HF propagation relies mainly on the F layer an ionized layer acting like a mirror at an altitude of 200 to 400 km.



The ionization results from solar activity. The more active the sun, the better the mirror works. Sun's activity has a more or less regular pattern known as the 11 years cycle. The bad news is that 2007 should be the minimum. The ionosphere is also very sensible to magnetic perturbations. The magnitude of these perturbations is indicated by the K index, it changes within 1 or 2 hours with pertutbations and poor conditions lasting for a few hours to several days.

The effective flux creating the ionization is monitored permanently and the K computed several times daily. High Flux and low K are requested for "good propagation". These informations are requested by prediction programs and assessing current bands conditions. They can be obtained on the net  http://www.wm7d.net/hamradio/solar/index.shtml

and  http://www.sec.noaa.gov/rt_plots/kp_3d.html

or on 30 m in Europe http://www.dk0wcy.de/ (10144kHz, CW, RTTY 20 th minute, BPSK31 35th minute, QPSK31 50th minute).

Ionograms are graphical representations of radio soundings of the inosphere. They measure the time needed by the wave to travel up and down to the layers. The upper maximal frequency at which reflection occurs is called critical frequency. Beyond this point the signal will leave the earth. The critical frequency is a fonction of the electron density. The higher the ionization the higher the critical frequency is : higher at noon, higher in sommer, higher on the best part of the sunspots cycle. Waves arriving at grazing incidences can be reflected at much higher frequencies. This is the difference between the critical frequency (vertical incidence) and the MUF (maximal usable frequency) for a 1 hop path (usually 3'000 km). During daytime, waves with frequencies lower than 5MHz are absorbed by the D layer which appears slowly after sunrise and disappears quickly after sunset as do the E layer. Below are 2 ionograms showing the difference between day and night conditions. The F layer is split in 2 during daytime. For the real time ionogram in Rome : http://ionos.ingv.it/Roma/latest.html Ionograms can be obtained for a lage number of locations. Take the time to observe their evolution during on 24 hours. If you are in a hurry click here http://dps-roma.ingv.it/Search.html and if you thinck that the ionosphre is something dull and static try this one http://www.ngdc.noaa.gov/stp/IONO/Dynasonde/TR/archive/index.html

Typical daytime ionogram

Nighttime, the E layer has disappeared, the F layer is higher and the max frequency is lower

DX on 160 and 80 m can be achieved only when most of the path is not illuminated by the sun. Several programs give an excellent representation of the earth to help planning contacts or understanding what's happening (http://www.dxatlas.com/).

Waves make single or multiple hops as they travel from the transmitting to the receiving antenna. In effect they are also reflected by the ground (sea being a much better reflector). The maximal range of single hop propagation is between 2'000 and 3'000 km depending on the F layer height. During daytime  a suplementary E layer can add complexity to the process.

The layers structure and refractive properties varies with daily,  seasonal and longer time pattern.   The structure of the ionosphere all over the world varies as no 2 points have the same exposure to sun radiation at the same moment. This makes propagation forecast impossible without the help of dedicated softwares. Accuracy requires skill and a deep understanding of the program. See the links for a survey of forecast programms.

Doing your own forecasts is a great pleasure. Try W&EL's software for an easy and (in my opinon) reliable start. VOCAAP is of course a must if you have time to master it.

The diagramms below were established using W6EL's software.

This is a link to a world wide forecast for the 160 m band
http://www.spacew.com/www/160pred.html

As atmospheric noise increases strongly with wavelength, the top band is very affected by it. This band is moreover very sensitive to remaining D layer absorption.

Absorbtion is moreover proportionnal to the inverse of the difference between the signal frequency and the electron gyrofrequency which rises to 1.6 Mhz around the magnetic pole.

DXing on topband requires a lot of patience as the required conditions are  seldomly present and difficult to forecast. An introduction to the physics involved in top band propagation is presented here.