Beside all the major components of an FPV gear, we have to mention those that are not always needed, but if so, then desperately. We are talking about minor electronics or passive components.
Many times we have a noise in our closed video system, or in the transmission. Most of the similar issues can be cured by positioning the electronic components a different way, or just by keeping enough space between them. This is, however hard to achieve on the medium and small multicopters. Here the various filters kick in.
- RC control signal affected
The worst kind of issue, that can put the whole flight into danger is when the radio signal transmission is affected. This must be solved at all costs. The reason is simple, all transmitters radiate also on higher frequencies, than the carrier. These higher frequencies are called the harmonics.
E.g. a vTX operating on 1280MHz will have its second harmonic at 2560MHz, which is very close to the used frequencies of the 2.4GHz RC control. The second harmonic radiation is much weaker of course than the first harmonic (1280MHz), but still enough to affect the 2.4GHz receiver, that can be fatal. It is easy to try: do a ground range test of the RC receiver with vTX switched on and switched off. The difference would be huge, the radio control range can be degraded even to 1/10 of its range, just because the harmonics.
The issue is getting worse by using stronger vTXs, with higher RF outputs.
To solve the problem, we use a passive device, the so-called “low-pass filter”. A low-pass filter is a filter that passes signals with a frequency lower than a certain cutoff frequency and attenuates signals with frequencies higher than the cutoff frequency. The amount of attenuation for each frequency depends on the filter design. Since an LP filter helps a lot in filtering the unwanted frequencies, it does not do a 100% job. Therefore, to have a clean configuration, it is highly recommended that the vTX operates on a higher frequency than the RC control system.
- Video transmission signal affected
Unfortunately, there can be a similar issue with the unwanted harmonics, when using higher frequency for the video gear than for the RC control. However, the issue occurs in the ground and so it is much easier to cure that any issue in the air. The problem usually occurs when using an LRS system for RC control.
Typically, when we use a relatively high powered 433MHz transmitter and an 1.2G/1.3G video gear, the video signal, reaching the video receiver (vRX) can suffer, because of the 3rd harmonic frequency (1299MHz) of the RC TX module. Usually, these modules are capable of putting out 150-1500mW of RF power, and if the vRX is not well filtered (stock) than we can have a problem.
The solution can either be to insert/replace the input filter of the vRX (so-called “SAW filter”), or to add a so-called “low-pass filter” to the output of the RC transmitter module (of course between the module and the antenna), that will filter any outgoing frequencied higher than 433MHz.
- Video picture affected
The video picture can generally be affected 2 ways, by EMI (electromagnetic interference, or RFI – radio frequency interference), coming from any onboard electronics device or by the unwanted voltage spikes, usually coming from the motors.
The latter can usually be cured (to a certain point) by using so-called “LC filters”. The “LC” comes from the names of the passive components used, capacitors and inductors. Making such a filter at home is relatively easy and very cheap, however, the result is not always satisfactory. A thumb of rule is to use as high capacitance and inductive components as possible.
Note that by using components with higher values the components sizes will be bigger as well, so if one wants a small and light solution keep that in mind. In some cases the filter will not work as expected (not filtering out the unwanted freqs). If that happen, there is still a solution: use a separate battery for the video gear, and be sure that there is no common ground for the video gear and the remaining electronics.
Curing the EMI issues can be a bit more complicated. Generally, a good solution is to find the component that “makes the noise” and shield it as much as possible. The shielding can be done the easiest way by using some copper or aluminium foils for wrapping. In addition to this, we can use ferrite rings of different shapes and sizes, and twist the wires going to the transmitter around them as many times as possible. In many cases, just simply twisting the wires, going between the video components can be sufficient, to filter the noise.
Antenna connectors and pigtail cables
It is impossible to put an FPV system together without these. The better quality the components are, the less is the signal strength loss, so there is absolutely no reason to buy any cheap connector or cable, because we can loose more on them, then gain on other components like vTX or vRX.
The two most used connector standards in the FPV world are the SMA and RP-SMA. The difference is in the position of the connector thread and the connector pin. Sometimes we also meet the connector called U-FL (also IPEX). the manufacturers usually use them in receiver and transmitter modules, for easy antenna replacement.
Before ordering any RF component, make sure that the connectors will fit to the application. There are different connector adapters, to fit any type to any type but by using any of these, there will always be a signal degradation. A rule of thumb is to avoid the adapters, and use as few connecting points and junctions as possible.
Sometimes it is simply not worth to avoid the adapters. A typical example is the racing quads. It is much easier to replace a broken antenna, than to resolder a broken connector to a vTX, so in these quads we use the 90-degree SMA/RP-SMA adapters, that connect the vTX and the antenna and is securely fixed to the quadcopter frame. In case of a crash, there is no force going to the vTXs connector so it remains intact for sure.
There is the same reason to use pigtail coaxial cables. However, we use them also widely for the ground installations, when connecting different components, like monitors, receivers, diversity receivers, antenna trackers, antennas and more. Two things affect the best the signal quality on these cables: the material (and shielding) of the wires and the connector material. Every manufacturer of a cable defines the attenuation value, that is loss in dB at a defined frequency, measured for a specific length of the cable. the lowest the attenuation for the used frequency, the lower is the signal loss, so it is always wise to choose a better quality cable, especially for bigger distances. The most used cable types for FPV are the RG-58, RG-59, RG-178, LMR-195, LMR-200. Beside the quality, rigidity of the cable is also important, mainly if using them in tight spaces.
Everyone who flies FPV knows that the biggest enemy, we are always fighting with is the range. The maximum range is affected by many factors, but some of them we are able to influence. For calculating the range, it is very hard and many times only theoretical, without using the appropriate measuring tools.
Unfortunately, many of the tools are not available to the hobby grade fliers, due to lack of financial funds. Professional equipment is too expensive and only the few chosen (working in places where they can meet these tools) are able to lay their hands on them. However, in the last few years the hobby shops realized the need for these, and some of the useful tools are already available for the public.
- RF power meter
It seems that Immersion was the first who realized the need for a reliable RF power meter, usable for all the frequencies, the FPV-ers use today. These are: 35MHz, 72MHz, 433MHz, 868MHz, 900MHz, 1.2GHz, 2.4GHz, 5.8GHz. The tool is essential during troubleshooting, when we need to know which component is faulty, the antenna, the TX/RX, the cable, the connector, the adapter or other in the chain. It can measures the output RF power of the vTX, which is useful in cases when fighting with range issues but unsure of the electronics. The tool is quite expensive (around 150USD) so not everyone can afford to have it at home. Note that outputs from transmitters, using the “frequency hopping” technique can not be measured by this method.
- Frequency counter
There are other devices that can help us in problem detection, the frequency counters. Although they are not able to measure the RF output of a transmitter, but are able to display frequency and strength of any caught continuous wave signal. They ususally operate in 50MHz-2.6GHz range and are relatively cheap (around 50USD). For fpv measurement purposes they are not so good but they can be used e.g. for getting information about which transmitter radiates stronger or which used antenna is more effective, based on the measure signal strength in the proximity of our transmitter.
- SWR meter
The standing wave ratio meter is designed to check the SWR value of different antennas. That value is important for antenna makers, and not many are manufacturing antennas at home yet, mostly because of the lack of other measuring devices, still needed for making quality antennas.