Setup and coordination of antenna-feeder devices
In the Preface to his book "Antennas", Rothammel from the first line repeated the well-known truth: a good antenna is the best high - frequency amplifier. However, many hams sometimes forget that to build a good antenna system costs the same, how much is a good transceiver and adjustment of antenna - feeder devices requires a serious approach and adjustment as a hub. Having constructed the antenna taken from somewhere description, the radio more often just build it with a SWR meter, or even rely on case and do not make any measurements. Therefore, in many cases you can hear the negative reviews are good antennas ,or for casual relationships are they not allowed capacity. Here an attempt is made in summary form to make the review easy ways of harmonizing and measurements in the ASF (antenna systems) in the form of guide books (hereinafter referred to as reference numbers):
and provides some practical the tips. So...
Why it is impossible to take seriously to setup the newly created antenna - feeder devices using a SWR meter? SWR meter shows the relationship (Work+Uотр) to (the Work-Uотр) or in other words how much different is the impedance of antenna-feeder path from wave the resistance of the device (transmitter output, for example). According to the testimony SWR meter cannot understand what it means VSWR=3 the resistance of the output stage 50 Ohm. The impedance of the antenna-feeder path in this case can to be purely resistive at resonance frequency ) and may be equal to 150 Ohms or 17 Ohms (both are equally probable!). Not at the resonance frequency resistance will contain active and reactive (capacitive or inductive )in the most different ratios and then it is unclear what to do - whether to compensate for the reactivity, or to agree the wave resistance. For accurate matching of AFD need to know:
- (a) actual resonant frequency of the antenna;
- b) the antenna impedance;
- in) impedance of the feeder;
- g) output resistance transceiver.
The aim of harmonizing the antenna is task fulfilment of two conditions connect the antenna to receiver-transmitter:
If these conditions hold in place powering the antenna (the connection point of the antenna with a feeder, the feeder works in the mode traveling wave. If you do the matching conditions at the connection feeder from the transceiver and the antenna impedance is different from the wave the resistance of the feeder, the feeder operates in the mode of the standing wave. However the operation of the feeder in the standing wave mode may result in the distortion diagrams orientation in directional antennas (due to the harmful radiation feeder) and in some cases may lead to interference of surrounding transceiver equipment. In addition, if the antenna is used for reception, then braid feeder will be accepted of unwanted radiation (e.g. interference from your desktop computer). Therefore, it is preferable to use the power of the antenna through a feeder mode traveling wave. Prior to sharing practical experience of matching antennas, a few words about the basic measurement techniques.
1. Measuring the resonant frequency of the antenna
1.1. The simplest method of measurement the resonant frequency of the antenna using a heterodyne resonance indicator (GAD). However, in multiple antenna measurement systems Gere is to perform difficult or even impossible because of the mutual influence of elements antennas, each of which may have its own resonant frequency.
1.2. Method of measurement measuring antenna and the control of the receiver. Against the measured antenna is connected the generator at a distance of 10-20l of the measured antenna is installed control the receiver with the antenna, which at these frequencies has no resonances (for example shorter than l/10). Generator pristraivaetsya in the selected area range, using S-meter control receiver measures the intensity of fields and construct the dependence of the field intensity on the frequency. The maximum corresponds to the resonance frequency. This method is particularly applicable for mnogoelementnykh antennas, In this case, the measuring receiver should be placed in the main lobe the directivity of the measured antenna. A variant of this method of measurement - use as a generator, peredatchika power of several Watts and simple field strength meter(for example , Figure 14-20.). However we must remember that when izmereniyah you will cause interference to others. Practical the Council for measurements in the range 144-430 MHz the measurements, do not hold in the hands of the field strength meter to weaken the influence of the body on the testimony of the device. Fasten the device above the floor at a height of 1-2 meters on diametrically the stand (e.g. a tree, a chair) and take readings, being at a distance 2-4 meters , not getting in the zone between the device and the measured antenna.
