Radio amateurs point their antennas skyward to talk via communications satellites built by groups of interested and talented hams.
At 18 minutes past the hour, radio stations WWV and WWVH broadcast the latest
solar flux number, the average planetary A-Index
and the latest mid latitude K-Index. In addition, they broadcast a descriptive account of the condition of the geomagnetic field and a
forecast for the next three hours. You should keep in mind that the A-Index is a description of what happened yesterday. Strictly
speaking, the K-Index is valid only for mid latitudes.
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, begins to be practical.
1.8-2.0 MHz (160 m) Top band, as it is sometimes called, suffers from daytime
D-layer absorption. Daytime communication is limited to
ground-wave coverage and a single E hop out to about 1500 km for well equipped stations (running the full legal limit, a
quarter-wave vertical with a good ground system, and a low noise receiving environment). At night, the D layer quickly
disappears and worldwide 160-m communication becomes possible via F2 layer skip and ducting. Atmospheric and
man-made noise limits propagation. Tropical and mid latitude thunderstorms cause high levels of static in summer, making
winter evenings the best time to work DX at 1.8 MHz.
3.5-4.0 MHz (80 m for the lower end, 75 m for the higher end) 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. 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 80/75 m DXer.
7.0-7.3 MHz (40 m) The popular 40-m band has a clearly defined skip zone during
the day due to insufficient ionization to refract high angles.
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). At night, reliable worldwide communication
via F2 is common on the 40-m band.
Atmospheric noise is much 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 amateursí
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
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.
The 10-m band is well known for extreme variations in characteristics and a
variety of propagation modes. During
solar maxima, long-distance F2 propagation is so efficient that very low power can produce strong 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 sporadic
E 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 propagation 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 propagation (TE), are rarely observed at VHF. Tropospheric propagation, which is not a factor at HF,
becomes increasingly important above 50 MHz.
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 (especially during the winter months), 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.
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
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 allwater
paths, such as that between California and Hawaii.
420-450 MHz (70 cm) The lowest amateur UHF band marks the highest frequency on
which ionospheric propagation is commonly observed.
Auroral 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.