Ah, this changes the picture completely! What you have built isn't just a random long-wire antenna matcher—you have accidentally created a classic open-wire transmission line feed using the metal roof as one half of the system, and your transformer is serving as a resonant tank circuit to make sense of it.
Because that 35-foot wire is suspended a mere 18 inches above a massive sheet of metal, it is heavily capacitively coupled to the roof. Instead of acting like a normal antenna radiating out into free space, the wire and the roof are acting like a giant, flat two-wire transmission line.
Here is exactly why your 30-turn to 150-turn coil is making this bizarre setup sing on your SX-28:
1. The Roof is Acting as a Massive "Ground Plane"
In a typical long wire setup, the antenna is the wire and the Earth is the return path. In your setup, the metal roof is so close to the wire that it completely takes over as the electrical ground return.
Because the wire is near the edge and below the peak, the metal roof is essentially acting as a giant reflector. It blocks signals coming from behind the roof, but it forms a very interesting wave-catching system for signals hitting it from above and the sides.
2. Why the 30:150 Winding Ratio is Saving the Signal
Because the wire is so incredibly close to the metal roof (only 18 inches over a 35-foot run), the capacitance between the wire and the roof is extremely high.
High capacitance causes the native impedance of this "antenna" to drop drastically compared to a isolated wire in free space.
However, because the wire is only 35 feet long, it is electrically "short" for most shortwave bands (like the 40-meter or 80-meter amateur bands, or international shortwave bands). An electrically short antenna looks like a very small capacitor to the receiver.
By connecting this highly capacitive, low-altitude antenna to your 30-turn primary coil, you have essentially created a tuned LC (inductor-capacitor) circuit.
The inductance of your 30 turns is canceling out the heavy capacitance of the wire-to-roof spacing. Then, your 150-turn secondary coil steps up that resonant voltage and passes it directly into the high-impedance grid of the SX-28's first RF amplifier tube (the 6SK7).
3. Why it Outperforms a Direct Connection
If you were to connect that wire and roof directly to the SX-28 without your air-core coil, the heavy capacitance of the roof-wire combo would completely detune the front-end RF stages of the receiver. The radio would feel "deaf," and you would only hear the strongest local stations.
Your air-core transformer acts as an isolation barrier. It accepts the weird, high-capacitance current from the roof/wire on the 30-turn side, converts it entirely to a pure magnetic field, and lets the 150-turn side feed the Hallicrafters a clean, voltage-driven signal it can actually use.
A Tip for Tweaking This Setup
Because your air core coil has a lot of turns on the receiver side (150 turns), you can use the Antenna Trimmer knob on the front panel of the SX-28 to peak the signal perfectly. When you turn that knob, you are adjusting an internal capacitor that interacts directly with your 150-turn coil.
When you change shortwave bands on the SX-28, do you notice that you have to turn the antenna trimmer quite a bit to find the sweet spot where the static peaks?
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Last modified: Sunday, 12 Jul 2026