1.3. Measuring with a generator and antennasia (for example , Figure 14-16). This method is applicable mainly on HF and will not give accurate results, but allow to evaluate and resistance the antennas. The essence of measurements is as follows. As you know, antennas.com allows you to measure the impedance (active+reactive). Because antenna usually zapityvat in antinodes current (low input impedance) and resonance frequency missing reactivity, at the resonant frequency of antennas.com will show the minimum resistance, and at all other frequencies most often it will be more. Hence the sequence of measurements - rebuilding generator, measure the input impedance of the antenna. The minimum resistance corresponds to the resonant frequency.For one thing - the need to connect antennas.com definitely right at the feed point of the antenna, and not through the cable! And practical observation - if you are near a powerful radio source (TV or radio), because of interference of antennas.com will never be balanced "zero" and to make measurements becomes almost impossible.
1.4. It is very convenient to determine the resonance the frequency of the vibrators using the meter response. Connecting the output of a meter Frequency and a detector head to the antenna, determine the frequencies at which visible dips in the frequency response. At these frequencies, the antenna resonates and is the selection of energy with the release of the device, which is clearly seen on the screen of the device. Suitable for measurements almost any measure of the amplitude a (X1-47, X1-50, X1-42, SK4-59). Option measurements using a spectrum analyzer (SK4-60) mode with a long the afterglow and external oscillator. As an external generator can use a generator of harmonics: on HF with a step of 10 kHz, for 144 MHz increments 100 kHz, at 430 MHz in 1 MHz steps. At frequencies up to 160 MHz the most balanced range with high intensity of harmonics gives the circuit of the harmonic generator integrated circuit IE . In the range of 430 MHz sufficient level harmonics can be obtained in the circuit with cumulative diode AB (scheme calibrator 50 MHz from SK4-60).
2. Resistance measurement at the antenna-feeder devices
2.1. The easiest (still available for the price) commercially available device for measuring resistance and the phase of the signal (and hence the reactance) is a measurement bridge. There are several modifications of these devices for use with 50 and 75 Ohm tract and at different ranges of frequencies up to 1000 MHz is measuring bridges P2-33 P2...-35.
2.2 In Amateur practice often use a simpler version of the measuring bridge, designed for measurement of impedance (antennas.com). The design, in contrast from bridges P2-33... very simple and easily repeated at home (, p. 308-309).
2.3 it is Useful to remember some comments concerning resistance in the ASF.
2.3.1. A long line with a characteristic impedance Zтр and with the electrical length l/4, 3 x l/4 etc will transform the resistance that can be calculated from the formula
either Fig. 2.39 . Private if one end of the l/4 cut open, infinite resistance at the end of the segment is transformed to zero at the opposite end (short circuit) and such device - properties used for transformation large resistances to small. Attention! These types of transformers work effectively only in a narrow frequency range, limited by shares percent of the operating frequency. A long line with an electrical length is a multiple of l/2 regardless of the wave resistance of this line transforms the input impedance to the output with a ratio of 1:1 and they are used to transfer sportivity to the required distance without transformation resistances, or for rotating the phase by 180°. Unlike l/4 lines line l/2 have more broadband.
2.3.2. If the antenna is shorter than you should, on your frequency of the antenna impedance has a reactive component a capacitive character. In the case when the antenna longer, your frequency antenna has reaktivnosti inductive nature. Of course your unwanted frequency reactance can be compensated by the introduction of additional reactance of opposite sign. For example, if the antenna longer than necessary, the inductive component can be compensated inclusion in series with the antenna power capacity. The value required the capacitor can be calculated for the desired frequency, knowing the value of the inductive component (see figure 2.38 ), or find experimentally, as described in paragraph 5.
2.3.3. The introduction of an additional passive members usually lowers the input impedance of the antenna (for example for square: with 110-120 Ohm 45-75 Ohms).
