Adjustment and coordination of antenna-feeder devices. Encyclopedia of radio electronics and electrical engineering

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Antenna matching

In the preface to his book "Antennas", Rothhammel in the very first line repeated the well-known truth: a good antenna is the best high-frequency amplifier. However, many radio amateurs sometimes forget that building a good antenna system costs as much as a good transceiver and setting up an antenna-feeder device requires the same serious approach as setting up a transceiver. Having built an antenna according to a description taken from somewhere, radio amateurs most often adjust it using an SWR meter, or generally rely on chance and do not make any measurements. Therefore, in many cases, you can hear negative reviews about good antennas, or that they do not have enough allowed power for everyday communications. Here an attempt is made in a brief form to review simple methods of matching and measurements in AFS (antenna-feeder systems) in the form of a guide to books (hereinafter referred to as references by numbers):

1. K. Rothammel "Antennas", M., "Energy", 1979 third edition
2. Z. Benkovsky, E. Lipinsky, "Amateur antennas of short and ultrashort waves", M., "Radio and communication", 1983

as well as some practical advice. So...

Why is it impossible to take seriously the adjustment of newly created antenna-feeder devices using an SWR meter? The SWR meter shows the ratio (Urect + Uref) to (Urect-Uref) or in other words, how many times the impedance of the antenna-feeder path differs from the wave impedance of the device (transmitter output, for example). According to the readings of the SWR meter, it is impossible to understand what SWR \u3d 50 means with an output stage resistance of 150 ohms. The impedance of the antenna-feeder path in this case can be purely active (at the resonance frequency) and can be equal to 17 ohms or XNUMX ohms (both are equally likely!). Not at the resonance frequency, the resistance will contain active and reactive (capacitive or inductive) in a variety of ratios, and then it is completely incomprehensible what needs to be done - either to compensate for the reactivity, or to coordinate the wave resistance. To accurately match the AFU, you need to know:

• a) the actual resonant frequency of the antenna;
• b) antenna impedance;
• c) wave impedance of the feeder;
• d) output impedance of the transceiver.

The purpose of antenna matching is the task of fulfilling two conditions for connecting the antenna to the transceiver:

1. to achieve the absence of a reactive component in the antenna impedance at the frequency used.
2. to achieve equality of the wave impedance of the antenna and the transceiver equipment.

If these conditions are met at the place where the antenna is fed (the point of connection of the antenna with the feeder), then the feeder operates in the traveling wave mode. If the matching conditions are met at the junction of the feeder with the transceiver, and the antenna impedance differs from the wave impedance of the feeder, then the feeder operates in the standing wave mode. However, the operation of the feeder in the standing wave mode may lead to distortion of the radiation pattern in directional antennas (due to harmful radiation from the feeder) and in some cases may lead to interference with the surrounding transceiver equipment. In addition, if the antenna is used for reception, then unwanted emissions (for example, interference from your desktop computer) will be received on the feeder braid. Therefore, it is preferable to use the antenna feed through the feeder in the traveling wave mode. Before sharing the practical experience of antenna matching, a few words about the main measurement methods.

1. Antenna resonant frequency measurement

1.1. The easiest way to measure the resonant frequency of an antenna is with a heterodyne resonance indicator (HIR). However, in multi-element antenna systems, it can be difficult or completely impossible to perform GIR measurements due to the mutual influence of antenna elements, each of which can have its own resonant frequency.

1.2. Method of measurement using a measuring antenna and a control receiver. A generator is connected to the measured antenna, at a distance of 10-20l from the measured antenna, a control receiver is installed with an antenna that does not have resonances at these frequencies (for example, shorter l/10). The generator is adjusted in the selected section of the range, using the S-meter of the control receiver, the field strength is measured and the dependence of the field strength on frequency is plotted. The maximum corresponds to the resonance frequency. This method is especially applicable to multi-element antennas. In this case, the measuring receiver must be placed in the main lobe of the antenna to be measured. A variant of this method of measurement is the use as a generator, a transmitter with a power of several watts and a simple field strength meter (for example [1], Fig. 14-20.). However, it must be taken into account that during measurements you will interfere with others. A practical tip when measuring in the 144-430 MHz band is to not hold the field strength meter in your hands when measuring, in order to weaken the influence of the body on the readings of the device. Fix the device above the floor at a height of 1-2 meters on a dielectric stand (for example, a tree, a chair) and take readings at a distance of 2-4 meters, without falling into the zone between the device and the measured antenna.

