I am going to test a stealth antenna hidden in the trees close to my condo building. For that, I am looking into half-square design which supposedly will be able to provide some directivity/rejection at low height I am limited to. Here in this post I will be dumping my finds and 4NEC2 models.
The basic design we'll start from:
The lenght of the longer horizontal line is L/2, where L is basically wavelength (1005/freq in MHz in feet or 306.3/freq in meters). The length of the radiating vertical elements is L/4, they don't touch the ground but could be placed fairly low so the total height of the system for 20m band will be around 5.2-6 meters (17-20ft).
The antenna can be fed with coax as the impedance at the feed point is (supposedly) close to 50 Ohm. Ok, let's start checking all of the facts above by building models in free 4Nec2 antenna modelling software. In my models, I am using 26 AWG wires and good ground, because that is what I am going to use for testing and that's the ground I have here in South Florida.
Here are the results for the 20m model (I do not have place for 40m, so I am going to start with 20m, but the model referenced below will accept any wavelength).
(1) Frequency sweep / SWR graphs (click image to zoom in):
(2) Far field data (radiation pattern, radiation efficiency and so on):
As you see, this is inherently lossy design (14% structural loss) and been so low above the ground, the antenna efficiency is even less in far field (Radiat-eff., the 4Nec2 parameter meaning the amount of energy in far field, is around 30%, with 70% lost on the way). This is the fact rarely mentioned in the publications on the subject; the most like to talk about radiation pattern, nice rejection properties and low SWR but not about inherent loss of the antenna at low height (the bigger the height, the lower the loss with maximum Radiat-eff. about 55% in far field at 60m / 200 ft., not a practical height for me).
Well, here is my take on it: since the antenna has very low take off angle (that is, low for such simple structure just 6m/20 ft high) and also nice radiation pattern (broadside to the feedline); all other things considered, it makes sense to give it a chance.
I also modelled a few variations of the design, adding resonators and feed lines, changing height, geometry and feedpoint location. I will share it later, but so far the consensus here is to start from the simplest (see the above and the 4Nec2 file reference below).
Here is the 4nec2 file for the model above; it has variables defined and can be played with using optimization engine of 4Nec2.
If you have anything to add, please do comment. Errors? Let me know.
73!
PS Few links to the "prior art"...
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