FPV video transmitter selection guide

FPV video transmitter selection guide

The video transmitter is a very important component to consider when building an fpv quadcopter, plane, or drone. It is visual link to your aircraft. And choosing the right one for the application can mean the difference between success in FPV and failure.

There are many choices available and trying to choose the right one can be daunting. After reading this post you should be better informed to make a decision regarding your FPV (Vtx) video transmitter.

choosing the right one for the application can mean the difference between success in FPV and failure.

Frequency selection f

 

electric vs magnetic field

electric vs magnetic field [1]

Fpv video transmitters transmit video in several different frequency ranges depending on what you choose. If you understand what frequency is you can skip to here.

What is frequency?

All wireless devices transmit electromagnetic radiation at a specific frequency range.  They use this frequency to transmit information by carrying signals on top of these frequencies.

Using a 5.8GHz transmitter, for example,  means is that the radio frequency of that specific transmitter oscillates at a frequency of five point eight billion times a second.  900 MHz means that the frequency oscillates at nine hundred million times a second.

These frequencies (e.g. 5.8, 2.4, etc) actually indicate that a range of frequencies can be used, and “5.8GHz” for instance means the center frequency of the band.

The 5.8Ghz ISM frequency band has a width of 150 MHz with 5.8 Ghz at the center.  This means that video transmitters that are 5.8 GHz actually operate between 5.725 GHz and 5.875 GHz.  It is this bandwidth that allows us to have separate channels on our 5.8 Ghz video transmitters.

If you look at a video transmitter, such as the TS58500, you will find that there is a way to change the channel that corresponds to different frequencies within the 5.8ghz band.

You can see from looking at this video transmitter’s frequencies that it is possible to use a frequency that is not in the ISM band, such as 5880, therefore it is important to make sure that you select a legal band that you have a license for when using this type of equipment.  More information about getting a ham license can be found here: http://www.arrl.org/getting-your-technician-license

Wavelength λ

Frequency determines what part of the electromagnetic spectrum the radio signal is in, but to understand why different frequencies act differently, we have to understand wavelength.

The wavelength determines several things such as how much energy is contained in a transmission, how the waves react with the environment effecting things such as range and penetration of radio signals, and whether or not the radio waves bounce off of the ionosphere around the earth, or just go off infinitely to space.


 What are radio waves?

I like to think of radio waves as a form of matter that takes on the same characteristics as light, except in a form that our eyes weren’t designed to see. (this is why radio is invisible)

Try to imagine antennas as giant flashlights to understand how radio waves work.  Think of high gain antennas such as microwave antennas as focusing a beam of radio frequency “light” towards another tower. Think about a a low gain antenna such as a router with dipoles attached, spreading radio waves “light” in a more general, broad fashion, similar to a flashlight with it’s beam spread wide to illuminate a broad area.

The high gain flashlight can allow you to see further, but only in a narrow spot, however, the low gain flashlight allows you to see a broader area, but you can’t see as far in front of you.

This “light” can carry information in the form of digital and analog signals.  These signals are sent by coupling information onto the frequency of the particular “color” (wavelength) of the “light” (radio wave)

Pretty much all FPV video transmitters at this point are analog, and they transmit video signals in analog.

 


 MATH

Radio waves travel at the speed of light and we need to know the speed of light and the frequency to determine the wavelength.

To figure out the frequency of a wave

$latex \LARGE f=\frac{C}{\lambda}&s=4$

where:

f=frequency in cycles per second, C=speed of light (299,792,458 m/s), and $latex \lambda$=wavelength in meters

converting this formula to figure out $latex \lambda$, we can arrive at the formula:

$latex \LARGE \lambda=\frac{C}{f}&s=4$

so, for 5.8GHz we can plug in our values to figure out $latex \lambda$

$latex 5.8 GHz(\frac{1,000,000,000 Hz}{1 GHz})=5,800,000,000 Hz&s=3$

-WOW! That is a very high frequency! five billion eight hundred thousand cycles per second!

Using the formula and the HZ calculation, we can determine:

$latex \LARGE \lambda=\frac{299,792,458}{5.8*10^9}=.05 meters&s=4$

$latex .05M (\frac{100 cm}{1m})=5cm&s=4$

The full length wave of a 5.8GHz frequency radio wave is about 5cm long.

That wasn’t to hard was it?

If we do the same for a 1.3GHz wave,

$latex \LARGE 1.3 GHz(\frac{1,000,000,000 Hz}{1 GHz})=1.3×10^9 Hz$

$latex \LARGE \lambda=\frac{299,792,458}{1.3*10^9}=.23 meters&s=4$

$latex .23M (\frac{100 cm}{1m})=23cm&s=4$

So the 1.3GHz wavelength is 23cm or nearly five times the size of the 5.8GHz wavelength.

This explains antenna sizing, and there is an obious advantage to 5.8ghz over 1.3GHz,  The antenna size is much smaller (it is proportional to the wavelength)

Trying to understand how these different wavelengths effect the way that the signal interacts with the environment is a little bit more complicated.  Many things effect these signals and how they propagate.  Higher frequency tends to be more sensitive to reflections and have a harder time penetrating things like buildings and trees vs. lower frequencies.  At the same time, the higher frequencies leak through holes a little bit more easily than the lower ones.

