All about standing wave ratio. Standing wave ratio Standards kbv ksv digital broadcasting radio broadcasting

Power entering the load

Return Loss
R.L.

Frequency range, MHz

Measured Characteristics

• electrical length (in feet or degrees);
• losses in feeder lines (dB);
• capacitance (pF);
• impedance or Z value (ohms);
• impedance phase angle (in degrees);
• inductance (µH);
• reactance or X (ohm);
• active resistance or R (ohm);
• resonant frequency (MHz);
• return loss (dB);
• signal frequency (MHz);
• SWR (Zo programmable).

200x100x65 mm

Range M1

M2 range

frequency range

1.8 - 200 MHz

140 - 525 MHz

Power measurement area

0 - 3KW (HF), 0 - 1KW (VHF)

Power measurement range

Power measurement error

±10% (full scale)

SWR measurement area

from 1 to infinity

Resistance

Residual SWR

1.2 or less

Insertion loss

0.2 dB or less

Minimum power for SWR measurements

Approximately 6W.

M-shaped

Power supply for backlights

11 - 15V DC, approximately 450 mA

Dimensions (data in brackets including protrusions)

250(W) x 93 (98) (H) x 110 (135) (D)

Around 1540

A device for measuring the quality of the match between the feeder and the antenna (SWR meter) is an indispensable component of an amateur radio station. How reliable information about the state of the antenna system does such a device provide? Practice shows that not all factory-made SWR meters provide high measurement accuracy. This is even more true when it comes to homemade structures. The article presented to our readers discusses an SWR meter with a current transformer. Devices of this type are widely used by both professionals and radio amateurs. The article gives the theory of its operation and analyzes the factors influencing the accuracy of measurements. It concludes with a description of two simple practical designs of SWR meters, the characteristics of which will satisfy the most demanding radio amateur.

A little theory

If a homogeneous connecting line (feeder) with characteristic impedance Zо connected to the transmitter is loaded with resistance Zн≠Zо, then both incident and reflected waves appear in it. Reflection coefficient r (reflection) is generally defined as the ratio of the amplitude of the wave reflected from the load to the amplitude of the incident wave. The reflection coefficients for current r and voltage ru are equal to the ratio of the corresponding values ​​in the reflected and incident waves. The phase of the reflected current (relative to the incident one) depends on the relationship between Zн and Zо. If Zн>Zо, then the reflected current will be antiphase to the incident one, and if Zн

The value of the reflection coefficient r is determined by the formula

where Rn and Xn are, respectively, the active and reactive components of the load resistance. With a purely active load Xn = 0, the formula simplifies to r=(Rn-Zo)/(Rn+Zo). For example, if a cable with a characteristic impedance of 50 Ohms is loaded with a resistor of 75 Ohms, then the reflection coefficient will be r = (75-50)/(75+50) = 0.2.

In Fig. Figure 1a shows the distribution of voltage Ul and current Il along the line precisely for this case (losses in the line are not taken into account). The scale along the ordinate axis for the current is assumed to be Zо times larger - in this case, both graphs will have the same vertical size. The dotted line is a graph of voltage Ulo and current Ilo in the case when Rн=Zо. For example, a section of a line of length λ is taken. If it is longer, the pattern will repeat itself cyclically every 0.5λ. At those points of the line where the phases of the incident and reflected coincide, the voltage is maximum and equal to Uл max -= Uо(1 + r) = Uо(1 + 0.2) = 1.2 Uо, and at those where the phases are opposite, it is minimal and is equal to Ul min = Ul(1 - 0.2) = = 0.8Ul. By definition, SWR = Ul max/ /Ul min=1l2Ulo/0I8Ulo=1I5.

Formulas for calculating SWR and r can also be written as follows: SWR = (1+r)/(1-r) and r = = (SWR-1)/(SWR+1). Let us note an important point - the sum of the maximum and minimum voltages Uл max + Uл min = Uло(1 + r) + Уло(1 - r) = 2Uno, and their difference Ul max - Ul min = 2Uлo. From the obtained values, it is possible to calculate the power of the incident wave Ppad = Uо2/Zo and the power of the reflected wave Pоtr = = (rUо)2/Zo. In our case (for SWR = 1.5 and r = 0.2), the power of the reflected wave will be only 4% of the power of the incident one.

