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Antenna System Return Loss/SWR/DTF Measurements With The WE-2705P

Most antenna systems consist of both an antenna and cable feed. The most important considerations are:
  1. Feeder cable loss and cable condition.
  2. Antenna return loss (or SWR).
  3. Complete system return loss (or SWR).

As the complete system return loss is the sum of the return loss due to the cable alone, and the return loss of the antenna alone, it follows that a total system measurement can lead to incorrect conclusions concerning the effectiveness of the system. For example, a lossy cable run connected to a faulty antenna could give an adequate total system return loss figure. A typical minimum total system return loss figure is 10 dB (SWR 2.0). This would be achieved with a cable run of loss 5 dB, connected to a faulty antenna (antenna open or short circuit, or considerably mis-tuned).

Return loss measurements of >25 dB are often meaningless at the end of a feeder cable (particularly at frequencies above 1 GHz) , due to connector and cable impedance discontinuities. From this, it can be deduced that measuring the antenna in isolation is necessary to confirm its condition, unless the cable loss is very low i.e. less than a 1 dB.

This antenna return loss (or SWR) can be measured directly with the WE-2705P by first calibrating the analyzer at the end of a short interconnecting cable (e.g LMR-195 of length less than 1m) and then connecting this cable directly to the antenna. With reduced accuracy it is not essential to calibrate the WE-2705P at the end of the cable as this intermediate cable loss is typically about only 0.8 dB at 2.4 GHz (1m of LMR-195 cable).

Refer to the specifications for the antenna to see expected return loss and SWR figures. If these measurements are worse than expected, this may be due to:

  1.  Faulty antenna.
  2. Antenna de-tuned due to its position ( i.e effected by nearby metal objects).
  3.  Antenna incorrectly mounted (e.g may require ground plane).
  4.  Antenna needs adjustment (depends on antenna type).

An absolute minimum value for an antenna is typically 6 dB return loss (SWR 3.0) and more typically 10 dB (SWR 2.0). In any case, the return loss measured for the cable and antenna combined should typically not be less than 10 dB, otherwise the performance of the equipment connected to this system may be degraded. This is all dependent on the radio communication system under consideration, and the requirement may be more stringent. Refer to the specification of the radio equipment for this requirement.

Return Loss and Distance To Fault measurements can be done on the disconnected feeder cable alone to access its condition.

If the cable is in good condition, the measured return loss in dB should be twice its expected loss in dB. The DTF reading should correspond to its length.

Faults in a cable run will show up on a complete antenna system, without having to disconnect the antenna from the feeder. They will show up as vertical lines on the DTF plot at distances less than the overall cable length. A vertical line will always appear at a distance corresponding to the total length of the cable, as the antenna is typically only matched to the cable over a narrow portion of the frequency range used during the DTF analysis in the WE-2705P.

Please note that small vertical lines in the DTF plot do not necessarily imply a fault. These often relate to negligible impedance discontinuities in the system. However, significant faults will always be revealed by the DTF plot, therefore the DTF feature is most useful for locating the position of faults in a known faulty system, rather than to establish the condition of a system.