Kyle Chavis wrote:
Sent: Tuesday, January 30, 2007 10:13 PM
Subject: Topband: Feeding a Cell Tower for 160?
>I have several cellular sites that I maintain and was wondering what would
> be the easiest way to shunt feed or some other means to feed a tower for
> 160. The two sites I'm thinking about are in large fields, one is a guyed
> 400' Rohn 80 with large cellular antennas on tops and several microwaves
> dishes top to bottom, the other a 300 free standing 2 sets of cellular
> arrays and 2 microwaves dishes. Climbing is an option but prefer not to.
> Each tower has extensive grounding. Any suggestions?
> Thanks, Kyle
My first comment is the 400 foot tower is over 3/4 wavelengths tall,
electrically, so any shunt feed system would send most of the RF energy
skyward, like off the end of a longwire.
For the guyed tower, if the guys are non-metalic or broken up in sections
short enough (maybe 80 feet each) or the 300 foot free standing tower,
either would be a good tower to hang a wire 4-square from... either ground
mounted 1/4 wave wires nearly vertical, (with ropes tied near the top of the
tower) or 1/2 wave verticals in either a sloper configuration or vertical
Vees where the feed point is pulled out about 1/4 wl from the tower and the
ends slope into the tower at the base and near the top per K3LR, etc.
The 300 foot tower being about 0.6 wl high could provide a large low lobe of
RF if properly fed with a shunt feed system. I wonder what the dimensions
of the free standing tower are and is it a tapered column of metal or a
structral tower like Rohn SSV? Someone would have to model it for you.
A shunt feed would require climbing to put up some stand-off insulators and
hang either a small cage of wires or sections of tubing for the shunt feed
system.
A sloper array, which could provide 4-5 db gain over a single vertical would
be my option on the 300 foot tower, but it requires climbing up about 125
feet to pull up the four feedlines for the wire antennas, and climbing to
about the 250 foot level to tie off the top of the 1/2 wl vertical dipoles.
73 & good luck
George K8GG
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