Some of you know that I've been experimenting (or maybe that's just 'dabbling') with various SDRs (ostensibly the Kiwi and the RSPdx, but also the HackRF One and Nooelec NESDR SMArTee RTL-SDR) running on a succession of semi-home-made magnetic loop setups. To clarify, what I mean by that is a re-working of the 'loop' element of a couple of off-the-shelf mag-loop pre-amps and Bias-T units placed in various locations around my QTH.
I thought I'd post a brief description, along with a few photos of the antenna I ended up with.
I've been tinkering for several months now, to see what sort of results I can get. I was hoping that the club would invest in a Wellbrook ALA1530 loop, which is the de facto standard if you like, so that I would have a yardstick with which to appraise my results. Happily, one now adorns the rear of Cyprus Hall.
I'm going to build my own Web-connected SDR (again, from off-the-shelf bits) running the same OpenWebRx system that the Kiwi uses, and then pop my homebrew effort on a big stick in Cyprus Road car park so that I can perform a direct comparison.
In all honesty, I don't really need another antenna, but (spoiler alert...) these loops do provide a pretty good HF reception solution in a compact and practical package. I can tell that already from comparisons with my main antenna.
So I started off by buying one of the cheap Chinese MLA30+ loops, via airmail from the source.
It was tiny. In fact, it came complete with Bias-T, USB lead, wire loop and maybe 15m of very thin coax which fed straight into the weatherproof pre-amp box - no connector, and it all came in an A5 padded envelope. I ran it like that for a while, but when I saw the modifications that other people were making to them, I decided to follow suit and snip off the coax (and SMA connector at the Bias-T end) replacing it with a chassis-mount BNC female connector and soldering the remaining short tail of coax to the appropriate points now on the inside of the enclosure. I re-sealed the entry-point with Araldite.
Pushing the 9mm drill through the side of the remarkably well-made enclosure without damaging the circuit board or the large coil you can see above was quite tricky and took quite a lot of concentration. The electronics are very well sealed in whatever goop that is, and that includes the coil and its windings, so I have no concerns around the longevity of the internals.
The big plastic mounting tabs that protrude from the top and bottom of the box had some useful mounting holes. Less usefully, they were only around 4mm diameter - not enough to get the common-or-garden 4.8mm wide cable tie through. They were soon opened out to 5mm.
Without touching on the actual performance of the unit in receiving RF signals, the only other slightly negative thing I have to say is that the attachment bolts for the loop itself (be it the supplied 70cm diameter stainless steel wire loop, or any other loop construction you wish to attach) are too small at 4mm diameter. In fairness, I suppose they're OK for the wire loop, but it wouldn't have hurt (or cost the manufacturer any more) to have fitted 5mm or preferably 6mm button-head Allen bolts. I didn't bother changing them out, as there isn't a lot of room going spare in there, but they could easily have been accommodated pre-assembly and prior to the tags being soldered.
Turning then to the loop itself, I played with a number of options. I started with some very promising composite tubing given to me by Phil G4UDU. It's a sandwich of PE-X/Aluminium/PE-X and on the face of it, it looked perfect. Neat, stiff, lightweight and pre-formed into roughly a 1m coil.
I cut a suitable length and started work on fabricating something to make the connections to the pre-amp attachment points. I started by trying to strip the outer sheathing of PE off the ends of the tube. Nightmare. It was all bonded together like you wouldn't believe. Next, I made some cranked brackets out of 2mm thick aluminium sheet with a 4mm hole at one end and a 14mm hole at the other. I then drilled out the bore of the pipe with a 14mm drill to remove the inner layer of PE and screwed a 14mm stainless steel nut and bolt up into the ends of the loop, securing the aforementioned brackets between the nut and the head of the bolt, leaving a good inch of thread in the tube (ooh err, look at me mixing my imperial and metric units...). That was obviously going to provide a solid connection, except that it didn't. It worked OK at first, but then stopped working. Intermittently infuriating, before being thrown up the garden in a childish rage.
