RSSI, dBm, Oh My!

Measuring receive signal strength is important to us amateur radio operators. On the HF bands, getting a 5-9-9 for you CW operators out there or "five nine" for those using voice modes  is what every operator wants to hear, but sometimes that 5-7 (or 5-7-7) report can get under your skin and cause you to rethink your antenna or overall ability to generate the maximum amount of power possible and ensure your 100% readable!

   (CB/HF Relative F/S Meter)

The R-S or R-S-T method is subjective and far from scientific. Same goes for a relative RF field strength meter. This article will cover "specific" signal reporting methods and how they compare to "less specific" or scientific measurement methods.

RST =  (R)eadability-(S)trength-(T)one 

This mostly unscientific method has been around a long time and provides uniform measurements, but is based on each station having exactly the same way of thinking. Because of this, the "RST" or "RS" method is not 100% accurate because not all radios or operators work hear the same. "RST" is defined as:

• (R)eadability is how well can you hear the other station with 5 being the best and 1 being the worst.

• (S)trength is the level of signal with 9 being the strongest your receiver can visually measure and 1 being the least.

• (T)one pertains more for Morse code operators with 9 being pure and clean and 1 being noisy based on the transmitter being used. Tone could even be adopted for modern digital communications.

Comparing the "S" in RST or RS to S-Meter Values

An "S" or "Signal Meter" displays a relative signal received by your receiver and is usually calibrated in "S" units, typically 0 to 9. Signals over S-9 typically read as S9+10, S9+20, S9+30, etc but how accurate is that? It  all depends on quality of the radio and calibration.

(Cobra 29 S-Meter and also found on many other "CB" radios)

One S-unit is typically equal to about 6 decibels or "dB".  Once you get past "S9" it changes from 6dB to 10dB. A signal in the 1-30 MHz range typically has a different power density compared to 30-300 MHz or even 300-3000 MHz and there are different ways to measure signals based purely on frequency. The below tables compares some common translations between signals levels.

         

        (For 3-30 MHz  calibration)                           (For 30-300 or 3000 MHz  calibration)

RSSI or S-Meter?

RSSI or "Receive Signal Strength Indicator" and an "S" meter are not the same. An "RSSI" is usually considered a higher calibration form of measurement compared to an "S" Meter. Some higher end receivers or those modified to show it, may show dBm, which is different than the relative intensity in the aforementioned charts. Here, we look at how dBm compares to where the baseline of S9 equals 0db which also equals 50uV or 5.0uV

(The Watkins Johnson 8711A is considered one of the best receivers and calibrated S-Meters before the age of SDR and DSP)

Conversion between power and HF S-units

S-reading	𝑃𝑜𝑢𝑡$P_{out}$ @50Ω	𝑉𝑜𝑢𝑡$V_{out}$ @50Ω	𝑉𝑜𝑢𝑡[1µV]$\frac{V_{out}}{\left[1\text{µV}\right]}$ @50Ω
S9 + 40 dB	-33 dBm	5.0 mV	74 dBµV
S9 + 30 dB	-43 dBm	1.6 mV	64 dBµV
S9 + 20 dB	-53 dBm	0.50 mV	54 dBµV
S9 + 10 dB	-63 dBm	0.16 mV	44 dBµV
S9	-73 dBm	50 µV	34 dBµV
S8	-79 dBm	25 µV	28 dBµV
S7	-85 dBm	12.6 µV	22 dBµV
S6	-91 dBm	6.3 µV	16 dBµV
S5	-97 dBm	3.2 µV	10 dBµV
S4	-103 dBm	1.6 µV	4 dBµV
S3	-109 dBm	800 nV	-2 dBµV
S2	-115 dBm	400 nV	-8 dBµV
S1	-121 dBm	200 nV	-14 dBµV

Conversion between power and VHF/UHF S-units

S-reading	𝑉𝑜𝑢𝑡$V_{out}$ @50Ω	𝑃𝑜𝑢𝑡$P_{out}$ @50Ω	𝑉𝑜𝑢𝑡[1µV]$\frac{V_{out}}{\left[1\text{µV}\right]}$ @50Ω
S9 + 40 dB	-53 dBm	0.50 mV	54 dBµV
S9 + 30 dB	-63 dBm	0.16 mV	44 dBµV
S9 + 20 dB	-73 dBm	50 µV	34 dBµV
S9 + 10 dB	-83 dBm	16 µV	24 dBµV
S9	-93 dBm	5.0 µV	14 dBµV
S8	-99 dBm	2.5 µV	8 dBµV
S7	-105 dBm	1.26 µV	2 dBµV
S6	-111 dBm	630 nV	-4 dBµV
S5	-117 dBm	320 nV	-10 dBµV
S4	-123 dBm	160 nV	-16 dBµV
S3	-129 dBm	80 nV	-22 dBµV
S2	-135 dBm	40 nV	-28 dBµV
S1	-141 dBm	20 nV	-34 dBµV

0 dBm: The starting point

Typically, a one meter long antenna is used as the RF sensing unit along with a known calibrated analog or digital meter. The circuitry for detecting the RF signal varies, but generally this is the calibration needed so that a 50 uV (microvolt) signal equals S9.

Examples:

• A strong signal with signal strength of S8 corresponds to received power of -79 dBm or 25 microvolts RMS in 50 ohms on HF

• A weak signal with signal strength of S2 corresponds to received power of -115 dBm or 0.40 microvolts RMS in 50 ohms on HF.

(Using a $20 SDR dongle with SDR# and a special plugin provides decent calibrated and relative signal measurements)

Modern Signal Measurement

Modern amateur radios usually have good accuracy for received signal accuracy and every radio should read the same. One radio having a "more sensitive" receiver has nothing to do with S-meter readings. Ideally, everything should be all the same, but it is not. Modern digital modulation radios using DMR, D-Star, Fusion and others make it easier to not only visualize signal reports, but also share them real time.

Blending the old and the new

When using newer voice modes such as DMR, its easy to look at RSSI values because of how repeaters can take user signal data and share it for all to see. This makes it very helpful when diagnosing issues such as readability since most "digital" signals are either 100% or not.  The "S" as found in the "RS" or "RST" is easily quantified!

(The Brandmeister Network shows signal reports via RSSI in S units and also dBm for stations talking via internet connected repeaters)

However, a digital signal can have both (R)eadability and (T)one issues, so hearing someone provide a 5-9 or 5-9-9 signal report actually makes a little sense. Much of these "R" and "T" issues sometimes have to do with internet based "propagation" issues, but is usually a non issue and much less often when atmospherics create bad band conditions on HF.

More recently, amateur radio operators move from HF to DMR or vice versa depending on communication needs at the time.

Conclusions?

Learning how to calibrate your radio to provide accurate "RSSI" , S-Meter or the "S" in R-S or R-S-T is something we should all explore. But, also add in the human elements as well as in the "R" and the "T" elements relating to providing signal reports.

DMR  is a great mode that blends both old and new thinking about communication technology because of the basic concepts of signal strength and related terminology.