Load Mismatch and Reflected Power Considerations

Power delivered to the load is maximized when the load is 50 Ohms, ideally matching the 50 Ohm output impedance of the amplifier. As the load impedance varies away from an ideal 50 Ohms, an impedance mismatch occurs and power is reflected back from the load to the amplifier.

The term “mismatch” refers to the situation where the source impedance and load impedance are not equal, resulting in power being reflected from the load. In the ideal case, all power from the source (amplifier) is transferred to the load. As the load impedance deviates from the source impedance, “mismatch” occurs, and power is reflected back from that discontinuity.

In the worst case condition of short circuit and open circuit, 100% of the power is reflected back to the source (amplifier). While most amplifiers are designed to withstand this condition, damage or degradation and reduced reliability can occur, especially at higher power levels, and should be avoided whenever possible.

The most common measures of mismatch are VSWR (Voltage Standing Wave Ratio) and RL (Return Loss). In addition, the term “reflection coefficient”, symbolized by the upper case Greek letter Gamma (Γ), is also used. The parameter VSWR is expressed as a ratio of the value to 1, where a VSWR of 1:1 is the ideal case of perfect match and a value of infinity to 1 is worst case.

Return Loss (RL) is express in dB (a ratio) where RL of infinity is the ideal case of perfect match and a value of zero is worst case. Reflection Coefficient (Γ) varies from zero (ideal) to 1 (worst case).

The relationships, in terms of VSWR:

Relationships in terms of VSWR

Typical range of mismatch in tabular form:

VSWR	RL (dB)	Loss (dB)	Trans. (%)	Г
1.1	26.4	0.01	99.8	0.05
1.2	20.8	0.036	99.2	0.09
1.3	17.7	0.075	98.3	0.13
1.4	15.6	0.122	97.2	0.17
1.5	14	0.177	96	0.20
1.6	12.7	0.238	94.7	0.23
1.7	11.7	0.302	93.3	0.26
1.8	10.9	0.37	91.8	0.29
1.9	10.2	0.44	90.4	0.31
2	9.5	0.51	89	0.33
2.5	7.4	0.88	82	0.43
3	6	1.25	75	0.50
4	4.4	1.94	64	0.60
5	3.5	2.55	55	0.67

Spanawave/Giga-tronics microwave power amplifiers are specified to withstand a maximum VSWR of 3:1 (25% of the power reflected back to the amplifier) without any damage or degradation in performance.

As a safety precaution, amplifiers should always be turned off when changing loads, including cables and antennas. The open end of a cable can radiate electromagnetic energy when disconnected from a load as may happen when changing antennas. The practice of “hot-swapping” antennas can cause electronic interference and possibly even radiation burns. A good safety precaution is to never look into the open end of a cable connected to a microwave signal source.