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Read That Mirrored Scale Correctly
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An engineer recently inquired about the mirrored scaling found on some meters — primarily RF power or AM ammeters — and what this scaling meant. It’s curious how many of us seasoned engineers just assume everyone else shares our knowledge.
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Fig. 1: Use the meter’s
mirrored scale to interpret meter indications accurately.
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So, old-timers, indulge me for a moment as I call your attention to Fig. 1.
Manufacturers added the mirror strip to improve reading accuracy. Especially with critical readings, as when measuring a station’s power, you want the most accurate reading. (You know: Keep the PD happy by squeezing every last legal watt out of that transmitter.)
If you look at a meter from an angle — not directly in front or perpendicular to the meter face — you will get an erroneous value, either too high or low depending on which side of the pointer you are viewing. When reading a mirrored scale meter, your eye is in the correct position when the pointer itself hides the reflection of the pointer, displayed in the mirror.
In Fig. 1, you can see the reflection of the meter pointer, so the camera was not aimed directly at the meter when the photo was taken. The picture shows the reflection off to the left slightly. If you move your head so that the pointer itself hides the pointer reflection, you are looking straight on and your reading will be accurate.
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Fig. 2: Align your eye so
the meter’s pointer completely hides its reflection in the mirror.
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Fig. 2 is a little better; it’s not easy to hold the camera straight and steady without a tripod. But in this photo, you can see that the reflection is nearly covered by the meter’s pointer. So the accuracy is improving.
Even with the linear meter scales of the Delta Electronics TCA Series AM ammeter, having the mirror is a big help in determining direct power measurements accurately. So keep this rule in mind when reading mirrored scale meters: If the reflection is visible, what you’re reading is wrong!
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If an engineer finds that his or her station budget is a bit short (show me one that isn’t) but would still like to implement a Caller-ID system, Wayne Eckert of Channel1Images recommends the Motorola/Freescale MC154447 IC, a Caller-ID data receiver in a 16-pin DIP package.
Though the product has been out of production for some time now, it is still available in single or small quantities for less than $10 each.
The output of the chip dumps data at 1200 baud; that data can be massaged into an app or simply dumped into a line printer such as an OKI Data DMP. One source is www.dpcav.com, selling them for $6.95 each at press time.
Our Radio World links page has the URL as well as a link to the data sheet for the chip. Visit radioworld.com/June-06-2012.
Thanks, Wayne, for a great budget-saving construction project.
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