Capacitor selection for Mag Loop TX/RX <100 watt

[jhansx]

Greetings all, Jeff KG7TXI here. I live in a mobile home (well engineered signal blocking tornado magnet) in a small MH park surrounded by lots of trees and power lines. Too old and busted uo to climb and hang a G5RV or equivalent and my MFJ-1799 scares away the straights but does little else well. I want to try a Mag Loop but do not want to have to hock a kidney to get one.
I have a Elecraft KX3 max output power 15 w which I can run through a tires AL-811 for <100 watts ERP. The question is about an air variable vs a vaccum variable cap. Trying to keep costs down  and truly do not expect to have gear that will ever exceed 100 watts. Looking for suggestions from the Mavens here with design experience in RF TX/RX mag loop systems. I expect to use the 10m - 40m bands with this system.
Thank you in advance. Please be safe, nothing matters if you don't make it home.
73
Jeff (KG7TXI)
 

[Andy Chee]

A quick calculation using RJELOOP suggests the peak capacitor voltage will be 3.5kV @ 15 watts.

Might also want to play with VK3CPU mag loop calculator

https://miguelvaca.github.io/vk3cpu/magloop.html

Either way, an air variable cap should be adequate, provided you give enough plate spacing.  Air variable caps used in vaccum tube linear amplifiers would likely have suitable plate spacing.

HOWEVER

Air variable caps for mag loop use must have low ohmic resistance connections.  In practical terms, this means the caps must be butterfly vane types.  Regular air variables use a shaft commutator, which is the enemy of low resistance!  Avoid commutators in your mag loop at all costs!

The upshot is that you may find that vacuum caps are more readily available than butterfly vane caps.

[radiolistener]

15W @ 50Ω = 39 Vpk

Magnetic loop for a short wave has Q up to 2000, so the capacitor needs to work for at least with 39*2000 = 78000 V
If your magnetic loop has worse Q, you can decrease voltage rating for capacitor.

The main issue for mechanical variable capacitor is Q factor, look for double capacitors which use rotor as connection between two capacitors, it allows to minimize resistance.Vacuum variable capacitors are much better for that, but they are expensive.

Also note, that using magnetic loop for transmission is not safe for your health, this is because magnetic loop antenna has very short size in comparison to wavelength and as result it needs to use high Q in order to compensate its small size. It means that such small antenna has extreme high H and E field strength near antenna which exceeds all safety limits many times...

I tried to play with magnetic loops in the past, it is funny to see how it fire all neon and gas-discharge lamps within 1-2 meters even if you put just 5-10 Watt. But then when I realize how strong E and H field it uses, I decided to not use it. It's better to use more large antennas, because they are more safe.

[mag_therm]

Magnetic loop for a short wave has Q up to 2000, so the capacitor needs to work for at least with 39*2000 = 78000 V

Maybe Q = 2000 with a super conductor!

Q = w*L / R_series
R_series includes the copper resistance of the turns.
You can calculate a reasonable approximation by R = rho* Length_turns /A ; rho =~ 21e-9 Ohm.metre
A is the width of the turn times the reference depth of penetration at the frequency.

Edit : Paper by KP4MD: "Magnetic Loop Parameters: 7-30 MHz"
https://www.qsl.net/kp4md/magloophf2.htm

[p.larner]

i wouldnt bother with loops,i built one for 80m with an arduino controlled cap,the bw was 1.5kc,no good for ssb but ok for narrow datamodes i guess.have fun.