There are three major reasons to VSWR or sweep test a coax and antenna system:
- System performance
- Coax performance
- Determining coax length to figure proper output power
The most important test is the overall system return loss test. This is performed by connecting all coax, jumpers, TMA’s (tower mounted amplifiers) or LNA’s (low noise amplifiers) and antennas to the test equipment and determining how much power is being reflected back to the transmitting equipment. This is usually the most important test that the transceiver equipment manufacturers require to maintain the equipment warranty. This value will usually need to be below (more negative than) –15 dB. Many manufacturers have more stringent values such as –17 dB. Many times this is the only test that is required; if the system test passes then usually all of the associated components of the system are installed and performing properly.
Sweep test pricing varies, based upon whether the technicians are already on site and the number of lines you will be required to sweep. In most cases, you will have to have two crew members on site. Many estimators will base their charge on an hourly rate for the anticipated amount of time that will be required or charge the client a flat fee of $150.00 per each transmission line. However, some sites have up to 36 lines so you will need to provide discounted pricing per line. Unless you are only sweeping one line, the height of the tower is typically not a variable in the cost. If the specifications call for specialized testing equipment, add the leasing cost to your quotation.
We recommend that you contact our listed sweep testing professionals to obtain additional information about their services, capabilities and experience.
Client/Contractor needs vary
Carrier/owner: Ensure that the contractor that will be testing your system is qualified to do so. Check to see if they have a manufacturer’s certification. Although this should give you an idea that this person is qualified, the lack of certification should not be used to disqualify a contractor. Probably the best way to make this determination is to see examples of other projects that the contractor has tested. Ask to see the documentation that they provided to their previous customer and have a knowledgeable party look it over.
Ask for and verify references for previous projects that this contractor has completed. Although not as cost effective, one way to verify the contractor’s results is to have an independent third party come in and do a verification of the tests by performing simple “system” tests through all components without needing a climber on the tower.
Contractor: Make sure to get a copy of your customer’s sweep testing procedures well ahead of time, if possible. Do not rely upon the client’s project manager’s assumptions or knowledge base. Different carriers have widely varying requirements, not only in procedures, but also in equipment. One carrier used to require that a contractor use an HP 8711,12,13, etc. This was mandatory and much more expensive than test equipment used today. Most markets do not have this requirement any more but there may be other issues.
Try to find out exactly what connectors the customer is using and which ones you will have to be able to connect your test equipment to at the bottom and your short/load at the top. Most customers would prefer, and it is good practice, to have all necessary adapters connected and calibrated out. The problem is, this could require you to carry four or more different types of short/open/load setups to be able to not only calibrate but then to use on the tower so as not to use any adapters. If you use quality adapters, you can usually get away with using one particular adapter that you are not able to calibrate through. Just be aware that more and more carriers and/or “turn-key” contractors are requiring this and it can be very expensive to have all of the proper RF cords, adapters and short/open/load setups. Find out their requirements ahead of time.
Return loss is second system test
The second test, to measure coax return loss, is mainly to determine that the installation crew did their job with properly installing, bending, grounding and installing the connectors on the coax itself. A coax return loss test is performed by installing a matching, typically 50 ohm, load to the end of the coax and determining how much power is being reflected back to the transmitting equipment. This value should be somewhere below –20 dB. This eliminates the possibility of the coax being installed improperly and, if the system test fails, will indicate that there is a defective antenna, jumper or TMA. If this is the case, then the installation crew will have to eliminate the components, one by one, to determine the failing component.
The last test usually required will be a DTF, or Distance To Fault, and/or insertion loss test. Sometimes it is incorrectly called a TDR test. TDR stands for Time Domain Reflectivity and is a system of testing coaxial, twisted-pair or optical cable by sending a timed pulse up the cable and determining how and where the reflected pulse is returned to the test equipment by time. This procedure is usually limited to twisted pair and optical cables. For coaxial cable used in wireless and other applications, most test equipment uses FDR, or Frequency Domain Reflectivity. This procedure takes a normal return loss test, as performed above with a system or coax return loss test, and mathematically determines where any occasions of signal reflection occur. These reflections usually happen at transitions between jumper and coax, jumper and TMA and coax or jumper and antenna. These reflections will be seen as “spikes” in a DTF test. These spikes should usually be less than –30 dB, except for possibly at the antenna and TMA. The rest of what should be only the coax should be less than –40 dB. Any spike that exceeds these values may be investigated for possible damage. If the overall system return loss test passes, however, and any “spike” is not greater than –25 dB then usually the system is determined to have “passed”.
Short circuit to identify length
The other method to determine coax length is by installing a short circuit on the end of the coax at the antenna. Prior to the common use of DTF this was the only method available to determine this value, especially when it was more important with analog cellular when setting proper output power of the transceivers was critical. This measurement determines how much power is being reflected back to the test equipment when it is completely “shorted”, meaning that all of the power that the test equipment is sending up the cable is being sent back to the equipment, minus the power that is being lost by the cable. This determines a “two-way” insertion loss, or cable loss. When averaged and divided by two this measurement gives a fairly accurate determination of power that is being absorbed by the coax in one direction.
VSWR or sweep testing is strongly suggested and almost universally required, to maintain the warranty of the transceiver equipment. It is good practice even if this is not the requirement of the manufacturer. Even the most qualified antenna crews can occasionally not connect components properly, not perfectly install connectors or never know if a factory jumper, TMA or antenna is defective. All of these failures can cause network performance problems and equipment damage.
Common return loss failures for a system sweep include:
- Improperly torqued or loose connectors
- Moisture in cable and connectors due to improperly applied or failed weatherproofing sealant
For additional information, please see RF Test Equipment.