TheRadioBoard

Strip cut from aluminum soda can.



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In the context of a magnetic loop antenna, one cause of loss when using an ordinary air variable capacitor is the mechanical, rotary wiper contact connecting the rotating shaft with the solder lug for the rotor contact. This mechanical joint introduces several milliohms of loss, critical for a low-loss small transmitting loop. Vacuum variable caps, butterfly capacitors, or split-stator capacitors are typically recommended alternatives.

It would be interesting if it would be possible reduce the losses in an ordinary air variable capacitor by soldering a heavy but flexible conductor (e.g. copper braid or copper strap) directly to the (aluminum) rotary shaft to avoid loss through the wiper contact. This might allow the use of an ordinary air variable capacitor without excessive loss compared to a butterfly or split-stator capacitor.

However I would wonder if an aluminum-copper solder joint is inherently lossier than a copper-copper joint. I would also think it might increase in resistance over time if exposed to the weather (corrosion).

Have not seen any aluminum shafts?

Some twenties caps had braid, or watchspring (phosphor bronze) connections either screwed, or soldered, to the (brass) shafts.

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Any idea what material this capacitor's shaft might be (and if it looks solderable)? I imagine it would conduct heat away too quickly to allow soldering.






Here's a close-up of the lossy mechanical wiper connection.

If it's not magnetic it's probably brass. Tarnished silver plating?

73,

_________________

http://kr1s.kearman.com/
http://qrp.kearman.com/

That big counter balance is begging to have a braid/litz connection.
ie. braid soldered into a small lug, screwed (self tapper?) to the counterweight.
(Though maybe not? at your frequencies?).

Looks a quality unit so it might be adequate as it is?

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You mean putting a new screw into the counterweight? My mechanical skills are practically nonexistent - is this an easy thing to do with hand tools, or is a drill required?

What frequency-dependent limitations are you hinting at with your comment about the frequencies?


Yeah, I've seen at least one transmitting magnetic loop design (G4ILO Wonder Loop: http://www.g4ilo.com/wonder-loop.html) that anecdotally claims a normal capacitor like the one I have works okay. More specifically, the designers of the "Midnight Loop" said they measured a maximum of 20 milliohms resistance from a wiper contact, so that's at least a starting point for trying to evaluate the possible effect of a wiper contact in the high-current path.

Just whether, or not, a braid or litz connection would be "hi-ohmic" at f's above 2MHz. Maybe the clock spring type would be better.

You have plenty of shaft to run a couple of extra wipers on, plus you could maybe run additional wipers on the counterweight somehow (with a bit of thought).

Wondering why, if you have zero mechanical skills, you persevere with home brewing/modifying hardware items? (Just asking).

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As enjoyable entertainment and self-improvement, of course. I did say "practically nonexistent" mechanical skills, not "completely nonexistent".

A year ago, I probably wouldn't have trusted myself to build a radio from scratch, but now, working slowly with simple hand tools and small wooden boxes, I've learned how to make an acceptable-looking radio with holes for the knobs and jacks and enough mechanical stability for comfortable listening and tuning. In the same vein, one of these days I hope to graduate on to working with metal, but I have little in the way of tools and knowledge right now.

Hi qrp-gaiijn,
I say jump right in! If you have a local scrap yard, go and pick a few small pieces of aluminum, iron, and brass. Get some decent drill bits and and a center punch to mark and aid starting the drill bit. You should get a viceand a hach saw.
A small drill press is nice, here's one for $70, I have about the same.
http://www.harborfreight.com/5-speed-dr ... 38119.html
Then experiment with the pieces of scrap and learn how different metals cut. I've been cutting, drilling and fileing metal since high school, so get started.
Oh, and I love working with aluminum!
I don't see your location, some of this may be easier said than done.
Mikek

Japan. I've not yet figured out here if there are scrap yards that actually sell stuff to people. I've seen junk collection areas where people drop off old analog TVs and radios for destruction (how I would have loved to raid the insides of those sets...), but I haven't seen a scrap yard around here that lets you pull out junk and haul it away for cheap. It seems there's a bit of a stigma here in second-hand goods of many sorts (e.g. there are strict rules against dumpster diving at basically all residential complexes, even going so far as to have locked metal cages around the areas where large garbage is dumped to prevent people - usually foreigners - from taking furniture or electronics goods from the garbage).

Thanks for the advice on the hardware. I was thinking too that a vice should be first on the list for safely and firmly gripping stuff in place as my amateur hands learn to wield power tools. I do have a hacksaw; I've only used it for cutting wood so far, but its blades can handle metal as well. Cutting straight is a bit harder than it looks though...

The strip loop, wide version:


Wide aluminum foil (450mm width, 0.040mm thickness, 7.5m length) forms the loop element and capacitor. Polyethylene packing foam (1m width, folded in half) forms a protective sleeve around the foil as well as the capacitor dielectric. The capacitor, at the bottom, is formed by overlapping the loop ends, held in place with clothespins. The capacitor is probably closer to concrete than is advisable, but this allows easier adjustment.

The main goal of the design was an easy-to-build and theoretically efficient magnetic loop, in order to evaluate if such a magnetic loop will work on my balcony given the concrete walls and aluminum railing. If it works, then I'll start to ponder a more permanent version with a solid frame and a remotely-tuned butterfly capacitor for easier wide-band operation. Connecting the capacitor to the aluminum foil loop element with minimum loss will be a challenge.

First tests indicate I can resonate the loop at least up to 14 MHz. 21 MHz may be a stretch.

Edit: Folding the loop ends back on themselves to make the loop shorter allows resonance at 21 MHz. For the first time I am hearing booming DX QSOs (China, Russia, Poland) on 15m at night. This is quite encouraging. Also, elevating or sinking the sag in the top part of the loop allows tuning the resonance by several kHz (at 21 MHz). This suggests a very simple remote control mechanism - a rope and pulley. Obviously this will not work well on a windy day when the loop is flapping in the wind.

Summer's coming, and soon the neighbors will be out on their balconies, wondering what that crazy gaijin is doing. Tell them it's an earthquake detector. Those power lines look nasty.

73,

_________________

http://kr1s.kearman.com/
http://qrp.kearman.com/

I originally was going to build a rectangular shelf-like frame for the antenna which is less conspicuous. I decided to suspend those plans until I was sure the loop would work in this location. It's looking promising so I may revisit the frame idea.


In terms of appearance, or in terms of possible RF pollution?

Both. They look like they could suck up some rf, too.

73,

_________________

http://kr1s.kearman.com/
http://qrp.kearman.com/