> 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.
While the Handbooks are generally pretty good, once in a
while some marginal stuff creeps in.
The problem is always sorting good ideas from bad ideas.
> 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
I've played with antennas like this many times over the
years, mostly when commercial interests want to market an
end-fed antenna. There is a major problem that is difficult
to solve. It's really common sense when you think about it,
which obviously many people writing articles and selling
antennas don't do.
The end-impedance of a 1/2 wave is many thousands of ohms.
The exact value of end-impedance depends on several factors,
the diameter of the conductor being a major player, but no
matter what we do it is pretty high.
To make the antenna radiate and NOT the feedline, we must
have the antenna near the feedline "float". This only occurs
when the isolation device has *many* thousands of ohms of
series impedance, and extremely low parasitic capacitance
coupling the antenna end to the coax.
Of course you can compensate that capacitance with an
intentional inductance, but you then have a trap with narrow
bandwidth.
A bundle of coax simply does NOT have enough reactance to
decouple the feedline shield, and you will never buy a balun
or line isolator that does that job either. The system need
perhaps 40,000 or more ohms common mode impedance just to
have marginal feedline isolation, because end-impedance of a
dipole is often 8,000 ohms or more.
You can get an idea of how difficult this is to do by
looking at the isolation impedance of traps that are
intentionally designed to sectionalize antennas.
See:
http://www.w8ji.com/traps.htm
You sure aren't going to do nearly as well with coax coils
or "choke baluns" as you would with a resonant trap, where
only Q limits impedance.
Of course a sleeve-feed could be used, but the choking
impedance of the sleeve is a function of the surge impedance
and loss of the transmission line formed by the sleeve. It
is nearly impossible to make a good sleeve with a large air
gap between the inner cable and the sleeve, and we sure as
heck aren't going to do it with the jacket of a coaxial
cable making up a significant part of the dielectric.
See:
http://www.w8ji.com/end-fed_vertical_j-pole_and_horizontal_zepp.htm
and
http://www.w8ji.com/Baluns/sleeve_baluns.htm
So there, in a nutshell, is the problem. The isolation
impedance offered by the sleeve, stub, or choke has to be
many tens or even hundreds of thousands of ohms over the
band, or the feedline shield is part of the system.
I should state clearly no having enough isolation does NOT
prevent the system from having a low SWR. It doesn't mean
you won't make contacts, and it certainly doesn't mean you
won't be happy with how it works.
What is does mean is that the feedline is part of the
antenna. It means you may have significantly more noise from
things in the house or power lines feeding the house, and
more RFI or RF in the shack. It means the antenna will be
quite a bit less predictable than a regular dipole. You
won't be able to copy or buy one and install it and have it
work the same as it does in other situations.
There are other variations of end-fed antenna with a
matching system. They are sold as various CB products
claiming to be end-fed 1/2 waves without radials, and even
in modifoed form as 5/8th wave antennas without groundplane.
One company markets end-fed 1/2 wave wires for the Ham
bands.
These systems are just as bad, if not worse. Every single
bit of current flowing into the antenna end is matched by
equal current flowing down into the tower, mast, and
feedline feeding the antenna.
My main point in bringing this up is if we are going to have
articles telling people how to build antennas like this,
people should be aware of the major pitfall. Then maybe the
manufacturers will learn how the products they sell actually
work, and improve the design.
> Of course VF was a consideration and as I recall, the
choke position
> required experimentation to get right.
I'll bet. That is because the feedline BOTH sides of the
choke is part of the radiating system!
The end result of a poorly designed sloppy feed system is
the antenna, while it certainly will "work", will be highly
dependent on how long the feeder is and what the feeder is
connected to. This is why some people swear by sloppy
designs, and other people swear at them.
If I was stuck using a system like this, I'd use the dipole
idea. I'd use the best choke I could manage at the antenna,
an air wound solenoid coil at or near self-resonance. I'd
also be prepared to add a second choke 1/4 wl or less down
the feedline towards the shack, if necessary. That would
pretty much guarantee a system that worked like a dipole.
73 Tom
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