Determining SWR by measuring the voltage distribution along a section of a line in search of the values ​​of Ul max and Ul min has been widely used in the past

not only on open overhead lines, but also in coaxial feeders (mainly on VHF). For this purpose, a measuring section of the feeder was used, which had a long longitudinal slot, along which a cart moved with a probe inserted into it - the head of an HF voltmeter.

SWR can be determined by measuring the current Il in one of the line wires over a section less than 0.5λ long. Having determined the maximum and minimum values, calculate SWR = Imax/Imin. To measure current, a current-voltage converter is used in the form of a current transformer (TT) with a load resistor, the voltage across which is proportional and in-phase to the measured current. Let us note an interesting fact - with certain TT parameters, at its output it is possible to obtain a voltage equal to the voltage on the line (between conductors), i.e. Utl = IlZo.

In Fig. Figure 1b shows together a graph of the change in Ul along the line and a graph of the change in Utl. The graphs have the same amplitude and shape, but are shifted relative to each other by 0.25X. Analysis of these curves shows that it is possible to determine r (or SWR) by simultaneously measuring the values ​​of Ul and UTL at any point in the line. At the locations of the maxima and minima of both curves (points 1 and 2), this is obvious: the ratio of these values ​​Ul/Utl (or Utl/Utl) is equal to the SWR, the sum is equal to 2Ulo, and the difference is 2rUlo. At intermediate points, Ul and Utl are shifted in phase, and they need to be added as vectors, however, the above relationships are preserved, since the reflected voltage wave is always inverse in phase to the reflected current wave, and rUlo = rUtl.

Consequently, a device containing a voltmeter, a calibrated current-voltage converter and an addition-subtraction circuit will allow you to determine such line parameters as r or SWR, as well as Rpad and Rotr when it is turned on anywhere in the line.

The first information about devices of this kind dates back to 1943 and is reproduced in. The first practical devices known to the author were described in. The version of the circuit taken as a basis is shown in Fig. 2. The device contained:

• voltage sensor - capacitive divider on C1 and C2 with an output voltage Uc, significantly less than the voltage on the line Ul. The ratio p = Uc/Uл is called the coupling coefficient;
• current transformer T1, wound on a carbonyl ring magnetic core. Its primary winding had one turn in the form of a conductor passing through the center of the ring, the secondary winding had n turns, the load on the secondary winding was resistor R1, the output voltage was 2Ut. The secondary winding can be made of two separate windings with voltage Ut each and with its own load resistor, however, it is structurally more convenient to make one winding with a tap from the middle;
• detectors on diodes VD1 and VD2, switch SA1 and voltmeter on microammeter PA1 with additional resistors.

The secondary winding of transformer T1 is connected in such a way that when the transmitter is connected to the connector on the left in the diagram, and the load to the right, the total voltage Uc + UT is supplied to diode VD1, and the difference voltage is supplied to diode VD2. When a resistive reference load with a resistance equal to the characteristic impedance of the line is connected to the output of the SWR meter, there is no reflected wave and, therefore, the RF voltage at VD2 can be zero. This is achieved in the process of balancing the device by equalizing the voltages UT and Uc using a tuning capacitor C1. As was shown above, after such a setting, the magnitude of the difference voltage (at Zн≠Zо) will be proportional to the reflection coefficient r. Measurements with a real load are carried out like this. First, in the position of the switch SA1 ("Incident wave") shown in the diagram, the calibration variable resistor R3 is used to set the instrument arrow to the last scale division (for example, 100 μA). Then switch SA1 is moved to the lower position according to the diagram (“Reflected wave”) and the value r is counted. In the case of RH = 75 Ohm, the device should show 20 μA, which corresponds to r = 0.2. The value of SWR is determined by the above formula - SWR = (1 +0.2)/ /(1-0.2) = 1.5 or SWR = (100+20)/ /(100-20) = 1.5. In this example, the detector is assumed to be linear - in reality it is necessary to introduce a correction to take into account its nonlinearity. With proper calibration, the device can be used to measure incident and reflected powers.

The accuracy of the SWR meter as a measuring device depends on a number of factors, primarily on the accuracy of balancing the device in position SA1 “Reflected wave” at Rн = Zo. Ideal balancing corresponds to voltages Uс and Uт, equal in magnitude and strictly opposite in phase, i.e. their difference (algebraic sum) is zero. In a real design, there is always an unbalanced remainder Ures. Let's look at an example of how this affects the final measurement result. Let us assume that during balancing the resulting voltages are Uс = 0.5 V and Uт = 0.45 V (i.e., an imbalance of 0.05 V, which is quite realistic). With a load Rн = 75 Ohm in a 50-Ohm line, we actually have SWR = 75/50 = 1.5 and r = 0.2, and the magnitude of the reflected wave, recalculated to intra-device levels, will be rUc = 0.2x0.5 = 0, 1 V and rUт = 0.2x0.45 = 0.09 V.

