The radio propagation
Before to extend our review on upper bands (V/UHF and sepcial modes), till stop a second in the HF spectrum. Instead of speaking of propagation in terms of ionospheric layers, that nobody can really materialize, amateurs and listeners are used to speak in terms of frequencies or bands. Working on 14.115 MHz or 14.340 MHz makes no difference except when you are searching for a free frequency to work or to schedule a QSO, Hi ! It is thus simpler to say that one works on the 20-meter band.
Each band shows some specific properties interesting to know when working on HF. Here they are.
160m band (1.8 MHz)
Not much shorter than medium waves, at daytime the "top band" is deeply affected by the D-layer absorption and only waves entering the ionosphere at very high angle can be reflected to the ground. At daytime this band is thus mainly dedicated to local QSOs by ground waves up to distances reaching about 120 km. It is thus relatively quiet compared to the other HF bands. Checking my logs, I worked on this band most of the time after the sunset. Indeed, at night the D-layer disappearing, low-angle signals reflect easier on the F-layer, and DX contacts are possible at several thousands kilometers at the condition to use a suited antenna system. So this is a band to mainly use at night, and if you can, in winter especially during the cycles of weak solar activities to reduce atmospheric noises. It is affected by the sunrise/sunset, weather conditions (noise of thunderstorm) and the electron gyro-frequency.
80m band (3.5 MHz)
This band is similar to the 160m but the frequency increasing, the D-layer absorption begin to decrease as it is proportional to the inverse square of the frequency. At daytime most contacts are worked with near countries, up to about 2000 km from your QTH. At night or using the gray line it is possible to exceed 9000 km (e.g. Europe to W, UA or JA). If you wake up in winter a few hours before the sunrise to avoid atmospheric noises you can work most DX stations (e.g. VK, ZL from Europe). In Region 1 the frequencies ranging between 3790-3800 kHz are usually dedicated to DX hunters.
40m band (7 MHz)
This band is still under the influence of the D-layer and is the lowest band showing an appreciable skip distance, up to 500 km at night. At noon it is hard to work station located over 800 km away, while working with the gray line or at night, this band is open to DX contacts. This band is slightly influenced by 11-year solar cycle. Atmospheric noises are still present but not as strong as on the lower bands. QSOs can however be difficult to confirm during the summer months but signals are not completely overriden by static. This is also one of the most crowded band and do not be surprised to find in Europe and during weekends each station within 1 kHz or so from each another.
Hopefully from March 29, 2009, in Region 1 this band will be accessible to amateurs up to 7200 kHz. Note that it is already open to amateurs to 7300 kHz in North and South America.
Conditions are similar to the 40m band but it works better in summer showing some of the properties of the 20m band. As communications reach 1600 km at daytime and 12000 km or more at night, this band is considered as open 24 hours a day. It is also band the least affected by variations of the solar cycle. However this band is affected by the ionization level of E and F-layers and at night, during the minimum of the solar cycle, it is regularly above the MUF for most DX paths, becoming thus the higher workable frequency for daytime communications. At very few exceptions (see next insert) this band is reserved to digimodes and CW.
20m band (14 MHz)
If you ask to active amateurs, including listeners, what is the band they use the most, taking all modes together and over a full solar cycle, without hesitation all will place the 20 meter band first for its "overall performance".
Indeed, the 20m band is the DX band per excellence and is considered by many hams as the most reliable band for hunting DX stations because atmospheric noises are weak.
At some rare exceptions (blackouts) whatever your position in the 11-year solar cycle, world-wide communications are open at daytime on the 20m band. It is practically usable all the day long as soon as there is propagation, and mainly in summer, and all the more during periods of high solar activity. This is only during the winter months in condition of low solar activity that this band closes down in the late afternoon and is unusable at night.
The 20m band shows an appreciable skip distance reaching about 700 km at daytime and exceeding 1600 km at night. It is thus not suited to local QSOs. This can partly be solved placing the antenna at low height to get a high takeoff angle.
17m band (18 MHz)
This band is similar to the 15m band in many respects although its activity is affected by the 11-year solar cycle but not as pronounced. During the maximum of the solar activity, this band is open all the day, up to well after the sunset. When the solar activity decreases this band closes earlier, just after the sunset. At the minimum of the solar cycle, this band opens to middle or equatorial latitudes, allowing north-south contacts but mainly around noon. This band is this mainly open at daytime and regularly opens before the others. The skip distance reaches 1200 km at daytime and disappears at night.
15m band (21 MHz)
This band works in the same conditions as the 17m band and is considered by most amateurs as a quieter alternative to the 20m band. It shows however a greater sensitivity to the fluctuations of the solar cycle. It is mainly used at daytime but during the peaks of the solar acitivity DX contacts can extend to the night. On the contrary during the minimum of the solar cycle, this band can be closed excepted for a few trans-equatorial paths. With the 12 and 10m this band is also subject to a weak E-sporadic activity mainly in early summer and mid-winter. The skip distance reaches 1300 km at daytime and vanishes at night.
