How to Calculate FPV Range with dB?

Top Source For All Things Drone Photography

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.

dB means decibels, we use this unit to measure 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 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.

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.

If you can calculate the total dB of your FPV system, and the signal loss for traveling in the air (free space path loss), we can estimate the range.

Maximum range is achieved when dB gets to 0. Therefore, a higher link budget means more range.

 

Although it is possible to calculate 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.

It is still a useful tool for working out how much improvement we can expect from using different components, such as increasing the output power of your VTX, and how antennas of different gains affect range

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 of the antenna.

Antenna gain shown in specs

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 a ton of online “mW to dB conversion calculators”. 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 a 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.

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).

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.

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

Here is how it works in case you are interested.

As mentioned, the absolute maximum range can be determined when the power of the signal drops to 0dB. But our videos will show static and becomes “unflyable” when the signal gets too weak, to ensure a reliable connection, you can assume a minimum level to maintain, i.e. the Link Margin. For example, stay above 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)

Using this setup as an example:

  • 25mW VTX (14dBm)
  • VTX antenna: Lollipop V2 Antenna (2.5dBi)
  • VRX: Typical RX (-85dBm)
  • VRX antenna: Lollipop V2 Antenna (2.5dBi)
  • Assuming a link margin of 10dB

Enter these numbers in our calculator, the distance is 0.21Km.

Now, in order to double the range, we have to somehow add 6dB to our link budget. 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, for example, the Menace Pico Patch (9.4dB)

Doing one of these things will theoretically double your range to over 420 meters. If you do both, it will quadruple your range to nearly 1km!

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 4km! 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.

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

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.

  • Mar 2019 – article created
  • Jan 2019 – revised



Source link

Leave a Reply

Your email address will not be published. Required fields are marked *