Mark Keith, NM5K wrote:
"I`ve had horizontal antennas that picked up horrible amounts of noise."

Yes, the protectection comes from noise beyond the line of sight range
but not so far away as to require aky wave propagation.

Propagation is a function of frequency. Below 100 KHz, gtound waves are
little affected by the earth`s attenuation and the sky wave is reflected
with little loss by the ionosphere. Waves travel up to 600 miles with
little perturbation from the time of day, season, or year, but at
greater distances, low frequency reception is better at night and in the
winter due to ionospheric changes affecting the reflected signal.. On a
yearly basis, signal strength over long distances correspond with the
11-year sunspot cycle. Low frequency signal strength changes only slowly
without rapid fades which characterize high frequency operation.

At frequencies above 100 KHz but below 535 KHz, ground wave attenuation
is greater than at frequencies below 100 KHz. Daytime ionospheric losses
are very high. Daytime ground wave propagation is better at the lower
end of this frequency range and over soil of higher conductivity.
Signals may extend to several hundred miles, where noise levels in the
receiving location are low. Nighttime transmission to distant points is
possible due to ionospheric reflection. Dependable daytime reception in
the 100 to 535 KHz range is bad due to lack of ionospheric propagation
and high attenuation of the ground wave especially at the higher
frequency end of this band over poorly conductive earth and during the
summer months when there may be thunder storms producing static eithin
ground wave range..

At frequencies between 535 KHz and 1600 KHz, only the ground wave is
useful in the daytime beyond the line of sight, as the sky wave is
completely absorbed. The higher the frequency in this range, and the
poorer the earrth`s conductivity,, the greater the attenuation of the
ground wave. High powered transmitters at the lower frequencies in this
range reach 50 to 100 miles over high conductivity soil. This may be
pessimistic. I listen 24 hours to 50 KW KKYX in San Antonio which is 200
miles to my west satisfactorily. It broadcasts on 680 KHz. My receivers
are quite ordinary and use internal loop antennas. The earth is highly
conductive but there is no sea water in the path. At night, other
stations
produce low frequency carrier beats with KKYX causing undesirable
automatic volume control action. but KKYX`s sky wave is stronger than
its groundwave and its reception is still acceptable.. Radio Havana is
one of its competitors. I hear all about "El Comandante" at times.

Sky wave goes far in the 535 to 1600 KHz band. During Hurricane Carla in
the 1960`s I listened to Dan Rather describe the storm blow by blow on
KTRH, Houston`s 50 KW outlet, from Tierra del Fuego where I was working,
and listening on a Hitachi pocket transistor portable radio with its
built in loop antenna. The path is about 6000 miles long but mostly over
the ocean. KTRH transmits on 740 KHz from the banks of Cedar Bayou. They
have a 4-tower directionnal array with a North-South bias. Reception was
good in Tierra fel Fuego as it is nearly at the Antarctic Circle and
there are no thunder storms there. It is too cold. Groundwave extends
hundreds of miles from KTRH, but not 6000 miles. My reception was shy
wave using several hops.. Broadcast transmitters concentrate energy
along the horizon so low elevation angles are favored.. This works well
for sky wave DX, especially over the ocean.

Sky wave attenuation in the 535 to 1600 KHz band is about the same
throughout the band, so nighttime coverage of broadcast stations in this
range is almost independent of frequency, while daytime ground waves
favor the lower frequencies. When I was a kid, I had a crystal set fixed
tuned to KTRH which directly drove a loudspeaker, if I could find a
sensitive spot on the galena. I lived almost in sight of the station.

At frequencies between 1600 KHz and 30 MHz, the ground wave attenuates
so rapidly as to be usseless except over very short distances.
Propagation is either line of sight or via ionospheric reflection or via
tropospheric scattering. Frequencies above 30 MHz are often used for
scattering ao that extremely high gain antennas are practical.
Scatterihg often uses brute force to extend the range of signals beyond
the line of sight.

Most long-distance short-wave communications result from ionnospheric
reflection. In the frequency range of 1600 KHz to 30 MH, a band of
frequencies can almost always be found that provides communications by
sky wave over a path between two points on earth.

The maximum usable frequency depends on the distance between the points
and ionospheric conditions. The minimum usable frequency depends on
ionospheric conditions, effective radiated power, and the noise level at
the receiver. Losses in the ionosphere increase with wavelength, so the
frequency which gives the best signal is usually the maximum usable
frequency. For communocations reliability, the maximum usable frequency
is often discounted by 15% to provide an "Optimum Working Frequency".

Daytime DX requires a high frequency. Shorter paths require lower
frequencies.

Typically 10 to 29 MHz during the day and 5 to 10 MHz, at night, are
best for transmission over transoceanic distances (thousands of miles).
Rember the Zenith portable? The best frequencies are usually higher
during the day for long paths than they are at night

Optimum frequency increases with the length of the path up to the
maximum distance for one-hop transmission, about 1200 to 2400 miles. Low
elevation-angle radiation such as 5 to 15 degrees is usually most
desirable. Radiation below an angle of about 3.5 degrees may be
absorbed by the earth near the transmitting antenna and wasted.

Frequencies above 30 MHz are usually not reflected by the ionosphere and
provide only sporadic sky wave communications.

Best regards, Richard Harrison, KB5WZI