Phasing up a pair of inverted delta loops for 40m, coax delay lines, current forcing, and OVF.
Inverting the loops allows them to hang between trees around 60ft apart and be fed at the bottom. This is convenient since there are no heavy baluns or coax hanging up high as there would be with end supported wire dipoles such as those I explored in Phased Arrays - 40m 2 Element Horizontal.
Modeling phased inverted delta loops turned out to be a little more challenging compared to verticals and dipoles..
In the models I created the inverted delta loops are closer to right angle vs equilateral. This makes the top wire longer which results in slightly more gain, broader F/B and SWR performance when used in a phased array. So far I got a model using OVF working.
Coax Delay Lines
Unable to find a solution that worked.
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.
Key details:
- Top wire height of 45ft.
- 8.9 dBi gain at 40 degrees elevation.
- F/B 15 dB or better below 45 degrees between 7.05 MHz and 7.25 MHz.
- Matched SWR 1.5:1 or better.
A bit more fiddling may improve it further..
Model file 40m_Delta_2El_OVF.ez.
Notes about the OVF model:
Each element has an electrical 1/2 wave coax line meeting at a common point, one line’s polarity must be reversed. VF/loss figures typical for LMR-400 as an exmaple. Current chokes are required at element feed-points if building this.
First L network is a series loading inductor, the shunt is not needed and is open circuit represented by a 1M ohm resistor in the model.
Second L network matches to 50 ohms, its output can be set to V1 or V2 which reverses the direction of the array as it simply switches which half wave line the loading inductor is inserted into.
With OVF arrays either a single or a pair of 1/2 wavelength lines can be used depending on what is more practical, what I have noticed in the models where one line is used F/B is better maintained, and the SWR is bandwidth is broader.
With inverted delta loops it could be done either way as one line will reach the other element, thou the loading/switching/matching network will need to be located at the feed-point of one of the loops. In the model I created the feed-points are about 17ft above ground, the loops can be reshaped to bring the feed-points closer to ground level by narrowing the top wire, when I tried this it appeared to trade away the improvement seen with the original model. Not to say it can't be made satisfactory with experimentation.
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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.