On 11/1/2015 2:38 PM, Jeff Liebermann wrote:
I've been thinking of building a rather different magnetic loop
antenna. Instead of the usual rigid loop and adjustable tuning
capacitor, I want to try a flexible loop and a fixed capacitor. The
idea is to eliminate the cost of the tunable vacuum capacitor or
cheese grater butterfly capacitor.

For a loop, I propose to use a flexible aluminum dryer vent hose:
https://www.google.com/search?tbm=isch&q=flexible+dryer+vent+duct
http://www.ipagepro.com/dryerventsolutionsllc/logos/BTD48.png
http://ace.imageg.net/graphics/product_images/pACE3-17458764enh-z8.jpg
I've used similar hoses in my giant inflatable rubber ducky antenna
experiments. The major point of failure was that the spiral steel
wire antenna inside the hose produced some hot spots, which melted a
hole in the vinyl jacket. However, with an aluminum jacket, all the
RF will be on the outside, and there's no vinyl to melt.

The 8ft dryer hose will (somehow) form a loop when inflated with air
from a bicycle or hand pump. Inside the dryer hose is a length of
small diameter bungee cord to help collapse the loop. When hose is
inflated, the loop expands, which also tunes the loop to the desired
frequency.

One nice feature is that with the loop deflated and collapsed, it's
sufficiently small for portable or stealth use. Fully inflated, an
8ft hose should produce a 31" diameter loop.

The rest is fairly conventional. A small wire loop inside the loop to
provide impedance matching and coupling for the 50 ohm feed. Maybe
some kind of tuning indicator (small fluorescent tube). I'm undecided
on whether to mount it vertically, which requires a prop, or
horizontally, which can be done on an insulated table surface.

Permission to steal and use this idea is hereby granted, as long as
you fail to mention my identity in the event that your sanity is
questioned by the neighbors or local authorities. No patents are
pending.

When you say dryer hose, you mean the corrugated aluminum tube that is 3
or 4 inches in diameter. That might work for a loop antenna, but I
think the corrugations are hard to collapse once you expand them. So I
doubt it will work as the tuning element unless you simply change the
shape of the loop rather than keeping it a circle with an adjustable size.

I used some of this stuff in a larger diameter to connect a humidifier
and it was flexible enough to extend and shape, but didn't go back
hardly at all.

--

Rick