1.3. Measurement using a generator and an antennascope (eg [1], Fig. 14-16). This method is applicable mainly on HF and does not give accurate results, but allows you to simultaneously evaluate the antenna impedance. The essence of the measurements is as follows. As you know, the antenoscope allows you to measure the total resistance (active + reactive). Because antennas are usually powered at the current antinode (minimum input resistance) and there is no reactivity at the resonance frequency, then at the resonant frequency the antennascope will show the minimum resistance, and at all other frequencies it will most often be greater. Hence the sequence of measurements - by rebuilding the generator, they measure the input impedance of the antenna. The minimum resistance corresponds to the resonant frequency. One BUT - the antennascope must be connected directly to the antenna feed point, and not through the cable! And a practical observation - if there is a powerful source of radio emission near you (a TV or a radio station), due to pickups, the antennascope will never balance "to zero" and it becomes almost impossible to make measurements.

1.4. It is very convenient to determine the resonant frequency of vibrators using a frequency response meter. By connecting the output of the frequency response meter and the detector head to the antenna, the frequencies at which dips in the frequency response are visible are determined. At these frequencies, the antenna resonates and energy is taken from the output of the device, which is clearly visible on the screen of the device. Almost any frequency response meters are suitable for measurements (X1-47, X1-50, X1-42, SK4-59). Measurement option - using a spectrum analyzer (SK4-60) in a mode with a long afterglow and an external generator. As an external generator, you can use a harmonic generator: for HF - with a step of 10 kHz, for 144 MHz - with a step of 100 kHz, for 430 MHz - with a step of 1 MHz. At frequencies up to 160 MHz, the most even spectrum with high harmonic intensity is given by the harmonic generator circuit on the 155IE1 integrated circuit. In the range of 430 MHz, a sufficient level of harmonics can be obtained in a circuit with a storage diode 2A609B (50 MHz calibrator circuit from SK4-60).

2. Measurement of resistance in antenna-feeder devices

2.1. The simplest (still affordable) mass-produced device for measuring active resistance and signal phase (and hence the reactive component) is a measuring bridge. There are several modifications of these devices for use with 50 and 75 ohm paths and for various frequency ranges up to 1000 MHz - these are measuring bridges R2-33 ... R2-35.

2.2 In amateur radio practice, a simpler version of the measuring bridge is more often used, designed for impedance measurements (antenoscope). Its design, in contrast to bridges P2-33... is very simple and easily repeated at home ([1], pp. 308-309).

2.3 It is useful to keep in mind some remarks regarding resistances in APS.

2.3.1. Long line with wave impedance Ztr and electrical length l/4, 3 x l/4 etc. transforms the resistance, which can be calculated from the formula

Ztr=Sqr(Zin Zout)

or according to Fig. 2.39 [2]. In a particular case, if one end l/4 segment open, then the infinite resistance at this end of the segment is transformed into zero at the opposite end (short circuit) and such devices are used to transform large resistances into small ones. Attention! These types of transformers work effectively only in a narrow frequency range, limited to fractions of a percent of the operating frequency. Long line with electric length multiple l/2, regardless of the wave impedance of this line, transforms the input impedance into an output impedance with a ratio of 1: 1 and they are used to transfer resistances to the required distance without impedance transformation, or to reverse the phase by 180 °. Unlike l/4 lines, lines l/2 have more bandwidth.

2.3.2. If the antenna is shorter than you need, then at your frequency the antenna impedance has a reactive capacitive component. In the case where the antenna is longer, at your frequency the antenna has an inductive rectivity. Of course, at your frequency, undesirable reactivity can be compensated by introducing additional reactivity of the opposite sign. For example, if the antenna is longer than necessary, the inductive component can be compensated by connecting a capacitance in series with the antenna feed. The value of the required capacitor can be calculated for the desired frequency, knowing the value of the inductive component (see Figure 2.38 [2]), or selected experimentally, as described in paragraph 5.

2.3.3. The introduction of additional passive elements usually lowers the input impedance of the antenna (for example, for a square: from 110-120 ohms to 45-75 ohms).

