Practical_Antenna_Handbook_0071639586

588 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
ers almost always include a wattmeter, or directional coupler, that is capable of indicating
forward (incident) power and either reflected power or SWR. If no external
power amplifier is present, the transceiver wattmeter should be perfectly adequate
for all measurements of antenna and transmission line performance that can normally
be made at the transmitter end of the line. Similarly, T/R switching—whether
at low or high power levels—is now found inside all transceivers and even a few
amplifiers. Nonetheless, whether performed internally or provided in individual external
boxes, all the functions shown are typically found in a modern-Âday antenna
system.
In this chapter we will explore some of the more commonly used instruments to
help us test and evaluate the “health” of our antenna system, which we define as including
not only the antenna itself but the entire RF path between the antenna and the
transmitter or receiver. Because the feedpoints of most antennas—especially on the MF,
HF, and VHF bands—are up in the air, far from our reach, many of the devices described
here are intended to give us some idea of conditions “up topside” while we are
at ground level.
When measuring, monitoring, or troubleshooting an antenna system, we are typically
interested in certain key attributes of the system components:
• Signal strength at a distance
• Impedance matching throughout the system
• RF attenuation (loss) throughout the system
• Useable bandwidth of the system
A Review of SWR
One of the most important characteristics of an antenna system we can measure is
the standing wave ratio (SWR or VSWR) at the input to the system. In our definition, the
input is the end of the system farthest from the antenna or radiating load when we are
transmitting RF energy to the antenna. (The system functions a little differently, and
we examine it differently, when the antenna is being used for receiving purposes.) As
we saw in Chap. 4, the SWR can be calculated in various ways, depending on the information
we have available to us.
In general, if the antenna load impedance (Z L ) is not equal to Z 0 we define SWR
from one or the other of the following:
If Z 0 is greater than Z L :
Z 0
VSWR =
Z
L
(27.1)
If Z 0 is less than Z L :
Z
VSWR = L Z0
(27.2)