www.webx.dk Start page
MY little Radio Control page
UHF RC system version 3
Explain Items and their names

Abbreviations and names often used on my pages and when talking about radio control and video links and FPV

RC - Radio Control, remote wireless control of something
RC unit - The radio control unit you hold in your hand, can be a Futaba, JR, Graupner, Multiplex and so on
Servo - when connected to rudders/ailerons/elevator they steer a plane.
Servo pulse - is a digital pulse width that hold position information, 1.5mS is centre position.
LRS - Long Range System, often a short for my system, sometimes also called UHF LRS
UHF - Ultra Height Frequencies, is 300-3000MHz, but radio amateurs often call UHF = 70CM band, also know as the 430-470MHz area
TX - Transmitter used to transfer a signal to a receiver
RX - Receiver used to receive the radio signal and output the signals, pulses, audio, video, data
RC RX - Radio control receiver, can be any type any brand and any coding system, they all output standard servo pulses
LRS TX - the metal box containing the 500mW transmitter used for my long range system.
LRS RX - the receiver located in the plane, this receiver is connected directly to servoes and 5V supply
RSSI - received signal strength indication, is often an analog voltage that goes up or down depending of radio signal level in a receiver
Video TX - Video transmitter, located on a plane, car, boat, helicopter or whatever, often using 900-1300-2400MHz
Video RX - Video receiver, located on ground, when connected to a TV screen you can see live pictures from the Video TX
Video Splitter - is an amplifier that will allow the user to distribute a video signal to several things at the same time.
PPM - Pulse Period Modulation, is the pulse system used in trainer/student systems, it contain high resolution information’s on all servo positions assigned.
PPM inverted - the pulse can be normal or inverted, some older systems do not handle both when connected as student/trainer
LOS - Line Of Sight, is the distance from ground to a plane with nothing in the way, not even ground.
Long Range - is normally not defined, but when a plane is not visible by direct sight it is normally called long range
BNC - is the connector name/type used for the TX and Booster for my LRS, same connector is used on Ethernet systems.
Booster - is an amplifier that will take radio signals and boost them up to a more powerful level.
LNA - Low Noise Amplifier is used in receivers as the front end stage, they improve the sensitivity and therefore also the range
Diversity - is often a double antenna and/or double receiver system with auto switching to the best signal, this improves the useable range allot
GPS - often we use a GPS receiver on planes to feed speed, position, and height information to OSD systems.
OSD - On Screen Display, will overlay interesting information to a live video signal.
Logger - will record data or measurements for playback / view later, some OSD systems can log some information’s too.
Modem - Modulator Demodulator, encode data into sound, and back again, can use audio line in a wireless video system to transfer data like GPS positions
Head Tracker - a unit mounted on a persons head, will then control remote located servos so a remote camera follow head movements, gives Virtual Reality experience to FPV.
FPV - First Person View, like pilot view out the front window.
UAV - Unmanned Aerial Vehicle, a UAV is a fully computer guided plane, not a radio controlled plane, if RC'ed it is an FPV or just a normal RC plane
Trainer - Most advanced RC units have trainer connectors with PPM in/out so they can be connected via a cable to a student RC unit.
Student - Most advanced RC units have trainer connectors that can be configured to output PPM signals for a PC simulator or trainer RC unit or LRS.
Patch - A patch antenna is a directional antenna that will when pointed to a plane improve the range
Yagi - A Yagi antenna is a directional antenna that will when pointed to a plane improve the range
Dish - A Dish antenna is a directional antenna with highest possible gain, will when pointed to a plane improve the range
Gain - antenna gain is often named in db, more db more gain, and also a more narrow beam, so pointing correctly is harder with high gain.
RF - Radio Frequency, any frequency that is not directly hearable audio
RF module - often a plug in box or module or printed circuit board that can be changed/added in RC units, normally a transmitter
BEC - Battery Eliminator Circuit, is a 5V-6V regulator often linear type that makes supply for RC RX and servos
SBEC - Switching Battery Eliminator Circuit, is a 5V-6V regulator switch mode type handles more input voltage and have lower loss
RC Receiver Battery or Supply, is normally 4 or 5 NIMH cells providing 4.8 or 6V of steady and stable supply, such a battery must be able to handle
 all servo max currents and still provide sufficient stable voltage, do not use spring loaded battery cassettes or weak current capable cells like normal alkaline types,
 Today more new and fancy battery types like lipo and lion and such can also be used if receiver and servoes can handle the different voltage range they provide.

