[BC] lpfm question (Pattern Measurements)

[Richard Fry]

RichardBJohnson wrote:
>The antennas are usually tested at a test site with it mounted on
>a tower section of the type you plan to use.... A transmitting
>antenna connected to a signal generator at a far distance
>(1/2 mile at the Jampro site) is used to illuminate the antenna
>being tested. The antenna being tested acts as a receiving antenna.
>These two patterns, elevation and circularity, should be available
>from any antenna vendor.
_______________

OEM pattern measurements are done for FM antennas only when required to 
develop and document a directional azimuth pattern, or when a "pattern 
study/optimization" for an omni antenna is ordered by the customer.  And 
usually in those cases only several bays of the complete antenna are 
installed on a vertical tower section similar to the one on which the array 
will be installed, which is rotated on its vertical axis to measure the 
relative field azimuth pattern of the assembly in the horizontal plane.

Elevation patterns normally are not measured.  The final gain of the array 
is assumed to be the measured pattern gain in the azimuth plane plus the 
theoretical, free-space elevation gain for an array of elements of the type 
and vertical spacing used in the array.  Absolute gain of the array is not 
measured.

After the pattern measurements (or if they are not taken), the complete 
array is assembled on a fixed, horizontal tower section which may or may not 
be of the same width as the customer tower, where the antenna input VSWR is 
measured and adjusted, and it is pressure-tested.  Then it is taken apart, 
crated and shipped.

In all my years at Harris and working with ERI as Harris' "private label" 
source and with other antenna OEMs including Dielectric, Jampro and Shively, 
there was only one case where the elevation pattern of an FM antenna was 
measured.  The antenna was a Harris/ERI FMH-7BC, 7-bay, full wave spaced, 
and it was measured in full scale when mounted on 100 ft of a 10 ft face 
tower sections at the Harris antenna test range (see link).  This complete 
assembly was rotated on its horizontal axis to take azimuth patterns, and on 
its vertical centerline to take elevation patterns at various azimuths.

http://i62.photobucket.com/albums/h85/rfry-100/AntennaToweronHarrisTT.jpg

RF