K4JRB wrote:
There are two ways to feed the bob-tail or half square.
At the top or at the bottom. ....
K4SAV:
I know I have seen antennas built and fed like this, but now that I am
thinking about it, it doesn't seem right.
If you feed the half square at the top corner, the phase of the currents
in the different sections of the antenna are going to be different.
When fed at the bottom, the phase of the currents at the top corner
(around the corner) will be in phase. If you feed it at the top corner,
then these sections will be 180 degrees out of phase. The phase of the
current in the vertical sections will change to being roughly 130 out of
phase, instead being in phase. This changes the radiation pattern to
omidirectional and lowers the gain. The antenna will now function
roughly like a single vertical.
Fed at the bottom, the antenna is bidirectional along the line of the
horizontal wire. It looks like two verticals fed in phase and spaced 1/2
wave apart.
Jerry, K4SAV
David Thompson wrote:
>A half square is the Bob-Tail as W6BCX envisioned it. The common bob-tail
>with three vertical wires is
>a double bob-tail or three element bob-tail. Somehow this gets lost in both
>the W6SAI and ARRL Handbooks.
>
>There are two ways to feed the bob-tail or half square.
>At the top or at the bottom. Feeding it at the bottom
>requires a matching network and radials. You can elevate it and still feed
>it at the bottom with the matching network. You still need radials and
>either a ground screen or perhaps using Christman's gull wing radials might
>work.
>
>The normal half square that is written about is fed at the top of either
>vertical leg making sure the coax comes away at at right angles to the leg.
>Get the vertical legs up at least 1/8 wave to minimize ground losses. A
>ground screen or radial systems probably won't help (see W4RNL's notes) but
>may make you feel better.
>
>Another version of the bob-tail is the robert-tail which is an upside down
>bob-tail with the horizontal either lying on the ground or elevated
>slightly. A SK W8 used this effectively but never saw any test data or even
>empirical results.
>
>Ground screens do work. I have seen rolls of chicken coop wire rolled out
>in a crossing pattern. There was an article in a late 1970's Ham Radio
>Magazine by a WB0 which is quoted most often. (see ARRL Antenna Book or
>ON4UN's notes).
>
>Sorry I cannot be more specific but an away from my source materials.
>
>Dave K4JRB
>
>
>_______________________________________________
>
>See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless
>Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any
>questions and ask for Sherman, W2FLA.
>
>_______________________________________________
>TowerTalk mailing list
>TowerTalk@contesting.com
>http://lists.contesting.com/mailman/listinfo/towertalk
>
>
>
_______________________________________________
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather
Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions
and ask for Sherman, W2FLA.
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|