1. This antenna is one of the all time classics.
Strictly speaking what you call the main radiator is really TWO items.
The radiation essentially gets done by the part of the piece which has
nothing parallel to it. What you call the stub, taken together with
the remainder of the long piece is a shorted open wire transmission
line.
The distance between the two has to do with the impedance of this
transmission line, which in turn allows a feedline tap position that
gives a match. Quite a range of different spacing/diameter/tap
combinations are possible. This design is chosen for the efficiency
and relative broadbandedness, and that obtainable from common plumbing
components.
A popular rollup version of this is made out of 300 ohm twin lead.
Just the thing for a traveling ham who wants to bring along an HT and
have an antenna with some efficiency as opposed to a rubber duckie. In
this case the bottom half being a shorted feedline would have been
obvious to you.
2. .33 would be a physical length of coax with .67 velocity factor
which in turn would be an electrical half wave. The other end of the
coax needs to be 180 degrees out of phase, hence the half wave which
does the inversion. If this is not done with .67 VF coax, the length
would be 0.5 x VF, whatever that turns out to be.
The isolation obtained in this balun is by not allowing the shield of
the coax to touch anything except dead center of the element and
having a coax center conductor fed tap on BOTH sides of the hairpin at
equal points. The taps on either side are 180 degrees out of phase,
hence the electrical half wave.
The taps each are at 100 ohms above dead center. Since they are out of
phase with each other, the tap to tap may be referred to as 200 ohms.
The left hand tap at 100 out of phase appears at the other end of the
coax as 100 in phase after the half wave, which also maintains the 100
ohms. The two 100's in phase, in parallel give you 50 ohms which
matches the feedline.
You COULD use a regular 4:1 balun, but why bother. This device is
inexpensive, more efficient, with superior isolation if carefully
constructed, and suffers from none of the power related malaise of the
beaded/torroid-wound versions. You also would have to find a VHF
version, since the core material used for HF is not suitable at this
frequency.
73, Guy
----- Original Message -----
From: "Dino Darling" <k6rix@arrl.net>
To: <towertalk@contesting.com>
Sent: Monday, December 23, 2002 10:56 PM
Subject: [Towertalk] WHY?!!!
> 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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