Subject: [NEXTEL-1] Propagation

From: [email-masked]
Date: Sun, 08 Jun 2003 16:04:26 -0400

Radio waves, like light waves and all other forms of electromagnetic radiation,
normally travel in straight lines. Obviously this does not happen all the time,
because long-distance communication depends on radio waves traveling beyond the
horizon. How radio waves propagate in other than straight-line paths is a
complicated subject, but one that need not be a mystery.

http://www.wulfden.org/NVQS/PropStuff/ARRLPROP.HTM
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Propagation

Radio waves, like light waves and all other forms of
electromagnetic radiation, normally travel in straight lines. Obviously this
does not happen all the time, because long-distance communication depends on
radio waves traveling beyond the horizon. How radio waves propagate in other
than straight-line paths is a complicated subject, but one that need not be a
mystery.

Medium Frequencies (300 kHz-3
MHz)

The only amateur medium-frequency band is situated just above the domestic AM
broadcast band. Ground wave provides reliable communication out to 150 km
(90 mi) or during the day, when no other form of propagation is available.
Long-distance paths are made at night via the F2 layer.

1.8-2.0 MHz (160 m)

The top band, as it is sometimes called, suffers from extreme daytime D-layer
absorption. Even at high radiation angles, virtually no signal can pass through
to the F layer, so daytime communication is limited to ground-wave
coverage. At night, the D layer quickly disappears and worldwide 160-m
communication becomes possible via F2-layer skip. Atmospheric and man-made noise
limit propagation. Tropical and midlatitude thunderstorms cause high levels of
static in summer, making winter evenings the best time to work DX at 1.8
MHz. A proper choice of receiving antenna can often significantly reduce the
amount of received noise while enhancing desired signals.

High Frequencies (3-30 MHz)

A wide variety of propagation modes are useful on the HF bands. The lowest
two bands in this range share many daytime characteristics with 160 m. The
transition between bands primarily useful at night or during the day appears
around 10 MHz. Most long-distance contacts are made via F2-layer skip. Above 21
MHz, more exotic propagation, including TE, sporadic E, aurora and meteor
scatter, begin to be practical.

3.5-4.0 MHz (80 m)

The lowest HF band is similar to 160 m in many respects. Daytime absorption
is significant, but not quite as extreme as at 1.8 MHz. High-angle signals may
penetrate to the E and F layers. Daytime communication range is typically
limited to 400 km (250 mi) by ground-wave and skywave propagation. At night,
signals are often propagated halfway around the world. As at 1.8 MHz,
atmospheric noise is a nuisance, making winter the most attractive season for
the 80-m DXer.

7.0-7.3 MHz (40 m)

The popular 40-m band has a clearly defined skip zone during the day. D-layer
absorption is not as severe as on the lower bands, so short-distance skip via
the E and F layers is possible. During the day, a typical station can cover a
radius of approximately 800 km (500 mi). Ground-wave propagation is not
important. At night, reliable worldwide communication via F2 is common on the
40-m band.

Atmospheric noise is less troublesome than on 160 and 80 m, and 40-m DX
signals are often of sufficient strength to override even high-level summer
static. For these reasons, 40 m is the lowest-frequency amateur band considered
reliable for DX communication in all seasons. Even during the lowest point in
the solar cycle, 40 m may be open for worldwide DX throughout the night.

10.1-10.15 MHz (30 m)

The 30-m band is unique because it shares characteristics of both daytime and
nighttime bands. D-layer absorption is not a significant factor. Communication
up to 3000 km (1900 mi) is typical during the daytime, and this extends halfway
around the world via all-darkness paths. The band is generally open via F2 on a
24-hour basis, but during a solar minimum, the MUF on some DX paths may
drop below 10 MHz at night. Under these conditions, 30 m adopts the
characteristics of the daytime bands at 14 MHz and higher. The 30-m band shows
the least variation in conditions over the 11-year solar cycle, thus making it
generally useful for long-distance communication anytime.

14.0-14.35 MHz (20 m)

The 20-m band is traditionally regarded as the amateurs’ primary long-haul DX
favorite. Regardless of the 11-year solar cycle, 20 m can be depended on for at
least a few hours of worldwide F2 propagation during the day. During solar
maximum periods, 20 m will often stay open to distant locations throughout the
night. Skip distance is usually appreciable and is always present to some
degree. Daytime E-layer propagation may be detected along very short paths.
Atmospheric noise is not a serious consideration, even in the summer. Because of
its popularity, 20 m tends to be very congested during the daylight hours.

18.068-18.168 MHz (17 m)

The 17-m band is similar to the 20-m band in many respects, but the effects
of fluctuating solar activity on F2 propagation are more pronounced. During the
years of high solar activity, 17 m is reliable for daytime and early-evening
long-range communication, often lasting well after sunset. During moderate
years, the band may open only during sunlight hours and close shortly after
sunset. At solar minimum, 17 m will open to middle and equatorial latitudes, but
only for short periods during midday on north-south paths.

