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)