Let's look again at Fig. 1, b, the curves on which are shown for SWR = 1.5 (the curves Ul and Utl for the line will in our case correspond to Uс and Ut). At point 1 Uc max = 0.5 + 0.1 = 0.6 V, Ut min = 0.45 - 0.09 = 0.36 V and SWR = 0.6/0.36 = 1.67. At point 2UTmax = 0.45 + 0.09 = 0.54 V, Ucmin = 0.5 - 0.1 = 0.4 and SWR = 0.54/0.4 = 1.35. From this simple calculation it is clear that depending on where such an SWR meter is connected to a line with a real SWR = 1.5 or when the length of the line between the device and the load changes, different SWR values ​​can be read - from 1.35 to 1.67!

What can lead to inaccurate balancing?

1. The presence of a cutoff voltage of a germanium diode (in our case VD2), at which it stops conducting, is approximately 0.05 V. Therefore, with UOCT< 0,05 В прибор РА1 покажет "ноль" и можно допустить ошибку в балансировке. Относительная неточность значительно уменьшится, если поднять в несколько раз напряжения Uc и соответственно UT. Например, при Uc = 2 В и UT = 1,95 В (Uост = 0,05 В) пределы изменения КСВ для приведенного выше примера будут уже только от 1,46 до 1,54.

2. Presence of frequency dependence of voltages Uc or UT. However, accurate balancing may not be achieved over the entire operating frequency range. Let's look at an example of one of the possible reasons. Let's say the device uses a divider capacitor C2 with a capacity of 150 pF with wire leads with a diameter of 0.5 mm and a length of 10 mm each. The measured inductance of a wire of this diameter with a length of 20 mm turned out to be equal to L = 0.03 μH. At the upper operating frequency f = 30 MHz, the capacitor resistance will be Xc = 1 /2πfС = -j35.4 Ohm, the total reactance of the terminals XL = 22πfL = j5.7 Ohm. As a result, the resistance of the lower arm of the divider will decrease to the value -j35.4 + j5f7 = -j29.7 Ohm (this corresponds to a capacitor with a capacity of 177 pF). At the same time, at frequencies from 7 MHz and below, the influence of the pins is negligible. Hence the conclusion - in the lower arm of the divider, non-inductive capacitors with minimal leads (for example, support or feed-through) should be used and several capacitors should be connected in parallel. The terminals of the “upper” capacitor C1 have virtually no effect on the situation, since the Xc of the upper capacitor is several tens of times greater than that of the lower one. You can achieve uniform balancing over the entire operating frequency band using an original solution, which will be discussed when describing practical designs.

3.2. The inductive reactance of the secondary winding T1 at lower frequencies of the operating range (~ 1.8 MHz) can significantly shunt R1, which will lead to a decrease in UT and its phase shift.

3.3. Resistance R2 is part of the detector circuit. Since according to the circuit it shunts C2, at lower frequencies the division coefficient can become frequency and phase dependent.

3.4. In the diagram of Fig. 2 detectors on VD1 or VD2 in the open state bypass the lower arm of the capacitive divider to C2 with their input resistance RBX, i.e. RBX acts in the same way as R2. The influence of RBX is insignificant at (R3 + R2) more than 40 kOhm, which requires the use of a sensitive indicator PA1 with a total deviation current of no more than 100 μA and an RF voltage at VD1 of at least 4 V.

3.5. The input and output connectors of the SWR meter are usually separated by 30...100 mm. At a frequency of 30 MHz, the voltage phase difference on the connectors will be α= [(0.03... 0.1)/10]360°- 1... 3.5°. How this can affect work is demonstrated in Fig. 3a and fig. 3, b. The only difference in the circuits in these figures is that capacitor C1 is connected to different connectors (T1 in both cases is located in the middle of the conductor between the connectors).

In the first case, the uncompensated remainder can be reduced if the phase UOCT is adjusted using a small parallel-connected capacitor Ck, and in the second case, by connecting in series with R1 a small inductance Lk in the form of a wire loop. This method is often used in both homemade and “branded” SWR meters, but this should not be done. To verify this, just turn the device so that the input connector becomes the output connector. In this case, the compensation that helped before the turn will become harmful - Uoct will increase significantly. When working on a real line with an unmatched load, depending on the length of the line, the device can get to a place on the line where the introduced correction will “improve” the real SWR or, conversely, “worse” it. In any case, the count will be incorrect. The recommendation is to place the connectors as close to each other as possible and use the original circuit design given below.

