Direction finding by magnetic antenna. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Miscellaneous electronic devices
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In recent years, portable camping radios have become very widespread. As a rule, all marching receivers use a magnetic antenna. This circumstance gives such receivers one very valuable quality - the possibility of direction finding, that is, determining the direction to any radio station they receive.
The magnetic antenna has a pronounced directional characteristic. Figure 1 shows the polar diagram of its sensitivity, which has the shape of a figure eight.
Ris.1
The volume of the radio station reception on the magnetic antenna does not have a sharp maximum, while the minimum (almost complete dip in audibility) is very distinct and allows you to determine it with a fairly high accuracy. The direction to the direction-finding radio station is determined by the minimum volume of its reception. In this case, the direction to the radio station coincides with the direction of the ferrite core (rod) of the magnetic antenna. As for the accuracy of the determination, it turns out to be quite sufficient for tourist purposes.
Thus, the experience of direction finding broadcast radio stations using such a common receiver as the "Tourist" showed that the accuracy of the determination (reading the angle in azimuth) is about 0,5 degrees (± 0,25 °). As the distance decreases, the accuracy of the determination increases proportionally. A very significant drawback of such a simple direction finder is the presence of two minima, shifted one relative to the other by 180°. Therefore, such a direction finder does not give a definition of the active side, that is, it does not indicate the true, only direction to the radio station. Unfortunately, there are no simplest ways to exclude one of the minima and obtain the sensitivity characteristic of a magnetic antenna in the form of a so-called cardioid. Therefore, the active side has to be determined indirectly, in accordance with the position on the ground, or simply remembering the direction in which the pedestrian was moving before.
However, there may be cases when the orientation is still completely lost and it is completely impossible to establish the true direction to the radio station with two minima. In this case, a rather simple method for determining the active side is proposed, carried out by a very simple maneuver. However, it should be borne in mind that this method is applicable only to nearby radio stations.
Ris.2
Having determined the direction of the beam of the direction-finding radio station using any of the minima (Fig. 2) and marking it with the compass, the pedestrian moves perpendicular to it at a distance of 100-200 m. The bearing taken then again will show some declination, which is easy to detect and determine by compass. Its direction will indicate the direction in which the direction-finding radio station is located. The figure below explains the essence of the described method. If the bearing angle has increased, it follows that the radio station is, as shown in the plan in the figure, to the left; if the bearing angle decreases when moving away from the course in the direction indicated in the figure, the radio station is on the right (the position of the plane of the direction finder receiver is indicated by a dotted line).
The required departure distance from the main course is determined by the range of the direction-finding radio station and is usually not large. So, with a distance to the radio station of 10 km, it is enough to move only 100 m, which already gives about 1 ° declination, which is well read on a tourist compass. At a distance of 20 km, the distance should be doubled, that is, move about 200 m away.
It should, of course, be borne in mind that when using the receiver as a direction finder, for convenience, a small compass should be mounted on it.
Author: I. Korsakov; Publication: N. Bolshakov, rf.atnn.ru
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