At 12:19 PM +0000 3/14/03, Robert Thain wrote:
>I would like to build an end fed 1/2 for 20M....But what I dont get
>is how to connect the BALUN to the vertical element. There is only 1
>element and you have 2 wires coming from the BALUN. 1 of the
>goes to the vertical element, where does the other one go ?
The best way, which has worked extremely well for me with a 40-m
half-wave antenna, is as follows:
As a combination balun and impedance transformer, use one-quarter
wavelength of roughly 600-ohm open-wire transmission-line; i.e., two
parallel wires, 14 AWG or 16 AWG, spaced about 5 inches apart. I
used Plexiglass rods as spacers.
Looking into the transmitter end of this line, you'll see about 50
ohms resistive. You can connect a 50-ohm coaxial cable directly to
this end without a discrete balun transformer. However, a coaxial
common-mode choke, or "1:1 current balun" here would do no harm.
At the antenna end of the quarter-wave, 600-ohm, open-wire line, one
of the two wires is connected to nothing at all. Its end should be
cut a few inches short and suspended in mid-air about five inches
from the antenna conductor (and five inches from the second wire of
the transmission-line). I used a Plexiglass rod here, too.
The second wire of the transmission-line should be connected to the
The point on the antenna conductor at which this one
transmission-line wire is connected should not be the end; it should
be about one-third of the way up from the bottom, in other words
two-thirds of the way down from the top.
Changing the position of this connection point will change the
impedance seen at the transmitter end of the quarter-wave, 600-ohm,
open-wire line; so you can adjust it if necessary to get exactly 50
ohms at the transmitter end, for a perfect match to 50-ohm coax.
Old-timers (or those who read old antenna books) will recognize this
off-center, single-wire-fed antenna as a "Windom" (or "Wyndom" in one
old book I've read); and they will recognize the quarter-wave,
parallel-wire, feedline with one wire end floating at the antenna as
a "Zepp" feed. The name comes from "Zeppelin," because this
arrangement was used to feed the end of a half-wave wire trailing
behind, or hanging below, the Zeppelins. All that I've done is to
move the antenna-connection point away from the end of the antenna
in order to change the impedance at the transmitter end of the
feedline to 50 ohms.
I modeled this antenna with NEC-4 before actually building it,
measuring its input impedance with a noise bridge, and then using it
on the air. The NEC-4 simulation was so accurate that, after cutting
all lengths in accordance with the simulation, I did not have to trim
The NEC-4 simulation showed that radiation from the feedline (due to
unbalance, in other words due to common-mode current on the feedline)
was very small, more than 20 dB down IIRC.
If anyone wants them, I have and will happily email my NEC-4 input
and output files.
73 de Chuck, W1HIS