2.3.4. Below are theoretical the values of the most common vibrators (the vibrators are in free from the surrounding objects and space), antennas and feeders:
- half-wave vibrator with washing in the current antinodes (middle) is 70 Ohms, when the detuning of +-2% reactive resistance iX varies almost linearly from -25 to +25 with zero on resonance frequency;
- half-wave vibrator with washing with the help of the T-shaped matching circuits -120 Ohms; - loop with a vibrator the same diameters of all conductors - 240..280 Ohms, when the detuning +-1% reactive resistance but no nastroyka more than 2% reactance iX sharply increases to + 50 (see Fig 2.93 );
- the folded dipole with different the diameter of the conductors (see Fig. 1.15  or Fig. 2.90 in ) - up to 840 Ohms; - double the folded dipole with the same diameter all the wire - nicks - 540...630 Ohms;
- double the folded dipole with different the diameter of the conductors (see tab. 1.16  or Fig 2.91 ) - up to 1500 Ohms;
- quarter-wave vertical dipole with counterweight at the angle of 135° relative to the vibrator 50 Ohms;
- quarter-wave vertical dipole with counterweight at an angle of 90° relative to the vibrator 30 Ohms;
- the vibrator in the form of a square of length l - 110..120 Ohm; - vibrator in the form of a square of length 2l (two turns) - 280 Ohms;
- the vibrator in the form of Treugolnik (Delta) - 120...130 Ohms;
- Inverded-V with an angle of 90° to 45 Ohms;
- Inverted-V with an angle of 130° to 65 Ohms;
- wave channel, maintainable at maximum gain - 5...20 Ohms;
- wave channel, maintainable on a better match to 50 Ohms;
- two-wire line (Fig 2.26 ) - 200..320;
- two parallel coaxial line Z=75 Ohm - 37.5 Ohms;
- the same, of a quarter-wave transformer Z Input=50 Ohm - Zвых=28 Ohms;
- the same, of a quarter-wave transformer Z Input=75 Ohms - Zвых=19 Ohms;
- two parallel coaxial line Z=50 Ohm To 25 Ohm;
- the same, of a quarter-wave transformer Z Input=50 Ohm - Zвых=12.5 Ohms;
- the same, of a quarter-wave transformer Z Input=75 Ohms - Zвых=8.4 Ohms
- the transformer of the three parallel lines Z=50 Ohm z input=50 - Zвых=5.6 Ω;
- the same Z=50 Ohm z input=75 - Zвых=3.7 Ohms;
3. Measurement of the degree of harmonization
These measurements it is desirable to do already after the agreement, described in section 5 to assess the quality of matching.
3.1. Devices for determining the degree approval of open-wire lines with antenna:
3.1.1. Conventional neon lamp or GEAR. If you move the light along the transmission line, the brightness bulbs should not be changed (traveling wave). Variant of measurements - a device consisting of a hinge connection, detector and switch indicator (see Fig. 14.8 ).
3.1.2. Dual-lamp indicator (see Fig. 14.7 ). Configuring trying to do is get a light bulb connected to the shoulder, close to the antenna, do not light up, but in the opposite shoulder glow was to the maximum. At low levels of capacity, you can use the detector and dial indicator instead of the bulb.
3.2. Devices for determining the degree harmonization in coaxial paths:
3.2.1. Measuring line - a device which is applicable to measure coherence in coaxial and waveguide lines starting with УSW and ending centimeter wave range. Design its simple - rigid coaxial cable (waveguide) with a longitudinal slit in the outer conductor along which the measuring head is moved with the measuring probe is lowered into the slot. Moving the measuring head along the route, determine the maximum and minimum readings, the ratio of which is judged on the degree of harmonization (mode traveling wave - indications not change throughout the length of the measuring line).
3.2.2. The measuring bridge (Fig.14.18 ). Allows you to measure SWR in lines transfer to 100 Ohms on HF and VHF when the input power is about hundred milliwatts. Very easy to manufacture design, does not contain a wrap-catch, structural assemblies, critical to precision.
