I've been experimenting with various antenna installations around my house for use with ham radio. Propogation is poor right now as we are at the bottom of the sun-spot cycle, so it is important to have an effective/efficient antenna if you want to be heard.
One type of antenna I've run across which seems interesting/unique is the magnetic loop antenna. These are typically constructed with a loop of conducting material formed so that the two ends of the loop are almost touching. In the space between the two ends a variable capacitor is placed and tuned to resonate at a specific frequency.
Magnetic loops are typically extremely high Q devices that are tuned for a very narrow range. This narrow operating range means the antenna naturally provides some amount of selectivity, but it comes at the cost of needing frequent re-tuning. Changing frequency by even 2-3kHz can shift you out of the operating band of the antenna.
I want my loop antenna to work well on 20m, and for conveience I'd like to assemble it from copper piping (0.75" OD, 10 feet long). I also want to be able to operate up to 100W. I put those parameters into the calculator here and got the following results:
Antenna efficiency: 60% (-2.2 dB below 100%)
Antenna bandwidth: 18.9 kHz
Tuning Capacitance: 62 pF
Capacitor voltage: 3,687 volts RMS
Resonant circulating current: 20.1 A
Radiation resistance: 0.074 ohms
Loss Resistance: 0.050 ohms
Inductance: 2.08 microhenrys
Inductive Reactance: 183 ohms
Quality Factor (Q): 742
Distributed capacity: 8 pF
See the capacitor voltage of almost 4kV? With an air-core capacitor it is likely that the high voltage will cause arcing across the fins of the capacitor. There are methods to avoid this (increase spacing, submerge in oil), but a common approach is to use a vacuum variable capacitor.
I got the following model from ebay: Comet CVBA-500BC:
This capacitor has a tuning range of 5-500pf, a max voltage of 5kV, and a peak operating current of 57A.
I designed and printed a plastic holder for the variable capacitor. I also added a geared stepper motor, mounting points for the copper pipe, and plastic pieces to mount the loop to a wooden dowel.
The printed pieces look like this:
The assembled antenna should look something like this render from Fusion360:
If you want to make any changes to the design or print it yourself, I am sharing the Fusion 360 project here: https://a360.co/2TtVUop.