Total posts : 45366
[quote:79056903b7=”PhilB”]Which of the following two installations would perform better:
1. A part 15 antenna essentially at ground level (maybe up on a short pipe of say 2 meters max.
2. An elevated part 15 antenna on a grounded pipe 7 meters long, but immediately adjacent to a fairly conductive building with aluminum siding or stucco on wire mesh.[/quote:79056903b7]
It is a “given” that adding any conductor between the chassis of a Part 15 tx located at the base of a 3-m vertical radiator and Earth ground will increase the radiating length of the antenna, and improve its efficiency (other things equal). The longer the added conductor, the greater the effect — at least until the total length exceeds 5/8 wavelength.
[quote:79056903b7]I realize that the effect of the building siding or adjacent electric wires in the building is a total unknown for modeling. You may be able to use the worst-case situation where the building is a perfectly conductive ground surface. What I am really asking is whether the part 15 antenna elevated to 7 meters with the grounded surface adjacent to the mast will perform better or worse than a ground level antenna out in the open.[/quote:79056903b7]
I added 5 vertical wires each 6mm OD, 6m high, and spaced 3m apart in a line passing 0.2m from the 6m elevated Part 15 antenna+its ground wire, and each having a 10 ohm path to Earth ground (which resistance is probably lower than reality). The middle wire was 0.2m from the ground lead to the Part 15 tx. At 1.7 MHz this configuration of wires represents a solid rectangular, conducting surface, such as aluminum siding on a house. This surface shields the antenna from the other three sides of the house.
Adding the 5 wires reduced the gain of the open-space sited, 6m elevated antenna+ 6m ground wire by about 0.4dB, and introduced about 0.01dB of h-plane directionality to it.
It appears that the nearby conducting surface(s) are too small in terms of wavelength to couple much energy from the driven radiator system.
[quote:79056903b7]My gut feeling is the elevated antenna is still better because the antenna will radiate its signal above other adjacent building roof lines.[/quote:79056903b7]
Not really, because a 3-m vertical radiator elevated to 6m, with a 6m “ground wire” from tx chassis to Earth radiates the majority of its field from the ground wire itself. The amount of current flowing over the length of a conductor is the effective source of radiation. If you look back at my PDF paper, the current on the upper 3-m of this system tapers to zero at its top. So the average value of current flowing along that relatively short length of radiator makes the 3m section not as good a contributor to the total radiation as the “ground wire” below it.