On Thu, 22 Oct 2009 05:17:37 -0700 (PDT), Dennis OConnor wrote:
>Bob, have you modeled it?
>I bet you will be surprised at what you find...
Good advice. But I would insert the beam into the model as being
connected to the tower, and the tower connected to the bottom of
the vertical on one end and to ground on the other, with the
transmitter connected at the bottom of the vertical. Note,
however, that the connection to ground doesn't do much for that
vertical -- it's the metal below the antenna (and close to it)
that matters most.
Most verticals DO need some form of radials (counterpoise) to
serve as the return for antenna currents AND the fields that the
antenna produce. Those fields and the currents form a CIRCUIT, and
if that circuit is not there or is poor, antenna performance will
be poor. While it may be far from ideal, the beam could perform
the function as a counterpoise sufficiently well to make the
vertical work well enough to make you happy.
Typical all-band verticals have the reputation of poor efficiency.
There are at least two solid technical reasons for this. First,
the loading coils and other networks designed into them to make
them work on a lot of bands (and on bands for which they are short
as a fraction of a quarter wave) are lossy. Second, many verticals
are installed without a sufficient radial system. Both of these
failings burn some of the transmitter power.
A few verticals are designed to work without radials, but most DO
need radials (or some form of counterpoise).
As to the antenna tuner -- yes, you are likely to need an antenna
turner to get the transmitter to put power into the transmission
line, and some of that power will be burned in the line if there
is a mismatch at the antenna. BUT -- an antenna tuner will NOT
make the antenna RADIATE, and it will NOT make up for losses in
the antenna due to those loading coils and the lack of radials.
What the tuner does is make the transmitter happy and allow it to
put power into the coax.
73,
Jim Brown K9YC
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|