To illustrate how much the reasons discussed above can affect the reliability of the SWR meter readings, Fig. Figure 4 shows the results of testing two factory-made devices. The test consisted of installing an unmatched load with a calculated SWR = 2.25 at the end of a line consisting of a number of series-connected cable sections with Zо = 50 Ohms, each λ/8 long.

During the measurements, the total line length varied from λ/8 to 5/8λ. Two devices were tested: the inexpensive BRAND X (curve 2) and one of the best models - BIRD 43 (curve 3). Curve 1 shows true SWR. As they say, comments are unnecessary.

In Fig. Figure 5 shows a graph of the dependence of the measurement error on the value of the directivity coefficient D (directivity) of the SWR meter. Similar graphs for KBV = 1/SWR are given in. In relation to the design of Fig. 2, this coefficient is equal to the ratio of the HF voltages on the diodes VD1 and VD2 when connected to the output of the load SWR meter Rн = Zо D = 20lg(2Uо/Uore). Thus, the better the circuit was balanced (the lower Ures), the higher D. You can also use the readings of the PA1 indicator - D = 20 x x log(Ipad/Iref). however, this D value will be less accurate due to the non-linearity of the diodes.

On the graph, the horizontal axis shows the actual SWR values, and the vertical axis shows the measured ones, taking into account the error depending on the D value of the SWR meter. The dotted line shows an example - real SWR = 2, a device with D = 20 dB will give readings of 1.5 or 2.5, and with D = 40 dB - 1.9 or 2.1, respectively.

As follows from the literature data, the SWR meter according to the diagram in Fig. 2 has D - 20 dB. This means that without significant correction it cannot be used for accurate measurements.

The second most important reason for incorrect SWR meter readings is related to the nonlinearity of the current-voltage characteristic of the detector diodes. This leads to a dependence of the readings on the level of supplied power, especially in the initial part of the PA1 indicator scale. In branded SWR meters, the indicator often has two scales - for low and high power levels.

Current transformer T1 is an important part of the SWR meter. Its main characteristics are the same as those of a more conventional voltage transformer: the number of turns of the primary winding n1 and the secondary winding n2, transformation ratio k = n2/n1, secondary winding current I2 = l1/k. The difference is that the current through the primary winding is determined by the external circuit (in our case, it is the current in the feeder) and does not depend on the load resistance of the secondary winding R1, therefore the current l2 also does not depend on the resistance value of the resistor R1. For example, if power P = 100 W is transmitted through a feeder Zo = 50 Ohm, current I1 = √P/Zo = 1.41 A and at k = 20 the secondary winding current will be l2 = I1/k - 0.07 A. Voltage at the terminals of the secondary winding will be determined by the value of R1: 2UT = l2 x R1 and at R1 = 68 Ohms it will be 2UT = 4.8 V. The power released at the resistor P = (2UT)2/R1 = 0.34 W. Let us pay attention to a feature of the current transformer - the fewer turns in the secondary winding, the greater the voltage at its terminals will be (at the same R1). The most difficult mode for a current transformer is the idle mode (R1 = ∞), while the voltage at its output increases sharply, the magnetic circuit becomes saturated and heats up so much that it can collapse.

In most cases, a single turn is used in the primary winding. This coil can have different shapes, as shown in Fig. 6,a and fig. 6,b (they are equivalent), but the winding according to Fig. 6,c is already two turns.

A separate issue is the use of a screen connected to the body in the form of a tube between the central wire and the secondary winding. On the one hand, the screen eliminates capacitive coupling between the windings, which somewhat improves the balancing of the difference signal; on the other hand, eddy currents arise in the screen, which also affect balancing. Practice has shown that with and without a screen you can get approximately the same results. If the screen is still used, its length should be made minimal, approximately equal to the width of the magnetic core used, and connected to the body with a wide short conductor. The screen should be “grounded” to the center line, equidistant from both connectors. For the screen, you can use a brass tube with a diameter of 4 mm from telescopic antennas.

