Posting Comment for "Bicycle Rim Antenna for 20 Metres"

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26th May 2012 11:40

qrp-gaijin wrote...

Might you provide a close-up image of how exactly you connect the loop element to the capacitor? That detail is obscured in the existing photos. You mentioned use of screws, thick wire, and star washers, and it would be instructive to see exactly how you have it set up.

I'm considering building something similar, but with the loop element detachable and replaceable, as in the MFJ-932 QRP loop tuner, which is basically just 2 variable capacitors connected in the matching arrangement you have shown, with two wing-nuts for attaching the loop element. Obviously the wing-nuts are a source of resistive loss, so I'm curious to see in detail how you minimized the loss in your mechanical connection to the loop conductor.

8th April 2012 11:10

qrp-gaijin wrote...

Did you ever build the EH antenna for comparison?

Also, do you have some idea of the efficiency of your loop antenna? I would have expected all of the mechanical connections to raise the loss resistance (higher than the miniscule radiation resistance) and thus kill the efficiency of the antenna.

4th March 2010 16:56

Ian J Downie wrote...

Have you considered the typical Yagii style co-axial type tube within a tube option?

27th March 2008 14:23

Alan Yates wrote...

Thanks Peter.

Alan Parekh shot me an email last night saying his Hacked Gadgets site would be carrying it Today. I imagine Make picked it up from there.



27th March 2008 09:26

marxy wrote...

Congratulations on being linked on Make!

25th March 2008 16:54

Alan Yates wrote...


Yeah, I may have a go at making my own caps. That's a great idea, thanks. I'd have to pick a dielectric that isn't too lossy, air is the obvious choice, but polymers have better constants (but more unknown losses). Measuring the Q of a capacitor is surprisingly non-trivial, I tried a few months ago. Its easy to make relative measurements (better/worse than a reference unit), but rather hard to absolutely measure the very small ESRs of a good RF capacitor with respect to its reactance.

I was thinking of homebrewing some compression trimmers. That would give me tuneability. I don't have any mica but plastic sounds do-able. Voltage rating might be a problem, but with big gaps it will be fine at the powers I run.

On the higher bands you could bend the two ends of the loop so they form a parallel plate capacitor. The geometry alone would resonate with all distributed elements, no lumped devices at all. With the springy nature of the metal it would be possible to implement a compression trimmer to fine-tune the loop (with the bulk of the capacitance in a lumped cap for the lower bands). Distributed elements tend to be more lossy than a good lumped element (compare a coaxial capacitor to a real transmitting mica), but as the cap wouldn't be too large with respect to the frequency of operation it shouldn't be too bad.

Something to try anyway. Antennas offer almost endless experimental opportunity. Looks like I'll be making a spot-welder at some point too...



25th March 2008 15:28

Alan Yates wrote...


Nice to hear from you again!

Yep, small TX loops like this show great promise for my limited antenna options, and anyone else in the same situation. Unlike the many EH/Cross Field "antennas" claiming to be efficient, loops can be quite efficient if you take pains to reduce their losses.

Unfortunately you trade bandwidth for that efficiency, but above 40 metres that isn't a big deal. If you use PSK31 or CW exclusively you can almost fix-tune your loop on the higher bands, and just visit it manually when you want to move around.

The bike rim must be fairly conductive at RF, its relatively smooth and Aluminium (not steel), so the losses shouldn't be too bad. I considered polishing it, but my connections to it are likely the most lossy part, so for now its just natural. I could work out its losses based on the Q - might do that...

There is no reason why you couldn't use a strip of Aluminium from the hardware store. Aluminium flashing could be used for a quick lash-up test, but it isn't very mechanically strong as it seems to come fully annealed. I have a few metres of 1.5 mm thick, 50 mm wide Aluminium extrusion that could be bent into a loop and used as well. Being able to spot and stick weld Aluminium would be useful to keep the losses as low as possible.

I'm tempted to build a few of the "EH" antennas (Isotron, etc) and compare them to the loop. Truth be told such antennas appear to be just top-loading for the feed line, but I'd like to prove that to myself experimentally. Feeding such an antenna balanced, with a choked feed, or with a battery powered TX right at the antenna sound like good experiments.



25th March 2008 15:04

Arv - K7HKL wrote...


It might be interesting to try building your own fixed capacitors for that bicycle wheel loop antenna. All you need is some metal foil or sheet and the plastic from a 2-liter soft drink bottle.

I would suggest either tin can metal or galvanized roof flashing from the local DIY store, because you can bend up a small corner or tab for soldering connecting wires. Use nylon screws to hold the metal on each side of a thin sheet of plastic.

If you first make a trial capacitor you can determine the approximate dielectric constant for your plastic insulation, then use this for designing a pair of caps for your loop antenna.

You could even make a variable section for tuning purposes, but that may be overkill.

Arv - K7HKL

25th March 2008 14:53

Arv - K7HKL wrote...


As usual, an excellent interesting and informative article. Bicycle wheels don't usually get considered as antenna material. This seems like an excellent stealth antenna project for apartment dwellers, or others with severe antenna restrictions.