Saturday, December 15, 2007

80m loop antenna experiment failure

Just a brief post about some experimentation that has failed.

I've been trying to build a loop antenna for 80m (3.5Mhz) to reduce local electrical noise.

The idea was a 2m diameter square loop built from 1m lengths of electrical conduit. First I built with 1 turn and a 300pF polyvaricon capacitor and it resonated nicely at about 20Mhz. Next went to 3 turns and it seemed to resonate around 9Mhz, then 4 turns and I can't get this thing to resonate at all.

No idea what I'm doing wrong or why this won't work. The family is a little bemused about the structure in the back yard - I tried to pass it off as my version of a Xmas decoration.

I did learn a bit about conduit - there are two main kinds around, electrical and water. Water pipe is designed to carry water under mains pressure so it's much stronger.

Update: Check out Alan's excellent response. I will get back to this project soon.


Peter Marks said...

By email:

Re your loop project; By my back-of-the-envelope calcs, 2 metres in
diameter and three turns should be about 43-47 uH, which will resonate
at 3.5 MHz with about 45 pF, less the distributed capacitance which can
be significant if the winding is bunched.

The single turn loop with 300 pF should have bottomed out a bit below 5
Mhz. If you put a fixed 270 or 330 pF across it with the same variable
cap it should tune about 3-4 MHz.


Anonymous said...


I did a bit of practical investigation on this today, to make sure the numbers I emailed you were sane:

Looks like I was in the right ballpark with the math. The resonant frequencies you observed appear much too high, which suggests you were seeing something else, maybe the transmission line?

Peter Marks said...

Wow! You sure did a great job. I urge readers to take a look at Alan's experiments.