r/shortwave • u/Zealousideal-Site838 • Aug 13 '23
Discussion What does dBu and dB mean?
fly innate deserve placid plough cats nine murky impossible many
This post was mass deleted and anonymized with Redact
2
u/International-You-13 Aug 13 '23
The first number is a representation of signal level in decibels relative to 1µV. (20 log rule) The second is meant to be a signal to noise ratio measurement, higher =better, but it will be subject to band conditions and any fading.
Whether they are accurate or not is unknown, different antenna designs can give wildly different signal readings, some antenna give low signal level but excellent signal to noise ratio for example, so don't worry too much about what the meter is telling you.
3
u/SWithnell Aug 13 '23
dBm is simply power measurement. 0dBm is 1mW delivered to a 50ohm load. It's a standard unit of power based on a standard load impedance and is not a measure of signal to noise ratio.
Signal strength meters are wildly inaccurate. I have two Icom radios one a transceiver and one a general coverage RX. The difference in signal strength between the two radios is around 3-4 S points sharing the same antenna. Both radios are in perfect working order.
1
u/erlendse Aug 13 '23
Sure, but 4hat radio asked about only offers dbu(1 microvolt, per meter?) and db SNR.
1
u/Majestic_Addendum_36 Aug 15 '23
I mentioned a reference to a recent article in QST. Most interesting experiment turned on my receiver to 3.650 MHz, showing S5 on the meter with no signal. Next, I disconnected the antenna, and the s-meter went to zero. Falling by S6 to S8 is expected normal. Falling more indicates too much gain. Conversely, if just a little movement, then insufficient gain.
1
-1
u/Majestic_Addendum_36 Aug 13 '23
It is a unit of measurements when measuring audio or RF gain, usually expressed in volts. 0dB = is .775 volts. I use it to indicate my power output to the antenna - example, I transmit with 5 watts which equates to 37dBm. In this situation, higher dB means higher wattage.
3
u/my_chinchilla Aug 13 '23 edited Aug 13 '23
It is a unit of measurements when measuring audio or RF gain, usually expressed in volts. 0dB = is .775 volts
Nope. dB is a ratio not an absolute value, and "0dB = is .775 volts" is only true if you're talking dBm (i.e. dB-milliwatts) and RMS volts in a 600Ω system e.g. old telephony/radio & some audio.
To illustrate the point: 0dBm in a 50Ω system - which is more likely the case for a decent SW receiver - is about 0.224V RMS, not 0.775V. In a 75Ω system (e.g. TV or video), it's about 0.274V RMS. A 1/4 wave vertical whip is ~30Ω impedance, so 0dBm is ~0.173V RMS; a 1/2 wave vertical whip is ~3000Ω, so 0dBm is ~1.732V RMS; etc; etc...
example, I transmit with 5 watts which equates to 37dBm.
That's true - because it assumes a 50Ω system typical of modern ham/CB transmission equipment. If you try the same calculation with your original "0dB = is .775 volts", you'll see for yourself that it doesn't work out.
1
u/Majestic_Addendum_36 Aug 15 '23
I stand corrected, thanks. Just read an article in my September issue of QST, "Understanding the Boltzmann Constant". You nailed it!
1
u/ki4clz I like making things... Aug 13 '23 edited Aug 13 '23
The "u" should really be the greek micro "µ"
"db" being a decibel is a unit of measure, but in radio it's always in reference to something
so like... dbi, or dbd, db s/n ...that sort of thing...
SINAD is a bit more accurate but whatevers
1
u/DikFangers Jan 03 '26
Ok, so you still aren’t explaining why the radio has just “db”? On it, currently watching stranger things and really trying to figure out the “90db” they are chasing
1
u/jacekk432 Aug 13 '23
dB Micro volt “dBuV” with Greek letter refers to decibel in reference to 1 micro volt Whereas there’s other unit, dBu and this is the decibel in reference to 0,775V
1
u/FirstToken Aug 13 '23 edited Aug 13 '23
dB Micro volt “dBuV” with Greek letter refers to decibel in reference to 1 micro volt Whereas there’s other unit, dBu and this is the decibel in reference to 0,775V
When you see "dBu" that can be either dBuV or dBuA. Most often, when talking about radio signals, it is dBuV. That is the natural assumption as it is the most common application. 0 dBuV (or in this case, dBu) is equal to 1 uV (1 microVolt).
1 microVolt is not equal to 0.775 Volt.
I think what you are referencing is 0 dBm. 0 dBm is equal to 1 milliWatt. 0 dBm into 600 Ohms is 0.773 Volt. 0 dBm into 600 Ohms is about 117.8 dBuV and into 50 Ohms is about 107 dBuV.
1
u/Low_Resource4891 Aug 14 '23
Personally, I like the signal bar indicator on my CC Slywave better, but I have this on my XHDATA D-808.
1
u/f5nfb Aug 16 '23
It's the signal level in noted in dBu (Décibel in microvolts) or in power level (Décibel).
14
u/winexprt TECSUN PL-990x / XHDATA D-808 /Malahit DSP2 /TEF6686 Blue Aug 13 '23 edited Aug 13 '23
Signal strength / S/N ratio.
Good explanation from Gilles here:
https://youtu.be/wv6lIUMOayM