Figure 4. Cross section of the Type 874-QV2A Adaptor to 5T.5 ohm, 134-inch rigid line.
between this type of line and General Radio Type 874 Connectors. A tapered section of line is again used for the transition element. However, since the Type 874 line has a 50.0-ohm characteristic impedance and the VHF line has a 51.5-ohm characteristic impedance, a small reflection is produced at the junction. In the adaptor, the change in characteristic impedance is made directly behind the polystyrene insulator in the Type 874 Connector, as shown in Figure 4.
Meter is used to measure either admittance or standing-wave ratio, using the null method with the meter connected directly to the end of the adaptor, the length of the 50 ohm line is so short that it has a negligible effect on the measurements over the VHF-TV band (less than .015 effect on the VSWR at 200 Mc when the VSWR is near unity and proportionately less at lower frequencies). Measurements can also be made with maximum accuracy using the voltage-ratio method with minor modifications.
The reflection introduced by the adaptor in addition to that introduced by the change in characteristic impedance produces less than a 1.02 VSWR on a flat line over the VHF-TV band.
The adaptors just described enable the television engineer to make wider applications of a large number of very useful existing devices and thus simplify his measurement problems.
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