On Mon, 2 Apr 2007 17:40:13 -0400, Thomas Giella KN4LF wrote:
>The 120 radial concept came directly from the FCC
There was considerable research on the topic in the early days of
broadcasting. An engineer named George Brown (no relation) was a
leader in this work. Standards and FCC Rules for AM broadcasting
were (and still are) aimed at wringing the last fractional dB of
performance out of an antenna system AND having an system that is
stable and predictable. A massive radial system helps provide that
stability.
The primary concern with stability comes from directional arrays,
which are used on the AM broadcast band NOT primarily to produce
gain, but to produce NULLS in the pattern that protect certain
azimuths from interference. These nulls are part of the station's
license. Because nulls are the result of cancellation between
towers in the array and some nulls need to be pretty deep (and not
move around with soil conditions), rather small changes in the
ratio between the signal produced by the various towers can make a
large change in the depth of the null. THAT'S why a very serious
ground system is especially important in AM broadcast arrays.
By the way -- when you bore DEEP into the history of the number of
radials and their length, you get into some very practical
limitations like the number that could easily be plowed into a
certain field, and that worked, so they stuck with it. Remember --
in those days (the '20's), AM radio was the HOT new technology,
and nearly infinite dollars were thrown at it.
BTW -- as an undergrad EE (early '60's), I worked in the broadcast
consulting office of Pete Johnson, who, along with Carl Smith, had
written the FCC's technical Rules for AM Broadcasting right after
WWII. My comments regarding directional arrays are the result of
that background -- Pete made his living designing these arrays,
and squeezing new licenses into a broadcast band that had been
full for 25 years. I was one of several grunts who crunched
numbers to plot the equations he wrote.
73,
Jim Brown K9YC
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