[BC] P.O.ed by transmission line damage

[Richard Fry]

Bill Croghan loteng at lvradio.com
Mon Jan 23 16:42:40 CST 2006
etc etc (clipped)

Paste #2
MY MISSING? POST of Monday, January 23, 2006 3:16 PM CST:

Mark Humphrey wrote:
>The downward angle of that null/lobe will shift slightly with
>FM modulation (if the antenna is fed from the bottom
>with bays tapped off the hardline, as most are) which makes
>the level of signal at the studio vary in strength with modulation
>-- so there's the AM.
___________

Here are some numbers for what to expect for this from a normal 6-bay FM
antenna with 1-wave bay spacing at the carrier center frequency, and 10%
residual field at -90 degrees elevation.

At a depression angle of 60 degrees, the calculated relative field value of
the elevation pattern is

Fc - 0.1 MHz = 0.126
Fc  = 0.130
Fc + 0.1 MHz = 0.133

So as a result of that, the carrier amplitude will vary by +2.3% to -3.1%
across that bandwidth (in theory).  Of course it already has some
sync/async AM on it from the transmit r-f  system itself.

There would be more relative change in/near the nulls in the elevation
pattern, but the nulls in an installed FM antenna almost never go to zero
due to reflections from the tower and ground.  In fact I have tried to find
them with an FI meter, and it is difficult, if even possible.

Also the sidelobe peaks and minima can be quite different in amplitude and
depression angle than shown in the "classic" elevation patterns supplied by
antenna OEMs -- due to pattern distortions from the tower, mostly.  And
those parameters also vary with azimuth.

Nothing is simple.

RF