To find the impedance of an antenna you need to use specific devices that are not part of the traditional equipment enthusiast.
In this paper, we propose a resistive bridge which not only allows you measure the impedance (ohms) of an antenna but you will also know the ratio of transformation of a balun or establish the exact length of coaxial quarter wave transformer used as impedance.
To measure the impedance of an antenna, it is often suggested the use of a resistive bridge built according to the diagram in Figure 3. This type of arrangement has numerous drawbacks.
Indeed, the resistor R3, connected in series between the input and output of the bridge, must necessarily be type non-inductive power and dissipate more power than that charged by the issuer.
Today, the supply of non-inductive resistors 52 and 72 Ω and a power between 50 and 100 W, is very difficult. In addition, these components have a further disadvantage. Operating at high power levels, they heat and its resistance decreases with increasing temperature.
In some bridges, this resistance is replaced by a potentiometer which, cons, can withstand very high power and accept that its input signals of 0.5 W max. Moreover, the graphite of the potentiometer being circular, it behaves like a coil and generates an inductance in series with the coaxial cable. This phenomenon distorts the impedance measurements performed on the antennas.
So the solution? Our antenna impedance!
Study schema
Bridge 4 is capable of accurately measuring an impedance value whatsoever.
In this circuit, the resistor R3 is a 500 Ω trimmer which has a very low inductance, allows for precise measurements, even on frequencies VHF.
But as we have just explained above, this type of circuit we can apply a signal power above 0.5 W. There is no question of this bridge to connect the RF signals from a transmitter but only signals from an RF generator. Indeed, the latter provides powers which are generally between 10 and 20 milliwatts.
As you can imagine, the output voltage of this bridge will be only a few millivolts. Even a multimeter set to its smaller scale could not read voltage values so low. To resolve this problem, must boost the voltage, rectified by diode DS1 and it is the role of the operational amplifier IC1 (see Figure 4).
With the given values of the resistors R11 and R10, IC1 amplifies approximately 9 times the voltage applied to its noninverting input. Thus, it provides at its output a voltage of about 3 volts.
This voltage value can be easily read by any multimeter.
The LED DL1, connected in series with the zener diode DZ 1, illuminates when the circuit is energized or when the battery supply 9 V is almost completely discharged.
Figure 1: View of the bridge component side. The battery must be inserted into the slot at the bottom of the housing.
Figure 2: View from the bridge on the tracks printed. 
Figure 3: Diagram of a conventional bridge which we do not recommend because it has many drawbacks (see text). 
Figure 4: As you can understand from this circuit diagram, a bridge capable of measuring the impedance of an antenna is slightly more complex than the bridge whose diagram is given in Figure 3. The RF signal is rectified by diode DS1 and then amplified by the integrated circuit IC1. On the entrance of the bridge, there is no question of injecting the signal, too strong, a transmitter but the signal from a RF generator. 
Figure 5a: Diagram of the impedance of printed circuit antenna at scale 1. 
Figure 5b: Implementation Plan components of the bridge that you can measure the impedance of all kinds of antennas. List components LX.1393
R1: 47 Ω
R2: 47 Ω
R3: 500 Ω trimmer
R4: R5
10 kW: 10 kW
R6: 1 kilohm
R7: 1 MΩ R8
: 220 Ω
R9: R10
10 kW: 10 kW
R11: 82 kΩ R12
: 1 kilohm
C1: 10 nF ceramic
C2: 10 nF ceramic
C3: 10 nF ceramic
C4: 10 nF ceramic
C5: 10 uF electr.
C6: 10 nF ceramic
C7: C8 100 pF ceramic
: 47 uF electr.
C9 100 nF ceramic
C10: 100 nF ceramic
JAF1: 10 uH choke
JAF2: 10 uH choke
DS1: 1N5711 Schottky diode
DZ1: Zener diode 5.1 V 1 / 2 watt
DL1: red LED
IC1: CA3130 integrated circuit
S1: switch
Practical realization of the bridge
All items listed on the site plan in Figure 5b, must be mounted on the printed circuit LX.1393 the design is given in Figure 5a.
