"christofire" wrote in message
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"Jim Lux" wrote in message
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Wayne wrote:
"Spin" wrote in message
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Wayne........That's interesting.....Can you elaborate on that 2 meter
antenna you had? I wonder if one were to make a longer version would
it have gain & a wider bandwidth?


Sure. However, I'm not recommending it for anything. As I recall it
was a quarter wave of coax terminated in a 10 watt dummy load. The
quarter wave was connected as previously described, with the shield of
the quarter wave connected to the center conductor of the feedline and
the shield of the feedline connected to the center conductor of tthe
quarter wave. I was just trying to get a dummy load to radiate enough
for a short path to a repeater. (Transmitter ran 10 watts)

At a different time, I simply terminated a feedline (low quality RShack
RG-58) with a 10 watt carbon resistor (unshielded and 3 inch leads). It
worked about the same.

However, remember that I was only trying to hit a single repeater, and a
whip antenna with just a few milliwatts would have worked on that
particular path. The "antenna" was very poor, but there may be paths
where it is an acceptable compromise.

Think of this as a variant on taking a 1/4 wave of wire and attaching it
to the center conductor of the feedline (or, just stripping 1/4wave of
shield off the coax) with no choke, balun, or anything else. It's a sort
of sleeve dipole: The "outside" of the feedline coax essentially acts as
the other half of the dipole.

Depending on where it's installed, it might work, might not. No
decoupling means that the whole feedline potentially radiates, etc.
Probably no worse than a lot of other improvised antennas.

Put a really good choke around the coax at the 1/4 wave point, and it
starts to look better, but, having the feedline essentially hanging off
the end of the dipole means that you've got conductors in the high E
field part of the antenna, so there will be capacitive coupling.


Doing the Coro style thing with the resistor, etc, in effect makes this
another of the many "resistively loaded dipole" schemes where you give up
some efficiency in exchange for a better match. No different in concept
(although different in design) from the T2FD sorts of things from B&W,
etc.


The section of line at the end, terminated with the resistor, will present
something like 50 ohms between its terminals (outer and inner) at the
point where it is connected to the feedline, even if its terminals are
reversed, so if the outside surface of its outer conductor and the outside
surface of the outer conductor of the feedline present some sort of dipole
to the same point, the ~50 ohms will shunt the terminal impedance of that
dipole. So if the dipole actually radiates in the manner of a half-wave
wire dipole its VSWR could be improved by omitting the resistor.

Working: VSWR = (1+{Rho})/(1-{Rho}) where {Rho} is the magnitude of Rho,
the reflection coefficient for voltage at the dipole's drive point and Rho
= (ZL-Z0)/(ZL+Z0) where ZL is the impedance of the load at the end of the
feedline of characteristic impedance Z0.

~72 ohms from a half-wave dipole alone yields Rho = 0.16 for Z0 = 52 ohms,
and so VSWR = 1.38.

~72 in parallel with ~50 = ~29.5 ohms for the parallel combination yields
Rho = -0.28 so {Rho} = 0.28 and VSWR = 1.78.

Of course, if the 'dipole' does not radiate in the manner of a half-wave
wire dipole then this may not apply (e.g. if it is buried).

Chris

.... and replacing it with a short circuit! (important point, omitted from
posting)