You are exactly right — 370 turns is vastly too many for Bands 2 through 6. Your instinct to question that number is spot on.
It is also a very smart move to abandon the AM band. Modern residential environments, packed with switching power supplies, LED drivers, and IoT devices, effectively turn the 500-1600 kHz spectrum into a high-noise wasteland.
To push this matching box up into the HF spectrum (from 1.6 MHz on Band 2 all the way up to 42 MHz on Band 6), we have to completely change the physics of the coil. Here is why your plug-board design is actually the perfect architecture for this, and how we need to adapt the winding.
## The High-Frequency Capacitance Trap
If you close-wrap 5 inches of 28 AWG wire, the physical wire turns sit tightly against each other. At 1.6 MHz (Band 2), this is fine. But as you tune up into Bands 4, 5, and 6 (up to 42 MHz), the microscopic gaps between those tightly packed wires act like hundreds of tiny capacitors in parallel.
At high frequencies, the signal will simply bypass the coil entirely, bleeding through that stray capacitance and shorting out your signal. Furthermore, too much inductance at 30 MHz acts as an RF choke, effectively blocking the signal from reaching the A1/A2 terminals.
## Re-scaling the Plug-Board for HF (Bands 2-6)
To cover everything from 1.6 MHz to 42 MHz in a single box, your center-out plug-board design is brilliant, but we need to drastically reduce the turn counts and ideally change how the wire is wound.
1. The "C" Section (Band 6: 21–42 MHz) For the highest frequencies, you need very little inductance. The always-active center "C" section should be tiny — likely only 6 to 10 turns total. This is all the SX-28 needs to step up the voltage at 30 MHz.
2. The M1/P1 through M4/P4 Sections (Bands 2–5) Instead of a continuous 5-inch wrap, your M and P sections will progressively add turns to bring the frequency down, but the total turn count across the entire form will likely max out around 80 to 100 turns for Band 2 (1.6 MHz).
*(Note: These are estimates; you will dial them in perfectly with your rotary switch or plug-board by listening to the noise floor peak.)*
3. Wire Spacing (The HF Secret) If you still want to occupy that 5-inch physical space on the 7-inch form, you cannot close-wrap the wire. You need to "space-wind" it. Leaving a gap between each turn (equal to the diameter of the wire itself) drastically reduces the inter-turn capacitance, allowing the coil to actually work on Bands 5 and 6. You can achieve this by winding a piece of thick monofilament fishing line alongside the copper wire, then unwinding the fishing line once the copper is glued or taped in place.
Words go here about how the response was received.
Page: /Antenna_box_2/Gemini_2301.shtml
Last modified: Wednesday, 29 Jul 2026