Practical_Antenna_Handbook_0071639586

C h a p t e r 2 7 : T e s t i n g a n d T r o u b l e s h o o t i n g 617
If necessary, these tests can be run with a single meter by alternately inserting it in
the signal path at each end of the coaxial cable being tested. This has the advantage of
eliminating any errors caused by differences in the calibration of two meters but has the
disadvantage of changing the test configuration in midmeasurement. When using the
single meter approach, it is extremely important that there be a very good match between
the cable and the termination—a very low SWR, as close to 1.0:1 as possible, in
other words.
In general it is wise to perform these measurements on at least two different frequencies—preferably
at the extremes of the frequency range over which the cable will
be used. If the cable length is 100 ft, the loss found from Eq. (22.1) is the loss in decibels/100
ft. But if length L is anything other than 100 ft, use the following calculation to
obtain the matched loss per 100 ft, since it is this number that you will want to compare
with the manufacturer’s published specifications:
Loss = Loss dB
dB/100ft
100
L( ft) × (27.20)
Of course, the same type of measurement can be performed on other types of coaxial
transmission lines as long as a good match between the nominal line impedance
and the termination is maintained. Remember, however, that virtually all commonly
available hardline has a characteristic impedance of 75 W, not 50 W.
Vector Network Analyzers
The most recent addition to the stable of affordable analyzers for antenna systems is the
vector network analyzer. Historically, precision antenna analyzers have been quite big and
quite expensive laboratory instruments, but VNA designs for the amateur marketplace
are now available on a card or in a small enclosure for under $1000—plus the price of the
associated PC or Mac, of course! Inherent in its name, a VNA provides resistance, reactance,
and the sign of the reactance. VNAs rely on an active USB connection to a PC or
Macintosh during their operation, so they are not easily taken to the top of a tower. Instead,
VNAs have a procedure for calibrating out the effects of a transmission line between
the antenna and the VNA—something not possible with the simpler analyzers
described in the previous section. Basic VNA theory can be found on the Internet by
googling “vector network analyzer basics” or “vector network analyzer tutorial”.
VNAs for the amateur can be built by experimenters with access to surface-Âmount
device (SMD) mounting capabilities, or they can be purchased assembled. The seminal
articles on building such a VNA are from Paul Kiciak, N2PK; his Web site is a good
starting point for anyone interested in this path. For a while, a VNA kit designed by
Tom McDermott, N5EG, and Karl Ireland was available from TAPR, the Tucson Amateur
Packet Radio group that produced some of the earliest terminal node controllers
(TNCs) for packet radio, but the organization has gone out of the VNA manufacturing
business, so only used TAPR units are available. More recently, rights to the TAPR design
have been licensed to Ten-ÂTec, and the company has added it to its product line—
fully wired and tested, not as a kit—as the Model 6000 VNA. A competing unit, designed
by W5BIG, is offered as the AIM 4170 from Array Solutions, Inc.
VNAs are not just useful for antenna and transmission line analysis, but that is the
focus of this book, so we won’t expand our discussion to include their more general