Part #6 of the "Roger Writes" series - September 2023
Background
I was looking for an easy to build, short length beam, with reasonable gain. The classic is the Yagi, but for decent gain you need many elements and the length adds up. For short beams the Cubical Quads offer more gain in a much shorter length. Upto 4 elements, the Cubical Quad offers good gain, after that adding elements doesn't increase the gain by much (around 0.2dB extra per element). For more than 4 elements, a combined quad + Yagi (aka Quagi) is worth looking at.
The Cubical quad has many options. For a given number of elements, by varying the loop circumference and the spacing, you can balance the bandwidth, length, the forward gain, and the front to back ratio.
Simulation
I've been using 4nec2 to simulate a few antennas. Until this project I had been experimenting with existing published designs, to get a feel for how they work, or to calculate dimensions for difference bands.
For the cubical quad, there are lots of parameters, and none of the existing web calculators did what I wanted. Given the simple nature of the elements, it was easy to create a model for 4nec2, with some optimising, this is the result for a 3 element beam for the 2 meter band, I'm pretty happy with 9.81dBi forward gain in 1.03meters.
The wireframe looks like this:
The gain plot looks like this (viewed from above):
With good SWR over the band (tuned for UK mid-band):
The build
It's one thing testing the model, but another to actually build it. Many of the designs I found used insulating supports with wires strung between them.
I prefer using copper pipe, as it's self supporting. Copper water pipe is quite expensive now, so I searched for something smaller diameter and cheaper.
I found car hydraulic line, it's copper/nickel plated, 3/16" steel pipe, cheap (30m/100ft for £17 delivered, from eBay), easy to bend by hand, holds its shape reasonably well, can be soldered, and doesn't rust (if the ends are covered).
These are the dimensions I used:
Element
Length in meters
Length in inches
Reflector circumference
2.14m
84.3"
Reflector to driven gap
0.44m
17.3"
Driven element circumference
2.06m
81.1"
Driven to director gap
0.59m
23.2"
Director circumference
1.98m
78.0"
Notes:
The reflector and director, are closed loops (solder the ends together)
The driven element is fed in the middle of one side (at the top/bottom for horizonal polorisation, left/right side for vertical)
The dimensions are designed so that the driven element ends are 50ohm matched, so can be directly connected to a coax
Best to use 3...6 turns of RG58 (about 6"/150mm in diameter) as an air choke
The corner bend radius isn't important, I used a radius of about 1.5"/40mm
For other beam lengths, bandwidths, or gains, load the 4nec2 software, and the model. Change the S to change the bandwidth, the D for overall length, then optimise for V & L to find a solution (I tend to use 100% SWR, 5% Gain, 2% Front/back).
I did a quick test with it connected to the VNA+PC, so just rested it on the landing (with lots of metal in the walls!)
This gave these results:
I took it outside:
It was down to 1:1.05, with a reasonably wide bandwidth.
What next
I was going to build a 4 element for 4m, but will probably get this up in the loft and pointing at a nearby repeater. I'll turn the driven element through 90 degress, so that it's vertically polorised.
The Roger Writes series
I research / dabble with lots of things, and figured that if I write my notes here, I can quickly reference them, also, sometimes, they are useful to others!
Here is what I have so far:
