While this may be TMI, while laying there on the couch during Christmas/New Years recovering from umbilical hernia repair, the Oxycontin-fueled fugue state propelled copious notes on the subject of antennas. This would be my Kubla Khan, dare I be so profane as to even utter this association with the great Coleridge. Besides, my prescription to Percoset ran out in two weeks while the great gothic poet’s never did and it was likely the death of him.
So, like I was sayin’ I had all these great ideas for antennas. Not huge arrays and multiple acre tracts. I have a normal backyard in an old suburb just several miles from downtown Nashville (right by the stadium, in fact). It has a garage and a few not-quite-tall enough trees and a power line that runs through it. You know, a normal 50 foot by 100 foot backyard. So, how much signal could I radiate from this normal backyard with normal wires? This would be my project.
My first experiment was to play with tapered segment modeling of radial ground systems for vertical monopoles. One of my drug-induced ideas was to explore if moving the vertical monopole from the center of the radial pattern would introduce any pattern anomolies. Of the three scenarios I modelled, I was surprised to learn the pattern did not significantly change – it was still a toriod with less than 3dBi change in symetry. I was so impressed by this I almost missed how the gain seemed to very quickly diminish as you moved the monopole towards the edge of the radial boundary. Reference model was 0.55dBi (quarter-wave monopole with 64 quarter-wave radials). As I modelled the monopole at 1⁄8 wave from center of radials, the gain dropped to ‑3.3dBi, then to ‑14dBi at the edge.
You may ask yourself, “Self, any dummy knows you stick the vertical in the center of the radials, so why would WF7T bother with this model?” Several reasons:
First…you just never know the constraints until you test where they are. I have some models that quantify now what happens when you move a vertical monopole off of center. Pattern is still good except gain drops significantly.
Second…Real world imposes itself. I already know I will be putting in a bunch of copper in the backyard, but I only want to do it once. What will be my best course of action? More to be learned here, but placing the feedpoint of the vertical monopole a the center of the radial field is clearly desirable, with steep penalties if you don’t. I will be spending time/effort/cash to make a radial field…I want it to work great.
Third…I am working towards understanding how the radial field MUST look for vertical parasitic elements/arrays (VPA). 40M 3el VPA is very doable, and perhaps even 80M assuming a workable radial pattern.
My notes and experiments will be much more organized, but wanted to dump my brain on these topics a bit.
Another thing: 64 Radials arranged in a square 128-foot on the side (varying lengths of 90′ at diagonal to 63-foot perpendicular to side) with a 1⁄4 wave vertical monopole at 1.8MHz model as 0.41dBi compared to the reference vertical with 64 1⁄4 wave radials at 0.55 dBi. How would the same radial field look if reduced to a rectangle of, say, 100’x50’? Stay tuned! I intend to blog out my various exploits of using EZNEC + along with my experiments.