How to Calculate FPV Range with dB?

This tutorial explains how to estimate FPV range using the dB values of your FPV equipment. Being able to calculate dB will help you understand the capability, performance and limitations of FPV components.

What is dB?

dB means decibels, we use this unit to measure the level of sound, but in FPV we also use dB to measure signal strength.

dB is on a non-linear, logarithmic scale. For example, by increasing dB by 3, the signal strength doubles. (note: signal strength, not range)

Apart from signal strength, both antenna gain and VTX power can be expressed in dB too.

Why use dB you might wonder. Well, for some applications, dB is much easier to work with, because the math is simpler. You just add or subtract numbers, there is no multiplication or division.

“dB” and FPV Range

The total dB in the FPV system is called link budget, and can be determined by:

  • Antenna gains
  • Transmitter power
  • Receiver sensitivity

FPV signal is attenuated as it travels through air – the dB of the received signal decreases as you fly further away, in the same way that sound is quieter at a distance.

Knowing your link budget, and the signal loss for traveling in the air (a.k.a. free space path loss), we can then estimate the range. Maximum range is achieved when dB gets to 0, and so a higher link budget results in more range.

Although it is possible to estimate FPV range using dB, in real world conditions the signal can be affected by far too many variables such as background noise, interference, humidity etc. It’s almost impossible to generate an exact figure of how far your signal will travel in feet or meters.

Anyway, it is still a powerful tool to help us understand how much improvement we can expect from using different components, such as increasing the output power of your VTX, and antennas of different gains.

Antenna Gain in dB

You probably already know that FPV antennas have gain, which is measured in dBi (decibel isotropic). You can normally find out about antenna gain on the package or product page.

Antenna gain shown in specs

Video Transmitter Power in dB

VTX (video transmitter) output power is usually described in mW (milliwatt), and you can easily convert between dBm and mW using these equations:

dBm = 10*log10(mW)
mW = 10^(dBm/10)

Not good at maths? Don’t worry, there are tons of online calculators for converting mW and dB. Even better, here is a table listing all the common wattage used on mini quad VTX and the corresponding dBm values.

mW dBm
1 0
10 10
25 13.98
50 16.99
100 20
200 23.01
300 24.77
400 26.02
500 26.99
600 27.78
800 29.03
1000 (1W) 30
1500 (1.5W) 31.76
2000 (2W) 33.01

The best thing to remember is, every 3dB increase, will double the signal strength, but an increase of 6dB is required to double the range. From this you can ascertain that replacing a 200mW VTX with a 400mW will NOT double your range, but increase it by approximately 50%. It is also worth noting that as the power goes up, the rate at which dB increases goes down, further diminishing returns.

Fun fact, if you have an ImmersionRC PowerMeter, you will see it reports both mW and dBm on the screen :)

Receiver Sensitivity in dB

Receiver sensitivity determines the minimum RF power the receiver can detect. The more sensitive it is, the more negative the number is. (Yes, it’s a negative number)

Often this number is not publicly stated, and when it is, we have no idea how accurate it may be as manufacturers are often optimistic about the capabilities of their products.

If you can’t find the number anywhere, -85dB is a good conservative guess for FPV video receivers (suggested by this post on FPV lab).

The RX5808 receiver module claims to have a typical sensitivity of -90dB according to Foxtech. I also asked FuriousFPV about the True-D’s sensitivity and they told me it’s -93dBm +/-5dBm.

If you know the sensitivity of your receiver, let me know in the comments, this might help someone in the future.

How to Calculate FPV Range with dB?

Just plug the numbers into this range calculator and it will return the distance in kilometers.

Here is the technical stuff in case you are interested.

As mentioned, the absolute maximum range can be determined when the power of the signal drops to 0dB. However, our video will show static and become “unflyable” when the signal gets too weak. To ensure a reliable connection, it’s a common practice to assume a minimum level of signal strength to maintain, i.e. the Link Margin. For example, to stay above 10dB or 12dB, or even higher if you are being conservative.

Using the Free Space Path Loss equation (reference), we can rearrange it which gives us:

Distance = 10^((FSPL-LM-32.44)-20*log10(f))/20)

Where

  • FSPL (Free Space Path Loss) = TX Antenna Gain + RX Antenna Gain + TX Power – RX Sensitivity
  • LM = Link Margin
  • f = frequency in MHz (mega hertz)

Real Life Example – dB is so Useful!

