If you take your S meter at face value, you're probably being lied to several times a minute. Most modern HF rigs have meters that wander, ignore the textbook 6 dB per S unit rule, and move more with AGC and noise than with actual signal strength. The good news is that you don't need a perfect meter to make smart operating decisions, you just need to know what it's really telling you.
📌 TL;DR — Your S meter is a storyteller, not a lab instrument
- Core idea: S meters are only loosely calibrated and are best used as relative indicators, not precision instruments.
- Why it matters: Misreading the meter leads to bad signal reports, wrong antenna decisions, and confusion about band conditions.
- Key benefit: Once you understand what the meter really measures, you can give better reports and do better station testing.
- Who it’s for: HF operators, contesters, new hams, and anyone comparing antennas, rigs, or preamps using S meter readings.
On paper, S units are simple. By convention, S9 on HF is 50 microvolts into 50 ohms, and each S unit is supposed to represent a 6 dB change. Double the voltage, go up one S unit. In practice, that pretty graph turns into a drunk snake the moment you touch RF gain, switch the preamp, change filters, or move to a different band.
Radio manufacturers know most of us care more about whether a signal is weak, fair, or loud than its exact microvolts at the antenna jack. So the S meter often gets tuned for "looks right" rather than "lab accurate." That isn't a problem unless you start treating those numbers like hard truth when you're giving reports, diagnosing noise, or comparing antennas.
What your S meter is actually measuring
The S meter doesn't measure "how strong the other guy is" in some pure RF sense. It's sampling signal at a certain point in your receiver chain, shaped and distorted by AGC, roofing filters, IF gain, and DSP. Depending on the rig, it might be looking at RF, IF, audio, or even post-DSP levels.
When band noise is high, your AGC is busy riding the gain, and small changes in signal level barely move the meter. When the band is quiet and you flip in a preamp, suddenly every little blip looks huge. Same station, same power, completely different S reading just because your receiver is sitting in a different part of its gain curve.
Textbook S meter specs vs real-world behavior
| Metric |
Value |
Why It Matters |
| Legacy S9 reference (HF) |
50 µV into 50 Ω |
Gives a common voltage reference so two receivers can agree what "S9" means in the lab. |
| Ideal S-unit spacing |
6 dB per S unit |
Each S step should represent a meaningful change in signal strength, but many rigs deviate by several dB. |
A quick sanity check for your S meter
You don't need a full RF lab to get a feel for how honest your meter is. With a simple RF attenuator, a stable local signal, or even a signal generator built into another rig, you can map out how your S units behave on at least one band and mode.
- Step 1: Find or create a stable signal (local transmitter into a dummy load and a bit of leakage, or a low-power signal from a nearby antenna) that reads somewhere around S5 to S7.
- Step 2: Insert known attenuation in the path in 3 or 6 dB steps and note how many S units the meter moves for each step at that point on the scale.
- Step 3: Repeat with and without preamp/attenuator and on another band to see how wildly the S-unit spacing changes across setups.
Why one S unit is not always 6 dB
On many radios, S units are reasonably close to 6 dB around S9, then get compressed above that and stretched below it. At the low end of the scale, AGC thresholds, noise blankers, and DSP noise reduction all push the meter around. At the top end, the IF chain is already close to its comfort limit, so another 10 dB in doesn't move the meter by two neat S units.
That means telling someone they're "S5 instead of S7" doesn't necessarily mean 12 dB weaker. On your particular rig, at that particular moment, it might only be 6 or 7 dB. Or the meter might not be moving because the extra signal is buried in a band full of noise and your AGC is clamped.
Reading S meters like a ham, not a lab tech
If you stop expecting the S meter to be a precision voltmeter, it suddenly becomes a lot more useful. Think of it as a relative indicator tied to your ears. When the meter jumps but readability barely changes, you're probably just seeing noise or AGC action. When a station is only 1 S unit louder yet goes from "struggle copy" to "armchair copy," that's showing you how important noise floor and QSB really are.