For SWR meters with transmitted power up to 1 kW, ferrite ring magnetic cores with dimensions K12x6x4 and even K10x6x3 are suitable. Practice has shown that the optimal number of turns n2 = 20. With an inductance of the secondary winding of 40...60 μH, the greatest frequency uniformity is obtained (the permissible value is up to 200 μH). It is possible to use magnetic cores with a permeability from 200 to 1000, and it is advisable to choose a standard size that will ensure optimal winding inductance.

You can use magnetic cores with lower permeability if you use larger sizes, increase the number of turns and/or reduce the resistance R1. If the permeability of existing magnetic circuits is unknown, if you have an inductance meter, it can be determined. To do this, you should wind ten turns on an unknown magnetic core (a turn is considered to be each intersection of the wire with the internal hole of the core), measure the inductance of the coil L (μH) and substitute this value into the formula μ = 2.5 LDav/S, where Dav is the average diameter of the magnetic core in cm ; S is the cross-section of the core in cm 2 (example - for K10x6x3 Dcp = 0.8 cm and S = 0.2x0.3 = 0.06 cm 2).

If μ of the magnetic circuit is known, the inductance of a winding of n turns can be calculated: L = μn 2 S/250Dcp.

The applicability of magnetic cores for a power level of 1 kW or more can also be checked at 100 W in the feeder. To do this, you should temporarily install a resistor R1 with a value 4 times larger; accordingly, the voltage Ut will also increase 4 times, and this is equivalent to an increase in the passing power by 16 times. The heating of the magnetic circuit can be checked by touch (the power on the temporary resistor R1 will also increase 4 times). In real conditions, the power on resistor R1 increases in proportion to the increase in power in the feeder.

SWR meters UT1MA

The two designs of the UT1MA SWR meter, which will be discussed below, have almost the same design, but different designs. In the first version (KMA - 01) the high-frequency sensor and the indicator part are separate. The sensor has input and output coaxial connectors and can be installed anywhere in the feeder path. It is connected to the indicator with a three-wire cable of any length. In the second option (KMA - 02) both units are located in one housing.

The SWR meter diagram is shown in Fig. 7 and it differs from the basic diagram in Fig. 2 by the presence of three correction circuits.

Let's look at these differences.

1. The upper arm of the capacitive divider C1 is made of two identical permanent capacitors C1 = C1 "+ C1", connected to the input and output connectors, respectively. As noted in the first part of the article, the phases of the voltages at these connectors are slightly different, and with this connection, the Uc phase is averaged and approaches the UT phase. This improves the balancing of the device.
2. Due to the introduction of coil L1, the resistance of the upper arm of the capacitive divider becomes frequency-dependent, which makes it possible to level the balancing at the upper edge of the operating range (21...30 MHz).
3. By selecting resistor R2 (i.e., the time constant of the R2C2 chain), it is possible to compensate for the imbalance caused by the voltage drop UT and its phase shift at the lower edge of the range (1.8...3.5 MHz).

In addition, balancing is carried out by a tuning capacitor connected to the lower arm of the divider. This simplifies installation and allows the use of a low-power, small-sized tuning capacitor.

The design provides the ability to measure the power of incident and reflected waves. To do this, using switch SA2, instead of the variable calibration resistor R4, a trimming resistor R5 is introduced into the indicator circuit, which sets the desired limit for the measured power.

The use of optimal correction and rational design of the device made it possible to obtain a directivity coefficient D within the range of 35...45 dB in the frequency band 1.8...30 MHz.

The following details are used in SWR meters.

The secondary winding of transformer T1 contains 2 x 10 turns (winding in 2 wires) with 0.35 PEV wire, placed evenly on a K12 x 6 x 4 ferrite ring with a permeability of about 400 (measured inductance ~ 90 μH).

Resistor R1 - 68 Ohm MLT, preferably without a screw groove on the resistor body. With a passing power of less than 250 W, it is enough to install a resistor with a dissipation power of 1 W, with a power of 500 W - 2 W. With a power of 1 kW, resistor R1 can be composed of two parallel-connected resistors with a resistance of 130 Ohms and a power of 2 W each. However, if the KS V-meter is designed for a high power level, it makes sense to double the number of turns of the secondary winding T1 (up to 2 x 20 turns). This will reduce the required power dissipation of resistor R1 by 4 times (in this case, capacitor C2 should have twice the capacity).

