[TowerTalk] half square ground screens

[K4SAV]

  OOPS!

Can I retract the last message.  It is all incorrect.  Half square works 
OK fed at the top.  Brain took day off today.

Jerry, K4SAV

--------------------------------
A closed mouth gathers no feet.
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K4SAV wrote:

>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.
>>
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>> 
>>
>>    
>>
>
>_______________________________________________
>
>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.
>
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>  
>