Practical_Antenna_Handbook_0071639586

618 P a r t V I I : T u n i n g , T r o u b l e s h o o t i n g , a n d D e s i g n A i d
network analysis capabilities. In addition to swept plots of VSWR, feedpoint R + jX,
and Smith chart displays of antenna performance across the entire band, VNAs can
function as time domain reflectometers (TDRs), providing a detailed graphical look at
pulsed waveforms sent down a transmission line. At each point along the line that an
outbound pulse encounters an impedance other than the Z 0 of the line, a reflected pulse
is created that travels back to the instrument, where it is detected and plotted, very
much consistent with the transmission line pulse analysis of Chap. 4. A VNA’s ability to
locate every connector, every splice, every transition in the feedlines, and—most important—any
potential fault (short circuit, open circuit, or other unexpected discontinuity)
between the radio room and the antennas hundreds of feet away is amazing!
Figure 27.23 shows a VNA trace of the entire RF path from the radio room to an
antenna, obtained while troubleshooting the author’s 20-Âm monoband gamma-Âmatched
Yagi. In the first 39 ft of the path, the outbound pulse encounters a series of short lengths
of 50-ÂW cable and a few inline devices along the path—in particular, connectors, a lightning
arrestor, and various relay boxes. While the lumped-Âcomponent devices and circuits
exhibit nominal 50-ÂW impedance at HF, the wide bandwidth of the VNA pulses
exposes their departure from 50 W at other frequencies. The last of the positive-Âgoing
impedance bumps on the left side of the chart, at 39 ft from the VNA, signals the junction
of 50-ÂW coaxial cable to 75-ÂW hardline. Near the top of the tower, at a total distance
of 159 ft from the VNA, the hardline connects to a 39-Âft “pigtail” and rotator loop of
50-ÂW cable that completes the path to the SO-Â239 connector at the feedpoint of the four-Â
element beam. (Little additional information can be gleaned from the wiggles and
bumps to the right of that spike.)
Because of their circuit implementations, the N2PK and AIM 4170 VNAs are reportedly
less susceptible to measurement errors from ambient RF in the vicinity of the analyzer
than the older SWR analyzers described earlier.
Figure 27.23 VNA TDR trace example.