I Need Help - AI Part 2300

Prompt: Let us switch to version two. I would like to describe what I intend to build and see what you have to say about it. Form length: 7 inches (was 7.5). The first coil on the form is the secondary because the wire is so fine and there are so many turns. Coil length: 5 inches (was 4.5). The secondary will have one always-active section in the very middle and sections on each end that are activated by means of a plug-board style interface. In my notes, I labeled the sections M4, M3, M2, M1, C, P1, P2, P3 & P4. All of these sections form a continuous close wrapped single layer occupying the 5 inch space. The plug-board style interface (banana plug sockets) are at the junctions of the M, C & P sections.

This plug-board approach transforms your matching box from a simple balun into a highly flexible, laboratory-style variable inductor. Using a center-out topology (expanding symmetrically from the "C" section) is a massive architectural upgrade for feeding the balanced A1/A2 terminals of the SX-28.

Here is an engineering breakdown of what this design accomplishes, along with one classic RF caveat to watch out for as you build it.

## 1. Perfecting the Balanced Feed

In your first version, tapping from one end of the coil meant the electrical center of your secondary shifted every time you changed a tap. This caused an imbalance in the capacitive coupling to the primary.

By making the "C" section your always-active core and adding matching "M" (minus/left) and "P" (plus/right) sections symmetrically via the banana plugs, you ensure that the electrical center of the secondary remains completely stationary. This guarantees that both the A1 and A2 terminals see an identical impedance and identical capacitive coupling to the active element, maximizing common-mode noise rejection from the metal roof counterpoise.

## 2. Coil Scale and Inductance

Assuming you are still using the 28 AWG enameled wire from version one, a 5-inch close-wrapped coil is a substantial winding.

This gives you an enormous maximum inductance when all M and P sections are patched in, which will be excellent for pushing the resonant frequency down into the VLF/beacon bands or the very bottom of the AM broadcast band, giving you plenty of runway to tune upward by bypassing the outer sections.

## 3. The "Dead-End" Winding Caveat

This is the most critical RF design factor to keep an eye on. When you patch into the M1 and P1 junctions, the remaining outer sections (M2 through M4, and P2 through P4) are physically disconnected from the receiver, but they are still magnetically coupled to the active part of the coil.

In RF coil design, these open-ended, unused windings act as distributed capacitance and can become series-resonant "traps." If the unused M3/M4 or P3/P4 sections happen to self-resonate at the exact frequency you are trying to receive, they will absorb the signal energy like a sponge, creating a "dead spot" on the receiver dial.

The Fix: If you notice sudden, severe signal drop-offs on specific frequencies, you can use shorting jumpers on your plug-board to completely short out the unused outer M and P sections (e.g., jumper M2 to M4, and P2 to P4) rather than leaving them open.

## 4. Primary Placement

Because the secondary is 5 inches long and the active portion expands outward from the center, the most efficient place for the 28-turn primary (antenna side) will be wound directly over the dead center of the "C" section, separated by a layer of insulating tape. This ensures the primary always maintains maximum magnetic coupling with the active portion of the secondary, regardless of how many M or P sections are patched in.

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Last modified: Monday, 10 Aug 2026