[BC] Base Insulator Elevation

[Mike McCarthy]

  We had a situation where the terrain delta was about 10 degrees across 
the array.  At 820, it's about 3.2ft. per deg. At the high end of the 
band, it's half that and some arrays could easily see 20 deg. separation.

The problem as others have pointed out is the vertical radiation 
component and potential to create unexpected results even at the ground 
in what should be the desired nulls. This is especially critical where 
an unpredicted lobe at an unusually high angle could create 
self-interference day or night.  Or worse, IX to someone else at night 
due to that un-calculated and uncompensated high angle lobe.

Our designer, Ron Rackley recommended tower base elevation differential 
be limited to 5 deg. maximum for the above reasons.  It goes to stand 
the Z plane or vertical plane (as looked at from a plan view) is just as 
important as the X and Y planes in the geometric spacing of the 
radiating elements. Those tolerances are obviously fairly tight and the 
Z plane is also not to be ignored for that reason.

Finally, on a project we just completed, we had terrain which would have 
resulted in in about a 7 deg. delta. We opted to raise the base pier of 
the lowest tower by 2 deg. to achieve that 5 deg. spec.  The MoM on that 
array coincided and the array was licensed using MoM.  I'm not sure the 
MoM would have favorably resolved within the FCC's specs had we not 
raised the pier to achieve that 5 deg. spec.

MM

On 11/12/2010 4:46 PM, Tom Dimeo wrote:
> What's the disadvantage of not having all the base insulators
> at the same elevation in an AM directional array?
>
> Thanks,
>
> Tom
>