Standing wave measuring device
A standing wave measuring device (English SWR meter ) is a device for measuring the standing wave ratio . With it, for example, the high-frequency waves running in a coaxial cable can be recorded separately according to their direction. It allows a statement to be made about the degree of mismatching of an antenna or a substitute load at the end of the cable.
The standing wave ratio ideally (i.e. no mismatch) is 1.
Measurement with directional couplers
A standing wave meter for very high frequencies consists of one or two directional couplers , rectifiers and a voltage measuring device, which is optionally connected to one of the two outputs, or two measuring mechanisms whose pointers cross each other (cross-pointer instrument).
One value is a measure of the tension of the incoming wave, the other a measure of the tension of the returning wave. The length A of the coupling wires cannot be chosen arbitrarily:
- A must be shorter than the wavelength λ of the measurement signal. The absolute upper limit is λ / 4.
- If A is too short and the decoupled voltage is only about as large as the threshold voltage of the diodes, the measurement error increases enormously. Directional couplers of this type are therefore hardly usable at wavelengths over 50 m.
Broadband directional coupler
At frequencies below 5 MHz, the pieces of wire are replaced by current transformers ( straight-through transformers ). This means that the measured voltage is almost independent of the wavelength.
One possibility is the Bruene directional coupler, which combines a current transformer with two adjustable capacitors.
In the directional coupler according to Sontheimer-Frederick, two identical current transformers are used to
- transform the current of the inner conductor in the ratio n: 1 with T1 and
- to transform the voltage between the inner and outer conductor in the ratio n : 1 with T2 .
This means that the impedance U / I is preserved. The coupling constant is calculated as C 3.1 = 20 · log ( n ). The two resistors R1 and R2 of the transformer T2 must have the same value as the characteristic impedance of the coaxial cable between P1 and P2.
Direct voltage measurement
With sufficiently short wavelengths and very high demands on measurement accuracy, the voltage curve on the inner conductor of a cut coaxial cable ( slotted line ) is determined. The (often too high) voltage of the inner conductor is not measured directly, but a small fraction is capacitively coupled out and thus the λ / 4 resonant circuit shown in orange is excited, at whose tap the HF voltage is rectified. Resonance is set with a movable short circuit at the left end of the oscillating circuit. The minimum length of the slotted line is λ / 2, which is why its applicability is limited to the VHF range.
Measurement method: You are looking for a point on the waveguide where a particularly large effective voltage U Max can be measured. At a distance of λ / 4 from this, the voltage U Min must be particularly small. The standing wave ratio sought is calculated as follows
calibration
The standing wave measuring device to be calibrated is switched between the transmitter and the equivalent load; the connecting coaxial cables should be considerably shorter than a quarter of the wavelength at which the measurement is to be made, in order to avoid distortions due to cable resonance . The standing wave ratio for different values of the equivalent load can be calculated and used to check the displayed values.
Where should the SWR be measured?
In the case of long cables, cable resonances are the reason that standing wave ratios that differ greatly in some cases are displayed at different measuring points . The optimal measuring point is at the connection point between cable and antenna, because then the actual base point resistance of the antenna is compared with the calibration value of the standing wave measuring device . However, this point is often inaccessible or inconvenient to reach. This is why the measuring device is usually connected immediately after the transmitter. The correct value is only measured at this point if the (mechanical) cable length is an integer multiple of the shortening factor × λ / 2, because then there is a 1: 1 transformation .
Enormous deviations from the true value occur when the cable length is an odd multiple of the shortening factor × λ / 4 and the SWR shows values over 2.
Each waveguide attenuates the waves passing through it by a certain percentage , which is why the measurement result at the beginning of the cable differs from that at the end of the cable. The often serious consequences can be shown with a simple example: A transmitter with a power of 100 W is connected to an antenna via a coaxial cable, the cable attenuates by 3 dB, and only 50 W arrives at the antenna.
- If the antenna is short-circuited or the connection is torn off, this power is completely reflected, which is why the SWR → ∞ is measured and calculated directly at the antenna.
- The reflected power is also attenuated by 3 dB, only 25 W arrive at the transmitter and here the measuring device shows SWR = 3. That may be enough for modest claims, but it is wrong.
- If the cable attenuation increases to 4 dB due to moisture, the SWR measured at the transmitter drops to the acceptable value 2.3 and signals a functional system, although the antenna does not emit anything.
Nevertheless, the standing wave measuring device is almost always operated directly on the transmitter because it is much more accessible there than on the antenna.
If the signal attenuation along the cable is sufficiently large, an acceptable SWR can be displayed despite the complete mismatch. For example, a 30 m long piece of RG58U coaxial cable at 432 MHz can be used as a dummy load up to around 200 W because the reflected wave is attenuated by a total of 20 dB. Therefore - regardless of the terminating resistor - a maximum of 1% of the fed-in power arrives at the transmitter, from which the very low SWR = 1.22 is calculated. For lower frequencies, a correspondingly longer cable must be used to achieve the required total attenuation.
application
The wave impedance of the high-frequency cable and the antenna or load must be matched to one another as well as possible in order to avoid unnecessary losses due to power reflected into it. The standing wave measuring device does not allow any statement as to how well the waves are radiated by the antenna. The impedance of the transmitter deviates strongly from the characteristic impedance of the cable at high outputs in order to enable an efficiency of over 50% (see line adaptation ).
Standing wave measuring devices are used in the construction and operation of high-frequency systems
- for impedance matching of transmitting antennas
- for monitoring transmission antennas during operation, here often provided with additional devices which reduce the transmission power or switch off the transmitter in the event of significant mismatching
- for monitoring devices for high-frequency heating, plasma generation and excitation of gas lasers (e.g. HF-excited CO 2 lasers ).
Web links
Individual evidence
- ↑ Bruene directional coupler (PDF; 250 kB)
- ↑ Bruene SWR measuring device
- ↑ Bruene-SWR with improved accuracy
- ↑ a simple SWR / wattmeter (PDF; 144 kB)
- ^ Thomas H. Lee, Planar Microwave Engineering: A Practical Guide to Theory, Measurement, and Circuits, Cambridge University Press, 2004, ISBN 0521835267
- ↑ SLOTTED LINE MEASUREMENTS (PDF; 1.7 MB) in English
- ↑ Measurement method "slotted line" from page 16
- ↑ HIGH FREQUENCY SLOTTED LINE AND REFLECTOMETER MEASUREMENTS (PDF; 37 kB)
- ↑ The Slotted Line ( Memento of the original from March 28, 2014 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. (PDF; 1.3 MB)
