7

^{becomes troublesome, making the reading of
min
unreliable.} For the same reason, the directivity of the hybrid tee should not be checked against the matched load. The directivity is obtained to the best available accuracy from a mismatched load of approximately 1.05 VSWR.

The accuracy of the hybrid tee decreases as the measured VSWR value increases. On 60 dB guaranteed accuracy hybrid tee reflectometers, VSWR values up to 1.20 are accurate with a 60 dB certainty. VSWR values up to 1.40 are accurate with a 50 dB certainty, and VSWR values up to 2.0 are accurate with a 40 dB certainty. Calibrating the hybrid tee to a short circuit is not accurate.

Although the Andrew hybrid tee reflectometer provides the basis for very accurate measurements of return loss, it depends critically on the ancillary equipment having sufficient dynamic range and being accurately calibrated. Care is needed to ensure that the detectors are frequently checked against the calibration procedures within the sweep equipment. The calibration of the sweep equipment itself should also be maintained.

Measurement Procedures For antenna/waveguide systems, antennas, or other components incorporating their own termination, the return loss is obtained directly by attaching the component or system input to the hybrid tee. The peak value may be read frrom the sweep display. An average r.m.s. figure may be deduced from the display by analysis of the display waveform.

For measurement of components not containing their own terminations (waveguides, connectors, etc.), an external matched load is used. Fixed loads with 1.01 maximum VSWR are usually adequate. Sliding loads can offer additional accuracy to the measurement.

By sliding a matched load through one-half wavelength and observing the maximum and minimum points, the reflection coefficient of the load may be separated from that of the waveguide or component under test, thus permitting a measurement to be made of the component alone.

Mismatch Conversion Table ^{This table shows the relationship over a wide range of values,
between return loss, VSWR, and reflection coefficient expressed
in percent (i.e.,
x 100).} The following formulas can be used for conversion values not listed in the table.

VSWR =	^{1 +}

^{1 -}

R.L. = -20 log ₁₀

⁼	VSWR - 1

VSWR - 1

⁼	antilog₁₀	- R.L.

20

Return loss, R.L., is the decibel power ratio of the power to the reflected power.

^{Reflection coefficient,
, is the
numerical ratio of the reflected voltage to the incident voltage.}

Voltage standing-wave ratio, VSWR, is the numerical ratio of the maximum voltage to the minimum voltage that would exist on the uniform reference transmission line.

Page 6 of 12 -- Page 7 of 12 -- Page 8 of 12