How to Calculate FPV Range with dB?

This tutorial explains how to calculate/estimate FPV range using the dB of your FPV equipment. Learning about this topics really help understand the capability and performance of FPV components, at least for me it was huge.

What is dB?

In FPV, we use the unit dB (decibels) to measure signal strength, on a logarithmic scale. Confused? Don’t worry, just know this, increase dB by 3, the signal strength doubles. (note: signal strength, not range)

And both antenna gain and VTX power can be expressed in dB too.

Why use dB you might ask. Well, for some applications, dB is much easier to work with, because the math is simpler. There are no multiplications, only adding numbers together.

“dB” and FPV Range

FPV signal is attenuated as it travels through air – the dB of the signal decreases as you fly further. In simpler words, by knowing the total dB in your FPV system, and the signal loss for traveling in the air (free space path loss), we can estimate the range.

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

  • Antenna gains
  • Transmitter power
  • Receiver sensitivity

The maximum range is when the dB drops to 0, or a certain minimum level. More link budget means more range. Every 6dB increase, doubles the range.

So yeah, we can estimate FPV range with dB, but it’s not saying we can work out exactly how many meters or feet our FPV system can achieve. The exact range can be affected by simply too many variables, such as background noise, interference, humidity etc.

But still, it’s a useful tool for working out how much improvement we can expect from using different components. like antennas of different gains, VTX of different powers, or receivers of different sensitivity etc.

Antenna Gain

You probably already know that FPV antennas have gains, 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

VTX (video transmitter) output power is usually described in mW (mill-watt), 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 a ton of online “mW to dB conversion calculators”. Even better, here is a look-up table listing all the common powers we use on mini quad VTX.

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

As you can see, every time VTX power is doubled, dBm increases by 3 (roughly). However, to double your range, you need 6dB increase remember? That’s 4 times the VTX power! And as power goes higher, the rate of increase in dB decreases, so it’s getting less and less effective for improving range.

Receiver Sensitivity

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 is as manufacturers are often too optimistic about it.

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

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 comment.

So, How to Calculate FPV Range?

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

Here is how it works in case you are interested.

The absolute maximum range is determined when the power of the signal drops to 0dB. But our videos normally start to show statics and becomes “unflyable” when signal gets weak. To ensure a reliable connection, we like to assume a minimum level we want to maintain – the Link Margin. For example, 10dB or 12dB. You can go even higher if you are being conservative.

Using the Free Space Path Loss equation from this page, 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)

Example – How dB Affects Range

Say, I have this setup here:

  • 25mW VTX (14dBm)
  • VTX antenna: Lollipop V2 Antenna (2.5dBi)
  • VRX: True-D (-93dBm)
  • VRX antenna: Lollipop V2 Antenna (2.5dBi)
  • Assuming a link margin of 10dB

Plug these numbers into our calculator, the distance is 0.52Km.

Now, if I want to double the range, what should I do? You need to gain 6dB to our budget right? So here are the options:

  • Use a 100mW VTX (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, such as the Menace Pico Patch (9.4dB)

Doing one of these things will theoretically double your range to over 1km. If you do both, it will quadruple your range to over 2km!

If this is still not good enough, using lower frequency provides more range assuming other variables stay the same. For example, using 1.3GHz instead of 5.8GHz, will give you over 10Km! 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 not going to be this good.

Other Signal Loss

The following can cause signal loss and reduction in FPV range.

  • 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, there are more than one way to increase your FPV range apart from VTX output power. Using the correct antennas as well as good practice, like matching polarization, good orientation etc, are far more important. And I hope you now have a basic understanding how different components can affect your FPV range, and use dB to estimate your range.

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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