We recommend you solder first support for the integrated circuit IC1 and then all resistance. Continue the assembly by welding trimmer R3, diode DS1, directing his black ring to the ceramic capacitor C2 and zener diode DZ 1, directing his black ring around the resistor R8 (see Figure 5b).
the Schottky diode can be replaced only by its equivalent BAR10 or HP8052. Solder
now all ceramic capacitors and electrolytic both C5 and C8, paying attention to their respective polarities.
After soldering the coils and JAF1 JAF2, insert the integrated circuit IC1 in its support, leading its slot-keyed to the ceramic capacitor C7.
After installing the card is complete, attach the two BNC connectors in the two side holes of the metal casing.
Then mount the two banana sockets for chassis output to the meter (see Figure 7).
Finally, mount the switch S1 and insert the circuit inside the housing, aligning the hole on the upper surface and the cursor trimmer R3.
Now solder the ground points of the printed circuit alongside the box (see Figure 6).
Connect, using cues resistors or small pieces of tinned copper wire, two BNC connectors, two banana sockets and switch S1 to locations on the PCB.
Complete the assembly by welding the two son taking battery and the LED, by folding the L and with as shown in Figure 5b.
When the 9V battery is installed, to operate the bridge, just switch the inverter S1 so that the LED lights.
Before use, the case must be closed by its two covers.
Figure 6: As you can see from this drawing, the mass of the PCB must be welded at several points in the metal case. 
Figure 7: The insulating rings of banana jack for output meter must be placed on the inside of the housing. 
Figure 8: Pinout of the operational amplifier IC1 CA3130 drawing of the LED. As you can see in Figure 5b, the longest of the pin diode, indicated with the letter A, should be directed to the switch S1. On the test bench
To verify the proper functioning of the device, use its input signal from an RF generator and connect a multimeter set to level 3 volt sockets for banana output (see Figure 9).
Then, adjust the amplitude of the signal generator RF output to what the meter shows a voltage of about 2 to 3 volts.
Note: a voltage of 1.5 volts is already sufficient.
Now, connect a resistor from 47 to 56 Ω on BNC output (see Figure 10) and turn the cursor trimmer R3 until the meter needle scale rapidly to 0 volts.
Then unplug the RF generator and resistance, switch the meter on the scale "ohm" and, by connecting its probes on the soul of the BNC input and output of the bridge (see Figure 11), measure ohmic value of the trimmer R3. If this procedure
was properly followed, the value of the trimmer must exactly match the value of the resistor used for calibration (between 47 and 56 Ω).
Our instrument is ready to measure the impedance of all kinds of antennas!
Figure 9: To calibrate the bridge, connect an RF generator at its input and adjust the amplitude of the output signal until the meter needle is positioned at the bottom of a ladder or 1.5 V. 
Figure 10: Once this is done, connect a resistor between 47 and 56 Ω at the output of the bridge and turn the cursor trimmer R3 to tip the needle on the meter position 0 V. 
Figure 11: Disconnect the RF generator and the resistor connected to the output of the bridge. After switching the meter to ohms, connect its probes on the soul of the BNC input and output to read the value of the trimmer R3. This value is the resistance value previously used. Using the same principle, you can easily measure the value of impedance of an antenna on its frequency. 
Figure 12: To measure the impedance of a dipole antenna, set the generator on the center frequency of the antenna and turn the cursor trimmer R3 until the needle goes down to 0 V. Next, measure the value of the trimmer R3 (see Figure 11) which correspond exactly to the impedance of the dipole antenna.
Figure 13: To see if the doors of a multiband dipole have been calculated correctly, turn the cursor trimmer R3 to obtain a value of 52 Ω. Then, sweep band with RF generator, starting from the minimum frequency to go to the maximum frequency. You will notice that every time the generator will go on a tuning frequency, the needle of the meter will switch quickly to 0 V. 
Figure 14: For the tuning frequency of a mobile vertical antenna, it must first be established. Then connect to the impedance, the HF generator and the coaxial cable from the antenna. Adjust the trimmer R3 on 52 Ω and search on the HF generator, the tuning frequency which will deflect the needle of the meter position 0 V.
Figure 15: To control the transformation ratio of a balun, you must adjust the trimmer R3 on 52 Ω. Then, connect the primary of the balun on the output of the bridge. Then send an RF signal having the same frequency as that of the working frequency of the antenna. Then turn the cursor 500 Ω trimmer connected to the secondary of the balun, until the needle switches to 0 V. The value of 500 Ω trimmer is used to find the transformation ratio of the balun. 
