Antenna Hallucinations

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 18 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 14 wave ver­ti­cal mono­pole at 1.8MHz mod­el as 0.41dBi com­pared to the ref­er­ence ver­ti­cal with 64 14 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.

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