(2) Mismatch Error. The following discussion uses the terms conjugate

power, Zo available power, conjugate match and mismatch, and Zo match and

mismatch. These basic terms are defined as follows:

Conjugate power is the maximum available power. It is dependent on

a conjugate match condition in which the impedance seen looking toward the

thermistor mount is the complex conjugate of the impedance seen looking toward

the RF source. A special case of this maximum power transfer is when both the

RE source and the thermistor mount have the same impedance as the transmission

line.

Zo available power is the power a source will deliver to a Zo load.

It is dependent on a Zo match condition in which the impedance seen looking

into a transmission line is equal to the characteristic impedance of the line.

(3) In a practical measurement situation, both the source and thermistor

mount have SWR, and the source is seldom matched to the thermistor mount

without the use of a tuner.

The amount of mismatch loss in any measurement depends on the total SWR

present.

The impedance that the source sees is determined by the actual

thermistor mount impedance, the electrical length of the line, and the

characteristics impedance of the line, Zo.

(4) In general, neither the source nor the thermistor mount has Zo

impedance, and the actual impedances are known only as reflection coefficients,

mismatch losses or SWR.

These forms of information lack phase information

data. As a result, the power delivered to the thermistor mount and hence the

mismatch loss can only be described as being somewhere between two limits. The

uncertainty of power measurement due to mismatch loss increases with SWR.

Limits of mismatch loss are generally determined by means of a chart such as

the Mismatch Loss Limits charts in Figures 4 and 5.

(5) An example may explain how imperfect match affects the uncertainty

of power measurement. A typical Zo available power measurement situation can

involve a source with an SWR of 1.7 (ps = 0.26) and a thermistor mount with an

SWR of 1.3 (pm = 0.13). Figure 6 shows a plot of power levels and mismatch

power uncertainties that result from source and thermistor mount mismatch. The

source Zo mismatch results in a power loss of -0.29 dB from the maximum power

that would be delivered by the source to a conjugate match. The power level

that results from this loss is the Zo available power. The thermistor mount Zo

mismatch causes an additional power loss of -0.07 dB.

However, on the

thermistor mount Zo mismatch loss is an uncertainty resulting from the unknown

phase

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