Further reading here: http://www.dxfm.com/Content/propagation.htm

The electromagnetic spectrum

The Electromagnetic Spectrum in terms of radio waves for R/C and FPV frequencies

The Electromagnetic Spectrum in terms of radio waves for R/C and FPV frequencies

The electromagnetic spectrum is the spectrum of radio frequencies that include Bluetooth, wifi, television, am fm radio, cellular, and all other radio communications. Light is also part of the electromagnetic spectrum. The reason that we can have all of these communication technologies; wifi,video satellite, fpv, etc is because all of these technologies operate in a different part of the electromagnetic spectrum, on a different frequency and therefore do not interfere with each other. If they were all operating on the same frequency there would be terrible interference and communication would be shotty at best

United_States_Frequency_Allocations_Chart_2003_-_The_Radio_Spectrum

This image shows all of the radio frequency allocations in the USA as of 2011. As you can see, the electromagnetic spectrum is a very crowded natural resource [2]

Old r/c radios were a good example of this. Early in the days of r/c. Radios had exchangeable crystals in them that changed the frequency slightly. When pilots used to fly,they would all have to get together to make sure that they weren’t using the same frequencies so that they didnt get interference from one radio to the next. There is nothing worse than losing control of an aircraft because some guy at the park just blatantly turns on his radio without checking what frequencies are already in use.

Luckily for us today, we have radio technologies such as accst frequency hopping that allows many pilots to fly in the same frequency band without the need to worry too much about who else is flying.

When it comes to fpv though, we encounter the same problem that those early r/c pilots encountered. FPV video is an analog signal and if there are multiple pilots in the same area flying on the same video frequency there will be interference.

Follow this link for an excellent introduction to how wireless devices work: http://www.wired.com/2010/09/wireless-explainer/all/

The analog video reality

 

Video transmitters for fpv quadcopters transmit an analog television signal. This signal includes video and audio. There is a lot of hype right now about HD goggles and high definition feeds, but the reality is that we are transmitting an analog tv signal and therefore any HD that we use will get downgraded to analog tv.

I’m sure that the future of FPV will involve some digital transmissions of tv signals through the same frequencies as the analog counterparts thus allowing for HD FPV but we aren’t there as of 2015. (note to self… consider doing this for a senior project)

SELECTING FPV VIDEO FREQUENCIES

Different frequencies have different characteristics and there are pros and cons to each frequency range for fpv video.

THE BASIC FPV FREQUENCIES ARE:


 Fpv Frequency advantages & Disadvantages

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5.8ghz-

Advantages.

  • Small antenna size -perfect for mini quads
  • not used for many other communications other than wireless networks.
  • Fairly open channels.
  • License needed for higher transmission power >10mw
  • the typical transmitter for a drone is 250-1000mw and therefore a license is required.
  • Works well with 2.4 ghz control systems
  • Integrated into most FPV goggle headsets.

 

Disadvantages:

  • Poor penetration through objects such as trees & buildings.
  • Expensive

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Fatshark Attitude Camera & VTX top

Fatshark Attitude Camera & VTX top

A TS58500 5.8ghz FPV Video Transmitter for Drone or Quadcopter

A TS58500 5.8ghz FPV Video Transmitter for Drone or Quadcopter

 

TS5823 mini fpv video transmitter for mini quad

TS5823 mini fpv video transmitter for mini quad such as the Qav250

 

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2.4ghz-

advantages:

  • better penetration through objects
  • Ism open frequency (license required >10mw transmit power)
  • Smaller antenna size than 1.3ghz.

Disadvantages:

  • Larger antenna size compared to 5.8 ghz
  • Interference with 2.4ghz control systems.
  • So you can’t use a 2.4ghz radio.
  • Crowded channel- most r/c control systems, wifi Bluetooth baby monitors – pretty much all wireless devices that we use occupy this frequency. For more on that check out this article on wired. http://www.wired.com/2010/09/wireless-explainer/all/

 

1.3 ghz

Main competing devices-amateur satellite

Advantages:

  • Excellent penetration through trees and objects
  • Not many competing frequencies

Disadvantages.

  • FCC ham license required at all power levels
  • Very large antenna size. (Almost half the size of a Qav250
  • Low pass filter (LPF) Required such as this one when using a control frequency of 2.4ghz.
  • Large antenna sizes tend to lead to the antenna breaking off on crash thus causing video transmitter to blow its amplifier.

Power levels:

Power output for video transmitted

 

1.2Ghz FPV TX 800mw Transmitter shown with and without heatsink

 

1.2Ghz FPV TX 800mw Transmitter shown with and without heatsink

 


900 MHZ

Advantages:

Low frequency, excellent penetration and range

Disadvantages:

  • FCC ham license required at all power levels
  • extremely large antenna size.
  • Low pass filter (LPF) Required such as this one when using a control frequency of 2.4ghz.
  • Large antenna sizes tend to lead to the antenna breaking off on crash thus causing video transmitter to blow its amplifier.

 

In conclusion,

for most fpv, a ham license is required.

There are many different frequencies to choose from.

5.8ghz is an excellent frequency for park fliers and noobs Because of its open channels and small antenna size.

Lower frequencies such as 1.3will have better penetration through objects and will travel further vs 5.8 ghz on the same power level.

Please leave comments to let me know how I can improve this article and subscribe to the blog for future updates.

 


sources:

http://sunearthday.gsfc.nasa.gov/2010/TTT/71.php

http://www.arrl.org/getting-your-technician-license

http://www.bb-elec.com/Learning-Center/All-White-Papers/Wireless-Cellular/10-Commandments-of-Wireless-Communications.aspx

U.S. Frequency Allocation Chart – National …

 

 

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