Microwave power handling

Updated September 10, 2008

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When power handling limits are exceeded, all Hell breaks loose!

When we talk about power handling, we mean power levels beyond which damage occurs (the magic blue smoke is released), usually well beyond the dynamic range of a system. Typical power handling issues are receiver protection during transmit for a system that the transmitter and receiver are duplexed to the same antenna. But any lab course should consider this subject, because exceeding power handling limitations of expensive test equipment can cost a lot of money!

Two limitations generally exist.

• Average power kills through heating effects
• Peak power kills through breakdown effects (like arcing)

By the way, this seemingly simple subject of power handling quite often requires very complex analyses, you can make a career out of it!

Average power

In this case we need to consider the combined heat due to average RF and DC power forms, and heat transfer to a heat sink. The better the heat sink, the more power you can apply.

The analysis of heat transport is a complex subject, and we are not going to get into it in any detail now...

Peak power

In this case we need to consider the combined maximum voltage due to RF and DC power forms, while voltage limits must be applied to peak waveforms.

Ionization breakdown in air is function occurs when a critical field strength is exceeded. Field strength is specified in volts/meter or volts per centimeter. In air, at 1 atmosphere, the critical field is about 3,000,000 volts/meter, or 30,000 volts/cm, or about 76 volts per mil. Generally, this breakdown voltage is NOT a function of frequency.

One way to greatly improve power handling is to add a dielectric in the place of air. So coax cables with PTFE have far greater power handling than air coax. The same same goes for stripline. For high-voltage applications that will see high altitude (such as microwave tube power supplies for airborne radar) the entire part is often "potted" with a dielectric filler, this often makes repair impossible.

One thing to look out for is that when you mate connectors, even if they are filled with dielectric, the interface can have just a slight air gap. Guess where voltage breakdown is going to occur?

Paschen's Law

This dates back to the 19th century, named after Friedrich Paschen, and is often referenced when in voltage breakdown calculations. For now look it up on Wikipedia!

The equation that Paschen came up describes the relationship between breakdown is a as a function of distance and gas pressure.

Vb=(apd)/(ln(pd)+b)

The constants have take on different values for different gases. V is the breakdown voltage in Volts, p is the pressure, d is the gap distance. The constants a and b depend upon the composition of the gas. For air, a = 43.6*106 and b = 12.8, when p is the pressure in atmospheres and d is the distance in meters.

Peak power handling in coax

This topic is discussed on this page.

Peak power handling in waveguide

Waveguide power handling is subject to both heat (average power) and voltage breakdown (peak power)

Voltage breakdown in rectangular waveguide can be predicted by the following equation

P=600,000ab(Lambda/Lambdaguide) (watts)

If you plug in the dimensions for WR-90, the power handling is in excess of 1 megawatt. Yikes!

For circular waveguide, the equation becomes:

P=450,000d^2(Lambda/Lambdaguide) (watts)

These two equations came from Gershon Wheeler's excellent book, Introduction to Microwaves, circa 1963. Look for it on our book page!

Multipacting

Coming soon!

Semiconductor failure

Failure can occur in semiconductors, when breakdown occurs in the "channel" or "junction". Voltages must be kept below a critical threshold in semiconductor devices or failure results. The voltage is instantaneous voltage, the sum of DC and RF components.