Wave catcher （1）

Published:2011/7/25 22:46:00 Author:Phyllis From:SeekIC

By Burkhard Kainka

Aerials and preselectors for AM and DRM

For initial experimenting and radio listening with a DIY short-wave or DRM receiver, a simple rod aerial or piece of wire is sufficient. But it’s only natural to want something better. This article gives you the right tips for better reception.

In the UHF band, radio waves propagate essentially in a line-of-sight manner, similar to light, and this limits their reception to a range of around 100 km, depending on aerial height. However, radio waves at frequencies below 30 MHz show completely different behavior, which makes a significantly larger reception range possible. Of course, the complex prop-" ms in this frequency range also lead to special problems, such as dependence on the time of day, field strength fluctuations (fading) and selective fading.

Radiation and propagation

The detective role in the propagation of radio waves in the short-wave bands is played the ionosphere, a set of weakly layers in the atmosphere at great height that are solar ionization. The ionosphere results from collisions between particle radiation and gamma radiation from the sun and air molecules, which become ionized with the release of electrons. Within certain frequency bands and at certain incident angles, the free electrons generated in this manner act like a mirror. At large incident angles and high frequencies, the ionosphere is instead transparent. A short-wave transmitter can be received in the nearby region via ground-wave propagation over a range of only around 30 to 100 km, depending on the height of the aerial. At greater distances, the transmitter disappears behind the horizon due to the curvature of the earth, making a direct link impossible. However, radio waves reflected from the ionosphere can reach receivers located in a region starting at certain minimum distance away for the transmitter, as illustrated in Figure 1.




Between the limit of reception of the ground wave and the start of reception of the reflected signal, there is a region called the ’skip zone’ where the signal cannot be received. The reflection angle that would be necessary for the signal to be received in this region is too large. Of course, the transmitter also radiates energy at this angle, but it leaves the atmosphere without being used, perhaps to be received sometime later on by one of our neighbors in outer space.

The minimum distance between the transmitter and the receiver varies with the time of day for each frequency, and it also depends on the level of solar activity, which varies over time. High frequencies can only be reflected at very shallow angles. Consequently, the ground distance spanned by the reflected signal is generally greater in the higher-frequency. The skip zone is also correspondingly larger; during p to around 200 km at 6 MHz and around 1000 km at 15 MHz.

The skip zone expands at night and with it the reception range. As a result, it often happens that a particular transmitter can be initially received quite well near dusk but then suddenly disappears, since in a manner of speaking it has slid into the skip zone. If the same programme is also being broadcast on other frequency bands (as is customary with BBC, DW, etc.), it is recommended to change to a lower frequency when this happens.

Generally speaking, radio waves arrive at a receiver via more than one path. The differences in the path lengths give rise to phase differences, which lead to partial reinforcement or cancellation of the waves. Especially in the short-wave band, rapid fluctuations in field strength are common. This frequently causes selective fading, which is particularly noticeable with AM transmitters in the form of unpleasant distortion resulting from nearly total loss of the carrier signal, causing it

owed by the sidebands. DRM (the new digital transmission technique) is also affected by this fading, but the modulation and coding methods used for DRM are especially robust and can tolerate partial loss of data. Thanks to effective error handling, even deep dropouts in the DRM spectrum, such as those caused by cancellation at certain frequencies, generally do not interfere with reception.




DRM restores excitement to radio listening on the medium-wave bands. There are already quite a few stations available (see Table 1), and new transmitters are constantly being added to the list.

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