Total posts : 45366
[quote:0ef891959c=”PhilB”]Rich, I want to take this opportunity to thank you for your excellent analysis of the elevated Part 15 antenna.[/quote:0ef891959c]
And my thanks to you, scwis, mlr and others who have commented favorably on my posts. One prominent manufacturer of Part 15 AM txs has told me I’m all wrong, though.
[quote:0ef891959c]There has been persistent discussions here in the past of the benefits of an elevated ground plane to enhance the performance of an elevated antenna. What do you think of that concept?[/quote:0ef891959c]
Sidestepping for the moment the legal issues about adding radiating lengths of any kind to the Part 15 AM antenna configuration defined and permitted by the FCC, I used NEC-2 to model a ground plane antenna having a 3m vertical radiator and four 2m radials, all elevated 6m above a perfectly conducting Earth. If this antenna configuration is driven by a battery-powered tx connected through a loading coil of 10 ohm resistance to the base of the vertical section (that is, there are no wires between the tx and the ground), then the peak h-plane gain of this configuration is -14.3 dBi. This value is about 10.5 dB lower than the 3m vertical at 6m elevation with a 6m “ground lead” leading to a 10 ohm ground — as shown in my earlier PDF paper at http://filebay1.home.comcast.net/Elevated_Part_15_AM_Antennas.pdf .
Of course in a real installation there will be wires carrying DC power and program audio from the ground to the tx, and they will conduct RF current which will add to the radiation of the 3m part of the antenna system. Assuming that these wires are 6m long and terminate directly under the 3m radiator, and that they provide a connection to Earth ground having an impedance of 100 +j200 ohms (a total guess), the gain of the antenna system becomes
-11.3 dBi, or about 3 dB better than without the wires.
So the pattern gain of both of these ground plane configurations is well below that of a 3m vertical+6m “ground lead” radiator connected to a 10 ohm Earth ground.
[quote:0ef891959c]I think many elevated antennas consist of a long mast pipe or ground lead right up against the side of a building with the antenna protruding above the roof line. It seems the benefit of the long ground lead radiation may be negated by the close proximity to the building. Negative factors would be conductive siding and close proximity to house wiring. Your analysis is valid for an elevated antenna well clear of obstacles, but not so much for a “typical” elevated installation.[/quote:0ef891959c]
The “ground lead” will still radiate, but the fields will be distorted and the net radiation pattern of the antenna plus house wiring etc will be somewhat directional. The nearby wires also can change the feedpoint impedance of the antenna system seen by the tx, and possibly reduce radiation efficiency.
BTW, I have no objection to direct e-mail if there are any questions: firstname.lastname@example.org