2.3.4. Below are the theoretical values ​​of the most common vibrators (vibrators are located in a space free from surrounding objects), antennas and feeders:

• half-wave vibrator powered in the current antinode (in the middle) - 70 Ohm, with a detuning of + -2%, the reactance iX changes almost linearly from -25 to +25 with zero at the resonance frequency;
• half-wave vibrator powered by a T-shaped matching circuit -120 Ohm; - a loop vibrator with the same diameters of all conductors - 240..280 Ohm, with a detuning of + -1% there is no reactance, but with detunings of more than 2%, the reactance iX increases sharply to + - 50 or more (see Fig. 2.93 [2] );
• loop vibrator with different conductor diameters (see tab. 1.15 [1] or Fig. 2.90c [1]) - up to 840 Ohm; - double loop vibrator with equal diameters of all conductors - 540...630 Ohm;
• double loop vibrator with different conductor diameters (see tab. 1.16 [1] or Fig. 2.91 [2]) - up to 1500 Ohm;
• quarter-wave vertical vibrator with counterweights at an angle of 135° with respect to the vibrator - 50 Ohm;
• quarter-wave vertical vibrator with counterweights at an angle of 90° with respect to the vibrator - 30 Ohm;
• vibrator in the form of a square l - 110..120 Ohm; - a vibrator in the form of a square with a length of 2l (two turns) - 280 Ohm;
• triangular vibrator (delta) - 120...130 Ohm;
• 90° Inverded-V - 45 Ohm;
• 130° Inverted-V - 65 Ohm;
• wave channel optimized for maximum gain - 5...20 Ohm;
• wave channel optimized for the best matching - 50 Ohm;
• two-wire line (Fig. 2.26 [2]) - 200..320;
• two parallel coaxial lines Z=75 Ohm - 37.5 Ohm;
• the same, quarter-wave transformer Zin=50 Ohm - Zout=28 Ohm;
• the same, quarter-wave transformer Zin=75 Ohm - Zout=19 Ohm;
• two parallel coaxial lines Z=50 Ohm - 25 Ohm;
• the same, quarter-wave transformer Zin=50 Ohm - Zout=12.5 Ohm;
• the same, quarter-wave transformer Zin=75 Ohm - Zout=8.4 Ohm
• transformer of three parallel lines Z=50 Ohm Zin=50 - Zout=5.6 Ohm;
• the same Z=50 Ohm Zin=75 - Zout=3.7 Ohm;

3. Measuring the degree of agreement

It is desirable to make these measurements after the matching described in paragraph 5 to assess the quality of matching.

3.1. Devices for determining the degree of matching of open two-wire lines with an antenna:

3.1.1. Ordinary neon light bulb or GIR. When moving the light bulb along the transmission line, the brightness of the light bulb should not change (traveling wave mode). The measurement option is a device consisting of a communication loop, a detector and a pointer indicator (see Fig. 14.8 [1]).

3.1.2. Two-lamp indicator (see Fig. 14.7 [1]). The setting is made so that the lamp connected to the arm close to the antenna does not glow, and in the opposite arm the glow is maximum. At low power levels, you can use a detector and a dial indicator instead of a light bulb.

3.2. Devices for determining the degree of matching in coaxial paths:

3.2.1. Measuring line - a device that is applicable for measuring the degree of matching in coaxial and waveguide lines from VHF to centimeter wavelengths. Its design is simple - a rigid coaxial cable (waveguide) with a longitudinal slot in the outer conductor, along which the measuring head moves with the measuring probe lowered into the slot. By moving the measuring head along the path, the maxima and minima of the readings are determined, the ratio of which is used to judge the degree of agreement (traveling wave mode - the readings do not change along the entire length of the measuring line).

3.2.2. Measuring bridge (Fig.14.18 [1]). Allows you to measure SWR in transmission lines up to 100 ohms on HF and VHF with an input power of about hundreds of milliwatts. Very easy to manufacture design, does not contain coil catches, structural units that are critical to manufacturing accuracy.

3.2.3. SWR meters based on reflectometers. Many designs of these devices are described (for example, Fig. 14-14 [1]. They allow you to monitor the state of the AFS during operation on the air. 3.2.4. SWR meters based on frequency response meters. Very convenient for studying the quality of matching at any frequency, up to up to 40 GHz Measuring principle - the measuring set of instruments consists of a frequency response meter and a directional coupler, connected in the following circuit:

1 X1-47

>--------------------->3 2 <--------------------<1

3 For example rep

2><-------------------\|/ Ant. 4

where 1 - frequency response meter (X1-47); 2 - low-resistance detector head from the X1-47 kit; 3 - directional coupler, for example, NO 144-991 from the kit for the SK03-4 device is suitable for the 60 MHz band; 4 - measured antenna. The high-frequency signal from the X1-47 output goes to pin 3 of the directional coupler and then only goes to pin 2 of the directional coupler. The signal is then transmitted to the measured antenna. At frequencies where the antenna has a high SWR, energy is reflected and returned to pin 2 of the directional coupler. In this signal direction, energy is transmitted from pin 2 to pin 1 only, detected by the detector head, and the reflected signal level is displayed on the X1-47 screen depending on the frequency.

