> FEED SYSTEMS - THE SIMPLER, THE BETTER!
> Despite popular belief, linear radiators, normally employed in 2 and 3
> element parasitic arrays, have a driving point impedance of close to 52 ohms
> when open at the center. To feed such a radiator it is only necessary to
> connect a 52
> ohm line at this point to achieve the best possible match of line to antenna.
Nope, depending on the spacing of the parasitics, it will range from
generally 20 to 30 ohms. And optimized, the figure is about 20 ohms.
You have to have some pretty wide spacing between the Reflector and DE
to get 50 ohms. And there is certainly nothing unstable about a gamma
or hairpin match. And if you find this on a trapped antenna, you can
bet your boots the traps are adding both resistance and reactance to
the system.
Now, I have 3 long boom 5 element yagis on 20, 15, and 10 on separate
towers. They all are directly fed through a balun, and they all have
an impedance of 50 ohms, but, this was a design I developed myself (
by mistake ) and the first director is so close to the driven element
you wouldn't believe it. About 18 inches on the 15. None of these
yagis have a F/B of less than 50 db at resonance. The greatest
determining factor in the impedance, besides that 1st director, is the
spacing and length of the 3rd director.
And believe me, the spacing between elements is critical. I have a
1000 mm long rule obtained from McMaster Carr, and those placings are
set to within .5 mm.
Have purchased 1 commercial antenna in almost 50 years, a
TH11-DX...and don't ask.
That thing is an absolute killer on 17, but I have severe doubts it
gets much further than a couple hundred yards away on all the others.
73
Ed
_______________________________________________
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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