The capacitance of each of the capacitors C G and C1 "can be in the range of 2.4...3 pF (KT, KTK, KD for an operating voltage of 500 V at P ≥ 1 kW and 200...250 V at lower power). Capacitors C2 - for any voltage (KTK or other non-inductive, one or 2 - 3 in parallel), capacitor C3 - small-sized trimmer with capacitance change limits of 3...20 pF (KPK - M, KT - 4). The required capacitance of capacitor C2 depends on the total value of the capacitance of the upper arm of the capacitive divider, which includes, in addition to capacitors C "+ C1", also capacitance C0 ~ 1 pF between the secondary winding of transformer T1 and the central conductor. The total capacitance of the lower arm - C2 plus C3 at R1 = 68 Ohm should be approximately 30 times more than the capacitance of the upper one.Diodes VD1 and VD2 - D311, capacitors C4, C5 and C6 - with a capacity of 0.0033... 0.01 µF (KM or other high-frequency), indicator RA1 - M2003 with a total deviation current of 100 µA, variable resistor R4 - 150 kOhm SP - 4 - 2m, trimming resistor R4 - 150 kOhm Resistor R3 has a resistance of 10 kOhm - it protects the indicator from possible overload.

The value of correction inductance L1 can be determined as follows. When balancing the device (without L1), you need to mark the positions of the rotor of the tuning capacitor C3 at frequencies of 14 and 29 MHz, then unsolder it and measure the capacitance in both marked positions. Let's say that for the upper frequency the capacitance turns out to be 5 pF less, and the total capacitance of the lower arm of the divider is about 130 pF, i.e. the difference is 5/130 or about 4%. Therefore, for frequency equalization, it is necessary to reduce the resistance of the upper arm by ~ 4% at a frequency of 29 MHz. For example, with C1 + C0 = 5 pF, the capacitive resistance Xc = 1/2πfС - j1100 Ohm, respectively, Xc - j44 Ohm and L1 = XL1 / 2πf = 0.24 μH.

In the original devices, the L1 coil had 8...9 turns with PELSHO 0.29 wire. The internal diameter of the coil is 5 mm, the winding is tight, followed by impregnation with BF-2 glue. The final number of turns is determined after it is installed in place. Initially, balancing is carried out at a frequency of 14 MHz, then the frequency is set to 29 MHz and the number of turns of coil L1 is selected such that the circuit is balanced at both frequencies with the same position of trimmer C3.

After achieving good balancing at mid and high frequencies, set the frequency to 1.8 MHz, temporarily solder a variable resistor with a resistance of 15...20 kOhm in place of resistor R2 and find the value at which UOCT is minimal. The resistance value of resistor R2 depends on the inductance of the secondary winding T1 and lies in the range of 5...20 kOhm for its inductance 40...200 μH (higher resistance values ​​for higher inductance).

In amateur radio conditions, most often a microammeter with a linear scale is used in the SWR meter indicator and the reading is carried out according to the formula SWR = (Ipad + Iref) / (Ipad -Iref), where I in microamperes is the indicator readings in the “incident” and “reflected” modes respectively. In this case, the error due to the nonlinearity of the initial section of the diodes’ current-voltage characteristics is not taken into account. Testing with loads of different sizes at a frequency of 7 MHz showed that at a power of about 100 W the indicator readings were on average one division (1 µA) less than the real values, at 25 W - 2.5...3 µA less, and at 10 W - by 4 µA. Hence a simple recommendation: for the 100-watt option, move the initial (zero) position of the instrument needle one division up in advance, and when using 10 W (for example, when setting up an antenna), add another 4 µA to the reading on the scale in the “reflected” position. Example - “incident/reflected” readings are respectively 100/16 µA, and the correct SWR will be (100 + 20) / (100 - 20) = 1.5. With significant power - 500 W or more - this correction is not necessary.

It should be noted that all types of amateur SWR meters (current transformer, bridge, directional couplers) give values ​​of the reflection coefficient r, and the value of the SWR then has to be calculated. Meanwhile, it is r that is the main indicator of the degree of coordination, and SWR is a derivative indicator. This can be confirmed by the fact that in telecommunications the degree of agreement is characterized by the attenuation of inconsistency (the same r, only in decibels). Expensive branded devices also provide a reading called return loss.

What happens if silicon diodes are used as detectors? If a germanium diode at room temperature has a cutoff voltage, at which the current through the diode is only 0.2...0.3 μA, is about 0.045 V, then a silicon diode is already 0.3 V. Therefore, in order to maintain the accuracy of the reading when switching to silicon diodes, it is necessary to increase the voltage levels Uc and UT (!) by more than 6 times. In the experiment, when replacing diodes D311 with KD522 at P = 100 W, load Zn = 75 Ohm and the same Uc and UT, the following figures were obtained: before replacement - 100/19 and SWR = 1.48, after replacement - 100/12 and calculated SWR=1.27. The use of a doubling circuit using KD522 diodes gave an even worse result - 100/11 and a calculated SWR = 1.25.

