Here is the breakdown of those three excellent bench questions, starting with the physics of the winding direction.
## Winding Direction
For this specific receive-only application, the winding direction does not matter at all.
If you wind the primary in the opposite direction of the secondary, it simply shifts the phase of the RF AC signal by 180 degrees. Because you are feeding a single receiver and the primary is completely electrically isolated from the secondary (no shared ground), the SX-28 cannot detect a phase inversion. A positive voltage swing peaking slightly earlier or later makes zero difference to the audio output. Wind it in whichever direction is easiest for your hands.
From an RF standpoint, the bare copper touching the sticky adhesive will not cause any noticeable capacitive or dielectric losses. The tape is mostly air, and at these receive currents, the signal will pass flawlessly.
The only minor issue is long-term chemistry. Some adhesives in foam tape are slightly acidic and will eventually react with bare copper, turning it green (verdigris) over the years. Given that you are sealing this inside a project box, it is primarily a cosmetic issue, but if you want to prevent it, a quick spray of clear acrylic or a wipe of flux paste on the bare wire after winding will protect it.
Trying the 16 AWG insulated house wire is a great experiment, especially since wrestling 8 turns is far more manageable than trying to wrap 80 turns.
Here is what you will likely find with the physical fit: Standard 16 AWG THHN or similar PVC house wire usually has an outer diameter of around 0.1 to 0.12 inches due to the thick jacket.
It will likely spill slightly over your 0.6-inch "C" section, but electrically, this is perfectly fine. Having the primary overhang the center active section by a fraction of an inch will not negatively impact the magnetic coupling. The only real battle will be keeping that stiff solid PVC wire perfectly tensioned over the squishy foam tape without it trying to spring loose before you can glue or tape it down.
If the 16 AWG fights you too much, the 18 AWG bare copper spaced cleanly over the foam will perform beautifully.
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Last modified: Sunday, 2 Aug 2026