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RE: Cross Boom



Roy Welch wrote:

An added benefit [of a metal cross boom] is not having those cables dragging 
around off the back of the antennas, loading up with ice, dragging the roof, 
etc.  

The weight of  the cables hanging on the back of the antenna booms
represents a constantly changing torque on the elevation motor as it goes 
through elevation rotation.

Wayne replies: 

I use an 8-foot solid 1.5 inch fiberglass crossboom.  My cables don't drag on the roof because the cables are strapped to the mast just below the azimuth rotor.  The total length of "suspended" cable to my 7x7 VHF yagi and 15x15 UHF yagi is about 15 feet.  Yes, the cable torque varies with elevation, causing the front/rear balance to vary somewhat with elevation.  But I'm certain that my antennas are close enough to being balanced that there is no undue stress on the elevation rotor.  My point is that "suspended" cables don't have to drag on the roof.  In my opinion, cables should never drag on the roof.  That damages shingles over time, and a cable could get snagged under a shingle.

I don't need a counterweight for my 2-foot G3RUH dish.  I simply mounted my yagis with more antenna/cable weight to the rear of the crossboom to compensate for the dish's weight in front of the crossboom.  Twisting of the crossboom isn't an issue with my comparatively small antennas.  I can see how it would be a concern for a 6-foot dish!  But for dishes of 3 feet or less, I suspect it's okay to balance the elevation rotor by offsetting yagis to the rear to compensate for the dish.  Just remember to include the weight of dangling cable when setting up the balance.  I recommend balancing the array at low elevations, where the dangling cables exhibit maximum torque on the elevation rotor.  The torque becomes gradually more imbalanced at high elevations, but that is offset by the fact that the total torque on the elevation rotor decreases by the cosine of the elevation angle.  At 90 degrees elevation there is NO torque on the elevation rotor, regardless of the front/rear weight!
 d!
istribution (assuming no antennas are offset from the crossboom).

I never had antennas drop onto the roof either, with a severely cracked old KLM hollow fiberglass crossboom, or with my newer Max-Gain solid fiberglass crossboom.  Fiberglass is incredibly strong stuff.  Also, a cable could get almost as much ice buildup when strapped to other tubes as it would when dangling in the air.

Wayne Estes W9AE
Mundelein, IL, USA
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