Max Range Estimator

Background

Picking the right FPV setup can be a daunting task, and picking the wrong one can waste time and money. The other issue is who do you believe, the random guy that post on a forum; the vendors website that can't even get the type of connector right; or maybe the guy at the club?

So this why I created a tool that allows you to calculate the max theoretical range you can expect out of various setup. I know what you're thinking now, why should you believe the random article author guy? Answer: Science. I leave all formulas open for you to see and welcome any comments on how to make it better or correct a mistake. Below is a walkthrough on how to utilize it and some of the theory behind it.

Download here: RF Range Calculator

Excel 97-2003: RF Range Calculator

The Theory

This calculator is based on the Free Space equation. It uses dB for simplicity, and I have included a few conversion tools in case dBs are not readily available. 

There are 5 key parts to the Free Space Equation.

1. The first is the power output of your transmitter, or Pt. We will use a generic value of 250mW as it is fairly common. This equates to 23.98dBm. The 'm' of dBm only indicates we are looking at mW and is not a unit per say.
    
2. Then we have to consider the gain of the antenna, or Gt, as it allows to focus the Radio Frequency (RF) energy provided by your transmitter. It is a measure of the power that is gained in comparison to an isotropic antenna, or an antenna that would distribute the energy in a perfect sphere. This is important because gain does not necessarily equate to directionality. In other words you can have an antenna that radiates omnidirectionaly, or in a donut shape around the antenna, which has a similar gain as a directional antenna, or in a conical shape in one direction. For example, we will use 1dBi for our Gt as being on an airplane, we want to have the most coverage to compensate for manoeuvering. Note that the 'i' of dBi stands for isotropic.
    
3. The biggest effect in our equation is from Space Loss, or Ls. It represents the energy that is lost through the medium, air in this case, due to the atmospheric properties and contaminants, and from the signal spreading over a distance. To name only a few effects, water vapour will absorb enrgy while dust and other particles will scatter it.The main item to remember is that the loss is dependent on frequency and range. The lower the frequency, the lesser the effect. The greater the distance, the greater the effect. This is why it is preferable to always have a video frequency higher than your controller frequency. Of note, the distance is a straight line between the two, therefore, the higher you fly, the shorter the ground distance will be. As there are entire books dedicated to the subject, the calculator will use a simplified estimate, which still provides an accurate answer. 
   
   
4. We then have a second antenna on the receiver, or Gr. Similar to the first one, the antenna collects the energy before sending it to the receiver for processing. In this case, you can either have an omni if you fly like me, often finding yourself wandering why the runway is on the other side of the parking lot, or a directional antenna if you know what your doing and are able to maintain the plane within the cone. We will assume the latter and use a 5 dBi.
    
5. Finally, we have the received power, or Pr. Which equates to the power that is finally received by your system. Now there are two ways of interpreting this value. But first, we need to know what the receiver needs as a minimum, which is known as sensitivity, or Smin. This is one of the most commonly 'forgotten' information when looking at receivers from vendors. As a personal rule, if the vendor is not able or willing to provide this, I will not buy it. With the sensitivity in hand, we can either provide a range to get the received power, or we can play with the equation to figure out at which range power and sensitivity will be equal. We will use -90dBm as it is again a fairly common sensitivity for FPV equipment. 

The Calculator

There are three main parts where the magic happens.

First, there is a converter section, which allows to go back and forth between miles and km for our American friends to understand the range, and a dBm and mW, as well as tables with some of the most common values. You will use this section mainly to convert the transmitter mW value to dBm prior to placing it in the calculator. 

Secondly, the calculator requires you to give some simple info. First, the transmitter power calculated above or given by the vendor. The gain of your transmitting antenna, which is the one on your vehicle for FPV, or the ground for your controller. Then the gain of your receiving antenna which is the one on the ground for your FPV or the vehicle for control. Finally, the frequency you are using. 

You now have two options. Either you input the distance at which you want to fly and verify that the power received will be more than your receiver sensitivity (which in negative (-) dBm is a smaller number) or you input your receiver sensitivity and get max range. Done.

The other sections have various uses. The ERP (or EIRP) stands for Effective (Isotropic) Radiated Power, which is the addition of transmitter power and transmitting antenna gain. It is mainly a way to compare the total transmitted power of a system rather than its components. I.e.: get a 1000mW transmitter with a 1 dBi antenna or a 250mW transmitter with a 6dBi antenna? A 250mW transmitter with 6dBi will have a higher ERP and use less electrical power to get more range, as long as the antenna radiation pattern works for your application.

There is also an antenna gain estimator. This relies on a few assumption and should be used as a ballpark figure. Assuming a vendor only gives you the following: "Beamwidth 35 degrees horizontal and vertical" you will be able to input the number and get an estimated gain in dBi. It is an estimate only as it relies on an assumption of the antenna's efficiency. The ones I tried where within 15% or so of the value advertised.

Lastly, there is a look at the effect of mismatching antenna polarization. Although I will not get into the details, you should always ensure you are using the appropriate antenna polarization otherwise the result may be a near total drop in range from kilometers to a few hundred meters.

Conclusion 

Please keep in mind this tool is an approximation only, which relies on some fundamental RF principles. The range will be affected by your specific environment, which includes urbanization, pollution, weather, and even time of day to name a few. You should always fly within a safe distance and reduce your range if you get indications the signal is dropping. At the very least though, you will be able to have a way of comparing setups in a quantitative manner. Please leave comments/questions below. 

Cheers,

ALF