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Video link (vRX, vTX)

There are many aspects that we need to consider when choosing the video gear for our FPV system.

These are:

  • distance (range) we want to fly
  • penetration
  • components size/weight
  • legal/illegal usage
  • urban/field environment (noise levels)
  • higher/lower frequency than RC link (additional filtering)
  • GS (ground station solution)/mobility
  • general purpose of flying (AP, racing, LRS)
  • antenna sizes
  • component powering (battery cell number, voltage)
  • modularity
  • module frequency selection range
  • … many more

 

As one can see choosing the right video gear for FPV is not as easy as it seems to look. By doing a good planning by the beginning, much money and time can be saved.

FPV gear

Basically, the video gear consists of an fpv camera, a video transmitter (vTX) and an antenna. This can be extended by an OSD (on screen display), that is able to project different on-flight information into the camera picture, getting a HUD-style picture at the end. However, when talking about the video link usually we mean the vTX itself and the vRX on the ground.

 

vTX (video transmitter)

The video transmitter is the device that transmits an analog (some digital devices exists already) signal of the fpv video camera to the ground, where a video receiver catches the signal and processes it.  The signal is transmitted in SD quality, that means 576 horizontal lines. That is why many says that there is no use to buy fpv cameras with higher resolution, as the signal can not be transmitted natively (see fpv camera section).

Video transmitter 5.8g

The main attributes of a vTX:

  • frequency/band
  • RF power
  • input voltage
  • antenna connector type
  • size/weight

 

As mentioned above there are many aspects to consider when choosing a vTX.

 

Range

We need to know what should be the maximum distance we want to fly. Both RC and video link should be able to handle it.

For better understanding, here is a short article about the “FPV budget” that may bring some light into this issue:

 

In this paper, I’m going to explain the RF link « budget ». Where is that budget, how it
can be calculated and what affect it for better or worse.
Your budget is spread over 3 things : Tx power, Rx sensitivity and antennas gains.
The RF currency is very often the dB (decibel, for one tenth of a bel) What is a bel? A
bel is a ratio of 10 between a quantity and a reference level . The bel is seldomly used in
favor of the decibel, 3 dB being a ratio of 2 following the logarithmic scale of that unit.
Example : We often set 1mW as a reference in RF power, this is our starting point or
0dBm (dBmiliWatt) 3dBm is 2mW, 10dBm is 10mW, 20dBm is 100mW… Same can
apply to Volt (dBV) or Amp (dBA)
What about Dbi? We often sa that spooky unit on antenna spec sheet. « i » is for
« isotropic » an elusive perfect antenna that spread power evenly over a perfectly round
sphere. So, 0dBi is a perfect sphere, 2dBi is squished a little, 20dBi is totally squeezed in
one direction, having 100 times more power in a single direction than the perfect sphere.
We have a 500mW Vtx, that’s 27dBm, install a dipole : add 2dbi we’re at 29dB. Now on
the Rx side we have a patch with 8dBi with a Rx sensitive to -85dBm, that’s another
93dB added to our budget for a total of 122dB. You can see that most of the budget is
found in the Rx and its antenna.
Now that our budget is known we can go spend it, how? Free space loss. As the Tx is
moved away from the Rx a loss occur in the distance that separate the two. This loss in
dB is calculated like this :
32.44 + 20*log(F(MHz)) + 20*log(D(km))
Don’t let it scare you, if math is not your cup of tea, there’s plenty of online calc for this :
http://www.microwave.gr/content/jsffield.htm
We see MHz in the formula and it is in a similar expression than km, yes frequency play
an equally important role. Twice the frequency, four times the free space loss or 6dB.
Same with distance, twice as far, four times the loss or 6dB.
At 1280MHz, the loss over a km distance is 94.6dB (that’s free space, mean no air no
humidity, no house, no aliens) Bummer! Only one km and most of my budget is spent!
No, remember that the dB unit is logarithmic and like said above 6dB is twice the range.
Let’s travel another km and we have 100.6 out of 122dB spent. Still over 21dB to go,
divide that by 6 and we got the number of times we can double our distance. 3.5 times?
Let’s keep it at 3 times for some overhead ☺ We’re at 2km, double #one, we’re at 4km,
double #two, we’re at 8km, double #three, we’re at 16km, end of our journey, we spent
all the RF link!Many others things can tax your RF link, solid objects between Tx and Rx will eat a lot,
multipaths and noise are some other examples.
How can you improve your budget? You saw how the budget is divided :
-More power? To double your range, you need an extra 6dBm from that 500mW Vtx,
that make a monster 2W
-More dBi? At the Tx it won’t make much sense as you need an even radiation in order
to bank and turn your aircraft. At the Rx, it’s easy, switch to a 14dBi patch.
-More sensible Rx? This is also a good one, switch to a -90dBm Rx and you nearly
double your range. However, Rx sensitivity is rarely given as spec, let alone honest
rating of the sensitivity.