Figure 16: To find the impedance at the output of a piece of coaxial cable 1 / 4 wave, we must first adjust the trimmer R3 until the meter indicates 52 Ω. Then set the HF generator on the working frequency of the antenna. Then turn the cursor 500 Ω trimmer connected to the cable outlet, until the meter needle moves to 0 V. The value that will be measured on the 500 Ω trimmer will help you calculate the output impedance of the coaxial cable. Note: the value of the impedance of the coaxial cable 1 / 4 wave is calculated with the formula given in the text.
Figure 17: Our fat monitor allows you to check a piece of coaxial cable cut on 1 / 4 wave folded and U form a transformer impedance of 52 Ω to 200 Ω. First set the trimmer R3 on 52 Ω. Then connect on the bridge entrance, the RF generator set to the working frequency of the antenna. On output, connect your coaxial impedance transformer. Turn the cursor 500 Ω trimmer until the multimeter needle tilts 0 V.
The reading across the trimming of 500 Ω to match the output impedance of the coaxial balun.
How to change the impedance of an antenna
All enthusiasts know the area of radio communications by changing the physical length of an antenna, it also changes its impedance. In the case of a directional antenna, composed of several parasitic elements, we may remove or allow the reflector or the first director of its dipole.
When you install an antenna whose impedance characteristic is given for 52 Ω, even if it is a commercial antenna and even if you paid dearly, she will always be a certain amount of standing waves.
Indeed, it has been calculated for ideal conditions, never achieved in real life a facility on a house roof.
The same holds for the antennas to be installed on vehicles. It is for this reason that the air always have a possibility of adjustment in length. Some models of vertical antennas for vehicles are fixed length but are equipped with a small metal disk that can slide along the entire height of the strand by acting as a capacitor (see Figure 14).
How to measure the impedance of an antenna
Above all, start by connecting the antenna to review the output of the bridge (see Figure 12), then select a frequency on the HF generator and adjust the trimmer R3 until the meter measures a voltage of 0 V.
Then, unplug the generator and the antenna of the bridge and measure the resistance value of R3 trimmer by placing the probes on the soul and out jacks (see Figure 11). The result of this reading will correspond to the value of the impedance of the antenna. For different values, simply varying the length of the dipole. Let us now
a concrete example of impedance measurement on an antenna 144-146 MHz.
Set the RF generator at 145 MHz (band center) and turn the cursor trimmer R3 until the meter needle moves to 0 V. Once the antenna is disconnected and the generator, multimeter (ohm switched on) will indicate a resistance value of 53 Ω. This means that the value of the impedance of the antenna is also equal to 53 Ω. When using
this bridge, you notice that when the frequency emitted by the RF generator increases, the needle of the meter falls below 0 V, but stabilizes at a voltage of about 0.5 or 0.6 V. But even then, you can still see clearly when the meter needle falls to the bottom.
How to control multiband dipole antennas
A number of commercial multiband dipoles are available on the market. It is also possible to build them yourself.
The realization of such dipoles beyond the scope of our article but know that "trap doors" are carefully placed on each of the strands forming the antenna. Their number varies depending on the number of bands to cover.
Note: The traps are LC circuits (self-capacitor) or LC equivalent and are intended to "cut" an antenna on electrically different frequencies. Of course, the value traps and their location on the antenna are calculated based on working frequencies to obtain.
To verify that the characteristics of these traps have been calculated correctly, adjust the trimmer R3 until a resistance value between 50 and 52 Ω. Then connect a coaxial cable from the antenna on the bridge output and the RF generator at its input (see Figure 13).
Suppose we wanted to verify the agreement of a multiband dipole on frequencies 14, 30 and 50 MHz. To do this, place the HF generator on the scale from 10 to 60 MHz and slowly turn the knob for adjusting the frequency.
If the traps have been calculated correctly, the needle of the meter reaches the value of 0 V when you spend over 14, 30 and 50 MHz. Note that if the needle moves to 0 V at frequencies different from those normally prescribed, it is necessary change the number of turns traps until you get a perfect synchronism.
