Understanding Cable and Antenna Analysis- Anritsu America
9/03A. VSWR, Return Loss and. Transmission Loss vs. Transmitted Power. Return. Trans. Volt. Power. Power. Return. Trans. Volt. Power. Power. Loss. Loss. Standing waves form when there is a mismatch between the line and the load, and is This calculator computes the VSWR, reflection coefficient, return loss and. are Voltage Standing Wave Ratio (VSWR) and Insertion Loss. These are loss of power due to inserting the cable between the source and the load. . of the two reflection coefficients, in relation to each other, also varies.
When a transmitter is connected to an antenna by a feed linethe driving point impedance of the antenna must match the characteristic impedance of the feed line in order for the transmitter to see the impedance it was designed for the impedance of the feed line, usually 50 or 75 ohms. The impedance of a particular antenna design can vary due to a number of factors that cannot always be clearly identified. This includes the transmitter frequency as compared to the antenna's design or resonant frequencythe antenna's height above the ground and proximity to large metal structures, and variations in the exact size of the conductors used to construct the antenna.
Installing the tuner between the feed line and the antenna allows for the feed line to see a load close to its characteristic impedance, while sending most of the transmitter's power a small amount may be dissipated within the tuner to be radiated by the antenna despite its otherwise unacceptable feed point impedance.
Installing a tuner in between the transmitter and the feed line can also transform the impedance seen at the transmitter end of the feed line to one preferred by the transmitter. However, in the latter case, the feed line still has a high SWR present, with the resulting increased feed line losses unmitigated. The magnitude of those losses are dependent on the type of transmission line, and its length.How to Measure Insertion Loss - N9344C, N9343C, N9342C Handheld Spectrum Analyzers - Keysight
They always increase with frequency. For example, a certain antenna used well away from its resonant frequency may have an SWR of 6: For a frequency of 3. However the same 6: Certain types of transmissions can suffer other negative effects from reflected waves on a transmission line. Analog TV can experience "ghosts" from delayed signals bouncing back and forth on a long line. FM stereo can also be affected and digital signals can experience delayed pulses leading to bit errors.
Standing wave ratio
Whenever the delay times for a signal going back down and then again up the line are comparable to the modulation time constants, effects occur. For this reason, these types of transmissions require a low SWR on the feedline, even if SWR induced loss might be acceptable and matching is done at the transmitter. Methods of measuring standing wave ratio[ edit ] Slotted line.
The probe moves along the line to measure the variable voltage. SWR is the maximum divided by the minimum voltage Many different methods can be used to measure standing wave ratio. The most intuitive method uses a slotted line which is a section of transmission line with an open slot which allows a probe to detect the actual voltage at various points along the line.
This method is used at VHF and higher frequencies. At lower frequencies, such lines are impractically long. Directional couplers can be used at HF through microwave frequencies.
Some are a quarter wave or more long, which restricts their use to the higher frequencies. Other types of directional couplers sample the current and voltage at a single point in the transmission path and mathematically combine them in such a way as to represent the power flowing in one direction.
Other types use a single coupler which can be rotated degrees to sample power flowing in either direction.
VSWR / Return Loss Calculator - Electrical Engineering & Electronics Tools
These measurements show the user the match of the system and if it conforms to system engineering specifications. If problems show up during this test, there is a very good likelihood that the system has problems that will affect the end user. A poorly matched antenna will reflect costly RF energy which will not be available for transmission and will instead end up in the transmitter.
This extra energy returned to the transmitter will not only distort the signal but it will also affect the efficiency of the transmitted power and the corresponding coverage area. While different systems have different acceptable return loss limits, 15 dB or better is a common system limit for a cable and antenna system.
The return loss displays the ratio of reflected power to reference power in dB. The return loss view is usually preferred because of the benefits with logarithmic displays; one of them being that it is easier to compare a small and large number on a logarithmic scale. The return loss scale is normally set up from 0 to 60 dB with 0 being an open or a short and 60 dB would be close to a perfect match. VSWR measures the ratio of voltage peaks and valleys.
Understanding VSWR and Insertion Loss Plots
If the match is not perfect, the peaks and valleys of the returned signal will not align perfectly with the transmitted signal and the greater this number is, the worse the match is. A perfect or ideal match in VSWR terms would be 1: Antenna manufacturers typically specify the match in VSWR. The scale of a VSWR is usually defaulted to setup between 1 and The trace in picture 1 shows a Return Loss measurement of a cellular antenna matched between MHz.
The Return Loss amplitude scale is setup to go from 0.
Understanding Cable and Antenna Analysis
The VSWR display in the right graph measures the same antenna and the amplitude scale has been setup to match the scale of the Return Loss measurement.
Return Loss display Picture 2: Cable Loss can be measured using the Return Loss measurement available in the cable and antenna analyzer.
By placing a short at the end of the cable, the signal is reflected back and the energy lost in the cable can be computed. Equipment manufacturers suggest to get the average cable loss of the swept frequency range by adding the peak of the trace to the valley of the trace and divide by two in cable loss mode or divide by four in return loss mode to account for signal travel back and forth.
This is usually the preferred method since it eliminates the need for any math. The graph in picture 3 below shows a cable loss measurement of a cable between and MHz.
The markers at the peak and valley can be used to compute the average. This particular handheld instrument computes the average cable loss for the user as can be seen in the left part of the display. Cable Loss Measurement Increasing the RF frequency and the length of the cable will increase the insertion loss. Cables with larger diameter have less insertion loss and better power handling capabilities than cables with smaller diameter.
The picture below illustrates how the cable loss changes the perceived antenna performance. Even though this is something system designers take into consideration when setting up the specifications of the site, it is important to be aware of the effects the insertion loss and also cable return loss can have on the overall system return loss.
A very good system return loss may not necessarily be a result of an excellent antenna; it could be a faulty cable with too much insertion loss and an antenna out of specification.
This would result in a larger than expected signal drop and once the signal reaches the antenna, a great portion of the signal is now reflected since the match is worse than expected.