 

Hard reading, harder to understand, huhh?

Simply spoken, the range is estimated by the whole chain we use: vTX (RF power) -> TX antenna (gain) -> RX antenna (gain) -> vRX (sensitivity).

If we need to increase the range, we can do it in a few ways:

  • increase vTX RF power
  • use higher gain TX antenna
  • use higher gain RX antenna
  • increase the vRX sensitivity

 

General rule of thumb is ALWAYS to increase the antenna gain/vRX sensitivity, and AVOID increasing the vTX RF power. The reason is simple, the high RF power levels can simply affect other devices onboard, even the RX and this can lower the range for the RC link which we need to avoid.

Also note that by increasing the vTX power it will get more and more hot, and as a result it can easily burn out. Today’s manufacturers tend to rate the same sized transmitters in different power ranges, which is a very bad practice. One can buy the same sized device which is rated for 200mW and 600mW. Think logically, how can the 3x powered device get not hot very fast when using the same electronics? This applies mostly for the very small, miniaturized transmitters that are selling recently.

So when needing bigger range, rather buy a directional antenna with higher gain, and/or exchange the vRX for a higher sensitivity one.

 

Penetration

A short test can be seen here:

By the stage of planning the FPV systems it is worth to decide WHERE we want to use it. We will not get the same quality reception when flying in an open area or flying in the forest, or behind a building or hill. This is where penetration comes to the scene.

Generally, the lower the frequency, the better is the penetration (possibility for the signal to get through). Of course again, by using high gain directional antenna, more sensitive receiver or higher RF output power will help making the penetration better.

The lower frequencies we use for good penetration are the 0.9GHz and 1.2/1.3GHz ones. Now I am exaggerating a bit but while we get a clear reception on these frequencies in a moderate distance, behind many trees, by using 5.8GHz at the same distance we loose the signal even if one leaf is between the receiver and the transmitter.

By using the 0.9GHz we can easily run into trouble when flying near cellphone towers, that radiate at many watts of power, killing our video signal completely. Choosing the 1.2G/1.3GHz frequencies is better, but still there can be many RF emitters on the same frequencies like TV companies or such. Of course it differs in every country.

There are also legal aspects when using these frequencies without HAM license.

 

Components size/weight

Different vTX are of different sizes, genarally the lower the frequency, the bigger and heavier is the unit.

 

Urban/field environment (noise levels)

Every place we fly has a so-called “noise level”, that means how noisy is the environment on a given frequency. Imagine using a 2.4GHz video link in the city, when there are Wifi routers everywhere around. Our signal will be lost on much shorter distances, then in an outer field, where there is no other radiation around. If we plan to fly in the city, we should choose a frequency that is barely used, to avoid signal loss problems. 1.3GHz can be a much better option in this case, than 0.9GHz, 2.4GHz or 5.8GHz.