21.0-21.45 MHz (15 m)

The 15-m band has long been considered a prime DX band during solar cycle
maxima, but it is sensitive to changing solar activity. During peak years, 15 m
is reliable for daytime F2-layer DXing and will often stay open well into
the night. During periods of moderate solar activity, 15 m is basically a
daytime-only band, closing shortly after sunset. During solar minimum periods,
15 m may not open at all except for infrequent north-south transequatorial
circuits. Sporadic E is observed occasionally in early summer and mid-winter,
although this is not common and the effects are not as pronounced as on the
higher frequencies.

24.89-24.99 MHz (12 m)

This band offers propagation that combines the best of the 10- and 15-m
bands. Although 12 m is primarily a daytime band during low and moderate sunspot
years, it may stay open well after sunset during the solar maximum. During years
of moderate solar activity, 12 m opens to the low and middle latitudes during
the daytime hours, but it seldom remains open after sunset. Periods of low solar
activity seldom cause this band to go completely dead, except at higher
latitudes. Occasional daytime openings, especially in the lower latitudes, are
likely over north-south paths. The main sporadic-E season on 24 MHz lasts from
late spring through summer and short openings may be observed in mid-winter.

28.0-29.7 MHz (10 m)

The 10-m band is well known for extreme variations in characteristics and
variety of propagation modes. During solar maxima, long-distance F2 propagation
is so efficient that very low power can produce loud signals halfway around the
globe. DX is abundant with modest equipment. Under these conditions, the band is
usually open from sunrise to a few hours past sunset. During periods of moderate
solar activity, 10 m usually opens only to low and transequatorial latitudes
around noon. During the solar minimum, there may be no F2 propagation at any
time during the day or night.

Sporadic E is fairly common on 10 m, especially May through August, although
it may appear at any time. Short skip, as it is sometimes called on the HF
bands, has little relation to the solar cycle and occurs regardless of F-layer
conditions. It provides single-hop communication from 300 to 2300 km (190 to
1400 mi) and multiple-hop opportunities of 4500 km (2800 mi) and farther.

Ten meters is a transitional band in that it also shares some of the
propagation modes more characteristic of VHF. Meteor scatter, aurora, auroral E
and transequatorial spread-F provide the means of making contacts out to 2300 km
(1400 mi) and farther, but these modes often go unnoticed at 28 MHz. Techniques
similar to those used at VHF can be very effective on 10 m, as signals are
usually stronger and more persistent. These exotic modes can be more fully
exploited, especially during the solar minimum when F2 DXing has waned.

Very High Frequencies (30-300 MHz)

A wide variety of propagation modes are useful in the VHF range. F-layer skip
appears on 50 MHz during solar cycle peaks. Sporadic E and several other E-layer
phenomena are most effective in the VHF range. Still other forms of VHF
ionospheric propagation, such as field-aligned irregularities (FAI) and
transequatorial spread F (TE), are rarely observed at HF. Tropospheric
propagation, which is not a factor at HF, becomes increasingly important above
50 MHz.

50-54 MHz (6 m)

The lowest amateur VHF band shares many of the characteristics of both lower
and higher frequencies. In the absence of any favorable ionospheric propagation
conditions, well-equipped 50-MHz stations work regularly over a radius of 300 km
(190 mi) via tropospheric scatter, depending on terrain, power, receiver
capabilities and antenna. Weak-signal troposcatter allows the best stations to
make 500-km (310-mi) contacts nearly any time. Weather effects may extend the
normal range by a few hundred km, especially during the summer months, but true
tropospheric ducting is rare.

During the peak of the 11-year sunspot cycle, worldwide 50-MHz DX is possible
via the F2 layer during daylight hours. F2 backscatter provides an additional
propagation mode for contacts as far as 4000 km (2500 mi) when the MUF is just
below 50 MHz. TE paths as long as 8000 km (5000 mi) across the magnetic equator
are common around the spring and fall equinoxes of peak solar cycle years.

Sporadic E is probably the most common and certainly the most popular form of
propagation on the 6-m band. Single-hop E-skip openings may last many hours for
contacts from 600 to 2300-km (370 to 1400 mi), primarily during the spring and
early summer. Multiple-hop Es provides transcontinental contacts several times a
year, and contacts between the US and South America, Europe and Japan via
multiple-hop E-skip occur nearly every summer.

Other types of E-layer ionospheric propagation make 6 m an exciting band.
Maximum distances of about 2300 km (1400 mi) are typical for all types of
E-layer modes. Propagation via FAI often provides additional hours of contacts
immediately following sporadic E events. Auroral propagation often makes its
appearance in late afternoon when the geomagnetic field is disturbed. Closely
related auroral-E propagation may extend the 6-m range to 4000 km (2500 mi) and
sometimes farther across the northern states and Canada, usually after midnight.
Meteor scatter provides brief contacts during the early morning hours,
especially during one of the dozen or so prominent annual meteor showers.

