SWR meters bridges for tuning antennas. Cold Antenna Tuning Method

Working on the air for quite a long time, often becomeyou appear as a voluntary or involuntary listener or participant in conversations about amateur radio antennas. Unfortunately, most radio amateurs, for a number of reasons, do not know how to correctly evaluate them and setting.

The main reason, in our opinion, is the lack ofV teaching skills and specialized equipment. Apart from widely well-known SWR meter and GIR, there is an undeservedly forgotten (as it seems to us) device for tuning antennas - measuring noise bridge, the advantage of which is the ability to define many parameters of interest without radiation into the air.

This instrument can be used to determine a wide range ofmy antenna parameters, for example, such as:

Impedance (wave impedance) of the antenna and the nature of the wavevogo resistance (inductive or capacitive);

The resonant frequency of the antenna, and not only simple oneelemental antennas, but also multi-element multi-band antennas.

Using this bridge, you can determine the length of the communication line (phidera) and pick it up, if necessary, with a multiplicity of half a wave or quarter wave.

This device is so simple that its assembly is within the power of anyone for the sake ofamateur and he can take a worthy place in the home laboratory.

High frequency noise measuring bridge MFJ — 202B .

J. Schultz, W 4 FA.

Abridged translation by A. Wimboim.

When carrying out maintenance work on equipment communication noise measuring bridge is used as a device for measuringrhenium and testing the parameters of various antennas, communication lines, opdetermination of the elements of resonant circuits and their characteristics, measurement of antenna impedances, etc.

The scope of this device can be significantly expandedren with a fairly close acquaintance with the principles of its work.

A high frequency noise bridge can be used instead of a goethenative resonance indicator (GIR-a), but at the same timesignificantly higher measurement accuracy. The reason is the fact that the noise bridge is used simultaneously with the communication radio receptionnickname, the scale of which has a much more accurate graduation than GIR.

For example, almost all communication radios have a resolution1 kHz or more, while GIRs, say, at a frequency of 21 MHz do not have resolution even at 500 kHz. Such accuracy is not very important in the rough determination of the components. Lor C, but extremely useful when tuning antennas or resonant L- Cchains, where traditionally GIR was used.

This publication briefly describes the device noise bridge, its characteristics, methods of use and the possibility ofchanges.

The main features of the noise bridge.

The noise bridge, as its name suggests, is a classic bridge-type device.

The high-frequency noise source reproduces a wide frequency spectrum and provides the equivalent of a signal generated in the 3 to 30 MHz range, covering all shortwave amateur radio bands, but in practice it is much wider.

The device is operated in conjunction with a communication radio receiver used as a detecting device, and receptionThe nickname ultimately determines the quality of the measurements made.

When balancing a bridge consisting of an internal measuring platecha "resistance / reactivity" and a shoulder connected in parallel to the terminals of the measured "unknown" (unknown) component, noise on radio output becomes minimal.

When the bridge is unbalanced, the noise signal heard in the radioke, will increase sharply. The accuracy with which the unknown is measuredvalue depends on the graduation of the scale.

Of course, the bridge can also be used in reverse.

When determining active resistance, the variable shoulder is set to some specific value, for example 50 ohms, and The "unknown" measured shoulder, at the same time, gives a minimum of noise. TaThus, it is consistent with the value on the scale of the device, onwhich the regulator was installed in the variable arm.

In most practical noise bridge designs, there is symmetrical high-frequency transformer, on which the limits of the output signal range. In addition, the device usesThere is a little trick that allows you to measure both inductive and capacitive reactance, despite the fact that in the measuring armthere is only a variable capacitor.

There is a constant capacitor in the arm of the measured object half the capacity than the variable. In this case, the zero reactionthe activity will be in the middle of the noise bridge scale, i.e., correspondinglycorresponds to the average position of the variable capacitor.

Rotation of the capacitor C12 to one side from the middle position determines the capacitive reactance Xc or the minus sign, and during rotation to another - inductive XL- plus sign. Operation of the noise bridgevan on the classical principle of the Winston bridge.

