Bird Technologies 4304A - Manual (Page 22)

11
Chapter 3 Installation
When transporting the Bird 4304A, turn the element so that the arrow points
down and set the range switch to 5 W. This will shunt the meter circuit and pro-
tect the meter by dampening needle action.
Connections
Insert the Bird 4304A in coaxial transmission lines of 50
Ω nominal impedance.
The RF source can be connected to either side of the wattmeter without affect-
ing readings.
If non-50
Ω cables are used, a mismatch will occur causing inaccuracies in read-
ings. It is strongly advised that this condition be avoided. If this is not possible,
refer to "Impedance Mismatch" on page 9 for instructions on how to calculate
the actual VSWR.
Load Matching
When a Bird 4304A is used to tune a load to a transmitter and a good match is
obtained, removing the wattmeter will not change the match quality. A 50
Ω
load can terminate a 50
Ω transmission line of any length without altering condi-
tions at the transmitter. The wattmeter is just an additional length of 50
Ω line.
When the load is not well matched, e.g. an antenna with a VSWR of 2.0, the line
length between load and transmitter will transform the impedance as seen at
the transmitter. Removing the wattmeter shortens the total line length by four
inches plus two connectors. This is still not significant at low frequencies where
four to five inches is a small fraction of a wavelength, but at higher frequencies
the transmitter’s frequency or power output may be affected.
Transmission line theory shows that if the line length changes by exactly
1 ⁄
2
wavelength, the impedance at the transmitter will not change. So, to have iden-
tical match with the Bird 4304A in or out of the circuit, it is necessary to insert or
remove
1 ⁄
2
wavelength of line (including the wattmeter). Use a length of cable
which, when added to the wattmeter, equals
1 ⁄
2
wavelength at the frequency of
interest. If more than one frequency is involved, a separate cable length is
required for each. See Figure 5 for sample cable lengths.
WARNING
Never attempt to connect or disconnect RF equipment from the transmission
line while RF power is being applied.
Leaking RF energy is a potential health hazard.
CAUTION
Do not drop. Calibration could be disturbed or the meter could be damaged as
a result.