12m band (24 MHz)
This band is very depending of the solar cycle and combines the best from the 15 and 10m bands. One year before the paroxysm of the solar activity and up to one year after this peak, this band allows DX contacts until after the sunset with practically any kind of antenna. On the contrary, when the solar activity slow down, this band is open at mid and low latitudes but only at daytime with very few openings after the sunset. Near the minimum of the solar cycle this band becomes unusable, excepting at daytime for the highest latitudes with some north-south openings. This band is open to E-sporadic traffic between the late spring and early winter. The skip distance reaches 1600 km at daytime and vanishes at night.
10m band (28 MHz)
This band works in the same conditions as the 12m band but is characterized by a great variability according the solar activity. During the peaks of the solar activity DX contacts can be established with very low power and over 12000 km away. It is mainly a daytime band but remains open a few hours after sunset. During moderate solar activity this band open near noon for some trans-equatorial communications but is closed for the higher latitudes. During the minimum of the solar activity this band is straight out dead. There are however some exceptional openings for ionoscatter, meteor scatter and E-sporadic. This latter occurs mainly between April and early August allowing multihop communication up to 4100 km. This band is relatively quiet compared to the 15m band for example but it works globally very well for DX communications throughout the solar cycle. Unfortunately more and more CB, pirats and intruders occupy the lower part of this band (11m or ~27 MHz). The skip distance reaches 2000 km at daytime.
The Electromagnetic spectrum, from 31.2 mHz to 6.52 EHz
Radio spectrum bandplan, from 3 kHz to 30 GHz
Traffic via the E-Sporadic
With the E-sporadic traffic we enter the world of VHF communications although some "openings" also exist in the upper HF frequencies. Traffic via the E-Sporadic layer (Es) is an ideal experience for the 6 m band where it displays all its potential, still more than on the classical 2 m band, and it is open earlier too : if there is no Es activity on the 6 m band don't hope for a chance on the 2 m. The origin of these sporadic clouds of ionized particles is unknown and several theories have been suggested so far.
Whatever the source of the phenomenon, usually traffic via Es works best in early summer and mid-winter in the fore or afternoon and may last from a few minutes to several hours. Usually this is a single hop communication but exceptionnally we can operate multi-hop Es. It can occasionally be observed in HF bands between the 15m and 10m band but with an activity less pronounced than on VHF. Sometimes Es allows to work until past midnight on VHF, propagating signals extremely loud and sudden on a regional scale, but it disappears suddenly too.
When E-sporadic layers decay we observe also the raising of Aurora or E-layer FAI.
Field Alignment Irregularities, FAI
Another amazing propagation is the Field Alignment Irregularities, FAI for short, that mainly concerns VHF. It finds its origin in irregularities in the ionization of the E and F-layers (above 100 km aloft) that are aligned along the lines of the geomagnetic field.
These irregularities parallel or adjacent to the geomagnetic equator mainly occur late in the afternoon when the F-layer begins to decay together with the MUF, which in turn allow to free electrons to align themselves along the geomagnetic field lines, which are nearly horizontal in this region.
Like in FAI found in Aurora, E and F-layers FAI exhibit amazing characteristics with regard to VHF signals. These irregularities being rather strong, signals approaching these regions are scattered efficiently only they are parallel to the geomagnefic field lines. This occurs only for a signal path peropendicular to the geomagnetic equator, what is also true for VHF scattering from auroras. When these conditions of geometry and level of ionization are met, signals are reflected away in an specific azimut equal to twice the right angles of each FAI region. When this phenomenon occurs,amateurs working on VHF bands above 50 MHz observe that a directive antenna has to be shifted from the remote station azimut between 30° and about 110° in the anti-clockwise direction. The phenomenon is amazing and can confuse novice amateurs, thinking that their antenna system is faulty or that they are working a pirate station ! Imagine to contact Scandinavian stations pointing your beam to GB...very strange, doesn't it. Usual distances between north and south stations are ranging between 5000 and 8000 km.
Like in Aurora traffic, signals are distorted, weak, with fast QSB, and can be associated to a Doppler effect. Closer you are from the transmitter, higher will be the reflection zone. As these points of reflection don't move, you can steer your antenna over these locations in summer (between May and September) at the end of an Es opening to have some chances to work a FAI.
During exceptional VHF openings some amateurs worked DX stations located 8000 km away in crossing the equator. Imagine : from Mexico to Brazil on 2 m ! This phenomenon seems occur when both stations are located symmetrically to the equator and experimenting a high level of electronic density at fall and spring during periods of solar maximum activity. By a lucky combination of ionospheric reflexions and refractions, at the time of equinoxes such trans-equatorial (TE) traffic seems possible but we have few information about such experiences.
Not all F-layers paths crossing the equator propagate via the T-E FAI. The ionization level of F-layers at daytime has to reach a threshold over the two stations located respectively north and south of the geomagnetic equator, that corollary, shows the highest MUF values on any given day over the Earth. In these very special conditions some F-layers paths cross the equator, extending their ionization bands up to 4000 km on each side of the geomagnetic equator. Hower to make a contact the path between both stations must be parallel to the geomagnetic field, not perpendicular.
The stations located over 30° of latitude north (or south) are usually too far of the geomagnetic equator to make use of F-layer FAI. Sometimes however, these latitudes can be worked via a sporadic-E hop even if signals are usually weak and typically exhibit the fluttery and hollow like sound of pure FAI.
At last, in bands of 17m and below (HF), during the minimum of the solar cycle Trans-Equatorial FAI are open during the day.