Of course! You don't really need triax or twinax..(too early, and the
coffee hasn't taken effect yet)
I forgot, the inside and the outside of the coax are actually different
conductors for RF. The current will flow on the inside (where it's part of
the TL) up to the middle, then, move to the outside (where it's part of the
dipole). So, with suitable RF decoupling at the bottom (to set the "end" of
the dipole) it should work.
I suspect there might be a fair amount of "fiddling" required, but at least,
you only have two variables to work with.. the length of the "shieldless"
part, and the position of the decoupling.
Here's the other tricky part.. If you do the "coil of coax" approach, it
looks like a big inductor. Will that sufficiently decouple the end of the
dipole. If you used ferrite beads, then you've got a choice of materials...
Either it will look like a big resistor (lossy ferrite) or a big inductor
(not lossy ferrite).
This is clearly a case where an hour or two with a hunk of coax and an
analyzer would be a whole lot more productive than hours of trying to model
it.
You might want to think about what kind of coax would be best (loss, and
solidity of shield (so the inside, outside thing really works). I'm
thinking that since the impedance of the dipole is normally around 70 ohms,
a piece of Cable TV coax might be the optimum. It's lightweight, it has a
solid foil shield, etc. The shield's aluminum, but heck, people build
antennas out of aluminum all the time. Your top half of the antenna could
be soldered to a female "F" connector, and once assembled, the male F
connector on the coax mated to the female F should be weather proof. I
don't know that I'd run a kilowatt through it...(cheap and fun to try and
find out by a test to destruction, though?) You could use one or more of
those clamp on ferrite filters, at least for testing. (This would be the
lossy choke approach) As you slide it up and down, you should see a change
in feedpoint impedance. You will need a F to PL-259 adapter (or some sort of
funky pigtail)
Velocity factor shouldn't enter into it. You'll get some dielectric loading
from the insulation on the outside of both conductors.
Go for it.
Darn, if only it weren't before sunrise, and raining to boot; yes, in
southern California, land of eternal sunshine, our 180 day dry spell has
been broken ... I've been thinking about easily deployed dipoles for a while
now, and this scheme has real appeal.. (and it fits with N5BFs dictum for
antennas.. use only materials that can be purchased at Home Depot)
----- Original Message -----
From: "Kelly Taylor" <ve4xt@mb.sympatico.ca>
To: "K0PYK" <k0pyk@yahoo.com>; <towertalk@contesting.com>
Sent: Sunday, October 17, 2004 6:18 AM
Subject: Re: [TowerTalk] Vertcal Dipole
> There is in the ARRL handbook a design for an end-fed dipole just like Tom
> suggests. It was oriented horizontally, but I see no reason why vertical
> orientation wouldn't work.
>
> The antenna was made of coax, with 1/4-wl of sheild stripped from one end.
> At approximately 1/4-wl from that point (approx. 1/2-wl from the end of
the
> centre conductor), a simple choke was created by coiling the coax which
then
> could run intact all the way back to the shack. So the centre conductor
was
> one pole of the dipole and the braid from the feedpoint back to the RF
choke
> was the other.
>
> Of course VF was a consideration and as I recall, the choke position
> required experimentation to get right.
>
> Seems to me the big issue, once the electrical issues have been sorted
out,
> is water penetration, particularly with the antenna in a vertical
position.
> I'd be tempted in this case to get a continuous length of glue-lined
> heat-shrink to cover the top radiator and extend past part of the
feedpoint.
> Once the length was right, I'd use another piece of glue-lined heat-shrink
> to create a cap and upper mounting device.
>
> 73, kelly
> ve4xt
>
>
>
> ----- Original Message -----
> From: "K0PYK" <k0pyk@yahoo.com>
> To: <towertalk@contesting.com>
> Sent: Sunday, October 17, 2004 5:28 AM
> Subject: [TowerTalk] Vertcal Dipole
>
>
> > Tower Talk'ers,
> > Has any made a vertical dipole (lets say for 20
> > meters) where it is end fed with coax. I will try and
> > explain my thoughts;
> > * top half of radiator, i.e., 1/2 of dipole is a
> > single 14 guage wire.
> > * bottom half of dipole would be coax...the center
> > conductor would feed the top half inside the braid.
> > * the bottom half of vertical dipole would be the
> > outside braid of the coax.
> > * the bottom half of the dipole would be fed from the
> > bottom and end where the top half starts.
> > * a decoupling device may be needed at the bottom of
> > the dipole...1-1 balun???.
> > * I realize that normally dipoles are fed in the
> > center.
> > * A person could use some type of tubing for the
> > bottom half and still feed it in the center, but I was
> > looking for something that would be portable and you
> > could just hang up vertically or horizontally.
> > I look forward to your thoughts.
> > Thanks in advance,
> > Tom, K0PYK
> >
> >
> >
> >
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> > _______________________________________________
> >
> > See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
> Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with
any
> questions and ask for Sherman, W2FLA.
> >
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>
> _______________________________________________
>
> See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
questions and ask for Sherman, W2FLA.
>
> _______________________________________________
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_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
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