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Testing Antennas with a Network Analyzer
Since using a network analyzer is a mature process, there has been plenty of time to develop techniques to adequately characterize an antenna system. Both manufacturers and consulting engineers have contributed to this process. Although you will typically see variances in the plots presented, most operators familiar with an analyzer's use will tend to follow a certain pattern in the data they acquire.
When sweeping an FM system I will typically look at the VSWR and return loss across a 1MHz span centered on the carrier frequency as well as across the entire FM band. These techniques illustrate the system response within the occupied bandwidth of the FM channel in detail as well as illustrate that the system truly has the minimum VSWR near the frequency of operation and not somewhere else in the band. The frequency domain plots are then rounded out with either a polar return loss or Smith chart across 1MHz to demonstrate a decent match is occurring at both ends of the system.
Following these measurements, I switch over to the time domain to illustrate the condition of the transmission line. Under this technique a transformed span of 100MHz width is utilized as is a wider span of around 350MHz. Both spans will illustrate the magnitude of reflections at connectors, while the wider span will actually illustrate deformations to line or in the case of rigid line the standoff insulators in a stick. Normally the VSWR at these locations should be 1.008 or less.
To wrap up, the plots illustrate a portion of the data acquired during the recent checkout of an FM antenna system. Figure 1 illustrates the frequency domain VSWR of the antenna system across 1MHz centered on the frequency of operation. Figure 2 illustrates the wideband time domain VSWR of the system. Figure 1 illustrates that the carrier VSWR is 1.02, and demonstrates that within 500kHz either side of the carrier a value of 1.10 is not exceeded. It should be noted that the minimum VSWR lies a little above the carrier frequency, which is preferable in areas where antenna icing can occur. The time domain plot illustrates three small reflections near the midpoint of the transmission line. These anomalies are due to a slight deformation of the transmission line resulting from routing around an existing antenna. The maximum VSWR in this area of 1.004 is most acceptable, and is less than that which would typically result at the connection of two transmission line flanges. This plot does not illustrate regular repetitive flange reflections, which is consistent with the fact that the transmission line was semi-flexible coaxial cable.
The network analyzer is without a doubt one of the most elegant pieces of test equipment. It is undeniable that having your system initially and regularly swept is an expense. That expense is, however, good insurance, and will in the end be less costly than unscheduled system repairs.
Ruck is the principal engineer of Jeremy Ruck and Associates, Canton, IL.
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