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[Towertalk] WHY?!!!

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Subject: [Towertalk] WHY?!!!
From: k6rix@arrl.net (Dino Darling)
Date: Mon, 23 Dec 2002 19:56:21 -0800
I need your help!  After a short rant, you will understand my frustration 
and possibly be able to educate me...

I want to build a VHF antenna out of 1/2 copper pipe.  I have 2 designs I'm 
working with and the appropriate plans.  My problem is that I don't 
understand the "WHY" behind the design dimensions.  I have read article 
after article where the author fails to give the reasoning behind the 
calculations.

I understand "length" per frequency; diameter to frequency; how velocity 
affects "electrical" wave lengths; and (very) little about stepped 
elements.  I OWN, the ARRL Handbook, the ARRL Antenna Book (16th Ed.), 
Reflections by W2DU, the Physical Design of Yagi Antennas, and even a 
couple of books by Bill Orr and Stuart Cowan.  I read and read and have 
learned a lot!  Most is over my head as these fantastic publications fall 
short of explaining their text to the layman antenna 
builder/experimenter.  But I try!

Antenna #1.  A simple J-pole.  The design is straight forward.  A simple 
1/2 wave antenna with a 1/4 wave matching stub tapped at the 50 ohm 
location.  What I have not been able to find is information that fully 
explains the DISTANCE between the main radiator and the matching stub.  The 
following web site is one of the few that even addresses this distance with 
calculations but still doesn't say WHY (dimension D)...

http://www.packetradio.com/jpol.htm

Antenna #2.  A colinear VHF antenna.  This is a simple and effective 
antenna utilizing stacked 1/2 wave elements with a 1/4 wave matching 
stub.  The feed point is 200 ohms that requires a 4:1 balun.  The text says 
that the "balun" needs to be .33 wave length.  Why?!  Wouldn't a commercial 
built 4:1 balun work to match a balanced 200 ohm load to an unbalanced 50 
ohm feedline?  Maybe the "loop" is really a phasing line, but the text 
still lacks the supportive text to explain it.  The website is here...

http://w1.859.telia.com/~u85920178/antennas/6dbvhf0.htm

Any enlightenment would be appreciated!  Thanks and Merry Christmas!

Dino...k6rix@arrl.net 


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