The sensor housing in a separate version can be made of copper, aluminum or soldered from plates of double-sided foil fiberglass with a thickness of 1.5...2 mm. A sketch of such a design is shown in Fig. 8, a.

The housing consists of two compartments, in one opposite each other there are RF connectors (CP - 50 or SO - 239 with flanges measuring 25x25 mm), a jumper made of wire with a diameter of 1.4 mm in polyethylene insulation with a diameter of 4.8 mm (from cable RK50 - 4), current transformer T1, capacitors of the capacitive divider and compensation coil L1, in the other - resistors R1, R2, diodes, tuning and blocking capacitors and a small-sized low-frequency connector. T1 pins of minimum length. The connection point of capacitors C1" and C1" with coil L1 "hangs in the air", and the connection point of capacitors C4 and C5 of the middle terminal of the XZ connector is connected to the body of the device.

Partitions 2, 3 and 5 have the same dimensions. There are no holes in partition 2, but in partition 5 a hole is made for a specific low-frequency connector through which the indicator unit will be connected. In the middle jumper 3 (Fig. 8, b), foil is selected around three holes on both sides, and three feed-through conductors are installed in the holes (for example, brass screws M2 and MZ). Sketches of sidewalls 1 and 4 are shown in Fig. 8, c. The dotted lines show the connection points before soldering, which is done on both sides for greater strength and to ensure electrical contact.

To set up and check the SWR meter, you need a standard load resistor of 50 Ohms (equivalent to an antenna) with a power of 50...100 W. One of the possible amateur radio designs is shown in Fig. 11. It uses a common TVO resistor with a resistance of 51 Ohms and a dissipation power of 60 W (rectangle dimensions 45 x 25 x 180 mm).

Inside the ceramic resistor body is a long cylindrical channel filled with a resistive substance. The resistor should be pressed tightly against the bottom of the aluminum casing. This improves heat dissipation and creates distributed capacitance for improved wide-bandwidth performance. Using additional resistors with a dissipation power of 2 W, the input load resistance is set within the range of 49.9...50.1 Ohms. With a small correction capacitor at the input (~ 10 pF), using this resistor it is possible to obtain a load with an SWR of no worse than 1.05 in a frequency band of up to 30 MHz. Excellent loads are obtained from special small-sized resistors of type P1 - 3 with a nominal value of 49.9 Ohms, which can withstand significant power when using an external radiator.

Comparative tests of SWR meters from different companies and devices described in this article were carried out. The test consisted of connecting an unmatched 75 Ohm load (equivalent to a factory-made 100 W antenna) to a transmitter with an output power of about 100 W through the test 50-ohm SWR meter and making two measurements. One is when connected with a short RK50 cable 10 cm long, the other is via a RK50 cable ~ 0.25λ long. The smaller the spread of readings, the more reliable the device.

At a frequency of 29 MHz the following SWR values ​​were obtained:

• DRAKE WH - 7......1.46/1.54
• DIAMOND SX - 100......1.3/1.7
• ALAN KW - 220......1.3/1.7
• ROGER RSM-600......1.35/1.65
• UT1MA......1.44/1.5

With a load of 50 Ohms for any length of cables, all devices showed SWR “harmoniously”< 1,1.

The reason for the large scatter in the RSM-600 readings was found out during its study. This device uses not a capacitive divider as a voltage sensor, but a step-down voltage transformer with a fixed transformation ratio. This eliminates the “problems” of the capacitive divider, but reduces the reliability of the device when measuring high powers (maximum power RSM - 600 - only 200/400 W). There is no tuning element in his circuit, so the load resistor of the current transformer must be of high accuracy (at least 50 ± 0.5 Ohms), but in reality a resistor with a resistance of 47.4 Ohms was used. After replacing it with a 49.9 Ohm resistor, the measurement results became significantly better - 1.48/1.58. Perhaps the same reason is associated with a large scatter of readings from the SX - 100 and KW - 220 devices.

