The high frequency measurement bridge is a conventional Wheatstone bridge and can be used to determine the degree of matching of the antenna to the transmission line. This circuit is known by many names (for example, “antennascope”, etc.), but it is always based on the circuit diagram shown in Fig. 14-15.
The bridge circuit carries high frequency currents, so all resistors used in it must be purely active resistance 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 matter much. Under the assumptions made, the measuring bridge is in equilibrium (zero reading of the measuring device) with the following relationships between the resistors: R 1 = R 2 ; R 1: R 2 =1:1; R 3 = = R 4 ; R3:R4 = 1:1.
If, instead of resistor R 4, we turn on the test sample whose resistance needs to be determined, and use a calibrated variable resistance as R 3, then the zero reading of the bridge unbalance meter will be achieved at a variable resistance value equal to the active resistance of the test sample. In this way, the radiation resistance or input impedance of the antenna can be directly measured. It should be remembered that the antenna input impedance 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 characteristic impedance of transmission lines and their shortening factors.
In 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 “antennascope”).
Resistors R1 and R2 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 R1 and R2, as well as the capacitances of capacitors C1 and C2, are equal to each other. As a variable resistance, only non-inductive volumetric variable resistors should be used and in no case wirewound potentiometers. The 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 should be equal to the resistance of the test sample (antenna, transmission line). The additional resistance R Ш depends on the internal resistance of the measuring device and the required sensitivity of the measuring circuit. Magnetoelectric milliammeters with a scale of 0.2 can be used as a measuring device; 0.1 or 0.05 ma. The additional resistance should be selected as high-resistance as possible, so that connecting the measuring device does not cause a significant imbalance of the bridge. Any germanium diode can be used as a rectifying element.
Bridge circuit conductors should be kept as short as possible to reduce their own inductance and capacitance; When designing a 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 asymmetrical with respect to the ground. Therefore, the bridge is most suitable for measurements on unbalanced (coaxial) transmission lines. If it is necessary to use the bridge for measurements on balanced transmission lines and antennas, it must be carefully isolated 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.
It is quite sufficient to use a heterodyne resonance meter as a measuring generator that excites the measuring bridge. It should be borne in mind that the high-frequency power supplied to the measuring bridge should not exceed 1 W, and a power of 0.2 W is sufficient for 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 taken into account that if the coupling is too strong, the frequency calibration of the heterodyne resonance meter is slightly shifted. To avoid errors, it is recommended to listen to the tone of the measuring frequency using a precisely calibrated receiver.
The functionality of the measuring bridge is checked by connecting a non-inductive resistor having a precisely known resistance to the measuring socket. The variable resistance at which the measuring circuit is balanced must be exactly equal (if the measuring bridge is properly designed) to the resistance being tested. The same operation is repeated for several resistances at different measuring frequencies. In this case, the frequency range of the device is determined. 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 2 m range it can still be achieved by carefully constructing the bridge, then in the 70 cm range the considered measuring bridge is completely inapplicable.
After checking the functionality of the measuring bridge, it can be used for practical measurements.
In Fig. 14-17 show the antenna design proposed by W 2AEF.
Determining Antenna Input Impedance
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 on the measuring device; in this case, the read resistance is equal to the input resistance of the antenna. If the resonant frequency of the antenna is not known in advance, then the frequency feeding 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 scale of variable resistance 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 the specified input impedance of the antenna, monitoring it with an antenoscope.
If it is inconvenient to carry out measurements directly at the antenna feed points, then in this case, between the measuring bridge you can connect a line having an electrical length R/2 or a length multiple of this length (2 λ/2, 3 λ/2, 4 λ/ 2, etc.) and having any characteristic impedance. As is known, 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 resistance 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 antennascope.
A sufficiently long freely suspended section of line is short-circuited 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 to higher frequencies, until the balance of the bridge is achieved. For this frequency the electrical length is exactly λ/2. After this, it is easy to determine the line shortening factor. For example, for a piece of coaxial cable 3.30 m long at a measurement frequency of 30 MHz (10 m), the first bridge balance is achieved; hence λ/2 is equal to 5.00 m. We determine the shortening coefficient: $$k=\frac(geometric length)(electrical length)=\frac(3.30)(5.00)=0.66.$$
Since the balance of the bridge occurs not only with an electrical line length equal to λ/2, but also with 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 shortening factor of coaxial cables is approximately 0.65, ribbon cables are 0.82, and two-wire air insulated lines are approximately 0.95. Since measuring the shortening factor using an antennascope is not difficult, all transformer circuits should be designed using the method for measuring the shortening factor described above.
