RE: [TowerTalk] Radian/Rohn availability

[Jim Lux <jimlux@earthlink.net>]

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.
>
> Anyway, I figure if the tower twists that much with just two little
> reflectors on it, we got bigger issues.
>
> http://www.hyperlinktech.com/web/hg2424g.php
>
> thanks,

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. 


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