Brief technical characteristics of the device MFJ — 202B.

The bridge diagram is shown in Figure 10.

The overlapped frequency range covers a continuous section from 160 to 6 M, which is very useful for making measurements of amateur radio equipment, including WARCranges.

The limits of the measured resistance - from 0 to 250 ohms remain constant in the frequency range from 1 to 100 MHz.

Inductive and capacitive reactance depend on the measurement frequencynia, which is quite normal, although this is not alwaysrecognized by users of noise bridges. Reactivity regulator ( REACTANCE) has the scale Xc and XL measured object that does not correspond to the actual value of reactivity at a given frequencythose, but speaks only of a certain nature of reactivity.

The main limits of measurement of reactivity by the device MFJ-202V is sufficient for most applications, however, they can bewidened to a large extent with a "range extender" when connecting a 200 ohm resistor. This is especially evident when measuring impedances of the order of several thousand ohms. In practice this means that the high impedances of transmission lines and antennas, which usually not measurable on most types of noise bridges - can be measured by the instrument MFJ—202.

Zener diode type 1N753 is the actual noise source, which is amplified by three broadband cascades on a transistormax 2N3904.

The high-frequency transformer T1 is wound with three twisted wires (trifilar) on a toroidal ferrite core to ensure symmetry.

A variable resistor is located on the front panel of the device. R15" RESISTANCE", variable capacitor C12 "REACTANSE", "range extender" switch S2, connecting fixed resistor R16 200 Ohm, for extending the measuring range of active and reactive components up to several thousand ohms.

The device is assembled on a simple printed circuit board. Structurally executed in a small case on which coaxial connectors are installedTerminals for connecting measured "unknown" objects and a communication radio receiver.

The device is powered by an internal battery of the "KORUND" type, i.e. + 9 V at a current consumption of 17 mA.

MEASUREMENT OF ANTENNA PARAMETERS.

The most common application of noise meterbridge is to determine the impedances and resonant frequencies atemo transmitting antennas.

To do this, to the measuring bridge using a short coaxialcable with a wave impedance equal to the wave impedanceto the feeder of the measured antenna, a measuring receiver, and the antenna to be measured is connected to the other connector.

DEFINITION OF IMPEDANCE

bridge potentiometer RESISTANCEis set to the position corresponding to the impedance (wave impedance) of the antenna kawhite (50 or 75 ohms for most applications).

variable capacitor REACTANSEis set to the middle position (zero). The receiver tunes in to the expected rezone frequency of the antenna. The bridge is turned on and somery level of the noise signal. Using a variable resistor, try to tune in to the minimum noise level. With a capacitor REACTANCEbeforefurther reduce the noise level. These operations must be repeated several times, because. controllers influence each other.

An antenna tuned to resonance must have zero reactance, and an active one must correspond to the wave resistance.resistance of the cable used. In real antennas, the resistance, both active and reactive, can differ significantly from the calculated ones.

For this, certain methods of coordination are used. In this case, several options for instrument readings are possible:

1. If the active resistance is close to zero, then a short circuit in the cable is possible; if the active resistance is close to 200 ohms atdisconnected "range extender", then a break in the cable is possible.

2. If the device shows inductive resonance, then the antenna is toocom is long, if capacitive, then short.

The length of the antenna can be adjusted. For this, it is determined real resonant frequency Fpe3.

DETERMINATION OF THE RESONANT FREQUENCY.

The receiver is tuned to the expected resonant frequency. Pebelt resistor RESISTANCE set to 75 or 50 ohms. Capacitor REACTANCE is set to zero, and the receiver is slowly tuned to obtain the minimum noise signal.

If the antenna has a high quality factor, then at least it is easy skip when hopping in frequency.

The receiver must be tuned down in frequency with inductive impedance and up in frequency - with capacitive until a minimum noise signal is obtained. By adjusting the bridge regulators, it is necessary to additionally achieve noise reduction.

