In a message dated 10/12/01 7:54:26 AM Pacific Daylight Time,
brewerj@squared.com writes:
<<
Why do certain yagis used an insulated dipole (TA33) as the driven element,
while
others feed through some sort of matching (gamma, hairpin, etc)?
What are the different mechanical issues that force the use of each?
Thanks!
John
>>
The main reason is to get R's close to 50 for a direct match with coax with
no balun. The coax shield has RF spill over and excites the tower also
vertically polarized which ups sets F/B. To get a reasonable match the
parasitic elements are detuned from max gain and even past best F/B. So you
have a very low gain beam with fair bandwidth at a low cost.
In actuality using a balun on the coax, the TA33 will do a fair job at about
40' only because the vertical pattern is very very broad and almost
completely fills the high angle reflection factors and will do a fair job
stateside even at the lower gain. Any TA33 will need a complete overhaul
before installing and that is a job. There is a way to Grid Dip the coils
but you need to know what frequency they need to be resonant and how to do
it. There are no replacement parts as far as I know.
I'd retire it and look at the M2, Raibeam and Antenna Mart Quads. You will
set your Ham Radio back if you install the TA33 again. I've checked out many
of them. Start with a 2 element at 40'. Small rotators can be used and
light towers. It's a great investment and all joints will be new and clean.
k7gco
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