[BC] lpfm question

[RichardBJohnson at comcast.net]

The antennas are usually tested at a test site with it mounted on
a tower section of the type you plan to use. The antenna and tower
section ate mounted HORIZONTAL on a turntable. So-called "space cloth"
is spread on the ground under the antenna to simulate the antenna being
vertical in free-space. This was charred burlap at one time, now carbon
fiber. The idea is to absorb everything and not produce reflections.

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. It is rotated in two
planes to determine both circularity and and pattern shape. The gain
is calculated based upon the signal received in the major lobe.

These two patterns, elevation and circularity, should be available from
any antenna vendor. Over 30 years ago, Jampro provided continuous charts
produced by analog measurements so you got the entire picture, not some
piecewise approximations invented from a few discrete samples. Perhaps
an antenna vendor may comment upon how these antennas are measured now.

Cheers,
Richard B. Johnson
Website http://AbominableFirebug.com

----- Original Message -----
From: "Peter Moncure" <pmoncure at radiosoft.com>

Richard Fry wrote:
> ...test range measurements made by most FM antenna OEMs do not include 
> measuring the elevation pattern at any  (possible) azimuth, let alone 
> at enough azimuths to adequately characterize it.
How very true.  I can recall the astonishment when a ten-bay was 
measured by helicopter and the elevation pattern found at both 90° and 
270° from the boresight to be between 13° and 18° down, not good for a 
Class C FM in flat terrain!