Next I tried some beefy coax with an obscene amount of copper in it, also supplied by Phil, but it too wasn't really lending itself to the connection approach I wanted to use, plus it wanted to form any other shape than a perfect circle. When I started to see various animal-shapes in it, it followed the PE/ALU/PE loop into the herbaceous borders. I will retrieve it at some point, as I'm going to use it for yet another mag-loop in the loft, and I don't care what that looks like.
I'd watched a YouTube video of some guy that had formed a 1m loop out of two lengths of 15mm copper plumbing pipe. Very sensible, except that he had joined them with one of those plumbing connectors pre-filled with solder, and then used other connectors and joining methods to attach wires to go to the pre-amp lugs. Basically, he had taken a really nice, simple concept, and then proceeded to turn it into a right dog's dinner. Spray paint, tape and dribbly solder everywhere. OMG.
He obviously hadn't found the 3m lengths of copper pipe in Wickes DIY store in Burgess Hill. Other DIY emporiums are available.
So the diameter of a circle is 3m of copper pipe divided by π, which equals a 1m diameter mag-loop. Thank you Archimedes.
Time to get baking. Yes, baking. Wet sand is no good for filling copper pipe, so I heaped one of the GLW's baking trays with soggy sand and popped it in the oven at 170°C for 20 minutes. I told the wife that I was making a tray-bake. I don't know what that is, but I can tell you that when the beeper on the oven went, and she went to help herself to a tasty treat, I had to do some pretty fast talking.
I plugged one end of the pipe, filled it with sand, then plugged the other. The sand stops the pipe from deforming and collapsing when you bend it of course. Once I'd wrangled the 1m diameter round kitchen table out into the garden and duct-taped the pipe to the edge, I was easily able to bend the copper around it. It went remarkably smoothly, and no, you can't borrow my table.
After emptying the sand, I bent a couple of tabs on the ends (annealing the copper mid-bend to stop it from fracturing) and drilled a couple of 6mm holes in them. A bit of filing and so on, and we were done. Simplicity is nearly always the best policy.
Why 6mm holes? Ahh, revelations follow.
Not a bad job, if I say so myself.
I made the top mount out of a white 20mm electrical conduit tee with a 13mm wide slot cut in the top to snap the 15mm pipe into. This was in turn held in place by a white BMX handlebar grip, cut down to be a really snug fit in the end of a length of the ubiquitous 40mm waste pipe.
A decent new male BNC compression plug and some Mini-8 coax feeds the signal away and up into the shack. The loop is in a compromised location currently, and testing is therefore also compromised, but I can tell you that it's remarkably good. Proper comparative testing will follow, as hinted at earlier.
Job done then.
Well, not really. Of course I now have the same problem that I had when the KiwiSDR was located here. Namely, a nearby HF transmit antenna spitting out 100W (or 6.1W ERP if you're the ICNIRP police). I started looking at several solutions to this on the various forums, and didn't have much confidence in any of them, until I came across an apparent consensus that the pre-amplifier produced by Cross Country Wireless does what it says in the sales literature i.e. it "provides protection which allows it to be used very close to transmit antennas without damaging the amplifier or the attached receiver".
Now, these units get mixed reviews in the various loop shoot-outs that I've read, but I pulled the trigger at £55.20 plus shipping and I haven't been disappointed thus far. Again, some hard-nosed testing between the Wellbrook, the MLA30+ and the CCW amplifier will follow, with the last two using the exact same copper loop, but that's for another day. No mounting tabs/lugs on this unit you'll notice, but it does have 6mm loop mounting bolts and wingnuts!
If you fancy an unobtrusive Rx antenna for your shack or even your backpack, then a mag-loop may be the answer. You could potentially avoid having to spend £264 on a Wellbrook loop too, if you're not super-fussy.
If you can't wait for my testing results, there are a number of quite objective studies already out there on the Internet - the best of them on YouTube.
For the price (around £35 for the MegaLoop MLA30+) I don't know why you wouldn't have one at least on standby. OK, so it's receive-only, but what a great pair of ears, even when used with a random length of wire draped over a nearby thingamajig. They even work indoors.
M0XYF
