Saturday, July 18, 2009

The Wonder Loop

Today I finished a project that has been on the go for some time - the G4ILO Wonder Loop. The idea came to me several months ago while testing the Wonder Wand antenna. Regular visitors to the site will know of my interest in small, very portable HF antennas that can be set up without any supports, even indoors. Most will also know that these very small antennas tend not to be very efficient. I have been extremely impressed with the performance of the MFJ Magnetic Loop antenna, and wondered if a magnetic loop would make a better "wonder" antenna than trying to use a short telescopic whip. How small could you make a magnetic loop and still get better performance than a Miracle Whip or Wonder Wand?

The Wonder Loop is an attempt to make a QRP version of the MFJ Loop Tuner. Essentially it's a project box containing a variable capacitor whose terminals are brought out to two 4mm binding posts, to which are attached the two ends of the loop radiating element. The small coupling loop plugs into an RCA socket which connects it to an N7VE QRP SWR indicator built into the box. I'm grateful to one of my blog readers, Steve Silverman KB3SII, for this. Steve sent me the one he bought at Dayton after reading of my frustration when QRP Kits sent the wrong kit, thereby enabling me to complete the project. The transceiver plugs in to the BNC socket on the side. I'll probably publish more details, photos and results in the main part of the site later on, once I have done more tests.

The neat thing about the Wonder Loop is that it is very compact for transportation, because everything detaches from the box. You can also use loops of different sizes, for different frequency coverage or to get better efficiency at the expense of larger size. The loop support is a length of uPVC electrical conduit. The loop itself could be made of stout wire or coaxial cable.

Currently I'm using a loop just over 40cm in diameter, which gives a tuning range from 30m to 10m. It's on test using my FT-817 and WSPR as I write, with the Wonder Loop sitting on my shack desk, just like in the photo. So far my 1W signal has been heard in Germany by two stations on 20m, in the UK and Germany on 17m, and in France, Germany and Holland on 30m. Not bad in just an hour and a half!

Plans include trying an 80m loop, and one for 6m. So watch this space!


myles said...

soft annealled copper pipe is sold in coils, 1/2" and 3/4". It is very easy (by hand) to bend into a loop, simple size would be 1 metre in dia.
Capacitors to tune the loop can be 200pF variable with 1mm air gap between stator and rotor, good for 5 watts, 80M up to 12M.
I also experiment with materials.
I started with caps made from aluminium foil freezer bags and crdboard, for Rx, too easy, 200mm square to tune 7MHZ on a 1 metre loop made from RG213.

Next cap was made using PET the stuff coke plastic bottles are made from.

A 2 litre bottle provided ample supplies, then inserted a coke can into another insulated by the PET, 100 watts no heating, but not conclusive
and not easy to slide in and out.

Next is glass as a dielectric, good for 50KV, I bought many small picture frames 90c each , 150mm x 100mm with 1.5 - 2.00mm thick glass.

Some brass sheet from a remote controlled hobby shop, it's very thin I cut it with scissors, and made a sandwich.

2 outer with one inner separated by 2 sheets of glass, magic capacitor good for 100 watts for 5 minutes, still developing a variable cap, which will

have one piece (inner) for highest freq. and one beside it to fine tune, I just slide them in and out of the main sandwich.

I will supply pics if required.

I you have a ganged variable aluminium cap, to increase the working voltage just use 2 x gangs (stators) and not the rotor.

myles said...

the spirals of copper pipe are used for airconditioning installations, it is very soft. and easy to solder at the ends for connection to capacitors.

g4ilo said...

Thanks for the suggestions Myles. I did consider using soft copper pipe but the aim was to make a loop that could be quickly set up then taken down again after use and I was not sure how that material would stand up to being coiled up and uncoiled many times.

I have also seen some interesting ideas for homebrewing capacitors. Seems like too much work for QRP as a fairly ordinary capacitor seems to work fine, but if I wanted to make a loop for higher power then it might be worth trying.