The antenna analyzer is a device made up of two basic components:
- A sinusoidal radio frequency generator of known frequency
- An impedance bridge, where one of the four impedances of the bridge is from the antenna to be measured.
For a given frequency, the antenna analyzer has a series of commands that allow the three known impedances of the bridge to be varied, until the balance between the four impedances is obtained.
Once equilibrium is established, a simple mathematical relationship is established between the three known impedances and the unknown impedance, which allows the complex antenna impedance to be calculated.
Summary
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- 1 Components of an antenna analyzer
- 1 The sinusoidal generator
- 2 Reading the impedance value
- 3 Uses of the antenna analyzer
- 4 Limits of antenna analyzers
- 5 Sources
Components of an antenna analyzer
An antenna analyzer is made up of a sinusoidal radio frequency generator -of known frequency-, and an impedance bridge .
The noise bridge is a variant of the antenna analyzer, which instead of generating a single sinusoidal frequency generates them all.
The sinusoidal generator
The sinusoidal generator can be built with different technologies.
- Older generators used LC oscillators made with vacuum valves; they were later replaced by LC oscillators to transistors.
- The most recent generators use digital frequency synthesis circuits to be able to obtain very clean sinusoids – free of harmonics – with precision of the order of the hundredth of Hz; In these devices, the maximum usable frequencies are generally up to one third of the clock speed of the frequency synthesizer circuit.
The impedance bridge is an array of four impedances, two in series, in parallel with two others in series. A microammeter (a value of 500 mA is a typical value) links both branches at the midpoint of each branch. The impedance bridge is a generalization of the Wheatstone bridge .
For a given frequency, when the value of the current through the microammeter is zero, the impedance bridge is said to be balanced . In that case – and only in that case – a mathematical relationship relates the four impedances. It is then enough to know three impedances to be able to calculate the fourth.
Reading the impedance value
Older analyzers used a microammeter to measure the current at the midpoint of the impedance bridge. The operator manually varied the three known impedances until the current through the midpoint was canceled; At a given frequency, once equilibrium was obtained at the bridge, a simple mathematical operation allowed calculating the unknown impedance.
Some analyzers, such as the MFJ 259 and 269, introduce a microprocessor that automatically varies the three known impedances and calculates the unknown impedance. These antenna analyzers are fully analog, and only have an analog-to-digital converter (A / D) to allow a comfortable reading of the calculated value. For this reason they do not allow us to know the sign of the imaginary part of the impedance, which practically translates into the impossibility of knowing if the antenna is very short or very long with a single measurement.
Other more modern designs, such as the Palstar ZM-30, automatically calculate both the real part and the imaginary part of the antenna impedance, as well as the sign of the imaginary part.
Uses of the antenna analyzer
In addition to analyzing antennas, they can be used for other uses, such as:
- calculate inductances
- calculate capacitances
- calculate the speed coefficient of a cable (two-wire or coaxial). This value of speed coefficient is extremely important for the development of some antennas, such as the G5RV
- calculate the merit factor Q of a coil
- calculate the transfer function of a ladder filter to quartz, as a function of frequency
- sine frequency generator
Antenna analyzer limits
Antenna analyzers are exposed to various measurement disturbances:
- Existence of medium wave AM stations in the vicinity (verified in the MFJ analyzers)
- The existence of a contact between the body of the experimenter and the analyzer (verified in the Palstar ZM-30 analyzers)
- Errors in the calculation of the antenna sign (verified in the Palstar ZM-30 analyzers)
- Static electricity from an antenna (especially an antenna subjected to friction forces by the wind, and in particular, the antennas supported by a balloon or a kite) can destroy some devices such as the Palstar ZM-30; the manufacturer recommends connecting the antenna to ground before connecting it to the device.)
- In general, very large or very small impedances are difficult to measure, and their values must be taken with caution. The antenna analyzer is not a substitute for common sense.