The antenna scope 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; in this case, the variable resistance (in case the line has an electrical length exactly equal to λ/2) is equal to the resistance connected to the other end of the line.
Using an antennascope, 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 zero position of the variable resistance) the first balance of the bridge circuit is achieved. For this frequency the electrical line length is exactly λ/4. After this, the transforming properties of the λ/4 line can be determined and its characteristic impedance can be calculated. 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: $Z_(tr)=\sqrt(36\cdot(100))=\sqrt(3600)=60 om$. Thus, as we have seen, the antennascope, despite its simplicity, allows you to solve almost all problems associated with matching the transmission line with the antenna.
A noise bridge, as its name suggests, is a bridge-type device. The noise source generates noise in the range from 1 to 30 MHz. With the use of high-frequency elements, this range is expanded, and if necessary, antennas in the 145 MHz range can be configured. The noise bridge works in conjunction with a radio receiver, which is used to detect the signal. Any transceiver will also work.
The schematic diagram of the device is shown in Fig. 1. The source of noise is the zener diode VD2. It should be noted here that some examples of zener diodes are not “noisy” enough, and the most suitable one should be selected. The noise signal generated by the zener diode is amplified by a broadband amplifier using transistors VT2, VT3.
The number of amplification stages can be reduced if the receiver used has sufficient sensitivity. Next, the signal is supplied to transformer T1. It is wound on a toroidal ferrite ring 600 NN with a diameter of 16...20 mm simultaneously with three twisted PELSHO wires with a diameter of 0.3...0.5 mm; number of turns -6.
The adjustable arm of the bridge consists of variable resistor R14 and capacitor C12. The measured arm is capacitors C10, SI and a connected antenna with an unknown impedance. A receiver is connected to the measuring diagonal as an indicator. When the bridge is unbalanced, a strong, uniform noise is heard in the receiver. As the bridge is adjusted, the noise becomes quieter and quieter. "Dead silence" indicates precise balancing. It should be noted that the measurement occurs at the receiver tuning frequency. The printed circuit board and the placement of parts on it are shown in Fig. 2.
The device is structurally made in a housing measuring 110x100x35 mm. On the front panel there are variable resistors R2 and R14, variable capacitors C11 and C12 and a supply voltage switch. On the side there are connectors for connecting a radio receiver and antenna. The device is powered by an internal Krona-type battery or accumulator. Current consumption - no more than 40 mA.
The variable resistor R14 and capacitor C12 must be equipped with scales.
Setting, balancing and calibration
We connect the radio receiver with the AGC system disabled to the corresponding connector. We install capacitor C12 in the middle position. By rotating resistor R2, you should make sure that the generated noise is present at the receiver input on all ranges. We connect non-inductive resistors of the MLT or OMLT type to the "Antenna" connector, having previously measured their values with a digital avometer. When connecting resistances, we achieve by rotating R14 a sharp decrease in the noise level in the receiver.
By selecting capacitor C12 we minimize the noise level and make marks on the R14 scale in accordance with the connected reference resistor. In this way, we calibrate the device up to the 330 Ohm mark.
Calibrating the C12 scale is somewhat more complicated. To do this, we alternately connect to the “Antenna” connector a parallel-connected 100 Ohm resistor and a capacitance (inductance) of 20...70 pF (0.2...1.2 µH). We achieve bridge balance by setting R14 at 100 Ohm on the scale and minimizing the noise level by rotating C 12 in both directions from the “O” position. If there is an RC chain, we put a “-” sign on the scale, and if there is an RL chain, we put a “+” sign. Instead of inductance, you can connect a 100...7000 pF capacitor, but in series with a 100 Ohm resistor.
Antenna impedance measurement
We set R14 to a position corresponding to the cable impedance - for most cases this is 50 or 75 Ohms. We install capacitor C12 in the middle position. The receiver is tuned to the expected resonant frequency of the antenna. We turn on the bridge and set a certain level of the noise signal. Using R14 we adjust to the minimum noise level, and using C12 we further reduce the noise. We carry out these operations several times, since the regulators influence each other. An antenna tuned to resonance must have zero reactance, and the active resistance must correspond to the characteristic impedance of the cable used. In real antennas, resistance, both active and reactive, can differ significantly from the calculated ones.
