[TowerTalk] ground screens and bobtail antennas

[Craig Clark]

>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.

Jim Gray, W1XU wrote an informative article in Ham Radio Horizons on 
how to feed your antenna with coax at the top.

Here's some more information:

http://www.angelfire.com/md/k3ky/page39.html




>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).

Robert Sherwood, WB0JGP, "Ground Screen - Alternative to Radials", 
Ham Radio, May 1977, p 22




73, Craig Clark, K1QX



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