I Need Help - AI Part 2318

Prompt: I would like to discuss winding direction and polarity for a moment. I originally started this topic some time ago with the prompt "In an antenna balun, why does reversing one winding destroy the signal." With my first box (still functioning), if I connect the antenna to the primary one way and the secondary to the radio another way, it works. If I switch the polarity of one of the connections, it doesn't work at all. I thought at the time that the radio didn't 'know' what the antenna was doing and a phase difference would make no difference. It most certainly does. I can provide you with the answer you gave (it's now an HTML wrapped page) if it would be useful.

You are absolutely right to call this out. Your bench test proved a reality that textbook RF theory often glosses over.

In a "perfect" theoretical transformer with zero stray capacitance and a perfectly balanced load, phase inversion doesn't matter. But your matching box is not operating in a theoretical vacuum—it is operating in the real world, and your observation about the signal being completely destroyed is spot on.

I do not need the HTML page to validate your finding, because what you experienced is a classic, notorious RF phenomenon. Here is exactly why reversing the polarity on your first box killed the signal, and why it absolutely matters for your new build.

## The "Phantom Capacitor" Problem

When you wrap a primary coil directly over a secondary coil, you are doing two things simultaneously:

1. Magnetic Coupling: You are building an inductor that transfers energy via a magnetic field. 2. Capacitive Coupling: Because two physical copper wires are separated by only a thin layer of insulation (or foam), you have also accidentally built a capacitor.

Your antenna system is highly asymmetrical. The 35-foot wire is "hot" (carrying high RF voltage). The metal roof counterpoise is "cold" (acting as a massive ground plane with very low RF voltage).

When you connect the "hot" 35-foot wire to the primary coil, that specific end of the primary coil radiates a strong electrostatic field directly into the secondary windings directly beneath it.

## Phase Cancellation (Why the Signal Dies)

Because your transformer is transferring signal through *both* magnetism and stray capacitance at the same time, the phase relationship between those two paths becomes critical.

• In-Phase (The configuration that worked): The magnetic field induces a voltage in the secondary. Simultaneously, the stray capacitance bleeds a little bit of the "hot" RF voltage directly into the secondary. If both of these forces are pushing the electrons in the same direction, they add together. The radio hears a strong signal. * Out-of-Phase (The configuration that died): When you reverse the leads, you invert the phase of the *magnetic* transfer (shifting it 180 degrees). However, the *capacitive* transfer doesn't invert in the same way, because the physical "hot" wire is still just bleeding raw voltage into the secondary. Now, the magnetic wave is pushing electrons in one direction, and the capacitive wave is pushing them in the exact opposite direction. They cancel each other out. The signal drops to near-zero.

## The SX-28's True Balance

There is a second reason this happens. While the SX-28's A1 and A2 terminals are designed for a balanced line, the internal antenna coils inside the radio are rarely perfectly balanced relative to the chassis ground. One side of that internal link circuit often has a slightly higher capacitance to the steel chassis than the other.

If you wire the box so the capacitively coupled "hot" side of the primary ends up feeding the side of the SX-28 coil that has a lower impedance path to ground, the signal just bleeds into the chassis and disappears.

## The Takeaway for Box Two

Your real-world test proves that polarity absolutely matters for your specific antenna and radio combination.

When you wind this new 8-turn primary over the foam tape, the foam will significantly reduce that stray capacitance compared to close-wrapping it, but it won't eliminate it.

When you do your final installation, you should replicate your original test: connect the antenna and roof leads, tune in a weak signal, and then physically swap the primary connections. One orientation will likely produce a noticeably lower noise floor and stronger signal than the other.

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Last modified: Wednesday, 29 Jul 2026