3.2.3. SWR meters on the basis of the OTDR. He describes many of the designs of these devices (for example Fig. 14-14 . Allow to monitor the condition of the ASF in the process of work on the air. 3.2.4. SWR meters on the basis of measuring the frequency response. Very convenient for exploring quality agreeing on any frequencies up to 40 GHz. The principle of measurement - measuring a set of devices consists of a meter response and the directional coupler, United in the following scheme:
2><-------------------\|/ Ant. 4
where 1 - meter response (X1-47); 2 - low-impedance detector head from the kit X1-47; 3 - directional coupler, for example, for the 144 MHz band will fit BUT 991-03 supplied to the device SK4-60; 4 - measured antenna. High-frequency output signal X1-47 hits the output 3 of the directional coupler and later falls only on pin 2 the directional coupler. Further, the measured signal is transmitted to the antenna. At frequencies where the antenna has a high SWR, the energy is reflected and returns on pin 2 of the directional coupler. In the direction of the signal energy betrayed with output 2 output only 1, is detected by the detector head and the level of the reflected signal is displayed on the screen X1-47 based from the frequency.
Before the measurement, it is necessary to calibrate the circuit. To do this, instead the measured antenna connect a non-inductive dummy resistance 50 Ohms and make sure that no reflected signal(SWR=1). Further, ustikolina equivalent, note the signal level of SWR= infinity.All intermediate SWR will be displayed on the screen of the device position between 0 and a maximum value. Connecting equivalents antenna impedance 75 Ohm 100 Ohm 150 Ohm mark on the device's screen SWR respectively 1.5 , 2, 3.
As a response meter you can use a spectrum analyzer SK4-60 and external generator, depending on the wavelength range, which are manufactured measurements (G4-151 to 500 MHz G4-76 up to 1.3 GHz, G4-82 5.6 GHz, G4-84 10 GHz). At frequencies up to 500 MHz as an external oscillator can be used the harmonic generators described in claim 1.4.
- directional couplers make transient attenuation of about 15 dB for a signal source, so for measurements the required signal sources is quite high;
- the directional properties of taps (junction and direction) usually do not exceed 20...30 dB, so measurements need to execute in logarithmic and linear scale display.
4. Some useful methods of measurement
4.1. Measurements using antennasia(given in  pp. 308-312).
4.1.1. Determining the exact electric of length l/4 lines:
for this line with one end connected to Antonescu, and leave the second open-circuited. Further, by changing the frequency of the generator, determine the lowest the frequency at which a balance is achieved bridge with zero resistance. For this frequency the electrical length of the line is exactly equal to l/4.
4.1.2. Measurement of wave resistance of the line Zтр:
performing measurements according to claim 4.1.1., connect a 100 Ohm resistor to the free the end of the line and measure antennas.com resistance Zизм on the other end line. Calculated characteristic impedance of the line using the formula
4.1.3. Check dimensional accuracy l/2 transforming lines:
- the measured line connected to Antonescu, to the second end of the line, connect the 300 Ohm resistor
- Set the generator frequency so that the line l/2 must transform of 1:1.
- measure antennas.com resistance- it should be equal to 300 Ohms, if the line is exactly equal to l/2 for this frequency.
4.1.4. Opredelenie the velocity factor of the transmission line:
for measurements using a line segment with a length of several meters(length X).
- Close one end of the line and changing the oscillator frequency, find the minimum value of the frequency F at which antennas.com balanced - this will mean that the line transforms resistance 1:1 and for this frequency corresponds to the electrical length l/2 taking into account the velocity factor.
- Increasing further the frequency can be find the following balance of the bridge corresponding to 2 l/2 etc Length l/2 for frequency L=300/(2F), and the velocity factor K=X/L.
For example, if the length of the line X=3.3 meter and the balance occurred at a frequency of F=30 MHz, L=5 meters, and K=0.66. Normal values of the coefficients of shortening for coaxial lines - 0.66, for ribbon cables - 0.82, for open two-wire lines - 0.95 .
4.2. Measurements using the AFR meterare performed in the circuit shown in A3.2.4. 4.2.1. Localization of inhomogeneities in the feeder. If necessary to determine the distance to the fault in the feeder (short circuit or open circuit) without removing the feeder, you can do the following . When you open or short circuit in the feeder, maximum VSWR will be observed at frequencies where the line operates as a transformer l/2 and in multiples of frequencies regardless of the range selected for measurement. Feeder otstegivayut from the transceiver and connected to the pin 2 of the directional coupler. Set such a swing band, so it was convenient to measure the period of the CWS. The measured period in megahertz corresponds to the frequency at which the line works as l/2 segment based shortening. Suppose the frequency interval between the highs SWR is 3 MHz , then the frequency at which the line now works like a transformer l/2 equal to 6 MHz and is corresponds to the length wave 50 meters (i.e. to the heterogeneity of 50 meters without taking into account the coefficient shortening the line). Knowing the velocity factor of the line you can just say the actual distance to the fault. For example if the line is executed coaxial cable with cal. shortening 0.66, in our case the distance from the transmitter to the cliff (KZ) in the coaxial cable is equal to 33 meters.
