|To:||"Daron J. Wilson" <email@example.com>|
|Subject:||RE: [TowerTalk] Radian/Rohn availability|
|From:||Jim Lux <firstname.lastname@example.org>|
|Date:||Thu, 27 Jan 2005 10:15:55 -0800|
At 08:30 AM 1/27/2005, Daron J. Wilson wrote:
My bad, they are 8 degree bandwidth assuming I'm reading the page right, 24dbi gain reflectors with 2 SF of wind load. Thanks for all the calcs, wow I'm impressed. I figured someone has just convinced them that they need a Rohn tower. Personally, we try to shy away from them and prefer the old microflect four legged jobs for stability.
There's a fairly simple way to do these size/beamwidth kinds of calculations, as long as you're dealing with reasonably directive antennas (no fair trying this with a dipole..) and you're looking for ballpark numbers (as to see if the advertising literature is claiming to violate the laws of physics)
The beamwidth = 70/(diameter in wavelengths) is a really handy one.
20 dBi = 20 degrees beamwidth (approximately.. it's more like 18-19 degrees)
halving the beamwidth gives you 6 dB more gain (assuming that the beamwidth is same in both planes)
a tenth of the beamwidth (2 degrees) is 20dB more gain (i.e. 40dBi)
8 degree beamwidth would, by my rough calcs, be about 28dBi, but maybe the antenna pattern is fan shaped.
8 degree beamwidth would also be about 8-9 wavelengths (about a meter) across... Somewhat more than 2 square feet.
I see by looking at the data sheet for that antenna that it does have a different beamwidth in vertical and horizontal axes... about 16 degrees in vertical direction and 8 in the horizontal, so the 24dBi spec is moderately believable. (and, my SWAG of a meter across matches pretty closely to the datasheet.. the aperture is 100x60 cm)
20dB sidelobes tells you that it's pretty uniformly illuminated, but I'd be interested to know if those gain numbers are measured, modeled, or WAGs. It's pretty close to what you'd get with NO allowance for loss or illumination spillover.
See: http://www.mscomputer.com for "Self Supporting Towers", "Wireless Weather Stations", and lot's more. Call Toll Free, 1-800-333-9041 with any questions and ask for Sherman, W2FLA.
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