Wavemeter calibration for short waves. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Beginner radio amateur

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There is no need to expand on the need to have a wavemeter for everyone who works with short waves.

It is also obvious that the wavemeter must be as accurately calibrated as possible, otherwise it can only be misleading.

The calibration accuracy, which is quite sufficient for an amateur, should be expressed in approximately tenths of a percent.

The simplest and most accurate way of grading is grading with Lecher systems.

This method is already known to many, but, as practice shows, it is not enough to know the method or method: some more skill is needed, or if not, then knowledge of some details in which the covered method can give the desired results.

The purpose of this article is to communicate those few techniques and information that, if possible, will eliminate all the reasons that could give an incorrect calibration.

Let us briefly repeat the essence of the calibration method.

Assemble the generator G (see Fig. 1) or, which is the same, the transmitter according to some scheme. Putting it into action, we will receive oscillations in it with a certain wavelength unknown to us. With the generator, through the coupling coil L, two wires L are connected, forming the Lecher system. The same waves with which the generator oscillates will propagate through the connection along the Lecher wires. If now, at the beginning of the Lecher system, we put some indicator or indicator of resonance P, connecting it with the Lecher system, and from the coil L to the right to move a metal jumper along the wires - the bridge M, then it will be possible to find a point a at which: 1) segment L - a will be tuned into resonance with the generator, which will show the largest deviation of the arrow of the device P, 2) a standing wave appears in the segment (Readers are already familiar with standing waves in detail from special articles placed in the "Short Waves" section in the "Elements of Radio Engineering" cycle ), and the antinodes of the current will always be in the coil L and at the bridge M, 3) on the length of the segment from the middle of the coil L to point a, the half-wave of the generator will be located.

Rice. 1. Generator with Lecher system

Therefore, if we now measure in meters the length of the segment from the middle of the coil L to the bridge and multiply the resulting value by two, then we will determine in meters the wavelength that the generator oscillates. And having tuned the wavemeter to the generator, we will get a division on its scale, which will correspond to the wave we have defined. But here it becomes difficult to determine exactly the effect of the coil L on the length of the segment L - a, since the coil L shortens the length of the segment by some greater amount than the length of the wire of the coil itself. Therefore, in practice, they do this: having determined the place of the bridge at the first resonance, i.e., point a, they move the bridge further and look for point b, at which the indicator P will show the second resonance.

On the Leher segment L - a - b, just the whole wavelength fits, but we are interested in the segment ab, on which exactly half of the wave fits. This segment can be measured exactly (since it is not necessary to take into account the influence of the coil L here) and, therefore, it is possible to know exactly the wavelength to which the generator is tuned.

Further, by gradually changing the wavelength of the generator and determining its value in the manner described above, we can obtain for the wavemeter a series of divisions of the capacitor and the corresponding wavelengths, according to which the wavelength graph is plotted.

Now, remembering the method, let's move on to the details.

Generator. Any generator circuit can be taken, but the most convenient and simple is the three-point one. If it is possible to use two lamps, then the three-point dual circuit is used (see, for example, "RV." p. 510 - 511 No. 21 for 1927). The power of the generator should be as large as possible, since then it will be less affected by detuning. But in any case, you should not use lamps whose power is less than 10-15 watts.

The generator must be checked to ensure that the oscillations are stable and of sufficient power (no oscillation dips) over the entire required wave range.

Lecher's system is assembled from bare copper or bronze wires, with diameters of 1 or better - 1,5 mm. The distance between the wires is best taken at 5 centimeters. The length of the wires should be slightly more than half of the largest wavelength to which they want to calibrate the wavemeter.

As mentioned above, the coil L shortens the Lecher length l1, on which the first half of the wave fits. If a variable air capacitor C, shown in fig. 1 by a dotted line, then this length l1 will be shortened even more, i.e. the first position of the bridge M will be close to the coil L and therefore the entire Lecher length for the largest wave will be 0,6-0,7 of its length, instead of double.

For example, if you want to calibrate a wavemeter up to 50 meters, you need to take the length of the Lecher system of 30-35 meters.

Attention should be paid to the good insulation of the coil and wires of the system. - The end of the system behind the bridge (in Fig. 1 - right) may not be insulated.

The attachment of the Lecher system must be strong and rigid. It is convenient to use stakes with waxed boards, in the cutouts of which wires are laid (see Fig. 2).

Rice. 2. Suspension of the Lecher system

The coupling coil L usually consists of 2 or more turns. Its connection with the generator should be as small as possible, in which it is still possible to observe the readings of the indicator device. With strong coupling, the calibration accuracy will be less, especially if the generator is not powerful enough.

After choosing the appropriate connection, the coil L must be fixed absolutely firmly so that the movements of the bridge M cannot move it and thus change the connections.

Bridge. We have already clarified the effect of the coil L on the establishment of the wavelength in the Lecher segment. Therefore, if the bridge will have self-induction, then it will also reduce the accuracy of determining the wave. Therefore, a bridge of the following design will be good (see Fig. 3): two brass or copper plates P, having a semicircular or rectangular shape, are soldered to two brass corners U. A bolt and nut are passed through the corners, which makes a strong connection of the bridge with the Lecher wires. It is useful for wires to make small indentations. To move the bridge, slightly loosen the bolt nut.

