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.




--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

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

On Sun, 1 Nov 2015 18:31:55 -0500, rickman wrote:

When you say dryer hose, you mean the corrugated aluminum tube that is 3
or 4 inches in diameter.

Yep. Just like the stuff in the links provided.

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.

The bungee cord down the middle is suppose to help collapse the hose.
I must confess that I haven't tried it. If that doesn't work, then
some elastic cords. If that fails, a nylon rope and some external
springs.

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.

Yep. It won't go back by itself and needs some additional help.
Anyway, if you want to get your picture in QST as the building of the
worlds strangest and probably cheapest loop antenna, here's your
chance.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

On 11/1/2015 9:27 PM, Jeff Liebermann wrote:
On Sun, 1 Nov 2015 18:31:55 -0500, rickman wrote:

When you say dryer hose, you mean the corrugated aluminum tube that is 3
or 4 inches in diameter.

Yep. Just like the stuff in the links provided.

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.

The bungee cord down the middle is suppose to help collapse the hose.
I must confess that I haven't tried it. If that doesn't work, then
some elastic cords. If that fails, a nylon rope and some external
springs.

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.

Yep. It won't go back by itself and needs some additional help.
Anyway, if you want to get your picture in QST as the building of the
worlds strangest and probably cheapest loop antenna, here's your
chance.

Ok, so if you can force it to shrink with springs or ropes or whatever,
then something will be needed to force it to expand again. I'm having
trouble seeing how this will work without the antenna losing all shape.
These tubes are just not really easy to manipulate. They are intended
to be bent once with more than a little force but more importantly very
controlled force.

I'm not sure the inductance will change all that much. I have never
seen a calculation for the inductance of an accordion. It may have a
rather limited tuning range compared to a typical variable cap. At
least the frequency will scale the right way with size. Smaller loop,
lower inductance, higher frequency which will keep the radiation
resistance high.

--

Rick

On Sun, 1 Nov 2015 21:58:32 -0500, rickman wrote:

Ok, so if you can force it to shrink with springs or ropes or whatever,
then something will be needed to force it to expand again.

Yep. A bicycle pump, hand pump, crank pump, bellows pump, electric
pump, or pressure vessel will all inflate the antenna.

I'm having
trouble seeing how this will work without the antenna losing all shape.

Below some pressure level, it will probably flop over if mounted
vertically. That's why I mumbled that I wasn't sure if it should be
mounted vertically with a support pole, or horizontally on a flat
sheet of plywood. Both will work, but I'm not sure which is better.

These tubes are just not really easy to manipulate. They are intended
to be bent once with more than a little force but more importantly very
controlled force.

In other words, after a few inflation deflation cycles, it might fall
apart. I have a few that I bought for the inflatable antenna project.
It looked quite flexible to me but I'll test it to be sure.

I'm not sure the inductance will change all that much. I have never
seen a calculation for the inductance of an accordion. It may have a
rather limited tuning range compared to a typical variable cap. At
least the frequency will scale the right way with size. Smaller loop,
lower inductance, higher frequency which will keep the radiation
resistance high.

Good point. At one time, I was wondering how to increase the
bandwidth of a yagi antenna. I knew that rounding the ends of the
elements would increase the bandwidth because there was no single
length for which to consider the "end" of the antenna rod. Similarly,
when calculating the rod length of a yagi antenna, the RF path around
the center boom must be added to the rod length. That made me wonder
if I could roughen the antenna rod to produce the same effect. I
guess corrugation might be considered the ultimate form of antenna
"roughness". The question was would the antenna length be the
distance from end to end of the accordion, or would it be the distance
traveled across the surface along all the ups and downs of the
accordion.

What I found was that the effect varies with frequency and of course
the accordion geometry. At 1MHz, the resonant length was the surface
distance traveled. In other words, expanding the accordion had little
effect on the antenna resonance. At much higher frequencies (about
150 MHz), there was enough capacitance between the accordion "sides"
that the antenna was effectively shortened and the resonant frequency
was the end to end distance. However, that's not exactly true because
there were multiple path lengths which could be considered resonant,
much like the rounded end on the rod. So, at low frequencies, my
scheme probably won't work. At higher frequencies, maybe. Your task,
should you decide to accept it, is to try it. All it will take is a
length of flex aluminum dryer hose and an LRC meter.

Please note that my testing was not a proper lab test but more like
screwing around with a grid dipper, LRC meter, and network analyzer to
help settle a lunch time argument.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

On 11/1/2015 11:32 PM, Jeff Liebermann wrote:
On Sun, 1 Nov 2015 21:58:32 -0500, rickman wrote:

Ok, so if you can force it to shrink with springs or ropes or whatever,
then something will be needed to force it to expand again.

Yep. A bicycle pump, hand pump, crank pump, bellows pump, electric
pump, or pressure vessel will all inflate the antenna.

Ok, that might be workable. I think the tube will need a liner. I'm
not sure this stuff will be easy to seal.


I'm having
trouble seeing how this will work without the antenna losing all shape.

Below some pressure level, it will probably flop over if mounted
vertically. That's why I mumbled that I wasn't sure if it should be
mounted vertically with a support pole, or horizontally on a flat
sheet of plywood. Both will work, but I'm not sure which is better.