4.2.2. Measurement of velocity factor cable.
The measurements produce the same as in paragraph 4.2.1. but to the conclusion 2 directional coupler connect the measuring cable length of several meters. Suppose we measure the velocity factor of the cable length of 33 meters. The measured electrical cable length is 50 meters, then the coefficient shortening equal 33/50=0.66 .
4.2.3. Test cable 50 Ω on the absence of inhomogeneities.
Conclusion 2 BUT is connected check the cable, at the other end of which is connected agreed 50 Ohm load. On the screen the fixture should be smooth line, if the cable has no discontinuities.
5. How to configure antenna
As an example, a few words about how to configure the Delta antenna for the range of 80 meters, using the methods measurements given above. Need to agree on the output stage of the transmitter (50 Ohm) antenna cable 50 Ohms. If it is not possible to measure the resistance antenna and find the resonant frequency of the antenna by connecting straight to the point powering connected tranformers line l/2 between the devices and the antenna. Thus, using the transforming properties of the line (1:1) measurements not directly at the antenna, and at the other end of the line.
One of the described methods, measure the antenna impedance and resonance frequency. If the resonant frequency of the antenna is slightly offset by the change of the geometric the size of the antenna, to achieve resonance at the desired frequency. Usually the resistance antenna Delta equal to 120 Ohms and for matching the antenna with cable to apply the transformer is 1:2.4 . This transformer can be done using three-wire SPTL with respect Rвых/Rin=4/9 (Bunin, Yailenko "Guide ham-shortwave" Kiev, Engineering). After manufacturing transformer, connected to a high impedance input transformer resistor resistance 120-130 Ohms and connected to another input of the transformer antennas.com, measure its input resistance and the transformation ratio. Connecting the transformer between the PA and the power line, check the current in the antenna, using RF ammeter (Fig. 14-2 ). It is better to measure the current after PA using a calibrated RF ammeter and calculate the absorbed power. If, after the calculation will be, P=RII less than the equivalent antenna, means matching device makes reactivity and must be compensated. For this series with RF ammeter include variable capacitor (10-500 pF) and changing its value, we achieve maximum in the testimony of an RF ammeter. In the event that if the capacitor fails to increase the current in the antenna, it is necessary to replace the capacitor on the variometer and pick up a compensating inductance. After selection compensating reactivity, measure its value and replace element to a constant value.
After you configure matching device, it is placed in a sealed enclosure and transferred to the supply point of the antenna from the cable. In conclusion, once again check agreeing with one of the ways to measure SWR.
Tips connect computers
Many complain that their desktop the computer is a serious obstacle to the reception. The reason for this in most cases is poor alignment of the antenna. In this case, the enclousure antenna receives the radiation of the computer and they are in the form of interference appear on the receiver input. To test this hypothesis simply undock the cable from the input of the receiver, if the noise disappears, then the main path of the interference from the computer to the input of the receiver via the cable. After careful matching of the antenna using these methods, we can significantly get rid of interference on reception and from unstable operation of the digital nodes during transmission. The second necessary condition for the convenience of working with a computer - careful grounding all devices. Earthing pipe heating - not good! Third way to make all of the cable coming from the computer screen and it is very desirable skip every one of them through the ferrite ring 2000 NM (a couple of turns). You can also skip through the ring and the antenna cable (for additional balancing cable and eliminate the propagation of RF signals sleeving cable). Sometimes the source of interference is the monitor and the cables going to it. Try to turn-off the monitor from the network when running and loaded computer. If the interference level is changed, it is recommended to separately grounded the chassis of the monitor and the grounding point of the chassis, you must pick experimentally at a minimum interference.