Bird Technologies APM-16 - Manual (Page 20)

8
Low Reflection
 = 10% ( = 2) is the typical limit of antenna match. Further effort is frequently
not worthwhile because below this level reflected power is hard to measure,
and W
l
can not be significantly increased. TV and VHF transmitters are examples
of systems requiring lower reflected power but for reasons other than maximiz-
ing power transmission.
When the same element is used to measure both forward and reflected power,
meaningful readings are possible down to about  = 5% ( = 1.5). For accurate
measurement of very low levels of reflected power, i.e.  = 0.6% ( = 1.17), use
a second element rated at one tenth of the full scale power of the forward ele-
ment. This method should not be used with element ranges differing by more
than 10:1.
For example, consider an 80 watt transmitter and a Bird APM-16 with 100 and
10 watt elements. Measure W
f
with the 100 W element. Measure W
r
with the
10 W element (make sure the arrow points towards the transmitter). W
r
can be
measured down to at least 0.5 W, so that  = 0.5 / 80 or about 0.6%, corre-
sponding to  = 1.17.
Transmitter Monitoring
The Thruline Wattmeter can be used for continuous monitoring of transmitter
output or reflected power, for instance while checking intermittent antenna or
line faults.
Component Testing
The Bird APM-16 is very helpful in component testing, and may be employed in
several ways:
1.
Insertion VSWR or  can be measured by placing the component between
the wattmeter and a good load resistor.
2. Attenuation (power lost by heat in a line) as well as insertion VSWR or  can be
measured by inserting the unknown line between two Thruline Wattmeters, or
between a Thruline Wattmeter and a Termaline Absorption Wattmeter.
Note: Very small attenuations require allowance for normal instru-
ment errors. To correct for this without any calculations, connect the
wattmeters directly, with no line between them, and adjust their zero
settings.
CAUTION
For low reflection measurements, do not rotate the reflected power element
to read forward power. Damage to the element or wattmeter could result.