Fast Easy Guide to Decibels dB vs dBi in 6 Minutes

[@lederhudler]

A pretty decent armchair explanation for folks that aren't mathematically inclined. Perhaps a little extra would help clear up some of the confusion that some people expressed. The concept of decibels is tied to the mathematics of logarithms - which - don't get scared - is an easy way to change multiplication into addition and division into subtraction. The best way to explain logarithms is to construct this chart (showing pairs of numbers - the number and its associated "log"). log of 1 is 0 log of 10 is 1 log of 100 is 2 log of 1000 is 3 and so on. Also the log of 1/10 is -1 log of 1/100 is -2 So to add 1 to the logarithm of any number is the same as multiplying the associated number by ten - of course, you have to remember to come back out of the logarithm table the same way you went in - but in reverse. Likewise, to subtract 1 to the logarithm of any number is effectively dividing the associated number by 10. Now if you were to plot these pairs of numbers (all multiples of 10) and then draw a smooth curve between them, you would be able to figure out the log of any other number, not just multiples of ten. For example, the log of 2 is about 0.3. The log of 4 is about 0.6. So if I add 0.3 to 1 (=1.3) this corresponds to multiplying 2 X 10. (log 20 is about 1.3). Back before computers and calculators became commonplace, this made arithmetic simpler. You could add/subtrace numbers on the fly. And some physical phenomena actually are better characterized by a logarithmic relationship rather than a linear relationship. So a decibel is really nothing more than a logarithmic scale, divided by ten. We could have used a "bel" (almost no one does - again granularity - and tradition - dictate that a decibel is more convenient). Double the power - adds 3 dB. Quadruple the power - adds 6 dB. Tenfold times the power - equals 10 dB. One tenth the power is -10 dB. Two wrinkles to be aware of, so you don't trip over them. First, dB implies a ratio. Because it is based on a ratio of like quantities, it is typically considered unitless. This is useful to know, if you have learned the trick to "cancel units" (perform the indicated arithmetic operation(s) on units of an equation). If you use a variant of dB such as dBW (ratio compared to 1 Watt), dBm (ratio compared to 1 milliwatt), then you have consider the units. Otherwise, it is merely a number multiplying or dividing whatever units you started with. Also, In most arenas where dB is used, we are talking "power", not amplitude. For example, if you are in electronics, the power is related to the square of the voltage or the square of the current. So if you are working that realm, and are doubling the voltage (or current), you are quadrupling the power. So it is customary to incorporate that square term into the formula for dB. Hence power defines dB as 10log(ratio). But voltage or current defines dB as 20log(ratio).

[@DXCommanderHQ]

Write a book :)

[@Stephen_Orion]

Thanks Callum, now studying for my Foundation exam..These videos are very helpful. many many thanks..

[@M0JHN]

Absolutely brilliant Callum.

[@Siskiyous6]

I am sorry it took me a week to watch this. What a straightforward way to untangle the dB - We need a dBC measure.

[@JesterHellsingborg]

Thanks...have an amazing weekend...enjoy family time 🤙💚🙏

[@DXCommanderHQ]

Thank you! You too!

[@DK5ONV]

I always use green Marker on green Boards! Happy Weekend Lord Callum...😉💯🙋‍♂

[@DXCommanderHQ]

HAHAHA!

[@richb.4374]

Antenna manufactures love to use the DBI rating for their antenna's because it makes the antenna look 2.1db better than it actually is. Since none of us have antenna's in outer space using an isotropic rating is pretty much useless. Any time you see an antenna rated at dbi subtract 2.1 right off the top.

[@AndrewBryantPianoTuner]

I'm completely lost already!

[@Steve-GM0HUU]

Good explanation. Often bugs me when someone states states 10dB. I think dB with reference to what? Isotropic radiator power, dipole, mW, Volt, etc. This can be important. For example, +6dBmV is a doubling of voltage whereas +3dBmW is a doubling of power. With antennas, PAs, feeder, we are usually interested in power but for small signals in radio equipment it is often voltage that's important. To save folk looking it up: dB (power) = 10 x log (power_out/power_in). dB (voltage) = 20 x log (voltage_out/voltage_in). So, if power out is 200mW for 100mw in, we should say the gain is +3dBmW. If power out is 100W for 5w in, the gain is +13dbW. If RMS voltage out is 100mV for 50mV in we should say gain is +6dBmV. Also, be wary of antenna manufacturers who say things like their antenna has 1.5 dB gain. If they mean +1.5dBi, this is 2.15 - 1.5 or 0.65dBi less gain than a dipole. So, you might think you are buying an antenna that gives you more gain at a particular angle/direction but may actually perform worse than a basic dipole.

[@DXCommanderHQ]

Steve, how right you are!

[@markramsay6399]

Great Video Cal as always. If anyone is using dB in any exam I really recommend making up some of their own examples for adding and taking away dB. It is not a difficult concept, but it is odd if you have not come across it before. I found doing a load of simple calcs helps at loads. Mark, 2E0MSR

[@DXCommanderHQ]

That's a great idea!

