Yet another dodgy coax collinear

Robert Nickels, W9RAN shows a design for an “8 element colinear vertical” for 1090MHz ADSB in QST Jan 2014 in his article Virtual Radar From a TV Dongle.

Screenshot - 28_12_2013 , 05_54_45

Looking at the middle sections first, they are 4.5″ or 114.3mm or 182.45° if made from Belden 1694A RG6 coax. Lets call them a half wave interior electrical length.

For this type of antenna to be co-phased (or in-phase), the crossover points need to be at current minima on the exterior surface and an electrical half wave apart on the interior. Obviously that is not possible unless the coax has vf=1, and the designer has chosen a compromise where the drive phasing is approximately correct and the exterior sections are 82% of a half wave. This is a common compromise, and not too severe in the resultant degradation.

Next, the tip section is also 114.3mm which is 82% of a half wave, and there is no reason for it to be less than a half wave and better approach the objective of a current minimum at each of the 8 feed points that appear in parallel due to the interior electrical length of the sections and the transpositions.

As mentioned, ignoring the effects of coax loss, we effectively have 8 current minimum or high impedance feed points in parallel, and this would be the impedance seen by the feed line entering the bottom transposition.

Or is it???

Well, no, it isn’t. The bottom feed point is not like the others, rather than feeding one side of an almost half wave radiator on each side it has an indeterminate length of radiating feed line outer connected to the lower side. This severely compromises the design.

These types of antennas commonly suffer from problems in the compatibility of the top section and the bottom section with the phasing and current distribution of the middle sections, resulting in compromised gain and poor VSWR.

The author does not give input impedance, or even VSWR measurements, nor does he qualify the design as not suited to a transmitting application. Nevertheless, online discussion reveals that some hams are scaling the design for transmitting application even though it is severely flawed.

Some would argue that the feed point impedance is not important in a receiving application, but it is as the mismatch losses caused by the receiver load might well degrade the system gain to less than that of a simple quarter wave ground plane. The author provides no measurements comparing the design with a reference antenna.

One questions the method of joining the sections electrically. This type of antenna is renowned for noise problems when soldered joints between sections fatigue with flexure of the assembly in the wind and fail, and it seems likely that the construction used in the article will suffer these problems from the outset.

I wrote about these same issues in 9/13 on VK1OD.net (now offline) with an article in Silicon Chip magazine. Both articles may have been sourced from a single flawed Internet posting.