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Alex Loop (or other magnetic loop) Measurements

I gather the main advantage of silver over copper is that the tarnish on silver conducts better than the tarnish on copper…

…or is tha aluminium? (My brain’s not working well this morning…)

73, Rick M0LEP

In reply to all: Lots of good info here. Copper plated on steel gives the strength, and potentially a uniform high-quality conducting layer.
What about the capacitor, though. This needs to be a very low-loss device, and some of the references suggest that capacitor losses are significant. Very few of us can lay our hands on a vacuum capacitor. Some, probably most, surplus items are filthy with have surface corrosion on the vanes and the quality of the rotor connection is doubtfull. I had thought of making one from double-sided PCB material (only 'cos thats whats in my bits box), but fashioning a suitably rigid frame and rotor bearings needs some thought.

The article by Prof Mike Underhill in ‘International Antenna Collection’ (Published by RSGB 1993), and subsequent discussion in RADCOM over the years, gives food for thought.

Regards, Dave, G6DTN

In reply to M0DFA:

Very few of us can lay our hands on a vacuum capacitor.

Alex said a few years ago that you should not wait for any special vacuum capacitors, but just build something that works. The voltage on the capacitor can be high starting from one kilovolt on QRP power. The commercial loops are more expensive. See for example DF8PJ model from Wimo to get ideas for construction.

73, Jaakko OH7BF/F5VGL

In reply to F5VGL:

A “butterfly” variable capacitor does not have sliding contacts. That is the right choice for a small loop antenna. MFJ sells two butterfly caps: https://www.google.com/search?q=mfj+butterfly+capacitor

K6WRU

In reply to M0DFA:

There are plenty of sources for caps. For example:

http://mainline-group.com/index.php?option=com_content&task=category§ionid=12&id=40&Itemid=38

http://www.partridgeelectronics.co.uk/21Btransformers.htm

http://stores.ebay.co.uk/Michaels-Electronics-Emporium/HAM-RADIO-/_i.html?_nkw=vacuum+capacitor&submit=Search&_fsub=8&_sid=87175284

73
Mike G6TUH