[@brentjohnson6654]

Great discussion on “bananas” ! Haha. All the best!

[@DXCommanderHQ]

Yes!

[@acestudioscouk-Ace-G0ACE]

Thanks for the video Callum! Decibels is a subject which ties people in knots, I found it a little easier when I understood that it's a really useful way of comparing a really wide range of numbers, like when you start talking of plus or minus 30dB compared with 0dB.

[@DXCommanderHQ]

Great tip!

[@southernexposure123]

Thank you for the video and explanations. I've watched about a dozen dB videos and I can parrot a lot of what each says and each explanation is almost the same. I still have a few questions and I bet you're the only one who can touch me with some understanding of my questions. Mainly (that is to say my highest proprity at this time is) I'm trying to make an RF probe with which I can measure the output of a one transistor crystal oscillator, and then measure the subsequent stages to see the gain. I think I'll have to put amplifier stages in a box before I put the VOM leads at the oscillator output so I can get a reading on the VOM. I don't want to make one watt of RF power before I see the meter needle (digital meter reading etc) increase to zero. My question on that is how many stages (I suspect using small plastic RF capable transistors) of amplificaton do I need in line between the transistor output and the meter lead to get an indication on my VOM. I do understand that 0 dB is equal to 1 mW RF power. And I understand that doubling the power is 3 dB gain and all that other stuff about the S-Meter readings. Oh I don't care at this time about technical accuracy I just want the get a meter indication that I can see. My next question is secondary and relates (I suspect to Audio dB). I just got a few new VOMs and they have a dB scale. The manual says how to add a number to the AC meter reading get the actual dB. Is the the measurement made by placing the meter leads on the speaker terminals while the speakers are being played? I'm guessing that's right. Thank you very much for your time.

[@DXCommanderHQ]

RF Probe.. OK, not my thing sorry. I need to be honest!

[@southernexposure123]

@@DXCommanderHQ OK Thanks for the reply.

[@gaptastic]

It's logarithmic scale, no? But I did like the affix identifier explanation. Thanks Callum!

[@deankq4adj125]

You have helped clear the dB mystery for me. Thanks for taking the time to explain!!

[@DXCommanderHQ]

Glad to help!

[@davidhtims]

I am commenting here to seek an antenna solution. My location is in a shallow valley. It runs north and south for about 100 miles. The east and west ridges are 500 to 800 feet above my location. My property is 130 feet wide north and south. East and west is 600 feet. What antenna has a high takeoff angle to help clear the hills?

[@DXCommanderHQ]

I don't know how far away the peaks of your hills are.. You need to measure the angle. If they are above 20 degreees, then you're gonna be a bit screwed. However, if the peaks are a few miles away, you might be surprised.. They are probably under 10 degrees. NO antenna can go through hills so we are back to square one, which is what's the best take-off antenna there is? They will all do the same job with your hill there. Some better than others. I am about to do a comparison video between say a vertical and a dipole, so hang on!

[@davidhtims]

Thank you for your prompt reply. I just checked with a inclinometer. It’s about 6 degrees at my eye level. However above the first hill it rises again. I’m not doing terrible with a trap 80/20m dipole. It’s 30 ft up on fence rail poles. Can sometimes go from here to Florida or Canada or Minnesota. Heard Slovenia yesterday but no CQ. I did not mention I’m in north central texas. And have a ameritron Als 500m. Thanks again KI5TRF

[@davidhtims]

KI5TRF

[@BobMellor1954]

Thanks Cal

[@PatAutrey]

Field day is coming up pretty quick and I'm kind of concerned about operating in close proximity to other radios I was wondering if you have any recommendations to protect the radio so it doesn't get damaged I am using a 991 Yaesu

[@DXCommanderHQ]

The BEST way of doing this is coordinating with your friends to make sure you all don't end up on 20m! However, in the main, you should just be fine. I've done this a few times. COnsider ONLY double the frequency.. So if one guy is on 7.1MHz, watch the 2nd harmonic on 14.2, Same only with 20m band interfering with 10m.. So 14.2MHz will kick a strong harmonic at 28.4MHz. But assuming you have some separation on the antennas, you will hear the harmonics, so you'll move away!

[@PatAutrey]

@@DXCommanderHQ very helpful thank you sir!

[@mikeladd5880]

Any chance you can explain smith charts.

[@DXCommanderHQ]

This video does a good job: kzbin.info/www/bejne/pom6k5euhKaoetU

[@mikeladd5880]

@@DXCommanderHQ just watched that vid. Ty explaines pretty well. Kinda works to explain if i need capacitor at feedpoint etc. Using my vna. Also would explain if i need a different impeadance feedline

[@luiscosme1154]

@5:41 You said 13 is 2. Where did the 2 come from like magic?

[@DXCommanderHQ]

Because from 10 to 13 is a factor of 2 (again) difference is 3dB.

[@luiscosme1154]

@@DXCommanderHQ This is great! Got it! 3dB = x2, Thank you for clarifying! By the way, man that is a nice studio! Congratulations on your workspace!