Before starting measurements, you need to calibrate the circuit. To do this, instead of the measured antenna, a non-inductive equivalent of an antenna with a resistance of 50 Ohms is connected and make sure that there is no reflected signal (SWR = 1). Further, having undocked the equivalent, the signal level for SWR = infinity is noted. All intermediate SWR values ​​will be displayed on the device screen with a position between 0 and the maximum value. By connecting antenna equivalents with a resistance of 75 ohms, 100 ohms, 150 ohms, the SWR values ​​\u1.5b\u2bare marked on the screen of the device, respectively, 3, XNUMX, XNUMX.

As a frequency response meter, you can use the SK4-60 spectrum analyzer and an external generator, depending on the wavelength range in which measurements are made (G4-151 up to 500 MHz, G4-76 up to 1.3 GHz, G4-82 5.6 GHz, G4-84 10 GHz). At frequencies up to 500 MHz, the harmonic generators described in Section 1.4 can be used as an external generator.

Two points:

• directional couplers introduce a crosstalk attenuation of about 15 dB to the signal source, so fairly high-level signal sources are needed for measurements;
• The directional properties of the couplers (decoupling and directivity) usually do not exceed 20...30dB, so measurements must be performed not in a logarithmic, but in a linear display scale.

4. Some useful measurement methods

4.1. Measurements with an antenoscope (given in [1] pp. 308-312).

4.1.1. Determining the exact electrical length l/4 lines:

to do this, the line is connected at one end to the antennascope, and the other is left open. Further, by changing the frequency of the generator, the lowest frequency is determined, at which the balance of the bridge is achieved at zero resistance. For this frequency, the electrical length of the line is exactly l/ 4.

4.1.2. Measurement of line impedance Ztr:

after completing the measurements according to clause 4.1.1., connect a 100 Ohm resistor to the free end of the line and measure the resistance Zmeas at the other end of the line with an antennascope. The wave impedance of the line is calculated using the formula

Ztr=Sqr(100хZmeas)

4.1.3. Checking Dimensional Accuracy l/2 transform lines:

• the measured line is connected to the antennascope, a 300 Ohm resistor is connected to the second end of the line
• The generator sets the frequency at which the line l/2 should transform 1:1.
• measure the resistance with an antennascope - it should be equal to 300 ohms if the line is exactly equal l/2 for this frequency.

4.1.4. Determination of the coefficient of shortening of the transmission line:

for measurements, a line segment with a length of several meters (length X) is used.

• They close one end of the line and by changing the frequency of the generator, find the minimum value of the frequency F at which the antennascope is balanced - this will mean that the line transforms the resistance 1: 1 and for this frequency its electrical length corresponds to l/2, taking into account the shortening factor.
• By further increasing the frequency, it will be possible to find the next balance of the bridge, corresponding to 2 l/2 etc. Length l/2 for the frequency L=300/(2F), and the shortening factor K=X/L.

For example, if the line length X=3.3 meters, and the balance occurred at the frequency F=30 MHz, then L=5 meters, and K=0.66. The usual values ​​of the coefficients of shortening for coaxial lines are 0.66, for ribbon cables - 0.82, for open two-wire lines - 0.95.

4.2. Measurements with a frequency response meter are carried out according to the scheme given in clause 3.2.4. 4.2.1. Localization of inhomogeneities in the feeder. If it is necessary to determine the distance to the inhomogeneity in the feeder (short circuit or open circuit) without dismantling the feeder, this can be done as follows. In the event of a break or short circuit in the feeder, the maximum SWR will be observed at frequencies where the line acts as a transformer l/2, as well as at multiple frequencies, regardless of the range selected for measurements. The feeder is undocked from the transceiver and connected to terminal 2 of the directional coupler. The swing band is set so that it is convenient to measure the SWR period. The measured period in megahertz corresponds to the frequency at which the line operates as l/2 segment, taking into account the shortening. Suppose the frequency interval between the SWR maxima is 3 MHz, which means that the frequency at which the line is now operating as a transformer l/2 is equal to 6 MHz and this corresponds to a wavelength of 50 meters (i.e. up to an inhomogeneity of 50 meters without taking into account the line shortening factor). Knowing the coefficient of shortening of the line, one can accurately say the actual distance to the inhomogeneity. For example, if the line is made with a coaxial cable with a coefficient. shortening 0.66, then in our case the distance from the transmitter to the break (short circuit) in the coaxial cable is 33 meters.

