Digital Wattmeter Elementfor the Bird Model 43 Wattmeter

Long a mainstay in amateur radio shacks, the Bird Model 43 Wattmeterhas one very important weakness-the analog meter. Here WA8SMEdescribes how he rep laced the rather pricey meter w ith a digital meterelement he designed and built for his Model 43,

By Mark Spencer,' WA8SME

My Bird Wattm eter is tied for seco nd place as my mostimportant and most used piece of test equipment. Firstplace, hands down, goes to my voltmeter; my MFJ

Antenna Analyzer shares the second place slot with thewattmeter; and my oscilloscope comes in thi rd.

*ARRL Education and Technology Program Coordinator,774 Eastside Rd., Coleville, CA 96107e-mail: <mspencersehughes.net>

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The watt meter has been an invaluable asset in installing andmaintaining my satellite ground station antenna system. It isrugged, reliable, and fairly easy to use except that you have toenter the forward and refl ected watt readings into a form ula tocalculate the VSWR. This really isn't that big a deal and keepsthe dust off the gray maner . However, I did have a friend whodropped his wattmeter and broke the analog meter ele ment.That got me thinking: How much would it cost if my meterexperienced the same fate? \Vow . .. around 150 bucks toreplace the analog meter movement!

Th at though t was the cata lyst for the project detailed here, areplacement or add-on digital meter element that could moreaffo rdably bring a broken Bird Wattme ter back to life and dosome of the SWR and pow er-loss calcula tions for me. The dig­ital wattmeter element is designed to actu ally rep lace the ana­log meter element inside the Bird or be mounted in an externalenclosure to augment the standard Bird (figures I and 2).

Th e digital wattmeter eleme nt is based on a PIC® 16F688microcontrollerand an inexpensive LCD display module. Referto the circuit diagram of figure 3 for the following discussionof the circuit. The digital wattmeter element is powered by a 9-

Figures 1 & 2. The digital wattmeter element is designed toeither replace the analog meter element inside the BirdWattmeter (left) or be mounted in an external enclosure to

.......- augme nt the standard Bird (below). t

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LCD

I

..~... T-• •• " 211121, -~• ~ -~ 0W1 fIW2 SW3 ....~

~- ; -~ --I --I ..., -~

• "" "• "" i-'---- >-~ : >-

""" .~

""f'..--- ~~ ~ ;.~

"'",-'- ..... /77 ! ~~ ~~ ~-, ..'- >T

""......_.......,,,­

(~ tJoltIl­,-~.

Figure 3. Alternate Bird 'Wattmeter meter elemen t circuit diagram .

watts = 4.988 x !o-<> ADC2 = 1.652 x 10-3 ADC + 0.00538

With a calculated value for watt s, the PIC software co ntin­ues by calculating the VSWR and power loss using the fol­lowing formulas:

I generally program Pie s using ass embly language.However, these formu las wou ld have been a pretty da untingcha llenge to program in assembly. A C-co mpiler was the

WattsRe/

W attsFor

VSWR=

1 -

lossdB = - IOiog

the correlation between voltage and watts is not a linear func­tion, and this makes the mathematical conversion a bit morecomplicated. Figure 6 is the grap h that shows the relationshipbetween the measured watts and the ADC values . I used graph­ing calc ulator technology to do a curve fit of the wat ts to ADCgraph, and then in tum used the resulting function in the PICprogram to calculate the watt va lue from the measured ADCvalue. The curve fit function is:

1 +

vo lt battery with the voltage stepped down and regulated to 5volt s by one half of the OP A2340 .

The op amp is configured as a unity-gain buffer between theBird Wattmeter and the digital wattmeter element circuit. TheOPA2340 op amp requ ires a single voltage source, and this sim­plifies the circuit (at some added cost for the device). The inputprecision res istors that are shown in parallel with the analogmeter are inserted into the circuit by a jumper. These resistorsare required in the circuit to replace the resistance of the analogmeter element if the digital meter will replace the analog meter.but they are not required if the digital meter win be used in con­jun ction with the analog meter. The other half of the op amp isconfigured as a times- Hf) voltage multiplier. The voltage acrossthe analog meter (or the precision resistors) is on the order of 40mV at full sca le. The voltage multiplier boosts this voltage toapprox imately 4 volts to make it easier to measure with the PICanalog to digital (ADC) circuit.

Th e PfC is programmed to read the voltage from thewattmeter with the on-board IO-bit ADC , sca le and calculatethe watts being sensed (forward or -reflec ted). calculate the SWRand power loss, and disp lay the data on the LCD . A moredetailed ex planatio n of the pro gramming log ic will follo wshortly. The final component of the circu it is the LC D modulewhich includes a two -line eight-character LCD displa y and fourmomentary switches for user input, all in one convenient pack­age. Two LEDs round out the circui t and provide a visual indi­cation when forward power (green LED) or refl ected power(red LED) is being mea sured in rea l time.

The prototype of the digita l watt meter element mounte d in theBird analog meter mounting ring is shown in figures 4 and 5.

Using the PfC ADC to measure the voltage produced by thewatt mete r dur ing operation is fairly straigh tforward. However,

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Figures 4 & 5. The prototype of the digital wattmeter element mounted in the Bird analog meter mounting ring.

WallS Vs. ADC Value

Figure 6. Graph that shows the rela tionship between themeasured watts and the ADC values.

