Phasing up a pair of half square arrays on 40m. 5 dBi at 23 degrees elevation, 10 - 20 dB F/B, beam-width of 70 degrees, and electrically reversible from an antenna needing less than 40ft height, and no radials.
What is a half square? A pair of 1/4 verticals spaced 1/2 wavelength apart with a wire connecting the tops. They produce a low angle (~20 degrees) bi-directional pattern. They can be corner fed directly with coax or voltage fed at the bottom of one leg, and don't need radials. The current nodes are at the tops which makes them quieter on receive compared to conventional verticals. I built one in early 2020 - see 40m half square which also contains links to more information about them.
One downside with half squares is their narrow(er) bandwidth, and as a result when combined with a parasitic reflector or a pair phased together the F/B doesn't stretch as far compared to dipoles, loops or even verticals before it's half of the peak.
Due to the low angle pattern, and narrower bandwidth, they are perhaps better suited to the bottom half of the 40m band where the DX lurks.
Parasitic Array
While not a phased twin half square array, these have been built and used by at least one ham I know.
A cheap 40m DX machine: the twin half square array by VA7ST uses a parasitic reflector based on info by Cebik which can be found at https://www.antenna2.net/cebik/content/ao/ao11.html.
The simplicity of direct coax feeding is retained when using the second half square array as a reflector. The array is made reversible by having two equal length transmission lines meet at a common point, one is connected to the main feed-line, the other is shorted which resonates the other half square as a reflector. The lengths are critical, by using a model and the transmission lines function its easy to discover which length works best, the coax VF and loss figures in the model use the RG-8X spec as an example. In practice VF of coax being used needs to be measured with an analyzer.
Resonance and min SWR are at about 7.1 MHz with a 1.5:1 bandwidth of 70 kHz. F/B figures:
- 7.06 MHz - F/B 6.9 dB.
- 7.11 MHz - F/B 15 dB.
- 7.20 MHz - F/B 9.48 dB.
Phasing
Advantages of phased arrays include better F/B over wider bandwidths, broader SWR response, and like the parasitic array also electrically reversible.
Phasing adds complexity which will require test equipment, time and effort, possibly a helper to adjust and validate its working as expected.
With the help of tools like EZNEC and Arrayfeed1 or Feed2EL it's possible to create fairly accurate models of the complete system as it would be built. This enables the ability to see how the array performance behaves over its usable bandwidth, and know what to expect.
Coax Delay Lines
These can be calculated using a model with two sources where one has the desired phase shift, in EZNEC Source Data displays the driving impedance for each element, which can be input into Arrayfeed1 to calculate the line lengths.
In the case with this antenna, the driving impedances presented by the half squares result in "No Solution".
More about this feed system at Phased Arrays - Christman Feed System.
The other feed system type that can be calculated with Arrayfeed1 is L Network, which is known as current forcing..
Current Forcing
Current forcing uses 1/4 wave (or odd multiples of) coax lines and an L network to produce the phase shift. An additional L network can be used to match the system to 50 ohms. Since the coax lines meet the L network at a common point the array can be made electrically reversible.
A Half-Sqaure Array for 40 Meters by N2PD uses the current forcing feed-system, and includes info and diagrams for the L networks, and tuning the array.
I was curious to build a model of it and see how it might compare.
There is a process to work through in order to calculate the L network values which involves a few steps and is fairly easy to do:
- Start with two source (one source has the desired phase shift) EZNEC model to know the driving impedance at each feed-point via Source Data.
- Input Source Data into Arrayfeed1, calculates the L network values for the phase shift network.
- Model (or a copy of it) updated using transmission lines and calculated L network values connected via virtual connections.
- See if it works as expected, how the array behaves over a given bandwidth, and see the resulting current magnitudes and phase-shifts.
- Include L match matching network for how matched SWR response looks.
I created a model per N2PD's dimensions which were for the 40m CW sub-band. First thing to note is the F/B quoted in the article is optimistic :-) It may be possible to find a deep 30 dB null at a specific elevation angle but the rest of whats going on back there should be considered too, like maintaining an average across a given bandwidth below a given elevation angle.. This is where the fun starts, adjusting the values of things to find the compromise.
Model file 40m_THS_CF.ez.
Plots (7.00, 7.05, 7.12 MHz):
Current forcing offers better F/B compared to a parasitic array, and with a matching network broad SWR. 50 kHz from design frequency F/B falls to half. The phase shift L network has two values to adjust which could mean trial and error to find the best combination in practice.
Opposite Voltage Fed
Opposite Voltage Fed (OVF) arrays were developed by Pekka Ketonen OH1TV. His site contains several examples in different configurations, including details on direction switching and matching networks. OVF uses 1/2 wavelength lines, at a common point where they meet a loading inductor is put in series with one line which makes the array directional, and with a relay electrically reversible. An L match network matches to 50 ohms. The system is simple and can offer broader F/B and SWR performance.
A previous post Phased Arrays - Opposite Voltage Fed (OVF) using a pair of elevated 1/4 wave verticals attempts to explain how the transmission lines, loading and matching networks are "wired up" in the model with virtual connections.
Model file 40m_THS_OVF.ez.
Plots (7.00, 7.10, 7.20 MHz):
OVF appears to improve further, F/B peaks around 20 dB, and at 100 kHz either side of the design frequency the pattern is neat and F/B is around 13 dB at worst. This array would provide good performance between 7.0 and 7.2 MHz with low SWR when matched.
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Models are good starting point, and a way to investigate and better understand antenna systems. These tools can also help guide us to and validate the final result, if a good correlation is observed in the real world then we can have confidence the patterns and other information are accurate.
The models I have created and made available may contain errors, or overlook something someone more experienced can see. I don't claim to be an expert or authority on the subject of antenna modeling or phased arrays. I simply want to further my own knowledge and understanding of antennas which I find fascinating. Comments, suggestions, discussion are welcome - lonney@gmail.com.
This post is one of several on Phased Arrays.