Winter 2010 Issue

Digital Wattmeter
Element for the Bird
Model 43 Wattmeter


Long a mainstay in amateur radio shacks, the
Bird Model 43 Wattmeter has one very important weakness—the analog meter. Here WA8SME describes how he replaced the rather pricey
meter with a digital meter element he designed
and built for his Model 43.

By Mark Spencer, WA8SME

 

Figures 1 & 2. The digital wattmeter element is designed to either replace the analog meter element inside the Bird Wattmeter (left) or be mounted in an external enclosure to
augment the standard Bird (below).

My Bird Wattmeter is tied for second place as my most important and most used piece of test equipment. First place, hands down, goes to my voltmeter; my MFJ Antenna Analyzer shares the second place slot with the wattmeter; and my oscilloscope comes in third.

The wattmeter has been an invaluable asset in installing and maintaining my satellite ground station antenna system. It is rugged, reliable, and fairly easy to use except that you have to enter the forward and reflected watt readings into a formula to calculate the VSWR. This really isn’t that big a deal and keeps the dust off the gray matter. However, I did have a friend who dropped his wattmeter and broke the analog meter element. That got me thinking: How much would it cost if my meter experienced the same fate? Wow . . . around 150 bucks to replace the analog meter movement!

That thought was the catalyst for the project detailed here, a replacement or add-on digital meter element that could more affordably bring a broken Bird Wattmeter back to life and do some of the SWR and power-loss calculations for me. The digital wattmeter element is designed to actually replace the analog meter element inside the Bird or be mounted in an external enclosure to augment the standard Bird (figures 1 and 2).

The digital wattmeter element is based on a PIC®16F688 microcontroller and an inexpensive LCD display module. Refer to the circuit diagram of figure 3 for the following discussion of the circuit. The digital wattmeter element is powered by a 9-volt battery with the voltage stepped down and regulated to 5 volts by one half of the OPA2340.

The op amp is configured as a unity-gain buffer between the Bird Wattmeter and the digital wattmeter element circuit. The OPA2340 op amp requires a single voltage source, and this simplifies the circuit (at some added cost for the device). The input precision resistors that are shown in parallel with the analog meter are inserted into the circuit by a jumper. These resistors are required in the circuit to replace the resistance of the analog meter element if the digital meter will replace the analog meter, but they are not required if the digital meter will be used in conjunction with the analog meter. The other half of the op amp is configured as a times-100 voltage multiplier. The voltage across the analog meter (or the precision resistors) is on the order of 40 mV at full scale. The voltage multiplier boosts this voltage to approximately 4 volts to make it easier to measure with the PIC analog to digital (ADC) circuit.

The PIC is programmed to read the voltage from the wattmeter with the on-board 10-bit ADC, scale and calculate the watts being sensed (forward or reflected), calculate the SWR and power loss, and display the data on the LCD. A more detailed explanation of the programming logic will follow shortly. The final component of the circuit is the LCD module which includes a two-line eight-character LCD display and four momentary switches for user input, all in one convenient package. Two LEDs round out the circuit and provide a visual indication when forward power (green LED) or reflected power (red LED) is being measured in real time.

The prototype of the digital wattmeter element mounted in the Bird analog meter mounting ring is shown in figures 4 and 5.
Using the PIC ADC to measure the voltage produced by the wattmeter during operation is fairly straightforward. However, the correlation between voltage and watts is not a linear function, and this makes the mathematical conversion a bit more complicated. Figure 6 is the graph that shows the relationship between the measured watts and the ADC values. I used graphing calculator technology to do a curve fit of the watts to ADC graph, and then in turn used the resulting function in the PIC program to calculate the watt value from the measured ADC value. The curve fit function is:

watts = 4.988 ¥ 10–6 ADC2 = 1.652 ¥ 10–3 ADC + 0.00538


 

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