[GVARC] Antenna Downtilt calculator

[Pat Barthelow]

Hello Folks:

For years, I have heard ham radio repeater operators speak of the value of 
adjustable downtilts of the primary lobes of repeater antennas.  A long time 
ago, I did the trig, and couldn't reconcile the numbers with the claims of 
much better close-in coverage of repeaters that had the option of downtilt 
adjustments.  I saw some commercial antennas that had real trick allen screw 
adujustments for downtilt.  Of course you could do it by tilting the antenna 
from vertical also to do the same thing...

But the trig told me that it seemed that unless you were VERY VERY close to 
a VERY VERY high repeater, that you would almost always be within the 3db 
main horizontal lobe of any practical gain repeater antenna.  And if you 
were so close, then of course the signal was so strong, that downtilt still 
wasn't an issue.  Anyone have some tales?

Then I found a great website that has many and varied interactive RF 
calculations, including Radii of coverage of the + and - 3 db down points of 
the antenna pattern, given user gain, antenna elevation, and tilt 
parameters....

Also many other calcs, Fresnel zones, etc...

Check out:  http://www.ydi.com/downtilt-coverage.php

Figure out how far from, for example Loma Preita repeaters you have to be to 
be even -3db down, due to the antenna lobes being horizontal, or even 
downtilted...It will surprise you.

73, DX, de Pat AA6EG/N6IJ n6ij@hotmail.com
Local: RF or I-link to WA6HHQ, Aptos Ca
441.925 (+) PL 100
On the web: http://www.polkinghorn.org/n6ij, or   
http://communities.msn.com/n6ij
599 DX Drive Marina CA 93933


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