4.2.2. Measurement of the cable shortening factor.

Measurements are made in the same way as in paragraph 4.2.1., but a measured cable several meters long is connected to terminal 2 of the directional coupler. Suppose we measure the coefficient of shortening of a cable 33 meters long. The measured electrical length of the cable is 50 meters, so the shortening factor is 33/50=0.66.

4.2.3. Checking the 50 ohm cable for inhomogeneities.

The tested cable is connected to output 2 NO, at the other end of which a matched load of 50 Ohm is connected. A straight line should be observed on the screen of the device if there are no inhomogeneities in the cable.

5. Antenna tuning procedure

As an example, a few words about how to tune a delta antenna for the 80 meters band, using the measurement methods given above. It is necessary to match the output stage of the transmitter (50 ohms) with the antenna via a 50 ohm cable. If it is not possible to measure the antenna resistance and find the resonant frequency of the antenna by connecting directly at the power point, we connect the transforming line l/2 between instruments and antenna. Thus, using the transforming properties of the line (1:1), it is possible to carry out measurements not directly at the antenna, but at the other end of the line.

One of the described methods, we measure the antenna resistance and resonant frequency. If the resonant frequency of the antenna is slightly shifted, by changing the geometric dimensions of the antenna, resonance is achieved at the desired frequency. Typically, the delta antenna impedance is 120 ohms, and a 1:2.4 transformer must be used to match the antenna with the cable. This transformer can be made using a three-wire ShPTL with a ratio of Rout / Rin \u4d 9/120 (Bunin, Yaylenko "Handbook of a shortwave radio amateur" Kyiv, Technique). After the manufacture of the transformer, a resistor with a resistance of 130-14 Ohms is connected to the high-resistance input of the transformer and, by connecting an antenoscope to another input of the transformer, its input resistance and transformation ratio are measured. Connecting a transformer between PA and the power line, check the current in the antenna using an RF ammeter (Fig. 2-1 [10]). It is better to measure the current after PA with a calibrated RF ammeter and calculate the power absorbed. If after the calculation it turns out that P=RII is less than on the antenna equivalent, then the matching device introduces reactivity and it must be compensated. To do this, a variable capacitor (500-XNUMX pF) is switched on in series with the RF ammeter and by changing its value, a maximum is achieved in the readings of the RF ammeter. If it is not possible to increase the current in the antenna with the help of a capacitor, it is necessary to replace the capacitor with a variometer and select a compensating inductance. After selecting the compensating reactivity, its value is measured and replaced by an element with a constant value.

After setting up the matching device, it is placed in a sealed case and transferred to the antenna feed point from the cable. In conclusion, the agreement is checked again using one of the SWR measurement methods.

Computer Connection Tips

Many complain that their desktop computer interferes greatly with reception. The reason for this in most cases is poor antenna matching. In this case, the braid of the antenna power cable receives computer radiation and they enter the receiver input in the form of interference. It is easy to check this assumption - disconnect the cable from the receiver input, if the interference disappears, then the main way interference from the computer enters the receiver input is through the cable braid. After carefully matching the antenna using the methods below, you can largely get rid of interference on reception and unstable operation of digital nodes during transmission. The second necessary condition for the convenience of working with a computer is careful grounding of all devices. Grounding on the heating pipe - not good! The third way is to enclose all the cables coming from the computer into a screen and it is very desirable to pass each of them through a 2000 NM ferrite ring (a couple of turns). You can also pass the antenna cable through the ring (for additional cable balancing and eliminating the propagation of RF signals along the cable sheath). Sometimes the source of interference is the monitor and the cables going to it. Try turning the monitor on and off from the network while the computer is running and booted. If the noise level changes, it is recommended to ground the monitor chassis separately, and the chassis ground point must be selected experimentally to minimize interference.

Author: Alexander Doshchich, UY0LL, uy0ll@buscom.kharkov.ua; Publication: cxem.net

See other articles Section HF antennas.

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