FPV: Technical Area

VTX Power And Range

 

The power of your video transmitter is a common misunderstanding in the world of FPV. In the UK we have to adhere to regulations which restrict the power output of a video transmission. In the UK were limited to 10mw of power on a 2.4ghz signal. The reason for this is so that there is no interference from Bluetooth devices and other RC transmitters. The frequency of 5.8 is limited to 25mw. In the rest of the world these regulations do not exist so the equipment becomes legal to use and universally available.

On 5.8ghz we are regulated to 25mw of transmission power. Now 25mw sounds 2.5 times more powerful than the 10mw limit of 2.4ghz. Sadly not true, the 5.8ghz wavelength is half the length of a 2.4ghz wavelength so travels half the distance on the same amount of power. To increase the range to match the range of the 2.4ghz wavelength on 10mw it again needs 4 times the power to double the range. So to get the same distance of a 10mw 2.4ghz transmission the 5.8ghz needs to be broadcast at 40mw.

vtx power gauge

Vtx power rating

One common misunderstanding is the power of your video transmitter. The first thing to remember is here in the UK we are bound to regulations restricting the power outputs of our video transmission.

Why are we limited ?

We are limited in power levels for various reasons, for 2.4ghz for example in the UK wifi runs on this, so does bluetooth and of course RC control transmitters. In the UK we are limited to 10mw of transmission power on 2.4ghz so we do not interfere with any of these other services. On 5.8ghz we are limited to 25mw.

Why are there bigger transmitter available ?

In the rest of the world these regulations do not stand so the equipment is legal to use and widely available.

How do we understand power levels ?

The mw rating of a transmitter is the RF level emitted from the antenna into the environment, is measured in milliwatts or watts.10 milliwatts is ten one thousandth of a watt or 0.01 watts.

On 5.8ghz we are regulated to 25mw of transmission power. Now 25mw sounds 2.5 times more powerful than the 10mw limit of 2.4ghz. Sadly not true, the 5.8ghz wavelength is half the length of a 2.4ghz wavelength so travels half the distance on the same amount of power. To increase the range to match the range of the 2.4ghz wavelength on 10mw it again needs 4 times the power to double the range. So to get the same distance of a 10mw 2.4ghz transmission the 5.8ghz needs to be broadcast at 40mw.

So with the limits we have on our transmitter how do we improve the results ?

Most will increase the power of the transmitter, but to do that is illegal and its not very effective.

Take the 10mw transmitter for example. To double the range of that transmitter its not as simple as doubling the power. To double the distance of it you have to increase the power 4 fold. So to double the 10mw range you would need 40mw. To double the range of that you need 160mw and again to double that range ( still talking short range ) you now need 680mw.

As you can see that’s a lot of power for not a lot of gain. Coupled with the legalities its not the way to go about it.

How do we increase range then ?

To increase the range you increase the gain of the receiver antenna. By increasing the reception by 3dbi you have doubled the range, add another 3dbi and you have doubled it again. All without increasing the vtx power and staying completely legal to boot.

Your standard omni antenna had a typical dbi of 2.14 not 3dbi as advertised. So if you were to buy say a 17dbi yagi antenna you have now increased your receiver dbi from just over to 17, that’s 15dbi +/-. 15dbi divided by 3 gives us 5 times the range you think. Well no. 3dbi gain doubles your disance, another 3dbi on top doubles your new distance. So by increasing the gain from 2dbi to 15dbi actually increases the range by 32x its original distance ! All this without increasing the vtx power at all !

This is where some club fliers not in the know don’t understand and fear. Because you can receive from much further away you must have increased the power ! No you just improved your reception.

Look at your tv antenna on your house wall ( Its a high gain Yagi. )

If you have one of those cheap alien looking devices from argos sat on your tv box you picture is rubbish. Now plug in your tv arial attached to the house and suddenly perfect picture ! Has the transmitter suddenly increased their broadcasting power ? No. You just increased your antennas gain by swapping from the low gain poorly positioned antenna indoors to your high gain better positioned yagi outside. That’s all !

So using a high gain antenna such as the Yagi mentioned can increase your range by up to and over 30 times its original limit. For legal flying in the UK ( within LOS ) that’s all your going to ever need.

This is where some misunderstanding has been publicised in RC forums around the UK. Some LOS flyer’s have been reportedly had their 2.4ghz rc control planes dropped from the sky. FPV fliers get the blame. Its a case of not understanding how the power from vtx`s work and its limitations. Whats also sadly gets blissfully ignored is commercially available wifi boosters ( also illegal ) that operate on 2.4ghz used in homes surrounding flying fields. Also commercially available video transmitters on 2.4ghz and 1.2ghz ( second harmonic of 2.4ghz ) at illegal power levels. These are available online with power ratings typical of up to 4 watts in power. As has been stated the increase of power has not a lot of effect and very poor increase in range, but a 4 watt + transmitter within close proximity of a flying filed does have detrimental effects.

All we can do is stick to the regulations and use high gain antennas in a more efficient manner to get the results we require. That way we keep FPV in a good light.

View the next FPV Guide: Equipment for FPV >>>>>>