DIY "Wonder Loop" QRP Antenna

Submitted by gerry on Wed, 04/19/2017 - 16:26

I am a member of a group on Facebook that shares and discusses antenna designs, usually of a portable nature. Recently the topic of the WonderWand WonderLoop antenna came up, particularly how expensive it is relative to its apparent complexity. A number of members had posted pictures of a torn-down control box, and it really seemed to contain little more than a transformer wound on a mix-6 toroid and a variable capacitor. 

The general design is a wire loop, connected to 2 screw-terminals on the top of a control box. The control box has a knob to adjust the variable capacitor, and a PL259 to attach to a radio. The positioning of the PL259 indicates that this is intended primarily for use with the well-known all-band, all-mode Yaesu FT817 QRP Portable radio.

I thought it might make a nice Saturday morning project to try to replicate it, at least to prototype level, to see if it would be worthwhile to make a more productified version. I started out by digging in the junkbox to see if I had the necessary bits. I was going to build this on a piece of scrap PCB material, and had a FT130-6 toroid and a dual-gang 250pF polyvaricon I could use with it. Since I was only going to put 5W into it, I figured the PV cap would be sufficient - at least for a proof-of-concept.

Examining the pictures provided by the Facebook group members, and various other sources, I came up with the following schematic for the control-box. The toroid has 19 turns, and it tapped at the third.

schematic

I started out by doing a rough-and-ready measurement of the piece of PCB. Then, using a dremel, I etched away some copper to make two lands... one for the hot side and one for the groundplane.

pcb sketch

 

placement

I ended up etching out a bit more copper to ensure that stray capacitance was kept to a minimum. Once this was done, I drilled holes for the capacitor and SO239 connector and fitted them. I connected the dual-gangs of the capacitor in parallel, giving a maximum capacitance of ~500pF, and a minimum of ~50pF. I also soldered the trimmer tabs which would give a few extra pF of wiggle room.

I wound the toroid and fitted it also, with the ends of the coil going from hot to earth, and the 3-turn tap going to the centre conductor of the SO239.

front & back

 

I finally had another dig in the junkbox for some sturdy copper wire. I found a length of 1.5mm single-strand that had been cannibalised from a length of domestick mains wiring in the past. It held shape quite well, even in a breeze, as it turned out. I cut sufficient to make a loop of ~24" diameter and soldered the ends to the hot and earth of the PCB.

completed

 

I purposely placed the SO239 in such a way as it could be connected directly to the back of my FT817, similarly to the commercially produced antenna. I used a barrel adapter to make this connection. I initially tuned for most noise, and then fine-tuned with a 1W AM carrier and the SWR meter on the FT817. The exposed wiring made me realise that even at 1W, the voltages generated by this sort of antenna are not to be taken lightly. I got quite a nip on more than one occasion.

There was a contest on all bands that day, so I had plenty of variety to listen to, but contest situations aren't the best for testing a new antenna for sensitivity, since everyone seems to dust off their Legal Limit Linears and beams. Signals were very strong across the board. I tuned to 18m, where things were a little quieter, but the little loop performed well here too. Certainly within a couple of S-points of my main antenna, an Off-Centre Dipole at about 35'. I did take note of the directional capabilities of the loop too, with signals strongly nulled when broadside to the antenna.

using the loop antenna with ft817

Tuning was very sharp, especially on the lower bands, with the usable bandwidth often being less than 6KHz before the SWR rose to 3:1 or more.

I did attempt a number of calls, but had no replies. I suspect nobody contesting had any interest in wasting time digging a tiny QRP signal out of the noise. I was encouraged enough by this initial experiment. however, that I will build a more substantial magnetic loop in the near future. The ease of setting up alone will make it a worthwhile addition to the shack.

 

 

 

 

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