RF Thruline Wattmeter

When I talk to my RF customers, they are amazed at how long the Bird Wattmeter has been in the industry helping countless people. But that doesn’t mean I don’t receive lots of questions on how it works and how to use it! Let’s go over some of the finer points and common questions.

Elements, Slugs & Plug-ins

The Bird Model 43, and other thru-line wattmeters, are based on a “lumped constant” directional couplers. The directional coupler is called an element. I’ve heard many of my customers also call it a “slug” or a “plug-in”. Each plug-in element is such a coupler, sampling the voltage at the point of insertion and the current via a loop. Turning the element 180° reverses the loop (and consequently the current pick-up) while the voltage sample remains unchanged. By proper combination of the two samples we obtain an RF voltage proportional to the square root of main line power. If that is what you’re looking for, I’ve got some good news! Check out our directional attenuator elements which can deliver such a signal a given number of db down from the main line level.

In our standard elements, we rectify the RF sample and deliver a signal proportional to the envelope in level and form to the meter. When the Model 43 wattmeter is used on CW – its intended purpose – this signal is DC. It is not, however, when the carrier to be measured is modulated.

Measuring Power

I get a number of questions concerning the performance of the wattmeter in conditions other than its original design conditions. To help answer some of these questions, think of the plug-in elements as half-wave AM detectors – which is just what they are – connected to a d’Arsonval (average-reading) DC meter calibrated in RMS power. Passing through the rectifier diode, an unmodulated carrier will result in a DC voltage proportional to the peak-to-base value of the RF voltage, which will deflect the meter to a point on the scale that has been marked in watts corresponding to the power being sampled from the main line. If the same carrier is now amplitude-modulated at less than 100%, the rectifier will deliver the modulation signal riding on a DC voltage of the same value as before. Since a d’Arsonval meter reads average and the average of a symmetrical AC voltage superimposed on DC is the DC value, the meter reading will remain unchanged. (If the modulation frequency is very low, the needle will try to follow and quiver about the center reading).

You probably noticed we referred to a “symmetrical AC voltage”. If the modulation is other than symmetrical, such as pulsed or distorted by overdriving or in single side-bend, the meter indication of the “43” would be in error and you would have to use a peak-reading device.

There is one notable exception, namely the measurement of two-toned single side-band transmission. Since all the correction factors are known, they can be applied to the meter reading as follows: In two-tone modulation of equal magnitude, the true heating power is one-half the peak envelope power. The meter will read the average of the rectified pulsating DC output from the element, or .636 of the peak-to-base voltage. Since the dial is calibrated in power, it will indicate (.636)2 or .405 the peak power. For example, if the PEP were known to be 1 KW, the THRULINE meter should indicate .405 times 1 KW, or 405 watts.

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.