how does a bird wattmeter work

The “lumped constant” directional couplers serve as the foundation for the Bird Model 43 and other thru-line wattmeters. The directional coupler is called an element. Numerous clients of mine have also referred to it as a “slug” or “plug-in.” Every plug-in component functions as one of these couplers, sampling the current through a loop and the voltage at the insertion point. When the element is turned 180 degrees, the voltage sample stays constant but the loop (and subsequently the current pick-up) are reversed. When the two samples are properly combined, we get an RF voltage that is proportional to the square root of the main line power. Check out our directional attenuator elements, which can deliver such a signal a given number of db down from the main line level, if that’s what you’re looking for. I’m happy to hear that. We rectify the RF sample in our standard elements and provide the meter with a signal that is proportionate to the envelope in both level and form. This signal is DC when the Model 43 wattmeter is used for its intended purpose, which is CW. However, it isn’t when the modulated carrier to be measured

You probably noticed we referred to a “symmetrical AC voltage”. It would be necessary to use a peak-reading device if the modulation was not symmetrical, such as if it was pulsed, distorted by overdriving, or in a single side bend, as this would cause the meter indication of the “43” to be incorrect. The measurement of two-toned single side-band transmission is one notable exception. Since every correction factor is known, the meter reading can be adjusted in the following ways: The true heating power in two-tone modulation of equal magnitude is equal to half of the peak envelope power. The rectified pulsating DC output from the element’s average will be read by the meter, or 636 of the peak-to-base voltage. The power calibration of the dial means that it will show ( 636)2 or . 405 the peak power. For instance, the THRULINE meter should show if the PEP is known to be 1 KW. 405 times 1 KW, or 405 watts.

wave-form It should be stressed that this conversion factor only applies to measurements of two-toned modulation of equal amplitude of single side-band transmission. The entire wave-form discussion, however, is valid not only for our CW THRULINE wattmeters but also for the TERMALINE series The “43” is a “building block” instrument, meaning you can acquire the basic wattmeter with only one element at one time and buy additional elements later as needed. Any “43” element in any “43” wattmeter will meet published specifications of accuracy (±5% of full scale). However, our elements in other wattmeters or our wattmeters with other suppliers’ elements do not meet this accuracy.

I frequently field questions about how well the wattmeter performs under circumstances different from those in which it was designed. Consider the plug-in elements as half-wave AM detectors, which is exactly what they are, and connect them to a d’Arsonval (average-reading) DC meter calibrated in RMS power to help with some of these questions. The meter will be deflected to a point on the scale that has been marked in watts corresponding to the power being sampled from the main line when an unmodulated carrier passes through the rectifier diode and produces a DC voltage proportional to the peak-to-base value of the RF voltage. Should the identical carrier be modulated at a lower amplitude than 20100%, the rectifier will provide the modulation signal riding on a DC voltage of the same value as previously. The reading on the d’Arsonval meter won’t change because it measures average, and the average of a symmetrical AC voltage superimposed on DC is the DC value. (If the needle tries to follow and quivers around the center reading, the modulation frequency is too low.)

My RF clients are astounded to learn how long the Bird Wattmeter has been in business and how many people it has helped. Let’s go over some of the finer points and frequently asked questions! However, that doesn’t mean I don’t get a lot of questions about how to use it and how it functions.


What is the use of bird wattmeter?

The 43 Wattmeter is a field-proven portable, insertion-type instrument designed to measure both forward and reflected CW power in coaxial transmission lines under any load condition.

How accurate is the bird wattmeter?

Accuracy +/- 5% fs •Frequency Range 450 kHz to 2.7 GHz (elements are sold separately) •Power Range 100 mW to 10 kW (elements are sold separately) •Frequency & power range determined by optional plug-in elements •Quick Change QC…

How does a wattmeter work?

How does a wattmeter work? The wattmeter measures power by sampling amperes and volts several thousand times per second. These values are sampled and multiplied by one another to provide a wattage reading at any given time. There are two standard types of wattmeter, an analogue wattmeter and a digital wattmeter.

How does a RF watt meter work?

An RF power meter measures the power of the entire frequency range including all carrier waves, all harmonics and intermodulation products. To perform these measurements over a wide band specific sensors are used. The most common type today is the diode detector.