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Shortening Db-212 style antennas..

[email protected]
 

Hi Tedd,
I read your message with interest. I have a similar situation. I was given a
beautiful shiny UHF exposed dipole array with 8 dipoles... four on one side
of the mast and four more 180 degrees out... The problem is that the system
is cut for the low end of the UHF spectrum, in the 420 mhz range. It does
work as is with an acceptable reflected power, however the efficiency is real
poor. I am wondering if it is possible to shorten things somehow in order to
push the center freq into the FM portion of the UHF ham band? I suspect that
it is possible to do so, but would proceed with caution, as we may indeed get
the resonant freq moved up into the ham band by tweaking the physical length
of each element, but do not forget that the phasing harness is not going to
easily be adjusted. That would probably drag the efficiency of the re-tuned
dipoles down some, as well as produce uncontrollable effects on the radiation
pattern, gain, etc... I would be very pleased to hear what others think. Good
luck in your quest....

73, de Gary, W7NTF
 

Hi Guys:

I may have lucked out yesterday......I got an e-mail from
an ex-employee of a local 2-way shop. He heard that I have
a 6 meter repeater on the air (he's also a ham), and offered
the Sinclair version of a Db212-2 for my use!

Problem is, it's 400 feet higher than my present location.
Taking it down is not feasible, but getting a rigger to it
is.

Is it possible to put a "shorting bar" on each dipole
to bring it up to 53 Mhz, as it's presently on 43 Mhz.

Does it matter on which end the shorting bar is placed,
and about how much would have to be shorted?

Thanks for any and all help....


Tedd Doda CET CEO

Lazer Audio and Electronics

packet ve3tjd@va3sed
e-mail lazer@...
David Peterson
 

Gary,
check the spacing between the dipoles. Measure the spacing, and then
calc. the spacing for the frequency that you are using.
Dave

-----Original Message-----
From: w7ntf@... [mailto:w7ntf@...]
Sent: Thursday, April 15, 1999 6:18 PM
To: Repeater-builder@...
Subject: [Repeater-builder] Re: Shortening Db-212 style antennas..


From: w7ntf@...

Hi Tedd,
I read your message with interest. I have a similar situation. I was given a

beautiful shiny UHF exposed dipole array with 8 dipoles... four on one side
of the mast and four more 180 degrees out... The problem is that the system
is cut for the low end of the UHF spectrum, in the 420 mhz range. It does
work as is with an acceptable reflected power, however the efficiency is
real
poor. I am wondering if it is possible to shorten things somehow in order to

push the center freq into the FM portion of the UHF ham band? I suspect
that
it is possible to do so, but would proceed with caution, as we may indeed
get
the resonant freq moved up into the ham band by tweaking the physical length

of each element, but do not forget that the phasing harness is not going to
easily be adjusted. That would probably drag the efficiency of the re-tuned
dipoles down some, as well as produce uncontrollable effects on the
radiation
pattern, gain, etc... I would be very pleased to hear what others think.
Good
luck in your quest....

73, de Gary, W7NTF


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