[amsat-bb] Re: Mounting antennas on a tower

[John Kopala]

Eric,

Here is another approach to the problem.

1.  You need a rotor base plate that can be mounted at any level in the 
tower.  Basically, it should rest on and be secured to the cross braces 
between the tower legs.
2.  Next use a thrust bearing at the top of the tower.
3.  Your mast needs to be heavy enough to withstand whatever winds you may 
encounter from storms when extended several feet beyond the top of the 
tower.  At least half of the mast will remain in the tower.
4.  The mast can be positioned vertically within the tower using a pulley at 
the top of the tower and a rope secured to the mast near the base.  Helpers 
on the ground use the rope to raise and lower the mast while a person on the 
tower guides and secures the mast at the desired height using the thrust 
bearing.
5.  You then attach the elevation rotor with its cross boom and antennas in 
as many steps as required to do so.
6.  Using the rope to control the mast height, loosen the thrust bearing 
screws and raise the entire structure so that the satellite antennas have 
the necessary clearance above the HF beam.
7.  Secure the thrust bearing screws and attach the HF beam.
8.  Again raise the mast so that he HF antenna is at the desired height plus 
about 1 foot and secure it in place.
9.  Attach the rotor base and rotor to the tower just below the base of the 
mast.
10.  Lower the mast to the top of the rotor and secure it in place.

That is how I set up my current HF installation.  Rather than having a 
satellite setup on top, I have an Cushcraft R-7 vertical which extends 
another 22 feet above the top of the mast and is spaced about 5 feet above 
my triband beam.  You can put anything up there you want.  I was not 
thinking satellites when I set this up many years ago, so my satellite 
antennas are on a roof mount.

I prefer center mounted antennas because I can mechanically balance the 
entire antenna system.  I have balanced it front to back, side to side, and 
up and down.  Basically I mount my antennas on a fiberglass cross boom and 
adjust the antenna positions (with coax attached) so that the whole setup is 
balanced front to back.  Then I use another pipe and clamps to determine the 
rotor position so that the setup is balanced from side to side.  Finally, I 
mount the heaviest antenna on the bottom side of the crossboom and the 
remaining antennas on the top side to further eliminate imbalance as the 
antennas rotate vertically.  The idea is to minimize the stress on the rotor 
gears and bearings.  Ideally, the rotor is not lifting or supporting any 
extra weight when it moves the antennas.  The antenna structure could be 
moved with one finger if the rotor was not holding it in place.  With end 
mounted antennas, the rotor brake and  gears must support and lift the 
entire weight of the antennas every time the antenna elevation changes.

I'll also send you a picture of my current satellite setup.  This is NOT the 
ideal setup because the rotor is not mounted in a tower section and there is 
no lateral support above the rotor for the mast.  But it does give you a 
look at a setup with center mounted antennas on a cross boom with an 
elevation rotor all mounted above a 6 meter beam.  The satellite antennas do 
rotate a full 180 degrees of elevation and clear the 6 meter beam.  The 
longest antenna is an old KLM 2 meter circular polarized antenna that is 12 
feet in length.  Note that with the cables attached, the balance points move 
somewhat toward the rear of the antennas.  Also, note the preamps just below 
the azimuth rotor.

And DON'T FORGET the preamps.

John Kopala
N7JK


----- Original Message ----- 
From: "Eric Christensen" <eric at christensenplace.us>
To: "AMSAT-BB" <amsat-bb at amsat.org>
Sent: Friday, September 21, 2007 7:03 PM
Subject: [amsat-bb] Mounting antennas on a tower


> I'm contemplating antennas and a tower at the new homestead and I have a
> question.
>
> If I can figure out how to mount an elevation rotator at the top of a
> pole and have the azimuth rotator rotating said pole from the tower
> would I have any problems with putting a small HF tri-band beam on the
> pole, below the elevation rotator, and then mount all my satellite
> antennas to the elevation rotator's pole?  Has anyone done a combination
> like this?
>
> 73s,
> Eric Christensen, W4OTN
> AMSAT Area Coordinator - Southeastern Virginia USA
> AMSAT Member 35360