# [Antennaware] downtilt changing cable lenght

Terry Conboy n6ry at arrl.net
Sun Mar 19 01:48:07 EST 2006

At 11:55 AM 2006-03-18, you wrote:
>I am trying to obtain downtilt in an antena installed
>in a 1000 meters hill in order to cover a city
>situated at 24 km distance 100 meters over sea level.
>It has 4 bays and one splitter. All the coaxial cables
>from the splitter to the bays are actually same lengh.
>But a technician has comented that changin the lengh
>of the cables it is possible to obtain downtil. The
>problem is that he do not know if the longer cable
>goes to the upper bay or t he lower one.
>I hope you can give me an answer.
>J.A.Fuentes, Radio La Musical, Managua, NIcaragua.
>
>
>Jose Agustin Fuentes Sequeira

For downtilt, the longer cable goes to the bottom bay.  The signal
from the top bay must lead the phase of the bottom bay for the
wavefront to be below the horizontal.

I modeled a 4-bay colinear array with 1/2 wavelength vertical
elements spaced 1 wavelength on centers in EZNEC.

Phase delay per bay  Downtilt
===================  ===========
10 degrees           1.5 degrees
20                   3.1
30                   4.6
40                   6.2

What this means that for a "phase delay per bay" of 10 degrees, the
cable to the #2 bay (from the top) is 10 degrees longer than the
cable to the #1 bay (top).  The cable to the #3 bay is 20 degrees
longer and the cable to the #4 bay (bottom) is 30 degrees longer.

With smaller interbay spacing, the downtilt will be proportionally
smaller, etc.

Note that 40 degrees is 40/360 = 1/9 = 0.111 wavelength.
Remember to allow for the cable velocity factor (VF).

wl = VF * 299.79 / F   (wl in meters, F in MHz)

wl = VF * 983.57 / F   (wl in feet, F in MHz)

Good luck,

Terry