You are assuming it will maintain something remotely like a circle. I
don't see that happening. Have you worked with this stuff? Maybe what
you have is more pliable than the stuff I used.


These tubes are just not really easy to manipulate. They are intended
to be bent once with more than a little force but more importantly very
controlled force.

In other words, after a few inflation deflation cycles, it might fall
apart. I have a few that I bought for the inflatable antenna project.
It looked quite flexible to me but I'll test it to be sure.

I don't mean fall apart necessarily, but just not be much like a loop
antenna. I think the hard part will be shrinking it back down and
keeping its shape. Proof of the pudding...


I'm not sure the inductance will change all that much. I have never
seen a calculation for the inductance of an accordion. It may have a
rather limited tuning range compared to a typical variable cap. At
least the frequency will scale the right way with size. Smaller loop,
lower inductance, higher frequency which will keep the radiation
resistance high.

Good point. At one time, I was wondering how to increase the
bandwidth of a yagi antenna. I knew that rounding the ends of the
elements would increase the bandwidth because there was no single
length for which to consider the "end" of the antenna rod. Similarly,
when calculating the rod length of a yagi antenna, the RF path around
the center boom must be added to the rod length. That made me wonder
if I could roughen the antenna rod to produce the same effect. I
guess corrugation might be considered the ultimate form of antenna
"roughness". The question was would the antenna length be the
distance from end to end of the accordion, or would it be the distance
traveled across the surface along all the ups and downs of the
accordion.

There are helically wound antennas that have a similar issue. I have
yet to see any equations to model them. I wonder if they work or not,
in the sense of any better than a simple loop.


What I found was that the effect varies with frequency and of course
the accordion geometry. At 1MHz, the resonant length was the surface
distance traveled. In other words, expanding the accordion had little
effect on the antenna resonance. At much higher frequencies (about
150 MHz), there was enough capacitance between the accordion "sides"
that the antenna was effectively shortened and the resonant frequency
was the end to end distance. However, that's not exactly true because
there were multiple path lengths which could be considered resonant,
much like the rounded end on the rod. So, at low frequencies, my
scheme probably won't work. At higher frequencies, maybe. Your task,
should you decide to accept it, is to try it. All it will take is a
length of flex aluminum dryer hose and an LRC meter.

I don't have any equipment to date. I have a couple of projects ahead
of this if I decide to build something.


Please note that my testing was not a proper lab test but more like
screwing around with a grid dipper, LRC meter, and network analyzer to
help settle a lunch time argument.

You clearly have much more experience than I do. I wold barely know how
to use a SWR meter and don't have an LRC meter... I can't remember what
a grid dip meter is.

--

Rick

On 11/1/2015 10:32 PM, Jeff Liebermann wrote:
On Sun, 1 Nov 2015 21:58:32 -0500, rickman wrote:

Ok, so if you can force it to shrink with springs or ropes or whatever,
then something will be needed to force it to expand again.

Yep. A bicycle pump, hand pump, crank pump, bellows pump, electric
pump, or pressure vessel will all inflate the antenna.

I'm having
trouble seeing how this will work without the antenna losing all shape.

Below some pressure level, it will probably flop over if mounted
vertically. That's why I mumbled that I wasn't sure if it should be
mounted vertically with a support pole, or horizontally on a flat
sheet of plywood. Both will work, but I'm not sure which is better.

Hang it upside down.

Mikek

You analysis of the accordion roughness' effect is relative to the wavelength. I have no idea of the relationship, but my brother whom has designed antenna systems for military aircraft and patented antenna designs schooled on the fact that the formulas we all love and use are WRONG!

However, the formulas we use work well because we typically use relatively thin radiating elements compared to the wavelength, often significantly less than 1% of the operating frequency. The correct formula is surface area of the radiating element NOT LENGTH.

To you and I building wire, Yagi, and loop antenna with skinny radiating elements length ends up being very close to the surface area. However, design an antenna for a cellphone from a thin plate or part of an aircraft fuselage and the length formula starts falling apart, especially at the higher frequencies.

I didn't bother learning the correct formula as it is a little more complex.. I think it may come up when you start building fractal shaped antennas, such as were used in WWII by underground operatives using the lead frames stained glass windows in churches as radiating elements. The surface area formula may become relavent to magnetic loops with radiating loops with very thick radiators.

N4VEP

On 11/1/2015 11:38 AM, 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.




Probable should clear the lint out first!

Paul, KD7HB

On Sun, 1 Nov 2015 18:10:56 -0800, Paul Drahn
wrote:

Probable should clear the lint out first!
Paul, KD7HB

I know that hams are cheap, but I didn't realize that extended to
stealing the neighbors dryer vent hose in order to build an antenna. I
highly recommend purchasing a new hose, which comes pre-cleaned:
http://www.acehardware.com/family/index.jsp?categoryId=2627981

Incidentally, there's no requirement that the entire loop be made from
expandable hose. The loop could be in the form of a square loop,
where the flat base and flat top are rigid aluminum structures, and
the two vertical sections are flexible dryer vent hose. The weight of
the flat top would help collapse the antenna although I suspect that
springs or weights would work better.

It's not like inflatable antennas and structures are a new thing:
http://ltaprojects.com/towers/inflatable-antenna-video-towers/ham

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558