Sooner or later when you making your FPV setup will brought to the question of what frequency you choose and what antenna to use? Are antennas supplied with video transmitter and receiver optimal? Is it possible to improve their reach, or at least reduce their weight ?
These questions are difficult to find an answer for uninitiated. Although I have electrical training, but this area of electronics I've always been a bit strange, so i need help too. Fortunately, there is David from Sweden, who has done research and experiments, and from him we can learn a lot.
First we have to decide what frequency of video link to use. On the market are available video transmitters basically to four bands: 900MHz, 1.3 GHz, 2.4 GHz and 5.8 GHz. Every band has its advantages and disadvantages. The biggest question is about 2.4 GHz band which can be used to control the model. The general recommendation is: do not use it, although technically it is possible. Also the problem may occur if you use 2.4 GHz and 1.2 GHz system because it may cause interference. More information can be found for example here. In terms of interference appears to be the best use for the control system model of the old 35 MHz system.
Another parameter that should interest us is the transmit power. Its role in deciding plays laws, quite different for each country. Some bands are in some countries prohibited for free use, some only a limited transmit power, somewhere you must have a license.
What is the best procedure for chose the frequency?
First, find out which bands are legally free to use and what power is subject of restrictions. Asking an experienced colleague, asking for the Competent Authority or try luck on Google.
The next step is to decide which of the authorized frequencies for our purpose is the best. Those who used 2.4 GHz band to control model, the decision is easier. Others may decide according to this brief description from David:
900 MHz a 1,3 GHz
Now you must chose the transmitter power. On the market are available transmitters of various power, from 10mW / 2.4 Ghz up to several watt special equipment to a range up to 50 kilometers. There are the simple criteria. Like frequency, we first find out whether the law does not limit the power of the selected frequency.
Then, a transmitter with higher power will:
Because we talk about high frequencies, there are not linear relationships. It means that doubling the power will not double the range. With double power of transmitter the range increases by approximately 1,414 times. Therefore, it is much more efficient to increase the range by more powerfull antenna on receiver side, where we are not restricted by the size or weight.
Selection of power must also correspond to a range of control connection. It makes no sense to build video connection to a range up to five kilometers when we control the plane by common system with range up to 1km.
I chose 1.3GHz band transmitter with 300mW power. Together with patch receivers antenna can reach a range between 2 - 4 km which is the enough for start.
The construction and dimensions are key feature for good antenna. It must be in accordance with wavelength of signal that transmit / receive. Antenna should be tuned to a given frequency. The transmitter is shipped with antenna. Because transmitters usually have an optional frequency channel selector, the supplied antenna can't be tuned precisely and therefore some compromise to work for the entire range. By tuning the antenna exact to frequency, we can achieve better performance and less heating (losses) on the transmitter.
Another disadvantage of supplied antennas can be a big connector and slewing mount. We do not need those things in the air and so we not need to carry extra weight. If case of collision massive connector can breake the board which would have fatal consequences for the transmitter.
David discovered and experimentally verified, it is possible and beneficial not only to accurately tune the antenna to a particular channel, but make a completely new antenna without the use of heavy and massive joint and connector, plus a shorter length and improved mechanical properties especially in case of accident.
I decided to make this customization of transmitter, especially because the place where I plan to put the transmitter.
ATTENTION! The transmitters are manufactured by SMT technology and size of components used need good eyesight (magnifying glass) and high-quality tools for this technology. Forgot to your transformer soldering iron! Because we are in the RF technology, the connector for antenna is soldered directly to the output capacitor. High temperature and mechanical stress can permanently destroy all your efforts!
See photos of my proceeding. First I suck up the hot solder between the output connector and capacitor. Then I unsolder the connector from the shielding plate. Temperature of the soldering tip for soldering on the board should be about 280-300 degrees, for soldering the shielding plate 360-380 degrees.
Then I made a new antenna from copper wire with 0.5 millimeter of diameter. (Wire from the UTP cable). I double the isolation by silicone insulation for higher mechanical strength of the antenna. UTP cable wires are typically twisted into pairs and have to be straighten first. Given a sufficiently long wire, one end clamped in a vise and bend the other end until wire is flat. Take care to brake the wire.
Length of the antenna is calculated as follows:
l = ((c / f) * 1000 ) * 0,95
c = The speed of light = 299 792 458 m/s
l = antenna length in mm
f = frequency in Hz
Because impedanc adaptation we are using the antenna half or a quarter wavelength. Simply divide the resulting length by two or even four, depending on how we construct the antenna as half or quarter length.
Result - it works and is the main