 

Higher/lower frequency than RC link (additional filtering)Lowpass filter

It is a good practice to always choose higher frequency for the video than for the RC signal. The reason is simple, and called “frequency harmonics”. That means that a transmitter is not only radiating on its nominal frequency, but also on the harmonics frequencies (2nd, 3rd, …), e.g. an 1.2GHz vTX will transmit even on 2.4GHz and 3.6GHz frequency as well, of course the radiated power on those is much lower, but can be still significant in affecting our RC link negatively.

If for some reason we choose our video to transmit on lower frequency than our RC link, try to use a frequency, which 2nd harmonics differs from the freq of the RC link. If that is not possible, there is still a solution, although not 100% efficient. In these cases we use a so-called “lowpass filter”, which is a simple filtering unit that tries to dampen the “bad” frequencies.

A typical situation is when we use 2.4G for RC link (very widely used) and we need high penetration so we use 1.2/1.3G for video. In this case the vTX can (will) negatively affect the range of the RC link, that can get down even to 1/10 of its maximum range.

Note that the additional filter need an extra space on the multicopter and adds also extra weight, that can be very painful on small frames. Even, the filtering is never 100% so still the RC link will suffer and loose some range.

 

Antenna sizes

As mentioned earlier, the lower the frequency the bigger are the antennas used. While a CP antenna for 5.8G can fit in a child’s palm an 1.3G CP antenna is big as a child’s head. We can choose a different type of antenna for 1.3g (dipole) that is smaller but the multipath rejection will be worse of course.

Note that in a case of a crash, the bigger antenna is more prone to get destroyed. And when doing proximity flying it can more easily get hooked. The bigger antennas are even more expensive, especially the tuned ones.

Video transmitter powering

Component powering

There is a wide range of transmitters that are able to operate from 3.7V (1 cell low voltage) up to 26V (6 cell). It is always useful to check the input voltage of the module we plan to buy. E.g. if we plan to drive the copter by a 4 cell battery (16.8V), it is not the best idea to buy a vTX (or camera) that operates with max. 3 cell (12V) input as we will need an additional electronic (DC converter) for giving safe voltage to the transmitter and/or camera. Rather, buy a vTX and camera that is able to handle the 16.8V input.

There are also transmitters that can handle high input voltage but are able to prived 5V/12V output for the camera.

By careful planning, we can save money, time and also weight.

 

Module frequency selection rangeGoogle replaceable module 5.8g

Prior to buying the vTX, also check what bands (bunch of frequancies) it supports. Be sure the vRX will support the same band to avoid compatibility problems. The more bands, the better, because we may easily need to fly on another frequency later on and this way we do not have to buy another device for that. Typically, on FPV races there is always a frequency negotiation, and if somebody’s vTX does not support enough frequencies, he may not fly with the others.

 

vRX (video receiver)

 

The video receiver (vRX) is the device that catches the signal on a given frequency, renders it to video stream and sends it out to any device which is capable to display the picture.

The main attributes of a vRX:Video receiver

  • frequency
  • sensitivity
  • noise filtering
  • build (modular or cased)
  • antenna connector type
  • number of AV out connectors
  • diversity

 

Some aspects to consider when choosing a vRX:

 

GS (ground station solution)/mobilityGround station

A few years ago there was common to build a so-called ground station (GS), that usually consists of the following: receiver, antenna(s), monitor(s), battery pack, all integrated into a small unit, generally a suitcase. As the technology went ahead, receivers got smaller and many video goggles were developed, so we got into a point when there is no need for the GS anymore.

 

Today’s best solution is to buy video goggles that has a modular system, so the receivers are interchangeable. The best goggles even have an inbuilt

Video googles with module

DVR (digital video recorder) unit, and a headtracker as well. The antenna mounted on the goggles, especially for 5.8G is very small and handy.

By using this solution, we can get rid of all the cables, heavy components and gain high mobility, assuming that we are using small antennas that are directly attached to the goggles.

 

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