Note: On multi-band dipoles, we can see a very interesting phenomenon: the needle of the meter switches to 0 V at each frequency equal to 3 times that of agreement. In our case, we will move the needle on 14 x 3 = 42 MHz, another on 30 x 3 = 90 MHz and the last on 50 x 3 = 150 MHz.
How to grant a mobile
To find the frequency at which a mobile antenna has an impedance of 52 Ω, you must first establish the location expected.
Then you must connect the RF generator to the input of the bridge and the coaxial cable from the antenna on its output.
After adjusting the trimmer R3 a resistance value of 52 Ω, turn the RF generator until the meter needle, also connected to the bridge switches to 0 V.
If the antenna has been manufactured to operate on frequencies between 144 and 146 MHz and the needle drops to 0 V when the generator goes on 140 MHz, this means that the strand must be shortened slightly.
Conversely, if the meter needle is 0 V when the generator passes for 150 MHz, this means that we extend the strand of the antenna.
As we said above, some antennas have a bit of fixed length but have a small metal disk slide can be moved all the way up. By moving the disc that you seek the impedance of 52 Ω (see Figure 14).
Control of a balun
The bridge that you made, also gives you the ability to control the transformation ratio of a balun and evaluate its bandwidth.
Note: balun is a contraction English words Balanced-unbalanced, which means balanced-unbalanced.
Put simply, a balun is designed to achieve a match between a dipole symmetrical construction and coaxial cable, it is asymmetrical.
It can also be used as an impedance transformer.
Before this measure, turn the cursor trimmer R3 to obtain a resistance value between 50 and 52 Ω. At the entrance to the bridge, connect an RF generator while its output, you connect the primary of the balun. On the secondary, connect a little trimmer than 500 Ω (see Figure 15). After
chosen the frequency of the generator, turn the cursor slowly trimmer 500 Ω Balun until the needle of the meter switches to 0 V. Then measure the resistance value at the terminals of the balun trimmer. If the meter indicates
, eg 200 Ω, the transformation ratio of the balun is:
For a different report and able to tailor the 52 Ω coaxial cable to the values of impedance of 250 Ω or 300, must be added to the secondary windings of the balun.
After finding the resistance value of the trimmer at the terminals of the balun producing a 0 V output of the bridge, change the frequency of the HF generator.
If you used a ferrite core having an average permeability, you notice that the meter needle does not move between frequencies from 7-100 MHz.
If you need a balun can operate below 7 MHz or above 100 MHz, you must choose cores with different permeability and check the frequency range of operation of your balun.
transformer coaxial cable 1 / 4 wave
To match two different impedance values, using a piece of coaxial cable of length equal to 1 / 4 wave. To know what must be the value of the impedance of this cable, use the following formula:
The length of the piece of coaxial cable 1 / 4 l must then be multiplied by its velocity factor, which is equal to 0.66 for cables of 52 Ω and 0.80 for those 75 Ω.
Because tolerances coefficients velocity, it frequently happens that the coaxial cable is cut longer or shorter than its ideal size.
We can reassure you by saying that the lengths of cables, obtained using the theoretical formulas are still slightly higher than definitive. This way, you'll be able to change and achieve the exact values.
bridge in our possession, we can also verify that the cable used has the correct length. To check this, turn the cursor of R3 to obtain a resistance value equivalent to the down cable antenna, that is to say between 50 and 52 Ω. Connect the RF generator and the coaxial cable 1 / 4 wave on the end which you must solder a 500 Ω trimmer (see Figure 16).
After adjusting the RF generator to the center frequency of the antenna, adjust the trimmer until you get a 500 Ω voltage 0 V reading on the meter.
Now, disconnect the coax and measure the resistance value of the trimmer. If this is higher or lower than the impedance of the antenna, you must lengthen or shorten, respectively, the length of coaxial cable.
As you probably noticed, the bandwidth of this piece of cable is very low. This means that an antenna with its center at 30 MHz band, this cable adapter will not work properly on the frequencies between 28 and 32 MHz.
If you try to get out of this range, it will lead to a significant increase in the VSWR.
By changing the frequency of the HF generator, you can know the value of maximum and minimum resonance of the antenna, because if these values are exceeded, the needle of the meter will rise rapidly to full scale.
When you master the bridge, you will realize how easy it is to make the measurement of impedance of an antenna, about its central frequency of job change and the transformation ratio any of a balun in order to adapt to the value of the antenna.
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