One can only wonder how much the characteristics differ dipole and other antennas from the calculated ones, if they are located close from the surface of the Earth and any bulky objects.

DETERMINATION OF THE LENGTH OF THE COMMUNICATION LINE.

For some work on matching antennas, etc., cables that are multiples of a quarter or half wave at a specific frequency.

For this, the following method is used:

1. Install a shorting jumper on the test connector. Regulators RESISTANCE And REACTANCE get the minimum noise signal. Both regulators must be in the area of ​​zero scale positions.

2. Remove the jumper and connect the cable under test to the testrhetoric shoulder.

3. To determine the length of the cable, a multiple of a quarter wave, it is required to carefully shorten the cable until a minimum signal is obtained, at the open end.

4. To determine the length of the investigated cable, a multiple of half a wave, the cable is closed at the end during each measurement.

Literature

1. CQ—magazine, august 1984.

2. J. J. Carr. Two-way radio and broadcast equipment, N.J. USA

R— D

Figure 1 shows the RF bridge circuit, developed on the basis of the UA9AA design.

Fig.1

As a rule, the hinged mounting used in the manufacture of the bridge limits the operating frequency range of such devices to 140 ... 150 MHz. To ensure operation in the 430 MHz range, it is advisable to manufacture the device on a double-sided foil textolite. One of the successful mounting options is shown in Figures 2 and 3.

Fig.2

On the upper side of the board (Fig. 2) there are two non-inductive resistors R1, R2 with compensation capacitors C4, C5. On the underside (Fig. 3) the remaining parts of the bridge are placed. The installation was done on the "spots".

Fig.3

The distances between the "patch" are determined by the dimensions of the parts used. The circles indicated in the figures by dashed lines are interconnected through holes in the board.

In the manufacture of the bridge, special attention should be paid to the quality of the parts used. Capacitors C1, C2 - ceramic, leadless, type K10-42, K10-52 or similar. Reference capacitor C3 - KDO-2. Trimmer capacitors C4, C5-type KT4-21, KT4-25; the rest of the capacitors - KM, KTs. Resistors R1, R2 must be of the type MON, C2-10, C2-33 with a power of 0.5 W and have the same resistance within 20 ... 150 Ohms. If resistors of the MON type are used, then their conclusions are bitten off to the base, which is cleaned and tinned, and then soldered to the desired "patch". Resistor R3 - type SP4-1, SP2-36, non-inductive, with a graphite track. This resistor is mounted on the side wall of foil textolite, however, the foil at the place of its attachment is removed. The body of the resistor is not connected to a common wire, otherwise the bridge cannot be balanced. The handle fixed on the axis of the resistor must be made of insulating material. In addition to the resistor R3, CP-50 connectors are attached to the side walls. The joints (joints) between the side walls and the main board are carefully soldered.

The signal power from the generator should be about 1 watt. For example, IC-706MK2G, varactor tripler, etc. can be used as a generator.

When checking the balancing of the RF bridge in the VHF and UHF bands, only non-inductive resistors are used. Fine tuning of compensation capacitors (with the same load resistance) corresponds to a constant balance on several ranges (for example, 7 ... 430 MHz). If it is not possible to select a sufficient number of non-inductive resistors to calibrate the bridge, intermediate scale values ​​​​of the device can be calibrated on the low-frequency ranges using common resistors, for example, MLT or MT.

To measure the reactivity of the load, it will be necessary to replace the C5 capacitor with a variable one (with an air dielectric and a maximum capacitance of about 20 pF), however, the upper frequency measurement limit is limited to a range of 144 MHz, because fails to fully compensate for mounting capacitance.

If chokes with an inductance of 200 μH are used in the device, the frequency range of the bridge will be 0.1 ... 200 MHz.

The proposed design has a very good repeatability, in contrast to devices made using surface mounting.

