Wednesday, July 22, 2009

Size matters

What a difference an extra 14 inches (36cm) makes! I'm referring, of course, to the diameter of the radiating element of a magnetic loop antenna. I replaced the 40cm diameter loop of my Wonder Loop with a 76cm (2ft 6in) diameter loop made from RG-213 coaxial cable. Suddenly its performance is transformed from unquestionably poor to very impressive. Early tests show this is a desktop antenna that will blow Miracle Whips into the dust and give results not much worse than a full-sized dipole!


The revised antenna is somewhat larger and bulkier than the rather neat original. Accommodating the bigger loop created a few problems. An important factor in the design was to make antenna that can easily be taken to bits, packed up and transported in a suitcase or backpack. It was also intended to be free-standing. Therefore the support and loop need to be detachable and the loop has to be made of wire so it can be coiled up for transportation. The use of copper tube was out of the question - though obviously an option for constructors wanting to make a magnetic loop for home use only - while the solution adopted by G4TPH of using short lengths of aluminium strip bolted together to make a dodecahedron seemed cumbersome.

A length of uPVC electrical conduit was cut for the support, and a 240cm length of RG-213 was cut and the braid at each end soldered to two gold plated 4mm spade terminals. 80cm of stout wire was cut for the coupling loop. Because the cable would need to be removed from the support it is just hooked to the top of the support, and a self adhesive cup hook was attached to the top for that purpose.

The most obvious problem after hooking the cable to the support and attaching the terminals to the binding posts on the tuner was that RG-213 does not have sufficient stiffness to hold a circular shape in such a large loop. The shape could best be described as that of a saggy backside (younger readers may have to wait a couple of decades to see what I mean by that. :) )

Despite this, the antenna still pulled in some pretty strong signals, so I set it up with the FT-817 on 30m WSPR for testing and went downstairs for a cup of tea with the XYL. While we were half way through our tea we heard a crash from upstairs, and rushed to investigate. The self adhesive pad used to secure the hook had given way, allowing the loop to crash to the desk, leaving the FT-817 to transmit (of course, this happened during a transmit period!) into a virtual short-circuit. Fortunately, the radio survived the experience!

The support was re-made using superglue to attach the hook to the mast. I also solved the saggy backside problem by gluing two more hooks to each end of a length of the back part of the uPVC conduit. This sits resting on the top of the loop support, hooked round the loop on each side. The uPVC back strip is thin enough to curve slightly under the weight of the coaxial cable, allowing the loop to be more of a circular shape. It's a bit Heath Robinson, but it looks better and more importantly helps the antenna work better, because the ability to tune to a 1:1 SWR is dependent on the position of the coupling loop with respect to the main one. You can see this support piece in the photo, which shows the Wonder Loop sitting on top of the 2m transverter while I was making some initial tests on 30m WSPR.

One effect of using a larger loop is that the frequency coverage has changed. Whilst the 40cm loop covered 30m thru 10m, the larger one covers 40m to 15m.

Initial A/B receiving tests with the Wonder Loop positioned as shown in the picture compared with the MFJ magnetic loop in the attic above it suggest that on 40m it is down about 6dB compared to the MFJ, on 30m it is about equal, and on 20m received signals actually seemed a bit stronger.

As shown in the photo the Wonder Loop was only a few inches from the wall and also a conduit carrying two of my cables up to my attic antenna farm, but it actually tuned up on 30m in that position to give a 1:1 SWR. I then did over an hour of A/B testing using WSPR on that band. The results were interesting. My signals were copied well in Europe and even by several east coast USA stations. I was consistently getting reception reports 10dB higher from IK2CMN and IV3DXW when using the Wonder Loop than when using the MFJ! Some reports from other directions seemed a few dB lower, but reports under identical conditions from the same stations at different times can often vary by 10dB or more due to QSB so it is hard to attribute such small variations to the performance of the antenna.

These are not scientific tests, as the two antennas are in different positions and may be subject to different coupling effects or reflections from other objects. Nevertheless I think the point is made that the Wonder Loop with 76cm diameter element performs extremely well for an indoor desktop antenna.

I will be doing some further tests on as many as possible of the bands the Wonder Loop covers (it's hard to find any propagation on 17m or 15m so testing on those bands is difficult.) As time permits I shall also attempt to make some QRP contacts with the antenna. But I am extremely encouraged by the results so far, which suggest that this portable magnetic loop could equal or even outperform typical portable QRP antennas such as random wires, loaded verticals and low slung dipoles, while avoiding the need for supporting masts, ground wires and counterpoises.
Post a Comment