dgaldrich
Joined: 24 Apr 2009
Posts: 267
Location: Naples, Fl and Belfast, ME
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 Posted: Mon Dec 31, 2012 10:45 am Post subject: Antenna Question (Yet Another) |
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I am building a Pietenpol (wood fuselage) and will be installing 3 antennae. 1 VHF com, 1 ELT (121.5/406), and one transponder. The turtle deck is current uncovered so I can install them anywhere I want. The issue is ground planes and the question is can I use the same ground plane for the ELT and the comm with one facing up and the other facing down with the feeds less than say 6 inches apart. at the point where I'd like to put the two vhf antennae, the fuselage is about 12 inches wide but the ground plane could be maybe 3 feet long and I could get a wider ground plane by turning it 90 degrees and extending it down the side of the fuselage.
So here are the questions:
1. Can I do the back-to-back almost collocated trick with the two VHF antennae. If so, should they be mirror image (both angled back) or "concentric" (one angled back and one angled front). Would using both a ferrous ( window screen?) and non-ferrous (aluminum or copper) ground plane (sort of like a Faraday cage) provide enough isolation bearing in mind that the ELT would be used as transmit only when disaster strikes.
1a. Do I really care about the ELT antenna's capability on 121.5? If I go down, it's the 406 frequency that will get SAR to me. My thinking is that the ELT antenna's proximity to the VHF comm antenna shouldn't compromise the 406 signal.
1b. If 1a is true, then the only issue is the comm antenna radiation pattern. Locating the antennae 1/4 wavelength apart might even help, sort of like reflectors and directors in yagis.
2. How sensitive is the ground plane to width? Can I get away with 12 inches by 36? Is there a benefit in extending the ground plane down the side say 6 inches to get a 24 inch wide ground plane?
Lots of questions but it's not like the supposed question on an MIT physics exam. "Given Maxwell's equations, derive the universe. Give two examples."
Thanks in advance
Dave
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