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