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Re: Clarc yagis



David Carr (KD5QGR) wrote:
> I just finished construction on a WA5VJB 6 element 435Mhz beam.  I have 
> a question though about where to attach the coax.  I'm using a 1" 
> fiberglass tube as the beam and the geometric midpoint of the driven 
> element is currently in the middle (inside) of the hollow fiberglass 
> tube.  Is it okay to solder the coax connections on the outside of the 
> tube (.5" from the geometric midpoint)?  The pictures on the Clarc 
> website (http://clarc.org/Articles/uhf.htm) are ambiguous.

David, the article and figures seem pretty unambiguous to me.  Remember
that the original design used a solid hunk of wood as the boom, so, of
course, there is no way that the coax could run through the center of
the boom.  The drawing that precedes any of the pictures shows quite
clearly that the coax is soldered to the U-shaped part of the driven
element, immediately alongside that side of the wooden boom.  Think of
the driven element as a J-pole on its side.  So the attachment point
that you propose is indeed similar to the original design.

Of course, the caveat is that a proven design only works to spec when
you reproduce it exactly, and once you start making changes, all bets
are off.  The properties of wood (the original boom material) are not
the same as the properties of fiberglass, so that may necessitate some
tweaking of the design.  (Fortunately, they are both insulators, which
helps, because if the boom was conductive, a larger change in element
length would be required.)  Furthermore, the original boom was 1/2 inch
wide (thick), and your boom is 1 inch thick, so the coax attachment
will indeed be in a different spot than on the original design.

The best thing to do, if you aren't into antenna modelling software
like NEC (something I cannot currently do, but am dying to get some
free time to start playing with), would be to just build it (which
you apparently *HAVE* already done!), put it on an antenna analyzer,
and see how well it behaves.  Poke it, prod it, bend it, stretch it,
whatever, and see if it gets better or worse.

Good luck!

P.S., I've looked at that article quite a few times and recommended it
to quite a few hams.  But it WOULD be really nice if they could get
clearer pictures and drawings on the web site some day.  I'm *not*
complaining, mind you, since I know what a chore it is to keep a ham
radio web site up-to-date! Just sending *constructive* criticism (which
is something I always seek for the site I help to maintain).
   http://www.nlrs.org

73 to all
W0JT

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