Hi Dino: Your questions are certainly reasonable.
1. I do not believe dimension D is a critical one. The 1/4 wave open
transmission line tap point for a transmission line match might be at a
slightly different point if D is different. This design is not ideal and
should have a balun at the tap point on the coax feed line. The
alternate design is much better as the coax is inside the copper tubing
and leaves at the grounded end of the matching stub. I believe the coax
line should, ideally leave the antenna horizontally for some distance to
avoid current being induced into its shield by the vertical antenna.
Many antennas of this type have been built and work well even though
they are less than ideal theoretically.
2. The mysterious 0.33 wavelength dimension comes from the fact that the
coax loop must be an electrical 0.5 wavelangth long for proper phasing
to drive the other half of the dipole (180 deg). The velocity of
propagation for radio waves in solid polyethylene dielectric coax is
66%. 0.5 wavelength x 0.66 = 0.33 This design is a very good one
requiring no balun if the coax leaves at right angles to the dipole.
Commercial 4:1 baluns usually use ferrite core toroids and the ones
shown in Sevick's book"Transmission Line Transformers" roll off rapidly
above the HF range. At 100mhz they have around 1 1/2 db loss and it is
increasing rapidly with frequency.
GL with your project, 73, Dan, N5AR
Dino Darling wrote:
>
> 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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