Terms:
Decibel dB
dBi- gain of an isotropic antenna
dbd- gain of a dipole antenna
dbm- decibel milliwatt
dbw- decibel watt
MHz- megaherts
Attenuation- to reduce
Recently I was having a conversation about radio, imagine that. We were reviewing loss, signal degradation, more loss and the topic of decibels (dB) came up. They wanted me to convert a decibel to volts, amps or something they could see on their DMM. You can’t! A decibel is a comparison of 2 values, kinda dimensionless. But it’s one of the greatest systems to use.
Utilizing dB makes measuring overall systems easy as shit. You can take real small numbers or multiple small or large numbers, and use decibels so you won’t have to put in 9 zeros in front of something and screw it up on a calculator. The real beauty is that decibels can be added and subtracted easily, I said add and subtract, can’t be any easier! But here is the best part and memorize this: 3 dB is double the power, 10 dB is 10 times the power. Stick it in your brain hole forever, it’s golden.
Station Loss Using dB:
Let’s say you have a 100’ run of coax to your antenna. You’re a cheap bastard and don’t want to spend coin on good coax and would rather have cool colored knobs on your radio.
Stop being a knob! You purchase 100’ of RG 58, realize your 15’ short and make a jumper with a barrel connector. Let’s review your system: Radio-jumper-tuner-coax-jumper-antenna and for argument sake the radio puts out 10 watts. RG 58 at 30 MHz for 100’ attenuates approximately 2.7 dB depending on who’s chart you read. Time to add it up:
- First jumper: negligible
- Tuner: 0.5 to 1 dB loss
- Coax: 2.7 loss
- 20’ jumper: 0.1 loss
- Antenna: ½ wave monopole 2.14 dB gain*
- Worst case scenario: -1 dB -0.1 dB -2.7 dB + 2.14 = -1.57 dB loss (oh shit). Your 10 watt signal is now around 7.75 watts
Let’s do the same thing with LMR 400 coax. At 30 MHz at 100’ 0.7 dB loss
-1 dB -.7 dB -0.1 dB + 2.14 = 0.43 dB gain. Your 10 watt signal is now about 10.7 watts
dB makes it real simple.
*Really, really , really question the advertised antenna gain from the advertisers!!! I have seen an A99 advertised by some to have 9.9 dB gain antenna. Never has such bullshit been promoted!
How to turn dB into watts, volts and all the cool shit we like to measure.
The only exact things are you’re gonna know: pay taxes and die (Ben Franklin, a long time ago). As radio folk we like talking numbers and we like talking specifics. First off, lighten up. Any power meter, any, has a +/- 5% accuracy rate. Your digital multimeter (DMM) can be as bad as +/- 2% and that can change with temperature and humidity. So let’s settle on an average approach to our numbers and take a lot of arguments out of the equation. So if I say 10 watts but it could be 9.5 watts, don’t break my balls. If you gotta argue over 0.5 watts, you should get your prostate checked.
Decibels are relative but dBw and dBm are exact! An assumption has to be made in that all measurements are made into a 50 Ω load. It’s the way shit is made and completely applicable in our radio universe (50 Ω coax, 50 Ω radio). Decibel-milliwatt (dBm) gives a measurement with a reference to 1 milliwatt. The power level of 0 dBm = 1 milliwatt (mW). Here is a comparison:
- Home Wifi [2.4 GHz and 5.0 GHz]: (smallest acceptable signal with no errors) -85 dBm or 3.1 pico watts (pW). That 3.1 with 12 zeros in front of it! See I told you that you can screw it up on a calculator.
- Garage door opener [300-390 MHz]: +10 dBm, or 10 milliwatts
- Bluetooth [2.4 GHz] : -20 dBm or 10 micro watts (uW)
- Tire Pressure Monitoring System (TPMS) in your car [315 MHz]: -6 dBm or 250 micro watts(uW)
- CB [27 MHz]: +36 dBm at 4 watts, and+50 dBm at 100 watts
- Cell Phone [800 MHz, 850 MHz, 1700 MHz, 1900 MHz, 2100 MHZ]: +34 dBm or about 2.5 watts
Decibel-watt (dBW)
Easy. dBW = dBm – 30
dBd and dBi
I have listened to grown men lose their shit over dBi and dBd. More arguments on the radio than anyone should ever listen to. It’s not an argument. It is an engineering approach.
dBi is the measurement of an isotropic antenna that doesn’t exist, it is theoretical. Its purpose is to imagine electromagnetic waves travelling in perfect free space with no interference, in all directions. Like I said, it doesn’t exist. What does exist is dBd, decibel gain in comparison to a dipole antenna. It’s simple math:
dBi = dBd -+2.14
dBd = dBi – 2.14
When it comes to antennas and their advertised gain, truly do your research. And if someone ever states that their ¼ wave over a ½ wave monopole antenna has 9.91 gain, change the channel, turn the radio off, or walk away, it’s not worth the most miniscule amount of neural synaptic activity.
Never argue with an idiot! They will drag you down to their level and beat you with experience!”
"To those who fight for it, life has a flavor the protected never know."