Wave catcher （2）

Published:2011/7/25 22:53:00 Author:Phyllis From:SeekIC

By Burkhard Kainka

A longish wire

Strong short-wave transmitters can be received using an "aerial system’ (rod aerial or piece of wire) with a length of less than a meter. For long-distance reception, it is naturally much better to use a ’longish’ wire aerial, which preferably should be strung up outdoors - as widely separated from other objects as possible, as high as possible and sufficiently far house, in order to avoid the ’noise cloud’ emanating from the house.

In theory, a wire aerial has a resonant frequency at one quarter of the signal wavelength, although a good earth connection acting as a counterpoise is important for this. In practice, wire aerials with lengths of around 10 meters have proved to be satisfactory.

If the receiver is located close to a window or the outside wall of the house, it is sufficient to connect the end of the wire directly to the inner contact of the aerial socket. However, if the distance between the aerial and the receiver inside the house is relatively long, the connection should be made using coaxial cable, with the opposite pole being provided by an earth connection close to the feed point of the aerial (see Figure 2). Here it makes no difference whether you use 50-ft cable or 75-i2 cable. After all, the base resistance of the aerial varies with frequency, and it has a complex impedance with alternating capacitive and inductive components. The coaxial cable also has its own resonances, since it is not being used at its characteristic impedance, and it transforms the aerial impedance, with the net result that resonances other than those to be expected from the length of the aerial can also occur. However, this does not have much of a noticeable effect at the receiver, since signal level variations of around 10 dB hardly matter with DRM.




Outdoor wire aerials are normally made from stranded wire with a sufficiently large cross-sectional area, in order to obtain good mechanical strength as well as low ohmic losses. A lead from a standard mains cable or loudspeaker cable with a cross-sectional area of 0.75 to 1.5 mm_ is suitable for this purpose. It is also possible to use significantly thinner wire. A test using 10 m of thin magnet wire (0.3 mm diameter) yielded usable results for DRM reception, and it has the advantage of being quite inconspicuous. If you shy away from constructing your own aerial, in many cases you can make use of existing systems or cables. A typical aerial system provides not only television and L’HF signals. but also the entire AM range from long-wave to shortwave. It’s certainly worth trying.

In many cases, better results can be obtained with a rooftop aerial than with an indoor aerial. Old rooftop aerials are often no longer in use. but the aerial cables leading to the roof are still in place. The cable by itself can also be useful. Such a ’forgotten" aerial cable that has been converted into a "single lead", which means with the inner conductor and the screen shorted together, forms a usable vertical aerial. The cable usually runs all the way to the roof of the building and thus reaches a greater height than a horizontally strung wire aerial. Especially at relatively high frequencies (above 15 MHz), better results can be achieved using such an aerial than with an outdoor wire aerial.

Preselection

A ’longish’ wire aerial has a broadband characteristic and receives the entire AM radio band between 0.5 MHz and 22 MHz relatively uniformly, so no additional tuning is need for the aerial itself. However, preselection is worthwhile if reception is degraded by mirror frequencies.

Practically every receiver that works on the superhet principle has two reception frequencies: the intended frequency and the mirror frequency, which is separated from the intended frequency by twice the intermediate frequency. With the usual IF of 455 kHz (as used in the DRM receiver published in the March 2004 issue), this means the mirror frequency is located 910 kHz above the tuned frequency. For receivers having a switching mixer (such as the diode-ring mixer of the DRM receiver), the received signal is mixed with not only the fundamental frequency of the oscillator signal, but also (with a certain amount of attenuation) with all odd harmonics of the fundamental oscillator frequency. Primarily in case of reception in the medium-wave band, this can lead to interference from mixer products formed by harmonics of the mixer oscillator and strong short-wave signals. Consequently, a medium-wave preselector often provides significant improvement. A preselector connected between the aerial and the receiver is most commonly used, and such preselectors are generally tuneable. Suitable preselectors are available from specialist (amateur) radio shops, but you can also build your own.

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