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Old 02-27-2008, 04:25 PM
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DaveInDenver DaveInDenver is offline
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Join Date: Jun 2006
Location: Larimer County
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Quote:
Originally Posted by subzali View Post
But there are some people who run 1/4λ antennas, which would be around 19" tall. But also, as on CB, a 5/8λ antenna generally works better, thus the 4' 2m antenna. Can you wrap the conducting surface as they do with CB antennas? A 5/8λ CB antenna should be around 246 inches (20.5 feet) long, yet they shrink it into a 4' whip. I suppose you give up a lot of RF transfer by doing that...
Don't forget that an antenna does not have to physically be 5/8th of a wavelength long to look like it's that long. In the case of a CB antenna that's electrically 5/8-wavelength but only say 50" tall there is a loading coil at the base (or sometimes in the middle or even near the top) that makes it seem longer than it truly is to the radio. You give up surface compared to a real antenna of the same length, but you get the benefits of the directionality, for example. The downside is efficiency, which is why it's compromise. Efficiency is the measure of how much of your signal fed to the antenna translates into RF energy sent out. An antenna loaded with a coil to be physically shorter will lose energy in the coil and has a lot less radiating length, so the quantum world is happy in that you are not violating the laws of physics. A perfect antenna is physically the right length, but like you can see with longer wavelengths you have inconveniently long antenna whips. So we compromise to make them manageable.

BTW, my shortie antenna is 1/4 on 2m and it's 19" tall. It's a NMO2/70SH, short thing with a spring and big coil in the middle. It's low profile (looks sorta like a cell antenna) and so it doesn't attract attention in the city and doesn't hang up on stuff on the trail since it's in the middle of my roof.
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