Bird Technologies 4331 - Manual (Page 25)

be readily shown that reducing φ below 10 percent produces little gain in
the way of increased W
l
. TV transmitter antenna lines, and VHF omnirange
transmitters are among those systems that require much lower levels of
reflected power but for reasons other than simple power transmission. Note
that in figure 4, the very small level of reflected power, i.e., φ = 0.06 percent
corresponds to p = 1.05. While both forward and reflected power levels can
be measured with the same element, detection of reflected power is possible
down to about φ = 1 percent (p = 1.2), provided W
f
approaches full scale.
However actual measurement is possible only down to about φ = 5 percent
(p = 1.5). For more accurate measurements of reflected power levels, an
element rated at one tenth of the full scale power value of the element used
to measure forward power is recommended.
CAUTION
When making low reflection readings using a more sensitive
element, take care to insert the element so that it senses
reflected power only. Do not rotate the element in the socket so
that it is subjected to forward power. This can result in damage
to the Plug-In Element, the microammeter, or both.
Low reflection measurements may be extended below this with two elements.
Say 80 watts are available, and you have 100 watt and ten watt elements.
Measure W
f
with the 100 watt element. Remove the 100 watt element and
insert the ten watt element. The ten watt element must be inserted only in
the reflected direction, “arrow” toward transmitter. Insert and remove only
this way. Now read W
r
on the ten watt element.
In this case, measurement down to at least 0.5 watt reflected is possible,
which means to say:
φ = =
0 5
80
0 6% 116
.
. . or close to or to roughly p
Theory of Operation
11