Reading Analog Meters
Long before digital displays existed, every piece of test equipment used an analog meter movement — a pivoting needle deflected by current, reading off a printed scale. Analog meters are still used today in SWR meters, power meters, and signal strength indicators because they show trends and peak movements that a digital display, snapping between discrete numbers, cannot match. Understanding how to read one accurately is an essential skill for every radio operator.
The D'Arsonval Movement
The most common analog meter movement is the D'Arsonval (also called a moving-coil or PMMC — Permanent Magnet Moving Coil) movement. A rectangular coil of very fine wire is suspended on jewelled bearings between the poles of a powerful horseshoe magnet. When current flows through the coil, the magnetic force between the coil's field and the permanent magnet twists the coil against a hairspring. A lightweight pointer attached to the coil sweeps across the printed scale. The deflection angle is directly proportional to the current through the coil.
A typical analog multimeter face. Multiple arced scales carry different functions. The mirror strip (silvered arc below the pointer) eliminates parallax error by aligning the pointer with its own reflection.
View LargerThe full-scale deflection (FSD) current is typically very small — 50 µA to 1 mA. This is why analog meters need precision shunt resistors for current ranges and high-value multiplier resistors for voltage ranges: the basic movement only responds to the tiny current that actually flows through its coil.
Zero adjust: A small mechanical screw on the front of the meter, usually a slotted screw at the bottom center of the face, allows you to set the pointer to exactly zero with no input applied. Adjust this first whenever you pick up an analog meter you have not used before, and periodically when temperature changes.
Reading the Scales
An analog multimeter typically has three or four arced scales printed concentrically on the scale card, one above the other. They share the same physical arc but carry different graduations:
| Scale | Direction | Spacing | Notes |
|---|---|---|---|
| DC Voltage / DC Current | Left (0) to right (FSD) | Even (linear) | Use the numbers printed for your chosen range |
| AC Voltage | Left (0) to right (FSD) | Slightly compressed at low end | Diode rectification causes non-linearity at low readings |
| Ohms (resistance) | Right (0 Ω) to left (∞ Ω) | Non-linear — cramped at left, spread at right | Read right to left; most accurate in the center third |
| dB / Decibels | Left (−) to right (+) | Logarithmic | Used for audio level measurements; add/subtract range offsets |
To read a measurement, note which scale applies to your selected function, then find where the pointer sits on that scale. Multiply the pointer reading by the appropriate factor if your range differs from the scale's nominal. For example, if the DC scale shows 0–10 and you are on the 50 V range, each scale division is worth 5 V — pointer at "7" reads 35 V.
Reading = 2.4 × (50 / 10) = 2.4 × 5 = 12 V.
Parallax Error and the Mirror Strip
Because the pointer floats above the scale card, reading the meter from an angle makes the pointer appear to sit at a different position than it actually does — this is parallax error. A pointer that is truly at 7 may appear to be at 6.8 or 7.2 depending on the viewing angle.
Better analog meters include a narrow mirror strip running along the arc of the scale, directly behind the pointer travel path. To eliminate parallax: position your eye so that the pointer and its reflection in the mirror strip appear to overlap into one. You are now looking at right angles to the scale card and reading the true position.
The Ohms Scale
Resistance measurement works differently from voltage or current measurement. The ohmmeter uses an internal battery to drive a small current through the unknown resistor, then measures that current with the movement. The relationship between resistance and current is non-linear (Ohm's Law: I = V/R), so the ohms scale is non-linear too — values are cramped together at the high-resistance (left) end and spread out at the low-resistance (right) end.
Key points for using the ohms scale:
- Zero reads on the right. With the probes shorted together, the meter should deflect fully to the right (zero ohms). Open circuit (probes in air) gives no deflection — pointer sits at left (∞ Ω).
- Zero-adjust (ohms zero) before each range change. Short the probes together and adjust the zero-ohms control (usually a thumbwheel or small rotary knob) until the pointer reads exactly 0 Ω. This compensates for battery voltage and lead resistance. Do this every time you change ranges.
- Read in the center third for best accuracy. The ohms scale is most spread out — and therefore most readable — in its center section. If your reading is in the cramped left quarter (very high resistance) or very close to zero (very low resistance), switch to a different range.
- Power must be off. Like any ohmmeter, the analog meter supplies its own current. External voltage will give false readings or damage the movement.
