Thoughts on Vertical Coaxial Sleeve Dipole-Driven Slim-Jim Collinear Antenna

I think I may have found a good source of data to pull from in order to optimize the non-folded top-section collinear. I came across Cebik's article, "The Case of the Curly Collinear". He talks a great deal about the 1/2 wave over 1/4 wave collinear design and while he models a 1/4 wave ground plane as the base antenna element, he makes it clear that a dipole could be used in its place. He uses the same type of hairpin phasing stub in his model, but his is fairly tall in order to increase the vertical spacing between the 2 phased elements. Because of this, the tall vertical part of the hairpin radiates and distorts the pattern.

I'm going to elect to keep the hairpin vertically short to prevent the pattern distortion at the expense of some gain. I'm going to use a piece of 450 ohm window line shorted at the end for the hairpin and have it wrap around the antenna concentrically for stability. He lengthened the top element just like you did. His gain figure and radiation pattern is fairly consistent with yours, only his pattern is more distorted due to the longer length of V2. The feed impedance is close to 100 Ohms, so I'm thinking a 50 Ohm 1/4 wave transmission line transformer can help bring that down to better match the 75 Ohm RG6 coax I'm using to feed the antenna.

It is odd though that in the super j-pole designs, the top element is shortened for end effect, yet for these 1/2 over 1/4 designs, the top element is lengthened to help it match. I have one of those nifty Chinese micro VNA's to help me tune the final design in the field. I'm eager to see how the theory matches up to the real world.

As for the original design in that patent with the folded top element, it appears to be defective. Perhaps that's why the patent expired early. If a single dipole can match it in performance, it's clearly not worth the effort. I can see how they arrived at that design. I kept thinking about a way to create a slim jim style collinear and clearly they were as well. For whatever reason, it doesn't seem to provide a gain advantage like one would think. A valiant effort nonetheless.

I will take pictures of the final project and the VNA screenshots once I get the design built and optimized.

Standard 1/2-Over-1/4 Collinear Array

V3 length 13.0" AGT: 0.951 = -0.22 dB
V2 length 4.0"
V1 length 6.0"
Horizontal length 6.2"
Radial length 6.35"

Max. Gain: 7.33 dBi Gain Differential: 2.17 dB Mean Gain: 6.25 dBi
TO angle: 4.8 deg. Feed Z: 96.8 + j 0.4 Ohms

 

On a side-note, based on Cebik's article, I was tempted to lengthen the top element to 5/8 wavelength, but then I would have to create a phasing coil that matched it and Cebik shows really no gain advantage from doing so. I'm skeptical of 5/8 designs that lack a ground plane, unless it is a double 5/8 design like the isopole.

 

If the collinear 5/8th elements plus phasing stub total to 1.5 wavelengths it should be okay.