Measuring with an unmatched load using an additional quarter-wave 50 ohm cable is a reliable way to check the quality of the SWR meter. Let's note three points:

1. For such a test, you can also use a 50 Ohm load if you connect a capacitor parallel to its input, for example, in the form of a small piece of coaxial cable open at the end. The connection is conveniently made through a coaxial tee junction. Experimental data - with a segment of RK50 28 cm long at a frequency of 29 MHz, such a combined load had an SWR - 1.3, and with a length of 79 cm - SWR - 2.5 (connect any load to the SWR meter only with a 50 ohm cable) .
2. The actual SWR in the line approximately corresponds to the average of two measured values ​​(with and without an additional quarter-wave cable).
3. When measuring a real antenna-feeder device, difficulties may arise due to current flow onto the outer surface of the cable braid. In the presence of such a current, changing the length of the feeder from below can lead to a change in this current, which will lead to a change in the feeder load and the actual SWR. You can reduce the influence of external current by rolling up the feeder entering the room in the form of a coil of 15...20 turns with a diameter of 15...20 cm (protective choke).

Literature

1. D. Lechner, P. Finck. Kurzwellen sender. - Berlin: Militarverlag, 1979.
2. W.B. Bruene- An Inside Pictures of Directional Wattmeters. - QST, April, 1959.
3. D. DeMaw. In-Line RF Power Metering. - QST, December, 1969.
4. W. Orr, S. Cowan. The beam antenna handbook. - RAC, USA, 1993.
5. Beketov V., Kharchenko K. Measurements and tests in the design and adjustment of amateur radio antennas. - M.: Communication, 1971.

Often the client, especially if he is buying a walkie-talkie for the first time, is perplexed when it is mentioned that to use the walkie-talkie you need to set up an antenna, namely antenna SWR setting. What is SWR? This term is unclear to a person far from technical subtleties and sometimes even frightening. It's actually simple.

What is SWR? The antenna is tuned using a special device - an SWR meter. It measures the standing wave ratio and shows the power loss in the antenna. The lower this value (SWR), the better. The ideal value is 1, but in practice it is unattainable due to signal losses in the cable and connectors; a working value is considered to be 1.1 - 1.5; acceptable values ​​are values ​​from 2 to 3. Why are acceptable? Because if the SWR value is too high, your antenna begins not so much to radiate the signal into the air, but to “drive” it back into the radio. What does this mean and why is it bad, you ask? Firstly, you lose in communication range, because the efficiency of your walkie-talkie-antenna system decreases. Secondly, the output stages of the radio station overheat, leading to possible failure. That's why it's important adjusting the SWR of the antenna after installing it. One of the inexpensive SWR meters is the SWR-420 or SWR-430 manufactured by Optim. It can be used with radio stations in the 27 MHz range with transmitter output power up to 100 W. The measurement error is no more than 5%. Using this device, you can achieve SWR values ​​= 1.1 - 1.3, depending on the type of antenna chosen (mortise or magnetic) and its installation location. But there is no need to dwell on this. 1.5 is a completely working and safe value.

How it is produced setting the SWR of the SB antenna? The antenna is installed on the car body, preferably at its highest point. The installation location should be chosen carefully, since the antenna will have to be there permanently. When installing a built-in antenna, you should ensure normal contact of the antenna (or bracket) with ground and carefully monitor that there are no short circuits in the cable and the points where the cable is connected to the antenna and radio. It is important to understand that the body of your car is also an element of the antenna, so the installation location and the quality of contact with ground should not be neglected.

The SWR meter should be connected to the radio station via TX connector, connect the antenna to ANT connector and select the limit of the passing power level. To calibrate the device, you must set the switch to position F.W.D., turn on the radio station to transmit on the desired channel and set the indicator arrow SWR to the extreme division SET red scale. After this, the device is ready for measurements. To check the SWR on the current channel, move the switch to the position REF(the radio station continues to transmit) and look at the indicator readings on the upper scale, this will be the actual SWR value. If it lies in the range of 1-1.5, the setup can be considered complete and successful. If it goes beyond this value, then we begin to select the optimal value. To do this, we first find the minimum SWR value on various channels or even grids. We are guided by a simple rule: if the SWR increases with increasing frequency, then the antenna needs to be shortened, if it decreases, then lengthen. Having unscrewed the screws securing the pin, move it in the desired direction, tighten the screws and check the readings of the device again. If the pin is pushed all the way and the SWR is still high, you will have to physically shorten the pin by biting it off. If the pin is extended as much as possible, you will have to increase the length of the matching coil (in practice, in this case it is easier to change the antenna).

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