Range Selection
Analog meters are manual-ranging only. Choosing the wrong range gives a poor or unreadable result:
- Too high a range: The pointer barely deflects from zero, making the reading very inaccurate — you are trying to read a small deflection on a wide scale.
- Too low a range: The pointer slams against the right-side stop. This can damage the movement permanently if the overload is severe. Always start on the highest range for voltage and current measurements.
The optimal deflection for accuracy is between 50% and 90% of full scale — this is called the mid-scale region and is where the scale is most evenly spaced and easiest to interpolate between divisions.
Why Analog Still Matters
Given that digital meters are cheaper, more accurate and easier to read, why do analog meters remain in common use? Several reasons:
- Trend and rate of change. A swinging pointer makes it immediately obvious whether a reading is rising or falling, and how fast. Watching S-meter, SWR and power meter needles while tuning gives instantaneous feedback that a slowly updating digital display cannot match.
- Peak indication. The inertia of the mechanical pointer means it integrates rapid fluctuations and shows an average or peak that the eye can follow. RF power meters and S-meters exploit this — the needle settles on peaks in a way a digital display cannot track in real time.
- No batteries required for display. An analog meter movement needs no power for the display itself — the pointer is deflected directly by the quantity being measured.
- Clarity in noisy environments. Where LED or LCD displays are hard to read due to sunlight, vibration or distance, a well-illuminated analog panel meter is unambiguous.
Hands-On Experiment
⚖ Experiment: Reading an Analog Multimeter
If you have access to an analog multimeter (AVO, Sanwa, or similar), this experiment shows you how scale selection and range choice affect the accuracy of your reading, and demonstrates the ohms zero-adjust procedure.
- Analog multimeter with DC voltage and ohms functions
- 9 V battery
- Assorted resistors (1 kΩ, 10 kΩ, 100 kΩ)
- Short connecting wires
- With the meter set to its highest DC voltage range and probes connected to COM and V, touch red to positive and black to negative on the 9 V battery. Note the (tiny) deflection.
- Switch down through ranges until the pointer sits in the upper half of the scale. Note how much easier the reading is at the correct range.
- Switch to the R × 1 ohms range. Short the probes together and adjust the ohms-zero control until the pointer reads exactly 0 Ω (far right). This is ohms zeroing.
- Separate the probes and touch them across a 1 kΩ resistor. Note where the pointer sits on the ohms scale.
- Without re-zeroing, switch to R × 10 and read the same resistor. Note how the pointer moves to a different — more readable — position in the scale center.
- Re-zero the ohms on R × 10, then re-read the 1 kΩ resistor. The reading should now be accurate at the new range.
On the 1 kΩ resistor, R × 1 will push the pointer into the cramped left side of the scale (hard to read); R × 10 puts it near mid-scale (much easier to read). This proves that picking the range that puts the pointer near mid-scale is not just convenience — it is how you get an accurate reading from an analog meter.
Frequently Asked Questions
Why does the ohms scale read right to left when everything else reads left to right?
The ohmmeter passes a fixed voltage through the unknown resistance. Zero ohms (a short circuit) allows maximum current to flow, giving full-scale deflection (rightmost position). Infinite ohms (open circuit) allows no current, so the pointer stays at rest (leftmost position). The scale is therefore drawn right to left — a lower resistance produces a higher current and a greater deflection.
What does "full-scale deflection" mean and why does it matter?
Full-scale deflection (FSD) is the maximum reading the meter can display on a given range — the point where the pointer reaches the right-hand end of its travel. Exceeding FSD can jam the pointer against the mechanical stop and permanently bend the hairspring, ruining the meter's calibration. Always start on a high range to prevent this.
Do I need to re-zero the ohms scale on every range change?
Yes. The zero-ohms adjustment compensates for the meter's internal battery voltage and lead resistance. When you change ohms ranges, the circuit parameters change and the zero point shifts. Short the probes and re-adjust after every range change to maintain accuracy. Most analog meter manuals specify this explicitly.
Why are analog SWR meters and power meters still used in ham shacks?
The mechanical inertia of the pointer naturally integrates rapid RF fluctuations and displays a reading the eye can follow during tuning. Digital displays update too slowly and too jerkily to give useful real-time feedback while turning an ATU. The smooth needle movement of an analog cross-needle power meter is genuinely superior for this specific purpose, which is why it remains the standard format for in-line RF instruments.
Test Your Knowledge
Answer the questions below to check your understanding. Every answer can be found in the lesson above.