Determination of resonant frequency
The receiver is tuned to the expected resonant frequency. Variable resistor R14 is set to a resistance of 75 or 50 Ohms.
Capacitor C12 is set to the zero position, and the control receiver is adjusted in frequency until a minimum noise signal is obtained.
The noise bridge is used to measure and test the parameters of antennas, communication lines, determine the characteristics of resonant circuits and the electrical length of the feeder. A noise bridge, as its name suggests, is a bridge-type device. The noise source generates noise in the range from 1 to 30 MHz. With the use of high-frequency elements, this range is expanded, and if necessary, antennas in the 145 MHz range can be configured.
The noise bridge works in conjunction with a radio receiver, which is used to detect the signal. Any transceiver will also work.
The schematic diagram of the device is shown above. The source of noise is the zener diode VD2. It should be noted here that some examples of zener diodes are not “noisy” enough, and the most suitable one should be selected. The noise signal generated by the zener diode is amplified by a broadband amplifier using transistors VT2, VT3. The number of amplification stages can be reduced if the receiver used has sufficient sensitivity. Next, the signal is supplied to transformer T1. It is wound on a toroidal ferrite ring 600 NN with a diameter of 16...20 mm simultaneously with three twisted PELSHO wires with a diameter of 0.3...0.5 mm with 6 turns wound.
The adjustable arm of the bridge consists of variable resistor R14 and capacitor C12. The measured arm is capacitors C10, C11 and a connected antenna with an unknown impedance. A receiver is connected to the measuring diagonal as an indicator. When the bridge is unbalanced, a strong, uniform noise is heard in the receiver. As the bridge is adjusted, the noise becomes quieter and quieter. “Dead silence” indicates a precise balancing act.
It should be noted that the measurement occurs at the receiver tuning frequency.
Placement of parts: 
The device is structurally made in a housing measuring 110x100x35 mm. On the front panel there are variable resistors R2 and R14, variable capacitors C11 and C12 and a supply voltage switch.
On the side there are connectors for connecting a radio receiver and antenna. The device is powered by an internal battery or rechargeable battery. Current consumption - no more than 40 mA.
The variable resistor R14 and capacitor C12 must be equipped with scales.
Setting, balancing and calibration
We connect the radio receiver with the AGC system disabled to the corresponding connector. We install capacitor C12 in the middle position. By rotating resistor R2, you should make sure that the generated noise is present at the receiver input on all ranges. We connect non-inductive resistors of the MLT or OMLT type to the “Antenna” connector, having previously measured their values with a digital avometer. When connecting resistances, we achieve by rotating R14 a sharp decrease in the noise level in the receiver.
By selecting capacitor C12 we minimize the noise level and make marks on the R14 scale in accordance with the connected reference resistor. In this way, we calibrate the device up to the 330 Ohm mark.
Calibrating the C12 scale is somewhat more complicated. To do this, we alternately connect to the “Antenna” connector a parallel-connected 100 Ohm resistor and a capacitance (inductance) of 20..70 pF (0.2...1.2 µH). We achieve bridge balance by setting R14 at 100 Ohm on the scale and minimizing the noise level by rotating C12 in both directions from the “0” position. If there is an RC chain, we put a “-” sign on the scale, and if there is an RL chain, we put a “+” sign. Instead of inductance, you can connect a capacitor of 100.7000 pF, but in series with a 100 Ohm resistor.
Antenna impedance measurement
We set R14 to a position corresponding to the cable impedance - for most cases this is 50 or 75 Ohms. We install capacitor C12 in the middle position. The receiver is tuned to the expected resonant frequency of the antenna. We turn on the bridge and set a certain level of the noise signal. Using R14 we adjust to the minimum noise level, and using C12 we further reduce the noise. We carry out these operations several times, since the regulators influence each other. An antenna tuned to resonance must have zero reactance, and the active resistance must correspond to the characteristic impedance of the cable used. In real antennas, resistance, both active and reactive, can differ significantly from the calculated ones.
Determination of resonant frequency
The receiver is tuned to the expected resonant frequency. Variable resistor R14 is set to a resistance of 75 or 50 Ohms.
Capacitor C12 is set to the zero position, and the control receiver is adjusted in frequency until a minimum noise signal is obtained.
V. KISELEV (RA4UF), Saransk
Figure 1 shows a circuit of an RF bridge developed based on the UA9AA design.
