I Need Help - AI Part 2321

Prompt: Are there any options we should look at for the primary that we could put in the box? A high pass filter perhaps. There's a lot of 60 cycle noise on the lines from the roof. Some sort of variable capacitor? A switch to go from 8 turns to say 10 or 12? No is an acceptable answer but if I don't ask, maybe we miss an opportunity

"No" is exactly the right answer here, and your instincts to ask are excellent because looking for those exact opportunities is how good RF design happens.

However, in this specific architecture, adding components to the primary will actually degrade the box's performance. Here is why keeping the primary strictly as a "dumb," isolated 8-turn wire is your best move.

The 60-Cycle Noise (The Built-In Filter)

You do not need a high-pass filter because your air-core transformer is already an extreme high-pass filter.

At 60 Hz, an 8-turn air-core coil has virtually zero inductive reactance—it is effectively just a dead short wire. The magnetic field it generates at 60 Hz is so incredibly weak that it cannot physically jump the gap to induce a 60 Hz voltage in the secondary. The 60-cycle house noise coming off the roof will flow harmlessly straight through the primary wire and out the other side without ever crossing over to the SX-28. Adding discrete capacitors and inductors to make a high-pass filter would just introduce unwanted parasitic resonances in your target 1.6 to 42 MHz range.

A Variable Capacitor

Putting a variable capacitor in series with the primary is a classic technique used in transmitting tuners to tune out the reactance of the antenna wire.

In a receive-only setup, adding one would turn your primary into a series-resonant tuned circuit. While this *could* give you a slightly sharper signal peak on specific frequencies, the cost is high:

It adds a third knob you have to constantly fiddle with. * The capacitor introduces a physical piece of metal on the primary side that acts as a stray capacitance plate, bleeding that house noise directly into the secondary. * Your 80-turn variable secondary is already doing the heavy lifting for the impedance match.

A Switchable Primary (8 vs. 12 turns)

Changing the primary turns would give you a broader range of impedance transformation ratios, which sounds great on paper. In practice, the mechanical reality ruins it.

If you put a switch on the primary, you have to run tap wires from that 0.6-inch foam bed over to a front-panel switch. You immediately lose the beautiful, tight physical isolation we designed. Those primary tap wires will radiate their "hot" capacitive electrostatic field all over the inside of the box, bypassing the magnetic transformer and coupling directly into the secondary tap wires. Furthermore, the unused turns on the primary would act as a resonant trap, just like the ones we discussed managing on the secondary.

The Verdict

The sheer elegance of an isolated, fixed primary is that it acts purely as a magnetic pump. By leaving it alone, you maintain absolute galvanic isolation, reject the 60 Hz roof noise naturally, and keep the parasitic capacitance inside the box to an absolute minimum. Let the secondary switches do all the work.

Comments

Words go here about how the response was received.

here.

Page: /Antenna_box_2/Gemini_2321.shtml