Using this setup as an example:

  • 25mW VTX (14dBm)
  • VTX antenna: Lollipop V2 Antenna (2.5dBi)
  • VRX: Typical RX (-90dBm)
  • VRX antenna: Lollipop V2 Antenna (2.5dBi)
  • Assuming a link margin of 10dB

Enter these numbers in our calculator, the range is only about 0.37Km.

Now, in order to double the range, we have to somehow add 6dB to our link budget. So here are the options:

  • Increase VTX output power to 100mW (20dBm, 4 times the power)
  • Or, use a higher gain antenna on the receiver, it has to be 8.5dB gain or more. Only directional antennas have such high gain, for example, the Menace Pico Patch has 9.4dB gain

Doing one of these things will theoretically double your range to over 740m. If you do both, it will quadruple your range to nearly 1.5km!

If this is still not good enough, using a lower frequency provides more range assuming other variables stay the same. For example, using 1.3GHz instead of 5.8GHz, will give you over 6km! Here is a beginner guide to using 1.3GHz for FPV.

Of course, this is all based on the assumption that we are flying in perfect conditions. In real life the range is most likely going to be less than this.

Other Causes of Signal Loss

You are not getting as much range as the estimation, because there are many factors that can cause signal loss:

  • Interference and noise in the environment, or from other pilots
  • VTX power drops when it gets hot
  • Antenna orientation (how TX antenna and RX antenna are aligned)
    • At 45 degree = -3dB
    • At 90 degree = -20dB
    • Two linear antennas pointing at each other = -30dB
  • Antenna Polarization
    • Linear to Circular  = -3dB
    • RHCP to LHCP = -20dB
  • Loss in Coax cable and adapters of SMA, MMCX, UFL (Usually not a lot, e.g. 0.1-0.3dB)
  • Antenna radiation pattern – Omni antennas have weak signal directly above, and directional antennas are far less effective outside of its beam-width

* Signal loss figures are from this source

Conclusion

So now you know that there is more than one way to increase your FPV range, how to use dB to calculate an estimate, and how different components affect range. Following good practice, like using the correct antennas, matching polarization and ensuring good orientation of antennas on your craft & receiver, are far more important than outright VTX power.

Edit History

  • Mar 2019 – article created
  • Jan 2020 – revised

10 thoughts on “How to Calculate FPV Range with dB?

  1. Alex

    I share Hugo Chamberland’s sentiment. 2.5 dBi is the max.
    What you really need is the radiation plot which shows gain vs degrees. Your radiation plot may have significant nulls which would cause a negative link margin but entering in the dBi in the calculator until you have 0 margin would show you the link availability WRT the angles on your antenna.
    ALSO what the calculator is missing is the margin threshold to demodulate data. The question is what kind of modulation R/C tx/rx use and what is the threshold
    for example, for BPSK you need at least 11 dB of margin
    QPSK 5.5 dB, etc.

    Reply
  2. Hugo Chamberland

    You should correct that the lollipop gain is nowhere near 2.5dBi. It is an average of 1.5dBi, I say average because it wiggle wildly along the elevation (-3 to 2.5Dbi). That make in fact only -3dBi usable as the 2.5Dbi regions are just too small. Unfortunately Dbi is about the same as C rating on lipo, most of it is made up.

    Reply
  3. alan mccluskey

    lol just reading some other replies hehe,,
    very useful info, its easy to forget how to read dbi in terms of distance and having a table of mw/dbi s very handy thanks… i have no extremely complex questions on the matter hehehe

    Reply
    1. Oscar Post author

      Just leave it at 5800 for 5.8GHz frequency. Only change it if you are using other frequencies like 1.3G, 2.4G etc…

      Reply
  4. Dave

    What is the difference of dbm and dbi??
    On your distant formula for calculating FSPL :
    FSPL = 2.5 + 2.5 – 93 !!!?
    It doesn’t seem correct.

    Reply
    1. Oscar Post author

      In the calculator, they don’t matter, just put the numbers in.
      RX sensitivity is a negative number, so minus a negative number makes positive.

      Reply

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