Literature

1. Y. Selevko (UA9AA). Device for tuning antennas. Radio amateur, 1991, N5, S.32...34.

Standard signal generators (GSS) provide a voltage of 1 ... 2 V at a load of 50 Ohms, which is clearly not enough to work with bridge antenna resistance meters. In order to use conventional bridge resistance meters without modifying them, it is necessary to use a broadband power amplifier. A diagram of such an amplifier is shown in the figure.

The broadband amplifier provides at least 1 W of output power when operating in conjunction with the GSS in the frequency range from 1 to 30 MHz. If you reduce the supply voltage to 12 V and use the part ratings given in brackets, then the output power of the amplifier drops to 600 mW, which is enough to work with many types of measuring bridges. When assembling the amplifier from serviceable parts and setting the collector current indicated on the diagram, the amplifier is immediately operational and does not need to be adjusted. It is convenient to assemble the amplifier by surface mounting.

Transformer T1 is made on an annular magnetic circuit with dimensions K7x4x2 made of ferrite with a permeability of 400 ... 600. The windings contain 12 turns of PEL-2-0.35 wire, wound with a twist - one twist per centimeter. The ferrite ring can also be used in larger sizes. The amplifier can be assembled in a case made of foil fiberglass. Transistor VT1 is mounted on a radiator. High-frequency input-output jacks and amplifier power leads are output to the amplifier case.

Sometimes it is inconvenient to use the GSS in conjunction with a power amplifier. These may be cases of measurements in the field; with battery-powered GSS, etc. In this case, you can use a bridge with a high-frequency unbalance voltage amplifier.

The scheme of such a bridge is as follows:

Its difference from other circuits of bridge meters is that the high-frequency voltage is not detected and measured immediately, but is fed through the transformer T1 to the input of a transistor two-stage amplifier and then already detected. be assembled on any high-frequency transistors such as KT315, KT312. The frequency response of the amplifier is linear up to 40 MHz. Transformer T1 contains 22 turns of PEL-0.1 wire in each winding. The windings are located symmetrically on both halves of the ring with dimensions K10x7x4 with a permeability of 400 ... 600

Calibration of the device consists in marking the variable resistor R2 of the load resistance on the limb. This is best done using a digital ohmmeter. The indications of the limb when balancing the bridge will correspond to the resistance of the measured antenna.

The bridge meter is assembled in a case made of foil fiberglass. Its installation should be as compact and rigid as possible. The limb of the variable resistor to increase the measurement accuracy should have the maximum possible dimensions.

The high frequency measurement bridge is a conventional Wheatstone bridge and can be used to determine how well the antenna matches the transmission line. This circuit is known by many names (for example, "antenoscope", etc.), but it is always based on the circuit diagram shown in fig. 14-15.

High frequency currents flow through the bridge circuit, so all resistors used in it must represent purely resistive resistances for the excitation frequency. Resistors R 1 and R 2 are selected exactly equal to each other (with an accuracy of 1% or even more), and the resistance itself does not really matter. With the assumptions made, the measuring bridge is in equilibrium (zero meter reading) with the following ratios between the resistors: R 1 = R 2 ; R 1 : R 2 =1:1; R 3 == R 4 ; R 3: R 4 = 1: 1.

If, instead of the resistor R 4, the test sample is included, the resistance of which is to be determined, and the calibrated variable resistance is used as R 3, then the zero reading of the bridge unbalance meter will be achieved at a value of the variable resistance equal to the active resistance of the test sample. In this way, the radiation resistance or input impedance of the antenna can be measured directly. It should be remembered that the input impedance of the antenna is purely active only when the antenna is tuned, so the measurement frequency must always correspond to the resonant frequency of the antenna. In addition, the bridge circuit can be used to measure the wave impedance of transmission lines and their velocity factors.

On fig. 14-16 shows a diagram of a high-frequency measuring bridge designed for antenna measurements, proposed by the American radio amateur W 2AEF (the so-called "antenoscope").