As illustrated in the figures, there are four pushbuttons thatare part of the LCD unit (I' ll label them buttons I through 4,from left to right, for this discussion). The two middle buttons(2 and 3) are programmed to select either rellected or forwardpower as the active wattage being measured. When first turnedon, the display will have 0000 .00 F (for zero watts forward) onthe first line of the LCD, and an R (for rellected) on the secondline. The green LED will also be illuminated as a visual indi­cator of forward power. There may be some random noise onthe ADC that will give non-zero wattage indications whenpower is not applied to the Bird Wattmeter. When you pressbutton 2, the digital meter element will switch over to measur­ing rellected power, the green LED will extinguish, the red LEDwill illuminate, the last forward watt reading will remain on thetop line of the LCD, and the rellected power will be displayedon the second line of the LCD. To revert to measuring forwardpower, press button 3, the red LED goes out, the green LEDcomes on, the last reflected power read remains displayed onthe second line of the LCD, and the current forward power isdisplayed on the top LCD line.

The Bird uses interchangeable slugs that are calibrated forfrequency and power, and then the slug is rotated in its direc­tional socket on the front of the Bird to measure forward orreflected power. The Bird analog meter has three scales that areapplied during reading andlor calculations depending on thepower of the slug in use: ato 100 for 10-, 100-, and 1000-wattslugs; a to 50 for 5-, 50-, and 500-watt slugs; and a to 25 for25- and 250-watt slugs. The digital wattmeter element needs tobe set to the appropriate scaling to display and make accuratecalculations. The right-hand pushbutton (4) on the LCD unit isprogrammed to toggle through the various slug watt valueswhen pressed by the user. When button 4 is pressed, the wattscale for both forward and rellected readings is displayed onthe appropriate line of the LCD. Holding button 4 will stepthrough the various scaling factors for the active channel (for­ward or reflected); release the button when the desired scalingfactor is reached. This value is stored in EEPROM of the PIC

/"

S

4

~3

~ 2

00 100 200 300 400 SOO SOO roo BOO SOO

Rav.rADC Value

answer . The C-compiler allows the programmer to use thepower of the higher level computer language C and then trans­lates the C code into assembly and machine language code thatis used by the PIC. Unfortunately, these hefty formulas requirea lot of computer memory space to store and execute the code.I prefer to use PICs that have 2 kilobytes of memory (for costreasons, and also most of my projects do not require a lot ofmemory), but in this case more memory was needed (approxi­mately6 kilobytes) and the PIC 16F688 has this amount of mem­ory (with a modest increase in cost of a buck or two).

The best way to go through the program logic is to describehow the digital meter eleme nt is used in operation. When poweris first applied to the digital meter element, the PIC and theLCD are initialized, the previous scaling factors used arerecalled from EEPROM, and the meter defaults to reading for­ward power. The green LED illuminates and the digital meteris ready to go.

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Figure 7. While applying power to the wattmeter, press button1 to calculate the VSIVR and power loss. The top fine of theLCD will display the SIVR and the second line will display the

power loss in dB.

and is retrieved when the digital meter is subsequently pow­ered-up. Press the appropriate button to se lect the other mea­surement channel to set its scaling factor. During my typicaloperating, I use the IOO-watt slug to measure forward powerand the 5-watt slug to measure reflected power. so these are thescaling factors that I use most frequently.

A fter you have the scaling factors set up. you're ready to takesome S\VR readings. Have the appropriate slugs on hand. I'llillustrate with the 100- and 5-watt slugs. Install the loa-wattslug and rotate it to the forward direction. Tum on the digi talmeter element. Apply powerto the Bird Watt meter and the for­ward power will be displayed.While the forward power is beingdisplayed , press button 2 to switch the digital meter elementover to reflected power (the forward power is now stored anddisplayed on the LCD). Turn off the transmitter RF. Swap outthe slug to the 5-watt slug and rotate it in the refl ected direc­tion. Again apply transmitter power to the Bird Wattmeter andthe reflected power will be displayed on the LCD. While apply-

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Figure 8. Prototype of the PCB the author plans to havemanufactured f or the project.

ing power to the wattmeter, press button I to calc ulate theVSWR and power loss. The top line of the LCD will displaythe SWR and the second line will display the power loss in dB(figure 7). When you release button I, the digital meter elementwill revert back to measure reflected pow er.

Some of you may be concerned with the accnracy of the dig­ital meter eleme nt, and this is a legitimate concern consideringall the conversions of vo ltages to ADC values. converting inte­gers to float values and back again, curve fitting, and all the cal­culations going on and the rounding errors that result . Usingthe PIC and doing the calculations with limited variable sizesdoes introduce some rounding errors. but frankly so does usingyour eyes to read an analog meter. [ have found that the digitalmeterelement is about as accurate as·using the eyeball and cal­culator. You just have to recognize that even though the digi­tal meter element will display numbers to two decimal placesthat does not necessarily mean that the displayed results areaccurate to the hundredth.

SummaryThis little project has turned out to be pretty useful. I plan to

get a PCB manufactured for the project to make it more pro­fessional looking and more rugged. Figure 8 illustrates what [have in mind. ·The final cos t of the digital wattmeter elementturns out to be a fraction of the cost of a replacement analogmeter. The addition of the calculated VSWR and loss figuresis a nice feature. I encourage you to co nsider duplicating thiscircuit to enhance yourBird Wattmeter. repairyour Bird, or putinto service that good deal on a broken Bird you got at the lasthamfest. [ will conclude with a bad pun intended, "A Bird inthe hand.. ." Well, you know what I mean.

If you wo uld like more information about this project, fee lfree to co ntact me at <mspencer@hughes.net>.

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