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I'm building a sort of "screwdriver antenna". In my case I have a motorized roller inductor. This motor is placed near the bottom of the coil (the coil is placed near the center of the vertical antenna. In this photo of the antenna it's still missing a few more meters of top radiator. The red wire is the lower radiator. The black wire is DC power to the arduino).

As a test, I used a small but strong servo motor modified for continuous rotation. The antenna worked OK, I got it to tune in the calculated range. My idea is to replace that small servo with a decent stepper motor (so I can just count steps to jump between bands for rough tuning).

But having an arduino up there isn't a great idea. It's RF hell. There are rather strong E and B fields which would cause it to malfunction or just fry. I'm not sure if shielding it would help.

So I thought of just using very long leads (about 6-8M) between the stepper motor and its driver. I know it's not a good idea to do this, but, will this work? Would the driver get damaged?

Most antennas of this type are smaller than what I'm building. They use a mechanical linkage and keep the motor at the bottom. I want to avoid this as this would have other problems: finding a suitable material for the driving rod (I can't find fiberglass rod or tube long enough for my application), and the tolerances required for a mechanical linkage that needs to flex with wind (I will use ropes to hold the antenna but that doesn't mean there won't be some flex). Obviously, once finished, the coil will be covered from rain.

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  • \$\begingroup\$ Can you use a shielded twisted-pair (or twisted-pairX2) to control the motor? otherwise you can use inductors in the windings to exclude RF from the motor. Use inductors in BOTH ends of the leads. 1uH at 30MHz is +200 ohms. \$\endgroup\$ – analogsystemsrf Oct 8 '19 at 1:48
  • \$\begingroup\$ @analogsystemsrf like shielded cat5 for the motor leads? It would be great if I could use that since it's readily available. About the inductors, I think I'd need to go way bigger. this antenna is for 3.5 to 7 MHz (80-60-40M bands, maybe 160M if I add extra loading at the base). Will the stepper drivers care about the extra inductance? I suppose not, they are, after all, designed to drive inductances. \$\endgroup\$ – hjf Oct 8 '19 at 2:01
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    \$\begingroup\$ what is your power level? If you insert inductors every 1/20th wavelength, will the RF fields mostly be excluded from the motor? by the way, the volts/meter of low frequencies is not very high. And 150 micron thick copper foil will have SkinFrequency of 2MHz (4MHz for 35 micron foil), thus even magnetic fields will be partially excluded from a copper box. Thus 1/16th inch ( about 800 microns) copper foil, soldered at seams, may work fine even up on the Antenna. If you ran in a fiber optic control, you'd only need GND and MOTOR/MCU power. \$\endgroup\$ – analogsystemsrf Oct 8 '19 at 3:45
  • \$\begingroup\$ Use large CM chokes on STP cable for the motor drive. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Oct 8 '19 at 5:48
  • \$\begingroup\$ @analogsystemsrf well I can go with 2.4ghz wireless control. A 2.4ghz vertical should be immune to HF, and then I would only need power. RG58 coax could be used to power everything. Maybe even wired control inline with DC power along the RG58 coax? (Or even RG6, cheaper!) \$\endgroup\$ – hjf Oct 8 '19 at 11:38

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