Resistors R 1 and R 2 are usually chosen equal to 150-250 ohms, and their absolute value does not play a special role, it is only important that the resistance of resistors R 1 and R 2, as well as the capacitances of capacitors C 1 and C 2 are equal to each other. Only non-inductive bulk variable resistors should be used as variable resistances and never wirewound potentiometers. Variable resistance is usually 500 ohms, and if the measuring bridge is used for measurements only on transmission lines made of coaxial cables, then 100 ohms, which allows more accurate measurements. The variable resistance is calibrated, and when the bridge is balanced, it must be equal to the resistance of the test sample (antennas, transmission lines). Additional resistance R W depends on the internal resistance of the measuring device and the required sensitivity of the measuring circuit. As a measuring device, you can use magnetoelectric milliammeters with a scale of 0.2; 0.1 or 0.05 mA. The additional resistance should be chosen as high as possible, so that the connection of the measuring device does not cause a significant unbalance of the bridge. Any germanium diode can be used as a rectifying element.

Bridge conductors should be as short as possible to reduce their own inductance and capacitance; when designing the device, symmetry in the arrangement of its parts should be observed. The device is enclosed in a casing divided into three separate compartments, in which, as shown in Fig. 14-16, individual elements of the device circuit are placed. One of the points of the bridge is grounded, and therefore the bridge is not balanced with respect to the ground. Therefore, the bridge is most suitable for measuring on unbalanced (coaxial) transmission lines. If it is required to use the bridge for measurements on balanced transmission lines and antennas, then it is necessary to carefully isolate it from the ground using an insulating stand. The antennoscope can be used both in the range of short and ultrashort waves, and the limit of its applicability in the VHF range mainly depends on the design and individual circuit elements of the device.

As a measuring generator that excites the measuring bridge, it is quite sufficient to use a heterodyne resonance meter. It should be borne in mind that the high-frequency power supplied to the measuring bridge should not exceed 1 W, and a power equal to 0.2 W is quite sufficient for the normal operation of the measuring bridge. The input of high-frequency energy is carried out using a coupling coil having 1-3 turns, the degree of coupling of which with the coil of the heterodyne resonance meter circuit is adjusted so that when the test sample is turned off, the measuring device gives a full deviation. It should be borne in mind that if the coupling is too strong, the frequency calibration of the heterodyne resonance meter is somewhat shifted. In order to avoid errors, it is recommended to listen to the tone of the measuring frequency on a precisely calibrated receiver.

Checking the operability of the measuring bridge is carried out by connecting a non-inductive resistor with a precisely known resistance to the measuring socket. The variable resistance at which the measuring circuit is balanced must exactly equal (if the measuring bridge is properly designed) the resistance to be tested. The same operation is repeated for several resistances at different measuring frequencies. In this case, the frequency range of the device is found out. Due to the fact that the circuit elements of the measuring bridge in the VHF range are already complex, the balance of the bridge becomes inaccurate, and if in the range of 2 m it can still be achieved by carefully completing the design of the bridge, then in the range of 70 cm the considered measuring bridge is completely inapplicable.

Once the measuring bridge has been tested, it can be used for practical measurements.

On fig. 14-17 shows the design of the antennascope proposed by W 2AEF.

Antenna Input Impedance Determination

The measuring socket of the measuring bridge is directly connected to the antenna power terminals. If the resonant frequency of the antenna was previously measured using a heterodyne resonance meter, then the bridge is powered by a high-frequency voltage of this frequency. By changing the variable resistance, they achieve a zero reading of the measuring device; in this case, the read resistance is equal to the input impedance of the antenna. If the resonant frequency of the antenna is not known in advance, then the frequency supplying the measuring bridge is changed until an unambiguous balance of the measuring bridge is obtained. In this case, the frequency indicated on the scale of the measuring generator is equal to the resonant frequency of the antenna, and the resistance obtained on the variable resistance scale is equal to the input impedance of the antenna. By changing the parameters of the matching circuit, it is possible (without changing the excitation frequency of the high-frequency measuring bridge) to obtain a given input impedance of the antenna, controlling it with an antenoscope.

