"Cecil Moore" wrote in message
t...
James barrett wrote:
Hi, I just read instructions on building a "helically wound" antenna
using a broom stick.
http://www.hard-core-dx.com/nordicdx.../bromstik.html

After reading this, I had an idea. I'm doing more google searches for
this type of configuration, but wanted to ask here as well to get some
opinions. What if I took two broom sticks, and wound each with about
68 feet of wire. Each stick making half of a dipole antenna. Then
attach 450 ohm ladder line (or 50 ohm coaxial). How would this behave
when connected to a tuner? Would it behave like a 136 foot dipole, or
would it behave like a shorter dipole?

If you use the same amount of wire for the helix as you
would for the wire dipole, the resonant frequency will not
be the same.

Feedpoint impedance wise, you can get it to respond like a
1/2WL dipole probably with a lower feedpoint resistance.

Radiation wise, it will respond like a shortened antenna.
The currents on opposite sides of the coil are close to
180 degrees out of phase and, like a transmission line,
tend to cancel the fields. In addition, a broomstick is
not the best dielectric to use for your coil form.
--
73, Cecil http://www.w5dxp.com

Helically wound antennas used to be more common some years ago. An outfit in
CT used to sell HF beams that had elements slightly more than 1/2 normal
length. More common were CB mobile antennas, which are probably still made.
I have a CB vertical bought for RC use that is about 18 inches tall.
Supposedly a shortened helically wound antenna has a wider bandwidth than a
lumped inductor antenna of the same length.

Tam/WB2TT