As a rule, the suspended installation used in the manufacture of a 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 PCB. One of the successful installation options is shown in Fig. 2 and 3.
On the upper side of the board (Fig. 2) there are two non-inductive resistors R1, R2 with compensation capacitors C4, C5. The remaining parts of the bridge are located on the lower side (Fig. 3). The installation was carried out on the "spots".
The distances between the “patches” are determined by the sizes of the parts used. The circles, indicated in the figures by dashed lines, are connected to each other through holes in the board.
When making a 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. The reference capacitor C3 is KDO-2. Trimmer capacitors C4, C5-type KT4-21, KT4-25; the remaining capacitors are KM, KTs. Resistors R1, R2 must be of type MON, C2-10, C2-33 with a power of 0.5 W and have the same resistance within 20...150 Ohms. If MON type resistors are used, then their leads 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-induction, with graphite track. This resistor is mounted on the side wall made of foil PCB, but the foil at the place of its attachment is removed. The resistor body is not connected to the common wire, otherwise the bridge cannot be balanced. The handle attached to the resistor axis must be made of insulating material. In addition to resistor R3, CP-50 connectors are mounted on 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 W. For example, IC-706MK2G, varactor tripler, etc. can be used as a generator.
When checking RF bridge balancing in the VHF and UHF bands, only non-inductive resistors are used. Precise 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 for calibrating the bridge, intermediate values of the device scale can be calibrated in the low frequency ranges using common resistors, for example, the MLT or MT type.
To measure the load reactivity, you will need to replace the capacitor C5 with a variable one (with an air dielectric and a maximum capacitance of about 20 pF), however, the upper frequency limit of measurements is limited to the range of 144 MHz, because it is not possible to fully compensate for the installation capacity.
If the device uses chokes with an inductance of 200 μH, the frequency range of the bridge will be 0.1...200 MHz.
The proposed design has very good repeatability, in contrast to devices made using wall-mounted installations.
Literature
1. Yu.Selevko (UA9AA). Antenna tuning device. Radio Amateur, 1991, N5, P.32...34.
HF and VHF radio amateur. 2/2001, p.18 Related materials:
A simple method for matching HF antennas in “cold” mode.
Currently, antenna tuning and matching is carried out mainly using SWR meters, when a fairly large RF power is supplied to the antenna. At the same time, the antenna emits it, and since during tuning it is necessary to rebuild the transmitter several times within the operating range of the antenna, significant interference is created to other radio stations.
Meanwhile, there is another method of tuning antennas - using an HF bridge, it is described in the well-known Rothhammel reference book. But even in this case, the operation of the bridge requires significant power, which can provide sufficient current in the bridge arms.
However, if you slightly modernize the bridge, then you can use the signal of a conventional RF signal generator for tuning, with an output voltage of 0.5 - 1 volt. But for this it is necessary that the HF signal be modulated by a low-frequency signal of 400 -1000 Hz, and even better, that the generator operates in video modulation mode with pulses of this frequency.
Such modes are available in almost all modern signal generators.
The connection diagram for tuning the antenna to the desired frequency and matching it with a 50 ohm coaxial cable is shown in the figure. The RF generator is set to video modulation or AM mode with a modulation coefficient of 100% and connected to socket X1, the antenna - preferably first directly - is connected to socket X2. Headphones are connected to the HT sockets.
The generator is then tuned to the antenna frequency. If at the same time a low-frequency signal of the modulation frequency of the generator is heard in the headphones, it means that at this frequency the antenna has an input impedance different from the active 50 ohms. By adjusting the generator frequency in either direction from the set one, we achieve a loss of signal in the headphones. This will be the frequency at which the input resistance is active and equal to 50 ohms.
Depending on which direction and how different this frequency is from the desired one, we change the geometric dimensions of the antenna or the data of the matching elements, and again check the balance frequency of the bridge. Having achieved balance at the required frequency, we connect a 50 ohm feeder to the antenna, and perform a similar check of the entire antenna-feeder path.
If the feeder is in good working order and the settings are carried out correctly, after connecting the feeder there is no difference in measurements with or without a feeder, and connecting the SWR meter shows an SWR equal to 1, or close to it.
This method was tested when tuning antennas to a range of 14 MHz; both wire antennas were tuned for 160 and 80 meters, and a 4-element antenna for a range of 20 meters.
In all cases, it was possible to make adjustments quickly and accurately.