RF Analog Wattmeter Basics

RF Analog Wattmeter Basics

Wattmeters are used in a variety of electrical circuit measurement and debugging applications. For example, they can check the power consumption of electrical appliances. RF wattmeters are devices that measures power (watts) into a circuit or system such as a transmission tower. In addition to delivered power, they also measure reflected power, from which VSWR is easily calculated. This allows in-field troubleshooting. Depending on the selected scale, RF wattmeters can test high-powered transmission lines or tune low powered portable devices.

The most basic type of analog wattmeter is based on an analog current meter (the heart of all analog ohm, voltage and current meters), noting that power is proportional to current squared. A movable coil of wire (the potential or voltage coil) suspended with its axis at ninety degrees to a fixed coil (the current coil) and operates against a spiral spring of known constant. The current coil is connected in series with the circuit, while the voltage coil is connected in parallel.

When current is passed into the meter, it generates an electromagnetic field around the current coil that produces a torque that is proportional to the strength of the current. The voltage coil attempts to turn so that it aligns with the axis of the current coil but it stops when the electrical torque is equal to the spring torque. A stronger signal will cause more movement (compressing the spring more), a weaker signal less movement. In an analog wattmeter, the voltage coil has a needle attached to it that moves over a scale as the voltage coil moves to indicate how much power, in watts, is passing through the circuit.

Analog meters provide visual indication where it is easy to see subtle changes. When tuning a circuit, it is far easier to watch a needle move as an indication of change in reflected power, versus concentrating on a digital readout.

The spring in the meter has a fixed constant and is difficult to measure a wide range of power levels. However, the amount of electrical torque can be controlled for a given signal level. Multiple full-scale settings can be provided allowing a single meter to be used in many applications.

At radio frequencies, a rectifier is added to convert RF energy to a measurable DC voltage, which in turn is applied to the analog meter as a DC current to deflect the needle. A coupler is used to sample the power from the main line. Using a dual-directional coupler, Telewave’s analog wattmeter, the 44A, allows the user to look at incident and reflected power independently, with only a small penalty in main-line power.

Diodes can only handle a range of power; if the range is exceeded, the diode will be damaged. Using mechanically-switched variable attenuators at the coupled ports, the detector diodes can safely “see” sampled signals that are -20dB, -30dB, -40dB… down from the main line. These variable attenuators are what allows the meter to have selectable full-scale range. High attenuation is used for high-power, low attenuation for low power. The attenuators must be rated for the worst-case power that can be dissipated at the highest rated power of the analog wattmeter.

There are other methods for changing the range of power that a wattmeter can measure. If the meter has a current coil and two voltage coils, the two voltage coils could be connected in series or parallel change the ranges of the wattmeter. Or the space available for the voltage coil movement could be modified to change the meter's range.