If it is inconvenient to measure directly at the feed points of the antenna, then in this case a line can be connected between the measuring bridge having an electrical length R/2 or a length that is a multiple of this length (2 λ/2, 3 λ/2, 4 λ/ 2, etc.) and having any wave impedance. As you know, such a line transforms the resistance connected to its input in a ratio of 1: 1, and therefore its inclusion does not affect the accuracy of measuring the input impedance of the antenna using a high-frequency measuring bridge.

Determination of the shortening factor of a high-frequency transmission line

The exact length λ/2 of the line segment can also be determined using an antenoscope.

A sufficiently long freely suspended line segment is closed at one end, and connected to the measuring socket of the bridge at the other end. The variable resistance is set to zero. Then slowly change the frequency of the heterodyne resonance meter, starting at low frequencies and moving up to higher frequencies until the bridge balance is reached. For this frequency, the electrical length is exactly λ/2. After that, it is easy to determine the line shortening factor. For example, for a piece of coaxial cable with a length of 3.30 m at a measurement frequency of 30 MHz (10 m), the first balance of the bridge is achieved; hence λ/2 is equal to 5.00 m. Determine the shortening factor: $$k=\frac(geometric length)(electric length)=\frac(3.30)(5.00)=0.66.$$

Since the balance of the bridge takes place not only at the electrical length of the line equal to λ / 2, but also at lengths that are multiples of it, the second balance of the bridge should be found, which should be at a frequency of 60 MHz. The line length for this frequency is 1λ. It is useful to remember that the velocity factor for coaxial cables is approximately 0.65, for ribbon cables 0.82 and for two-wire air insulated lines approximately 0.95. Since the measurement of the velocity factor with an antenoscope is not difficult, all transformer circuits should be designed using the method of measuring the velocity factor described above.

An antennoscope can also be used to check the dimensional accuracy of the λ/2 line. To do this, a resistor with a resistance of less than 500 ohms is connected to one end of the line, and the other end of the line is connected to the measuring socket of the bridge; while the variable resistance (if the line has an electrical length exactly equal to λ / 2) is equal to the resistance connected to the other end of the line.

With the help of an antenoscope, the exact electrical length λ/4 of the line can also be determined. To do this, the free end of the line is not closed, and by changing the frequency of the heterodyne resonance meter in the same way as described above, the lowest frequency is determined at which (at the zero position of the variable resistance) the first balance of the bridge circuit is achieved. For this frequency, the electrical length of the line is exactly λ/4. After that, it is possible to determine the transforming properties of the λ/4 line and calculate its wave impedance. For example, a resistor with a resistance of 100 ohms is connected to the end of a quarter-wave line. By changing the variable resistance, the bridge is balanced with a resistance of Z M = 36 ohms. After substituting into the formula $Z_(tr)=\sqrt(Z_(M)\cdot(Z))$ we get: $Z_(tr)=\sqrt(36\cdot(100))=\sqrt(3600)=60 ohm$. Thus, as we have seen, the antenoscope, despite its simplicity, makes it possible to solve almost all problems associated with matching a transmission line with an antenna.

….. The problem of measuring SWR in the ranges of 1296 MHz and above is still relevant for many. This, in particular, is due to the high cost or small range of ready-made devices designed for this and the difficulty of making them at home.

Bridge #1

I encountered the same a few years ago when tuning the YAGI-DL6WU-mod antenna to 23cm. Having made several different designs of SWR meters (with communication loops, bridges ...) for this range, I was convinced that they all more or less “lie”. This was mainly manifested in the distortion of readings at low SWR. So, when connected to such a meter instead of a reference load antenna with a marked SWR = 1.05, they rarely “showed” SWR less than 1.3 ... 1.5.

