Bird Technologies 4331 - Manual (Page 32)

is less than 10 percent and the frequency is below 200 MHz, the four-inch
length will produce a mismatch that is not too serious. But going any higher
than these values, even if the transmitter is tuned up with the Model 43 in
place, the load impedance will be very different after the Thruline Wattmeter
is removed.
The Model 43 in a 50 ohm line will indicate zero reflection when the load,
at its load connector, is 50 ohms, pure resistive. However, even an ideal
condition on a 70 ohm line on the load side of the wattmeter will show three
percent reflected power and cause a VSWR of 1.4 to 1.0. (If Thruline
Wattmeter is 50 ohms and load is 70 ohms resistive, VSWR in the wattmeter
is 70/50 = 1.4.) The Thruline Wattmeter could also show this same reflected
percentage with a 50/1.4 = 35.7 ohms pure resistive load which could exist
with 10 percent reflected power on the 70 ohm line (VSWR = 2). From this
you can see that the 70 ohm line could actually have as much as 10 percent
reflected power and a VSWR = 2 even though the Thruline Wattmeter
indicates only three percent reflected power, and a VSWR = 1.4.
It should be especially remembered that with 70 ohm lines it is most
important to measure the reflected power and subtract it from the forward,
because of this factor being so much more critical here than with a 50 ohm
line.
Bird Model 43 Wattmeter
18