"Richard Fry" wrote in message
...
Below is a recent post to another server, which also should
be of
interest here.
The "SOMEBODY" identified below is well known in the amateur
radio
community.
RF
++++
“SOMEBODY” wrote: \\ As long as we are screwing up a
perfectly good
thread about VHF antennas with nonsense about BC antennas,
let's try
to understand why a 5/8th wave has fading on the fringe.
The reason is the ground losses are so high the skywave lobe
is
dominant. The skywave is dominant even though the skywave
path is
significantly longer and the ionosphere has a great deal of
attenuation. //
____
Not wanting to hijack this thread, but also not wanting to
let stand
the incorrect information posted here by “SOMEBODY”...
Probably most people will acknowledge that the _received_
daytime
skywave of a MW AM broadcast station is insignificant in the
coverage
area of the groundwave of that station, regardless of the
frequency
and propagation path losses for that groundwave.
The distance to a given AM BCB groundwave field intensity
over a given
terrestrial path is very nearly the same, day and night. In
the
daytime that groundwave signal can be received usefully for
more than
100 miles from the tx site, for some stations.
At night, the outermost part of the groundwave coverage area
also
receives a skywave signal from that radiator, which can
cause
interference to the groundwave (fading) if the two signals
have
comparable magnitudes, and are not co-phased.
The figure at the URL below was taken from Terman's Radio
Engineers
Handbook, 1st edition.
http://i62.photobucket.com/albums/h8...ermanFig55.jpg
Note that the single-hop radiation from a monopole that
serves
distances beyond about 500 miles leaves the monopole at
elevation
angles of less than about 12 degrees.
Yet a NEC analysis of all monopoles of 5/8-wave AND LESS at
an
infinite distance over real ground shows very little
radiation in this
elevation sector. This causes a lot of misunderstanding to
those who
believe that this NEC pattern is the pattern actually
launched by that
monopole over real ground.
But it _definitely_ is not. If such monopoles actually
launched
radiation patterns such as shown by NEC for an infinite
distance over
real earth, then daytime AM BCB service would be impossible.
There is a nighttime zone where the skywave and groundwave
from a
given radiator have nearly equal values. The distance to,
and the
width of that zone are dependent on:
1) Frequency
2) Earth conductivity
3) Radiator height in electrical wavelengths
4) Applied r-f power
5) Ionospheric conditions, and possibly,
6) Earth curvature
(“SOMEBODY” wrote) \\ This is because, even at a distance
of a
kilometer or less, even the best soil and at a frequency in
the
broadcast band where the soil is less lossy the earth still
has
significant attenuation to ground wave signals. //
Here are the field intensity values using the FCC's MW
propagation
curves for a 1 km groundwave path over a real earth of 8
mS/m, for 1
kW of power on 1,000 kHz, radiated by the stated monopoles:
1/4-wave = 295 mV/m
5/8-wave = 415 mV/m
The groundwave field from the 5/8-wave is about 40% greater
than from
the 1/4-wave. The field of the 5/8-wave is NOT redirected
from in and
near the horizontal plane to some high elevation angle as
stated by
“SOMEBODY”.
Note that both of these values are less than 6% below the
inverse
distance field for these conditions, over a perfect ground
plane.
Probably not enough of a loss (0.54 dB) to be called very
"significant. "
Also note that my example is for rather average conditions,
and not
for the "best soil and at a frequency in the broadcast band
where the
soil is less lossy..." as in “SOMEBODY” ’s description
above.
RF (ex-WJR, Detroit)
Hello ex-WJR, I am old enough to remember station breaks
running This is the Good Will Station WJR, Fisher Building,
Detroit.
The idea that coverage is maximum for the 5/8th wave
radiator is common but in practice, (maybe we are saying the
same thing) a straight 1/2 wave may have a smaller fading
ring because it does not have the high-angle lobe wich
appears on the 5/8th wave pattern. I certainly agree that
patterns calculated for "ideal" ground are not matched by
practical ground systems except, perhaps, sea-water grounds.
The fading ring is one reason very high power BC stations
don't work well at night. I don't suppose many AM
broadcasters think nighttime coverage is important these
days but in antidiluvian days it was considered very
important. Edmund A. La Port give all this good coverage in
his old book which is available on line from Pete Millett's
site at:
http://www.pmillett.com/
--
--
Richard Knoppow
Los Angeles
WB6KBL