You'll also notice that different radios, and even the same radio on different bands, give very different readings for the same on-air signal. That's why comparing "S7 on my radio vs S7 on yours" is mostly meaningless unless both receivers have been carefully calibrated against a known source. For everyday operating, focus on signal readability and relative changes on your own rig.
Common S meter questions
Should I ever trust my S meter numbers?
Yes, but only within a narrow context. If you keep the same rig, same band, same filters, and the same RF gain, then S meter changes are meaningful for comparing antennas, feedlines, or power levels. What you shouldn't do is treat "S7" as a globally valid number that means the same thing in every shack.
Why does my S meter move when no one is transmitting?
That's your noise floor and AGC at work. Atmospheric noise, man-made noise, band openings, and even your own switching power supplies all show up as signal at the receiver input. When that noise changes by several dB, the AGC responds and the S meter comes along for the ride, even with no strong stations present.
What about panadapters and SDR waterfalls?
Panadapters are much better at showing relative signal levels across the band, but they're still plotted through gain stages, AGC settings, and reference levels. Treat the dB scale on the waterfall the same way you treat the S meter: fantastic for comparison, suspect for absolute precision unless you've calibrated it yourself.
How to use S meter readings when testing gear
The place where S meters shine is A/B testing. If you're trying a new antenna, swapping feedline, or experimenting with preamps, the meter gives you a quick, repeatable way to see which setup is helping more on average. The key is to control all the other variables so you're actually comparing antennas and not AGC behavior.
Pick a band, pick a time when there's a steady stream of stations, and stick with a single mode and bandwidth. Keep RF gain, preamp, and attenuator settings fixed. Then move between antennas or configurations and log the average S reading and readability on several stations. Over enough samples, you'll see patterns that are far more reliable than any single S-unit report.
Why S meters and RST reports rarely agree
RST reports mix subjective readability (the R), tone or audio quality (the T), and a loose "how loud are they" guess. The S meter is only trying to talk about that last part, and not very precisely. You can have a signal that looks strong on the meter but is distorted, multipath, or heavily fluttery, which deserves a lower R value even if the meter says S9.
This is why "you're 5 by 9" gets tossed around even on weak, noisy signals. Operators default to a quick canned report that satisfies habit and contest macros. If you really want to help another ham, describe what your ears are hearing and use the S meter as a secondary data point instead of the headline.
Practical tips for living with a lying S meter
Instead of fighting your meter, treat it as a quirky but useful friend. With a little discipline and some repeatable habits, it becomes a solid tool for station building without pretending it's a calibrated instrument panel.
- Use the same rig and settings when doing comparisons so S meter changes reflect real RF differences as much as possible.
- Log both readability and S meter when testing antennas or new gear, then look for patterns across many contacts instead of one-off readings.
- Pair the S meter with other tools like reverse beacons, digital reports, or SDR waterfalls to confirm whether a change really improved your station.
If you're early in your ham journey and still sorting out what matters, check the Getting Started section for a bigger-picture look at building a first station. When you're ready to push deeper into antennas and propagation, the Antennas and SDR sections are worth a long read too.
So what should you believe: the meter or your ears?
Your S meter isn't useless, it just isn't the precision instrument the scale implies. Once you treat it as a relative gauge of "louder or quieter" instead of an absolute truth machine, it becomes a powerful tool for antenna work, troubleshooting, and on-air reporting. The trick is to lock down the variables and think in dB and trends, not single S units.
- Use S meter readings for controlled comparisons on one rig and one set of settings.
- Base signal reports on readability and audio quality, with the S meter as supporting info.
- Combine S meter observations with other data sources when you're making station changes.
If you want your signal reports and station experiments to actually mean something, start by treating your S meter as a relative tool and let your ears and experience have the final say. Then go spin the dial and see what your own rig is really telling you.