144-148 MHz (2 m)

Ionospheric effects are significantly reduced at 144 MHz, but they are far
from absent. F-layer propagation is unknown except for TE, which is responsible
for the current 144-MHz terrestrial DX record of nearly 8000 km (5000 mi).
Sporadic E occurs as high as 144 MHz less than a tenth as often as at 50 MHz,
but the usual maximum single-hop distance is the same, about 2300 km (1400 mi).
Multiple-hop sporadic-E contacts greater than 3000 km (1900 mi) have occurred
from time to time across the continental US, as well as across Southern Europe.

Auroral propagation is quite similar to that found at 50 MHz, except that
signals are weaker and more Doppler-distorted. Auroral-E contacts are rare.
Meteor-scatter contacts are limited primarily to the periods of the great annual
meteor showers and require much patience and operating skill. Contacts have been
made via FAI on 144 MHz, but its potential has not been fully explored.

Tropospheric effects improve with increasing frequency, and 144 MHz is the
lowest VHF band at which weather plays an important propagation role.
Weather-induced enhancements may extend the normal 300- to 600-km (190- to
370-mi) range of well-equipped stations to 800 km (500 mi) and more, especially
during the summer and early fall. Tropospheric ducting extends this range to
2000 km (1200 mi) and farther over the continent and at least to 4000 km (2500
mi) over some well-known all-water paths, such as that between California and
Hawaii.

222-225 MHz (135 cm)

The 135-cm band shares many characteristics with the 2-m band. The
normal working range of 222-MHz stations is nearly as good as comparably
equipped 144-MHz stations. The 135-cm band is slightly more sensitive to
tropospheric effects, but ionospheric modes are more difficult to use. Aurora
and meteor-scatter signals are somewhat weaker than at 144 MHz, and sporadic-E
contacts on 222 MHz are extremely rare. FAI and TE may also be well within the
possibilities of 222 MHz, but reports of these modes on the 135-cm band are
uncommon. Increased activity on 222-MHz will eventually reveal the extent of the
propagation modes on the highest of amateur VHF bands.

Ultra-High Frequencies (300-3000 MHz) and Higher

Tropospheric propagation dominates the bands at UHF and higher, although some
forms of E-layer propagation are still useful at 432 MHz. Above 10 GHz,
atmospheric attenuation increasingly becomes the limiting factor over
long-distance paths. Reflections from airplanes, mountains and other stationary
objects may be useful adjuncts to propagation at 432 MHz and
higher.

420-450 MHz (70 cm)

The lowest amateur UHF band marks the highest frequency on which ionospheric
propagation is commonly observed. Aurora signals are weaker and more Doppler
distorted; the range is usually less than at 144 or 222 MHz. Meteor scatter is
much more difficult than on the lower bands, because bursts are significantly
weaker and of much shorter duration. Although sporadic E and FAI are unknown as
high as 432 MHz and probably impossible, TE may be possible.

Well-equipped 432-MHz stations can expect to work over a radius of at least
300 km (190 mi) in the absence of any propagation enhancement. Tropospheric
refraction is more pronounced at 432 MHz and provides the most frequent and
useful means of extended range contacts. Tropospheric ducting supports contacts
of 1500 km (930 mi) and farther over land. The current 432-MHz terrestrial DX
record of more than 4000 km (2500 mi) was accomplished via ducting over water.

902-928 MHz (33-cm) and Higher

Ionospheric modes of propagation are nearly unknown in the bands above 902
MHz. Aurora scatter may be just within amateur capabilities at 902 MHz, but
signal levels will be well below those at 432 MHz. Doppler shift and distortion
will be considerable, and the signal bandwidth may be quite wide. No
other ionospheric propagation modes are likely, although high-powered research
radars have received echoes from auroras and meteors as high as 3 GHz.

Almost all extended-distance work in the UHF and microwave bands is
accomplished with the aid of tropospheric enhancement. The frequencies above 902
MHz are very sensitive to changes in the weather. Tropospheric ducting occurs
more frequently than in the VHF bands and the potential range is similar. At
1296 MHz, 2000-km (1200-mi) continental paths and 4000-km (2500-mi) paths
between California and Hawaii have been spanned many times. Contacts of 1000 km
(620 mi) have been made on all bands through 10 GHz in the US and over 1600 km
(1000 mi) across the Mediterranean Sea. Well-equipped 903- and 1296-MHz stations
can work reliably up to 300 km (190 mi), but normal working ranges generally
shorten with increasing frequency.

Other tropospheric effects become evident in the GHz bands. Evaporation
inversions, which form over very warm bodies of water, are usable at 3.3 GHz and
higher. It is also possible to complete paths via scattering from rain, snow and
hail in the lower GHz bands. Above 10 GHz, attenuation caused by atmospheric
water vapor and oxygen become the most significant limiting factors in
long-distance communication.

Copyright 1996, American Radio Relay League,
Inc.