Message-ID: <16548447.958758107528.JavaMail.imail@bubbles.excite.com>
Date: Fri, 19 May 2000 10:41:47 -0700 (PDT)
From: "john sexton" <computernetworks@excite.com>
To: rsgb_lf_group@blacksheep.org
Subject: Re: LF: Re: Puckeridge Decca station - Big & small antennas
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Hi Dave,
You wrote:
>Basically, as the antenna is set up, it appears to be a  vertical elongated

>umbrella structure with the eight downwires or 'top loading' wires extended

>down nearly to ground level, quite close in to the mast proper. The reasons

>for this were explained on the reflector some time back.

Perhaps I missed it, but I would be very interested to know why the umbrella
ribs are extended so far. My calculations show that the optimum length of
the ribs from the top is h(sqrt(1+n.sec(alpha))-1)/n, where h is the height
of the mast and alpha is the angle of the ribs from the vertical, and n is
the number of ribs, which for 10 degrees and 10 ribs (judging from the photo
of the mast) is approx 23%. By optimum I mean the length which makes the
Radiation Resistance a maximum. Of course that may not be the objective
here.

73s John, G4CNN


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