This is part 2 of my series on building a cheap loop antenna. If you haven’t read part one, it’s here. In this second part, I’ll cover the building of the tuning capacitor.

Obligatory Safety Notice

This article describes machinery and tools, workshop practice and construction that if not performed carefully and with the appropriate protective equipment (goggles, etc.) – could lead to serious injury. Please take care. I won’t be held responsible for your accidents!

Tools Required

In addition to keeping the material costs of the antenna as low as possible, it should also be buildable with a minimum of specialist tools. The following were used in constructing the capacitor:

  • Superfine permanent marker (Staedtler Lumocolor S)
  • Compass with universal adapter to take these pens (Staedtler Noris Club 550 01)
  • Metal / plastics drill (Dremel 3000)
  • Drill bit 3.2mm (Dremel)
  • Drill press (Dremel 220 workstation)
  • Thin circular metal file, flat metal file (Stanley warding file set)
  • 8mm spanners for M5 nuts
  • Junior hacksaw
  • Fret/coping saw (Stanley fatmax)
  • Centre punch
  • Two C-clamps for holding things whilst cutting (a vice would be far better)
  • Straight aviation tin snips

Materials

Item Cost each Total cost
9mm thick IKEA Legitim Chopping board £1.00 £1.00
Stainless steel washer M5 100 pack (Toolstation 79167), x2 £0.96 £1.92
Stainless steel Threaded Bar M5 (Toolstation 61139) £1.78 £1.78
Stainless steel Lock Nut M5 100 pack (Toolstation 64158) £1.58 £1.58
Stainless steel Spring Washer M5 100 pack (Toolstation 92679) £0.97 £0.97
0.9mm thick Aluminium (grade 1050) plate sheet, 300mm x 200mm, protective coating on one side £3.80 £3.80
£11.05

Well, there you go, I wanted CHEAP, and you can’t get much better than £11.05!

I chose M5 threaded rod, nuts and washers as it would be mechanically substantial, and not present too much resistance to RF. I considered M3, but it would introduce greater loss, despite being easier to drill for. The Dremel drill I use is excellent, but does not drill holes with a diameter greater than 3.2mm. I thought of drilling this size, then increasing the hole size with a burr, but this did not work out well, so I reverted to filing out to 5mm. Another reason for going with M5 hardware is that Toolstation in Tunbridge Wells stock M5, but not M3, and they’re really quite reasonable as the table above shows.

I initially considered Aluminium Warehouse for the plate, but their shipping cost was £15, so scratch that! I mentioned this to my wife, who suggested Amazon…. which turned out to be an excellent idea, as Hardware Outlet sell it for £3.80 with free postage. Nice.

Design

The diagram below shows the design of the rotor/stator vane plate (six of which are cut from the aluminium sheet), and end plate (two of which are cut from the 9mm thick cutting board). This layout comes from the article Magnetic Loop Antennas, by Tony ON4CEQ. The rotor and stator are cut from a single piece of aluminium with a minimum of tricky cuts. Other articles have separated them with Dremel cutting disks, which I had bought, intending to try – but managed to cut out Tony’s design easily.

Construction

In hindsight, it would have been better to make the end plate a few mm taller, as the rotor is exactly the same diameter as the end plate’s height (8cm), and if not raised, touches the desk as I rotate the finished capacitor.

I cut the end plates out using the above fret/coping saw, along with some extra pieces used to clamp things in my drill press. The holes for the threaded rod, forming the stator arms and rotor were drilled out with the Dremel’s 3.2mm drill bit then filed out to 5mm diameter.

I marked out the aluminium on its protective layer, using the microfine permanent marker and compass as shown. I left a 5mm gap between each plate (see detail below), thinking that the extra metal would prevent the drill from tearing the thin aluminium when drilling or increasing the size of the holes from 3.2mm to 5mm. I need not have done this; increasing the size of the holes was best done with a file, which was tedious. The vane panels could have been laid out directly next to each other; cutting each panel out was easy with the junior hacksaw. Each hole was centre-punched, which made drilling far easier.

Drilling was straightforward, although the Dremel workstation tends to allow the drill to move slightly. Centre-punching is essential.

The stator and rotor are cut from the panel easily; the diagonal cuts toward the centre are easily done with a junior hacksaw; the short cuts near the rotor axle, joining the diagonal cuts are done with the fret saw shown above (which is probably only supposed to cut wood; it handles 0.9mm aluminium well too).

Tin snips are then used to cut the unwanted parts of the rotor blade away, to yield its circular edges.

Burrs are filed off, edges and corners smoothed to remove any sharp points that could a) cut you and b) offer a point of voltage breakdown and arcing.

I then cut four 85mm lengths of M5 threaded rod for the four corner stator attachments, and a single 105mm length for the rotor. I then assembled the frame with the plastic, rod, spring washers, washers and nuts.

I assembled the rotor on the longer length of threaded rod. The washers I’m using to separate the vanes are exactly 1mm thick. Initially I used five of them, to make absolutely sure I’d have the inter-vane gap I’d need, but found the capacitance to be too low. So I rebuilt with four washers between vanes, and achieved a capacitance much closer to that needed.

The stators were assembled onto the frame, and roughly aligned with each other.

The rotor then fits snugly inside the stators and its axle fits in the centre hole. It’s hard to make out the end plate due to the perspective here, but it’s there.

Flat washers are fixed on either side of the rotor/end-plate mountings, in lieu of bearings. The top plastic end plate is then added, and everything tightened up. There’s a certain amount of adjustment of the stators and rotor to ensure that the rotor rotates freely within the stator plates, and that there’s an equal gap between rotor and stators. This adjustment is mostly done via the nuts mounting the stators, but there might also need to be gentle bending of the various plates.

Measurement

After construction, I measured the capacitance, and read 0.006nF (6pF) for completely unmeshed, increasing to 0.043nF (43pF) for completely meshed – close to the upper value I need – 46.311pF from the first article in this series.

Conclusion

There’s a bit more mechanical tweaking to do, but that’s the risky part of the magnetic loop project done. Next time, I’ll assemble the rest of the loop and think about manual/automatic tuning.

Here’s a short clip of the capacitor being tuned and measured…..

Stay tuned!

73 de Matt M0CUV

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