But with the antenna, it was easy to achieve SWR = 1.0, which was a mistake, because it meant that the antenna impedance, in this case, was simply “convenient” for balancing the circuit ... Of all the devices I tested, the design worked more or less well I. Nechaeva, published in the well. "RADIO" -12/2003 - “Bridge SWR meter”, and then, only after the bottom foil of the board was removed, and a small constructive capacitance was added to one of the arms of the bridge.

Before that, I already noticed that home-made SWR meters, assembled according to the RF bridge scheme, do their job better than others in the microwave ranges. The use of SMD components and printed wiring for their manufacture, it would seem, is an ideal solution, but the inductance and capacitance of printed tracks, or rather, their slightest difference in the arms of the bridge, on the microwave leads to an imbalance and requires measures to compensate for them, which complicates the manufacture and setting up such bridges at home.

Proceeding from this, my first “correct” microwave bridge for the range of 23 cm was subsequently made, all the details of which “hang” in the air and are fixed on the terminals of three “N” connectors, which, in turn, are simply soldered together by the ends. The fourth "wall" is a piece of tin (0.5 mm), with a pass-through capacitor installed on it and soldered to the ends of the connectors. The design, therefore, does not require the manufacture of a case (see Fig. 1), it is very simple, and the entire assembly can take 2...3 hours. The small size of the meter allows you to connect it through a short (and high-quality!) RF adapter directly to the antenna and check the SWR directly on its terminals without introducing noticeable effects.

The principle of measuring SWR is simple: We supply such power to the bridge that at its input it is within 0.3 .... 3 W, with Zx turned off. With the “Sensitivity” knob (Bl.Ch.), set the arrow to the last division (100 μA). Then we connect the investigated load (Zx) and read the SWR readings.
P.S. Here, a meticulous (and literate) reader will say: “Uh, here you are lying!” And he will be right! Indeed, when Zx is disabled, the signal source (transceiver) "sees" the bridge input impedance of about 100 ohms, and when Zx is connected, it is about 50 ohms. This changes the RF voltage level at the bridge input and the measurement results are distorted.

However, in practice, this is almost not noticeable, since, firstly, we connect the transceiver to the bridge input through a cable that has attenuation and, accordingly, acts as an attenuator to "improve" the SWR. So, 1296 MHz transceivers usually have Pout. about 10W, and if you connect it to the bridge through a cable like RG-58 (or RK-50-2-11) about 10m long, then the loss will be about 10dB and about 1W will come to the bridge. SWR with this cable at the point of connection to the transceiver will be close to 1.0 regardless of whether Zx is connected or disconnected.

In addition, a thin cable (dia. 4 ... 5 mm) is convenient when measuring SWR on the "antenna clamps", because does not exert a serious mechanical load on the antenna. But, as the test with exemplary loads (with SWR: 1.05 / 1.4 / 2.0) showed, shorter cables also do not lead to large errors in the measurement results.

Table #1

Practical results of measurements with bridge No. 1 and calibrated loads on the 1296 MHz band at different input powers

P(power)
SWR(reference)
SWR readings with bridge #1

For example, Table 1 is compiled when the bridge is connected to the TS-790S transceiver through a cable segment with attenuation of about 6 dB.

My TS-790S on the 1296MHz band has a minimum power of 1.2W, and a maximum of about 12W, so connecting it to the bridge through a cable with such attenuation provides the entire power range that the bridge eats with pleasure. Applying power less than 0.3W to the bridge is undesirable, as it can lead to underestimation of the readings (“improvement” of the real SWR), and above 3W it is fraught with overheating and failure of the resistors R1 ... R4.

About details:
N-connectors - imported, for printed wiring with flanges 17.5 x 17.5mm. The ends of the flanges are cleaned with a file to copper. The central conductors are bitten off and stick out to a length of 2 ... 3 mm (the fluoroplastic is cut off under the base and removed);

R1 ... R4 - OMLT-0.25W-100Ohm. The conclusions are shortened to 2…3mm;
C1, C2 - ceramic, NPO, the leads are shortened to the lengths required for the connections;
D1 - BAT-62-03W. Schottky (0.4pF/40v/0.43v). Purchased from "RFmicrowave.it" (0.3 Euro/piece);
Zo - exemplary load 50 Ohm with N-connector (DC-6GHz, 2W). Bought from "RFmicrowave.it" (COD: "TC-N-04"; 9.8 EUR/pc)

Rice. 2. RF bridge circuit for the 1296 MHz band.

