At the July meeting (Wednesday 20 July 2016) members Frank ZL2TTS and Doug ZL2AOV gave brief 15 minute talks about their favourite wire antennas. Frank told us about his 40m and up dipole (20m long) fed by open wire feeders. He showed how the feeders were routed between the antenna and the shack, and material used for separating the wires and anchor points. Doug talked about his all band (80m and above) end-fed antenna (40m in length) fed by a balun and coax (with a good earth connection at the feed point).
The talks provided for some useful thinking about alternative antennas at city sites where installation of a collection of antennas – one for each band is simply not feasible.
These ideas (open wire feeders and end-fed antennas) provide a means of avoiding the problems of lossy small diameter traps or large diameter traps that tend to come to grief in Wellington winds. We can all bemoan the lack of sunspots … but nothing beats a better antenna in getting a contact!
A four square antenna has now been built and assembled, assisted by a number of local amateurs (Doug ZL2AOV is photographer, Bernard ZL2BD, James ZL2ET, flatmate Josh, and Malcolm ZL2UDF and myself are in assembly mode).
Full testing wasn’t possible because 20m at the test location in Grenada Village was full of local QRN at S6+ and peaking over S9. It did, however, pull in stations on 15m, even if it was not designed for that band!
The array takes about an hour to assemble: four antennas, four feedlines, a control box, coax and an ethernet line back to the rig and remote control, and 32 radials!
I started this project with the intention of engendering interest in phased vertical arrays. I imagine that the club will get to play with this antenna (and hopefully others like it) in our annual efforts in Oceania DX Contest and CQ WW contests.
20m was picked as a starting band because element lengths are manageable (approx. 5m). If there is a band where your signal needs to stand out in a contest then it would be 20m! A four square has around 5.5dB of gain over a monopole, similar to a horizontal beam antenna. However, all assembly is done on the ground, there is no rotator and it is very quick to switch directions.
You can spot the 1/4 wave current forcing 75 ohm feed-lines from the controller box to each monopole.
The controller is not frequency dependent – containing only switching equipment (3 DPDT relays) and a broadband transformer.
To get the 90 degree offset for the centre radiators you use two pieces of coax, in a Christman matching arrangement. The lengths are calculated using a computer program downloadable from W7EL’s website at EZNEC.com. You simply input the impedances of the component monopole antennas measured using an antenna analyser, the configuration of the array, frequency and velocity factor of the coax. Out pops the lengths of coax to be cut in metres. The program solves two complex number calculations behind the scenes. Simple!
The two pieces of 75 ohm coax (one is 2.395m long, the other is 7.799m long) are joined together with a Tee connector creating a common voltage feedpoint where they are fed with 50 ohm coax (grey RG8U) from the rig. These feeders plug into the controller, giving 0 (rear element) and 90 degrees (centre elements) of phase difference. The 180 degree shift (front element) is achieved using a broadband transformer constructed on a FET240 type 43 ferrite toroid sourced from SICOM Ltd.
I am working on constructing a hybrid coupler as well, in order to compare the different feed arrangements. Hybrid couplers are frequency specific as they contain both inductance and capacitance. They are popular in commercial units as they are not as sensitive to the antenna feedpoint impedance. Some say they are more lossy than Christman feed. We shall see!
A special thanks to Malcolm ZL2UDF who provided the aluminium tubing for the four antennas (ex Quartz Hill 20m beam), Bernard for assistance in calculating the 1/4 wavelength feeder lengths and velocity factor of the 75 ohm coax and supplying the antenna analyser, and Doug ZL2AOV who reworked the “feet” and connection mechanisms for the antennas, and resolved the resonance issue so that they tuned up properly.
At the next club meeting on Wednesday 19 November at 7.30pm, you will hear me talk a bit more about phased vertical arrays in general. I will discuss why these arrays make sense for contesting, basic configurations (2, 3, 4 or more elements), switching and feed arrangements. I will also cover off practical design ideas for both this portable four square and my as yet unfinished two element multi-band array under construction.
As I mentioned at our last club meeting, I have been working on a paper that argues for a reconsideration of DX and contesting antennas for the low bands. This means that a smaller site would be required for any replacement station for Quartz Hill. I argue in the paper that phased vertical arrays make the most sense for 160m through 30m for a NZ DX location. Any site larger than about 1Ha would be adequate to build a collection of such arrays.