[@zeebraeend]

That's very clear indeed. Thank you

[@DXCommanderHQ]

Glad you think so!

[@aeron-mw7ofs]

The wife has quite a lot of dB's .

[@charleswoods2996]

Funny little discovery I made; I seem to get better audio quality (in single sideband) by lowering my output power in the transceiver. Also, better DXing!

[@DXCommanderHQ]

Well, I know why that is :)

[@richardnalisnick462]

A job well daoe!

[@DXCommanderHQ]

Ah thanks!

[@MIKROWAVE1]

dB or not dB - that is Attenuation!

[@DonzLockz]

dB is still hard for me to understand, expecially minus dB on my wifi signal on my router.🤔 How many "dB years" do I need to understand dB?😂

[@andye2005]

It's hard until the penny drops... the vid was a bit too basic in parts so this may help. dB without anything is a relative term, and because it was originally used to compare power without any suffix you assume power. dBs are simply a number raised to a power (and multiplied by 10) so they are logarithmic punch 3 divided by 10 into your calculator hit antilog or x^2 and you willl get 2 i.e. double When looking at a dB number if it is a single digit you just need to remember the double for every 3dB fact if the dB is a two digit number look at the first digit say 13dB the first digit is 1 so shift the decimal point one to the right i.e. you multiply by 10 then you can add the last digit 3 in this case so you double it and get the 20 If the dB value was 23 dB the first digit is 2, so shift the decimal point to the right by two places i.e. multiply by 100 the 3 doubles the result so thats 200 if the first digit was 3 then you multiply by 1000, 4 by 10000 and so on In the other direction you move the decimal point to the left, i.e. divide by 10 so -10db is 0.1 -20 dB is 0.01 and so on Because of history Db is relative too 1W but it can be stated to anything. 30dBm as an example. dBm is a reference to 1mW (it is sometimes stated as dBmW for clarity but not always) 1mW is 0.001W 30dBm the first digit is 3, so that shift the dp 3 to the right i.e. multiply by 1000 0.001 x 1000 = 1 so 30dBm is the same as saying 1W Go the other way at -30dBm -3 is shift the dp 3 to the left i.e. divide by 1000 1mW (the reference) / 1000 is one microwatt So if you see a warning label on your expensive Spectrum analyser saying do not exceed 20dBm input you will know that this is 1mW x 100 i.e. 0.1W So thinking about the first digit in the pair like this will get you into the right range. On your router if you see a figure of -16dBm Thats 1mW reference First digit is -1 so divide by 10 gives 0.1mW The last digit is 6 so that's normally double twice, however as this is a negative dB you DIVIDE by 2 twice so 0.1mW /2 = 0.05mW divide by 2 again 0.05 / 2 - 0.025mW So if your router power is stated as -16dBm that's 0.025mW or if you prefer 25 microwatts. Hope this helps a bit. Andy

[@jamesrkelly76]

@@andye2005 Great explainer Andy

[@DXCommanderHQ]

@Andy E - Brilliant reply..!!!

[@BobMellor1954]

@@andye2005 Thank you

[@bak2bassix]

@@andye2005 brilliant! Thanks. Previously I did struggle to understand how 10 was x10 relative to the other numbers going up in 3’s, each doubling the value. Maths was never my strongest subject but your explanation does the trick. Many thanks.

[@jackK5FIT]

Another great video. I had a basic understanding of dB but this video gives a simple, fun explanation for a subject that causes much confusion. Thanks!! Jack K5FIT

[@DXCommanderHQ]

Glad it was helpful!

[@jimmyjohnny3779]

So is 16dB x 40?

[@DXCommanderHQ]

Yes, perfect.

[@chrisbartlett6022]

Have spent a good amount of time trying to get folks not to use Green on Marker boards 🙂

[@forhadahmed4643]

mmmm such a cool guy

[@BetsyBeverly-e7d]

Erik Lodge

[@darrellcollins877]

where did all the bananas go ???

[@DXCommanderHQ]

They are on a picture in the office :) [this was supposed to be a serious video LOL!]

[@JasonPullara]

dB = bigger number make thing go louder

[@rrh2918]

Last part just confused me

[@DXCommanderHQ]

This is the dumbest / easiest video on dBs. So maybe either watch again or drop me an email so I can fix.

[@tomcass240]

How can you explain decibels without mentioning the word logarithm once? This really isn't an explanation of anything.

[@DXCommanderHQ]

Keep taking the pills - and the colouring pens are over there.

[@tomcass240]

@@DXCommanderHQ If you want to be a successful youtube creator you gotta learn to take criticism without getting upset.

[@Mystik-Kitty]

THANKS CAL for the great video!! SO NICE to watch you! This is Kathryn from Alaska, now living in Texas! Just got my 9 band Butternut erected at the new home, and perhaps one day will be able to DX with you! 88s! de KL4QZ (ps: still going strong with my DXC Classic for portable HF!!)

[@DXCommanderHQ]

Kathryn, yes I remember you my dear friend! Texas..? Oh maybe some day we can have some RF fun!