"Richard Fry" wrote in message
...
"Bill Turner" wrote
I have the same situation here in California. I'm about 100 miles from
KFI, another 50 kW station, and during the day reception is a bit weak,
but adequate. Come sundown however, reception begins fading and is
distorted. The reason is that the groundwave portion of the signal is
still present, but is now being interfered with by the skywave portion,
which is not present during the day. If I were to move farther away,
out of groundwave reception, the signal would become much clearer,
but only at night. During daytime it would not be heard at all.
______________

Various types of "anti-fade" radiators are used by many of the 50kW
stations
to try to minimize this effect.

There is a tradeoff in the electrical height of the usual AM broadcast
transmit antenna between producing the greatest groundwave and generating
an
excessive high angle skywave that can interfere with that groundwave at
night.

A 225 degree vertical is about optimum for groundwave, but has a high
angle
lobe that can cause this type of interference problem. A 195 degree
vertical has less groundwave but little/no high angle lobe, and so is used
by many full time 50kW stations. WJR, Detroit is an example of a station
using a 195 degree vertical (700 foot guyed tower).

Other antenna types also have been used for this, such as the "Franklin,"
which is a sectionalized antenna with one vertical radiator above another,
each driven separately.

Note also that the carrier frequency of the station, the conductivity
along
the groundwave path, and skywave propagation conditions will have an
affect
on the location of this zone, and the extent of interference there.

R. Fry
(WJR staff engineer, mid 1960s)


Richard,

Thanks for the response and the "inside", technical information about
Broadcast-band-AM stations. I'm happy to say that here in eastern
Pennsylvania, WJR has a beautiful night-time signal. About the only thing
that disturbs it is "local" weather-based QRM --- i.e. Mother Nature!!!. I
even recall listening to WJR in my college days in Charleston, SC in the
early 1960's (along with WLS in Chicage and WOR in NYC).

Without knowing, I would guess that WABC (770 KHz) has a rather high angle
skywave since its ground- and skywave seem in a constant "tug-of-war" at
night --- at least ~80 miles to the northwest of the xmitter.

Thanks,

Dave Pitzer
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