[Antennas] Re: Antennas digest, Vol 4 #150 - 2 msgs

[Robert Lay]

Dear Don Williams,

Now that we understand the motivation for using a balun, a few points come
to mind that may be worth your consideration.

1) The concept of matching the antennas feed point impedance with a balun is
quite valid - not to mention the side benefits of reducing the unwanted RF
current flow on the outer surface of the coax shield (note that I am now
satisfied that you do intend to use a coax feedline).

2) Not all folded dipoles use a resistance termination as does the B&W
creation - in fact, the resistance terminator is their unique creation, and
so far as I know is not marketed by anyone else.

3) Please do not lose sight of the fact that all of the watts dissipated in
that resistive termination convert to heat - not RF. The consensus of
opinion among knowledgeable contributors to this reflector and the r. r. a.
a. newsgroup seems to be unanimous in panning the B & W folded dipole.
However, if it is that important to have a low to moderate SWR without
having to deal with it by traditional means, then you have that choice.

4) Be aware of the fact that ANY dipole, regardless of its length, not
folded and without the terminating resistor will very likely perform as well
or better than the B & W design. Obviously, the closer you can get to a half
wavelength on 80 meters, the better it will perform on that band. With a
simple, inexpensive tuner you can stop worrying about SWR if you intend to
run barefoot. If you intend to use an amplifier, you will be much better off
right at the outset to go with open wire or ladder line feed and deal with
the "balanced to unbalanced" problems as best you can - using a balun,
tuner, or whatever.

Personally, I use the Johnson Matchbox with open wire line  and run the
legal limit on 80 and 40 to an inverted vee that is pretty close to half
wave on 80. I prefer an inverted delta fed in like fashion, for 20 meters. I
don't operate 10 m,  but there is no problem loading up almost anything
should the need arise.

Bob Lay in Dahlgren, VA
http://www.qsl.net/w9dmk