On fig. 3 shows other options for arranging bridges. The spring under the nut will be very useful: it will make it possible to easily move the bridge with constant good contact.

Rice. 3. Construction of bridges

Indicator, or resonance indicator, should be as sensitive as possible. The less energy it consumes, the more accurate the calibration will be.

In amateur practice, it is best to use a galvanometer with a detector for this (see Fig. 4). If there is no galvanometer, you can take a milli-ammeter, but for small milli-amperes (no more than 10 m / a). Since a milliammeter usually has a small resistance, it is useful to take a detector with a small resistance, for example, chalcopyrite, zincite, etc. To connect the device with Lecher, a frame is made - a coil of bare wire 1,5-2 mm thick. Finally, it is useful to shunt the device with a capacitor, the capacity of which is 200-500 cmXNUMX.

Rice. 4. General view and diagram of the resonance indicator

To communicate with Lecher, the indicator is installed at the beginning of Lecher (Approximately near the first antinode of the current, but so that the indicator acts from Lecher, but not directly from the generator) so that the top of the frame is parallel to one of the Lecher wires. The distance between Lecher and the frame should be as large as possible (20-40 cm), but, of course, such that the deviation of the instrument needle will be noticeable.

The general arrangement of all devices can be seen in Fig. 5.

Rice. 5. Arrangement of instruments during calibration

The order of work is as follows: having assembled the entire circuit, the generator is put into operation and the shortest wave is set in it, to which the wavemeter should also be calibrated. The wavemeter should catch this wave at the first degrees of its capacitor. Then they make a preliminary determination of the positions of the bridge, that is, they find points a and b. Work is more convenient for two persons.

While one observer, having connected, for a start, a strong connection of the indicator, observes its arrow, the second participant in the calibration, having set the compression of the bridge so that when good contact is made, the bridge can be moved along the wires, takes hold of its middle and leads very smoothly and slowly away from the generator to the right. At the same time, the worker himself must always be C3adi of the bridge, that is, between the bridge and the free end of the Lecher system, so that his body does not affect Lecher and, consequently, the setting.

At a certain position of the bridge, the first resonance occurs. The resonance is usually sharp and easy to pass without noticing why, for a start, they take the indicator's connection with Leher stronger.

Having found the first point, they notice it on the ground either with a line or with a peg and move the bridge further.

The second resonance is even sharper, and the deviation of the device is smaller. Usually it is enough to move the bridge from the resonance point by 2-3 millimeters, as the resonance can already be passed.

Having found the second point, mark it and proceed to the graduation as follows:

The indicator frame is bent as shown by the dotted line in Fig. 5. The frame is connected with Leher near the first point, but in such a way that the indicator is to the left of it. Then the observer, looking at the scale of the indicator, which is now facing him, becomes C3adi of the bridge and, moving it forward or backward, exactly finds the place of resonance. Immediately, the weakest connection of the indicator with the Lecher system is selected, in which the observation is easy.

Having installed the bridge at the resonance point, lower the plumb line and accurately mark point No. 1 on the ground (see Fig. 6). Then the device and the bridge are transferred to the point of the second resonance, and here, acting in the same way (and with a weak connection), point No. 2 is determined.

Rice. 6. Location of the bridge according to the Lecher system during graduation

Measure the distance between points No. 1 and No. 2, multiply by two and get the wavelength of Lecher, and hence the generator.

The calibrated wavemeter is connected very weakly to the generator and tuned to the generator wave, after which the degrees of the capacitor are recorded in the corresponding wavelength.

With a strong connection of the wavemeter with the generator, the latter can be upset and thus give the wrong intention.

Then they slightly increase the wavelength of the generator, adjust the wavemeter, make sure that the arrow of its capacitor has moved 15-20 degrees, transfer the indicator P and the bridge to a place slightly to the right of point No. 1, and according to the previous one, with a weak connection, find the point of the first resonance the second wave of the generator - No. 3.

If we measure the distance between points No. 1 and 3, equal to a, and set aside twice its value 2a from point No. 2 to the right, then we will immediately find the place where we should install the indicator and the bridge and look for the second resonance point of the second wave. Finding exactly this place, we get point number 4. By measuring the distance between points No. 3-4 and multiplying it by two, we get the second wave of the generator. We tune the wavemeter exactly to this wave, etc., etc.

A similar method can be used to calibrate the wavemeter, starting from the shortest (fraction of a meter) waves.

If there is an accurate wavemeter, then the calibration of the made wavemeter is carried out as follows: the generator is put into operation and, setting different wavelengths in it, they are measured with a weak connection with an accurate wavemeter, after which, with a weak connection, a calibrated wavemeter is tuned to the generator and thus the waves for a series of capacitor points.

Regardless of how the wavemeter is calibrated, the number of determinations of wavelengths (points on the capacitor) should be done more, for example, 10 (i.e., degrees through 15-20), since otherwise the graph curve may not be drawn quite accurately.

Author: S.I.Shaposhnikov

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