## Transmission Tabsheet

The Transmission tabsheet (Fig. 2) groups the quantities related to power transmission in the system, and the load-related powers.

Fig. 2. Transmission tabsheet.

Interference Factor

In the source-to-load power transmission formula (transducer gain), the interference factor accounts for multiple reflections between the source and the load.

If neither the source nor the load are perfectly matched, a wave travelling between them theoretically experiences an infinite number of re-reflections. The resulting wave propagating in one direction can be thought of as consisting of the primary wave and an infinite number of additional contributions with ever decreasing amplitudes. The magnitude of the resulting wave, and hence the transmitted power, both depend on the phases of the individual contributions. These phases, in turn, depend on:

- The source and load reflection coefficients.

- The electric length of the interconnecting waveguide.

This resonator-like effect, modifying the source-to-load power transmission, is expressed mathematically in terms of the interference factor.

If at least one of the transmission line ends is matched, the infinite series does not exist since the matched end completely absorbs the wave incident upon it, thus prohibiting the buildup of the series. In such case, the interference factor is unity.

The interference factor and its bounds for varying phases are displayed in the Interference Factor row of the Transmission page. The factor can be displayed as a linear magnitude (dimensionless), in dB, or in percent. To change the format, click a button in the Unit radio-box.

For background theory, please refer to the document suggested in the Introduction.

Actual Value, Min Value, Max Value

The Actual Value shows the actual interference factor for the current phases of the source and load reflection coefficients. (The phases also implicitly incorporate the phase shift caused by the interconnecting waveguide.)

If the phases were unknown (which is a common case in practice), we can only determine the limits between which the interference factor must lie given the source and load reflection coefficient magnitudes. These limits are the Min Value and Max Value. The Max Value is obtained by assuming that all higher-order reflection contributions are in-phase with the primary wave; the Min Value assumes the terms shifted by 180 degrees relative to the primary wave.

These values can be displayed as linear magnitudes (dimensionless), in dB, or in percent. To change the format, click a button in the Unit radio-box.

The bounds of the interference factor impose bounds also on the other quantities displayed on the Transmission page.

Brief information about the origin of these bounds can be invoked by clicking the Read Note button.

Transducer Gain

The transducer gain is the basic quantity describing the power transfer between a signal source and load. By definition, it is the ratio of the power absorbed in the load to source available power. It can be computed as the product of the source mismatch factor, the load mismatch factor, and the interference factor.

Transducer gain is displayed in the Transducer Gain text box. It can be displayed as a linear magnitude (dimensionless), in dB, or in percent. To change the format, click a button in the Unit radio-box.

Incident Power

The Incident Power row displays the power carried by the wave incident on the load. Also shown are its bounds, imposed by the variation of the interference factor.

The incident power can be computed as the product of the source available power, source mismatch factor, and interference factor.

Can the incident power exceed the magnetron power?

Absolutely! Please be aware that if both the source and the load are mismatched, the incident power may be greater than the “magnetron power” specified in the magnetron datasheets (which is the “power to match”). In this sense, the incident power is only a measure of the local field build-up due to the resonator-like effect mentioned above.

This effect may lead to an apparent paradox when you sample only the incident power (e.g., by a single coupler) and think of it as a measure of the power delivered to the load. You can easily simulate the situation using the PowTrans calculator.

The “real quantity” is the net power absorbed in the load (see below).

Reflected Power

The Reflected Power row displays the power carried by the wave reflected from the load. It also shows its bounds, imposed by the variation of the interference factor.

The ratio of the reflected to incident power is equal to the squared magnitude of the load reflection coefficient.

Absorbed Power

The Absorbed Power text box displays the net power absorbed in the load, which is the difference between the incident power and the reflected power. This, unlike merely the incident power, is the correct quantity for the evaluation of the power delivered to your process.

We need two couplers (or a device capable of measuring the incident power and the load reflection coefficient, such as a Homer Analyzer or Autotuner) to obtain the absorbed power.