Addition of the 1296 MHz band over the bridge.
For beginners, - calibration of the pointer device.
We carefully open the device so that there is access to the scale (or we compile such a scale in the form of a table). We mark the scale with SWR values ​​in accordance with the formula:

SWR=(A+B)/(A-B),
Where
A - readings of the entire scale (when Zx is disabled), for example: 100 uA.
B - SWR reading (when Zx is connected).

So, with a device scale of 100 μA, it will turn out:

SWR=1.0 → 0uA;
SWR \u003d 1.2 → 9.1 μA;
SWR=1.5 → 20uA;
SWR=2.0 → 33.3uA;
SWR \u003d 2.5 → 42.9 μA;
SWR=3.0 → 50uA;
SWR \u003d 5.0 → 66.7 μA.

Feel free to put these values ​​on the scale of the device. It is advisable to install a device with a large scale in the measuring unit - this makes it easier to read the readings when setting up the antenna - on the street, for example.

P.S. I think that the described bridge is also operable on other HF bands, but I did not carry out such checks, because. there are enough meters for the meter and decimeter ranges. I will be glad for any information.

Bridge #2

Bridge No. 2 "grew" from the first. “... Should I risk making a similar bridge for the 5.7 and 10 GHz bands ??” - somehow I thought. The result is in Fig. 4 and the diagram shown (Fig. No. 3).

I can’t say that this meter is as correct as the previous one, because, firstly, I don’t have calibrated SMA loads for these frequencies, and secondly, this design is too “daring” to qualify for this, and created rather as an experiment. But the fact that with the SMA load wound as Zx (the same as applied as Zo), the meter needle is set to SWR values ​​​​not more than 1.1 is a fact!

In addition, with the help of this bridge, the probes of my home-made irradiators with counter-reflectors on both ranges were checked and adjusted. The dynamics of the change in SWR is clearly visible, but isn't this often the main condition? .. I would be glad for additional information and experimental results.

About details: C1, C2, C3 - 1pF, "0806", NPO
R1...R4 - 100 Ohm, "1206", 0.25W
D1 - BAT15-03W (0.3pF / 4v / 0.23v), here, probably, BAT62-03W could also be used, but I decided to put a higher frequency one.
Zo - load 50Ohm, SMA (DC-18Ghz), 1W - purchased from "Rfmicrowave.it" (COD: "TC-SMA-11") 12.5 Euro / pc.

Rice. 3. RF bridge circuit for 5.7 and 10GHz bands

Bridges #1 and #2 use the same Measuring Unit, so bridge replacement is done using plug-in connectors (DB-9).

As you can see from the photo (Fig. No. 4), SMA connectors are soldered not at the ends, but somewhat closer - to ensure the minimum distances required for the installation of elements. Therefore, the accuracy of soldering here should be higher.

A picky reader will say that it is impossible to mount SMD components "in this way" - they will collapse when deformed! ... I know that it is impossible ..-but I really want to! ... At least - 100 times !! already twisted the connectors - so far nothing has fallen off!

But, of course, you need to be careful here, especially in preventing side loads on SMA connectors. R5 and R6 are installed with lower ratings than in bridge #1. This is done to reduce the lower limit of power during measurements, because DB6NT microwave transverters usually have about 200mW output, plus losses in the connecting cable.

C1 - reduces the input SWR of the meter.
R5 and R6 are connected to the circuit with pieces of thin copper wire (- strands from MGTF).

Rice. 5. General view of the measurement block.

Many thanks to Sergey, RA3WND, for help in preparing this article, and to Dmitry, RA3AQ
- for a great site! I wish you success and 73! Nikolai UA3DJG.