Advanced Marine Electrics and Electronics Troubleshooting A Manual for Boatowners and Marine Technicians

Standing Wave Ratio

The standing wave ratio (SWR), which is the

ratio of the maximum RF current to the mini-

mum RF current on the line, can be thought

of as a measure of an antenna system’s effi-

ciency. (By system, I mean the antenna plus the

coaxial cable between the antenna and the

transceiver.) The smaller the ratio, the more

efficient the system, so an antenna system with

an SWR of 1:1 is experiencing less loss than

one with a 3:1 SWR. Antenna design can have

a profound effect on the SWR. By integrating

various choke methods into the antenna, man-

ufacturers can contain more RF power in the

antenna element, as opposed to allowing it to

leak back into the cable. You can think of the

choke as a one-way valve designed to separate

your antenna from the feed line, thus deliver-

ing all the power to the antenna for maximum

power and directional control of the transmit-

ted signal. Amateur radio operators call these

chokes “baluns,” for balanced/unbalanced, as

176 marine electronics installation and troubleshooting

they balance the signal between the antenna

and the cable.

To test the SWR ratio of a system, we

again use the power standing wave ratio meter

shown in the photo on page 175. The steps are

as follows:

1 Install the meter in series between the

radio transceiver and the coaxial cable

routed to the antenna.

2 With the meter switched to the SWR

function, key the radio microphone and

observe the meter’s reading.

3 Use the graph on the back of the meter to

convert your reading based on the wattage

output you measured (see page 174).

Align your SWR reading on the X-axis

and with the Y-axis wattage reading to

find the corrected SWR value.

An acceptable SWR is a matter of opinion.
Some say that 3:1 is OK; others use a 1.5:1
ratio as their maximum. A value of 2:1 equals
about a 90% efficiency level, and this is quite
good. A 3:1 ratio represents about 75% effi-
ciency, which, in my opinion, is a barely pass-
ing grade. A quality installation will give you a
corrected reading between 1.2 and 1.6.
My Micronta (RadioShack) SWR/power
meter (shown in the photos) is a long-
discontinued model. Similar inexpensive
meters, such as the ART-2 from Shakespeare,
are available for about $50 through Boater’s
World stores (www.boatersworld.com). A
worthwhile step up for some readers might
be one similar to the Vectronics model listed
in the sidebar, available from RadioShack for
about $230. Some electronics specialists
might benefit from a high-end unit, such as
the one from Bird Electronic; however, at
prices ranging from $1,200 to $1,500, these
are not worth it even for advanced-level
The conversion graph on the back of the meter. boaters.