40m phased vertical array using a pair of verticals based on W6NBC's design from http://www.w6nbc.com/articles/2014-QST40mvertical.pdf. These are essentially a vertical dipole with loading in the bottom leg at the feed-point. In the model I created the total height is around 50ft with the bottom end 5ft above ground level. Not needing radials makes this an attractive design if the space available isn't suitable for radials.
Phasing a pair of them spaced 1/4 wavelength apart using OVF results in good performance with 3.2 dBi gain at 22.5 degrees elevation with over 20 dB F/B, and 10 dB F/B at 7.0 and 7.2 MHz. Matched SWR is 1.5:1 at the edges. I had tried closer spacings but the F/B and SWR bandwidth is significantly narrower
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 much broader F/B and SWR performance compared to coax delay lines or current forcing.
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 W6NBC_40m_Vert_2El_OVF.ez.
Plots:
What's often remarkable about OVF is how well the F/B and pattern is controlled either side of the design frequency.
In the model the first L network is the loading inductor for the "rear" element - no shunt is needed so a 1M ohm resistor represents an open circuit.
The direction of the array is switched by changing which side of the loading inductor is fed. The OVF array articles on OH1TV's site show examples. To reverse the direction in the model change V1 to V2 in the second L network. The second L network is for matching, by chance it only needs a 200 pF shunt.
Current chokes are needed where the feed-lines connect to each element in the array, and the polarity is reversed on one of the 1/2 wave lines.
Other phasing systems? Calculating coax delay lines resulted in line lengths too short to reach a common point to enable direction switching. A model using current forcing works but the F/B and pattern shape degrade quicker either side of the design frequency. An example of the difference between current forcing and OVF is shown in Phased Arrays - 40m Twin Half Square.
The Phased Arrays link at the bottom will show other examples using different systems and antenna types, and how they can compare.
Incidentally it was the idea and a QRZ post about phasing a pair of these verticals several months ago that got me started on the path to modeling and better understanding phased arrays and how the different feed systems work.
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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.