape to space the primary winding above the secondary and maintain whatever spacing between turns is needed.

The math for this primary coil works out beautifully because your variable secondary will naturally compensate for how your 35-foot antenna behaves across different frequencies.

I recommend an 8-turn primary coil.

Here is exactly why that number works across your entire frequency range, and how to use your foam tape to mount it.

## The Auto-Correcting Turns Ratio

Your 35-foot wire drastically changes its electrical characteristics depending on the band you are listening to. Because your secondary is variable, an 8-turn primary creates an adaptive impedance match:

• Band 2 (1.6 MHz): At this frequency, a 35-foot wire is electrically tiny (about 0.05 wavelengths). It presents a very low radiation resistance. With the secondary switched to the full 80 turns, your ratio is 80:8 (a 10:1 voltage step-up). This transforms the SX-28's $400\ \Omega$ input down to $4\ \Omega$, perfectly matching the low impedance of the short wire. * Band 6 (30+ MHz): At these frequencies, a 35-foot wire is approximately a full wavelength, presenting a much higher impedance. With the secondary switched down to just the 8-turn "C" section, your ratio becomes 8:8 (a 1:1 ratio). This $400\ \Omega$ to $400\ \Omega$ match prevents the severe mismatch that a fixed AM balun would cause on the high bands.

As you click the rotary switches through Bands 3, 4, and 5, the ratio smoothly bridges the gap between those two extremes.

## Winding the Primary

Your sticky foam tape is the perfect material for this. Foam is mostly trapped air, which means it has an exceptionally low dielectric constant—ideal for preventing stray capacitance between the primary and secondary.

1. The Foam Bed: Cut a strip of your foam tape and wrap it exactly over the center "C" section of your secondary coil. Since the "C" section is 8 turns of 22 AWG silicone wire, it will be roughly 0.5 inches wide. 2. Wire Selection: Since the primary current is still virtually zero, use whatever wire is easiest to mold and holds its shape well. A solid 20 AWG or 22 AWG bare copper or enameled wire is excellent here. 3. Spacing the Turns: Wind your 8 primary turns directly over the foam tape. Instead of close-wrapping them, space them out evenly so they span the entire width of the foam bed (matching the physical width of the "C" section underneath). This ensures the magnetic flux lines perfectly intersect the core of your secondary winding. 4. Securing: Once wound, you can secure the primary wire in place with a drop of superglue or a final wrap of tape so the spaced turns do not shift.

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Last modified: Sunday, 2 Aug 2026