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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