Fading - when the distance and positions and angles of a wireless system is changed the radio signal will also change
Multipath Fading - A direct signal and a reflected signal hit receiver antenna with -180 deg phase, creating a zero signal level
Nulling - is the same as Multipath fading
FHSS - Frequency Hopping Spread Spectrum, a way to send data using many different frequencies, makes a system immune to noise and jamming proof.

FPV general trouble shooting guide any systems with any kinds of products
read this first and try some of this before asking around :-)

Any FPV system car or plane or helicopter or whatever is often a compact system, containing:
transmitters, receivers, sensors, hi power pulsed electrical, vibrations, microcontroller electronic boards, video camera, switch mode supply
too be able to combine all this into a tiny lightweight platform is a huge challenge and to make all units perform perfectly
with out interfering each other is often an almost impossible task, even for skilled electronic educated persons.

Here are a few design hints:

Plane size small planes are cheap and easy to transport, but they are not optimal FPV platforms. The smaller type the more compromised performance you must expect to get

Receivers must be located as far away from transmitters as possible, at least a distance that can prove no degrade of the receiver system range when the tx is operating or not.
It is advised to keep receivers away from all electronics, in general, speed controllers and switch mode supplies are known to be able to jam.

Electric Motor controller a range check must prove no degrade/change when motor is off, halve, full power, this also apply for gasoline and methanol engines

Transmitters for live video or data back telemetry will always have harmonics, like 2x 3x and so on of their wanted frequency the level might be low and under demanded levels,
but when located near a receiver that may operate on a x2 or x 3 frequency it is doomed to fail! Plan your frequency bands

GPS are often small module receivers, designed to be cheap and just cheap!! That is what we like, but cheap come with a problem price!
They are not designed to be working near transmitters; some can not handle anything not even if it is even GHZ away!!
so a SAW front end is a must and a good high level capable front end is a must too, and even then some GPS modules do not like to be located
near 900 or 1300MHz video transmitters first harmonic of 900MHz is 1800Mhz, a GPS works at 1575MHz its front end system is often several hundred MHz wide,
even if they have a SAW, so 1300 and 1800 can jam it.
a 2.4GHz video transmitter have first harmonic at 4.8GHz so it is no big problem with the harmonics,
it is a known fact that most 2.4Ghz video transmitters are most likely better with most GPS units.

Video stripes is often seen when a power supply is not clean or ground / signal wires are shared with power or other units on the plane,
note if the stripes are constant or change with motors or servos or other items moving or operation, try to touch camera or video transmitter,

See if any change in the stripes.

Grounding items with analog signals specially video signals are super sensitive to grounding problems
you can not supply a pulsing current on a ground wire to a video connected device, if this ground wire is the same for video signal,
it is quite difficult to explain how to split signal ground with power ground. A “Star” kind of grounding have a zero current zero voltage in the centre point.

Power supply cleanness try to mount external battery on different items they are super clean, and is smart to try locate a problem

Switch mode, it is often seen SBEC or other switch mode regulators are designed to be cheap and have no really good filter,
extra 100uH coils and 470-1000uF on the input and output might help, input is just as important as the output.

Shielding some devices radiate magnetic pulses, specially SBEC and cameras, if they are contained in a closed metal case
the problem will be less, but weight will go up, iron cases are better for low frequency magnetic problems.

Camera some types radiate radio noise, uncased types are the worst! but cased types can also do it, case is maybe painted and therefore not fully shielded,
the wires from it works like radiating antennas, ferrite torrid and aluminum-foil is a good solution, and distance to GPS and other receivers is a good idea tool.

Servos some RC servos types are only designed to be used near receivers, clear enough, but when located near a transmitter they can be jammed,
moved or stopped or other weird things even periodic problems have been seen. Tricks/solutions, aluminum-foil around the servo, wire turned on ferrite torrid.

Electrical motors are also inside servos they will also radiate magnetic pulses when they move.
Some systems like video transmitters and cameras and osd systems do not like to be near this field.

Vibrations is a normal problem for receivers and transmitters, their coils and crystals and filters have microphone behavior, pack in foam and also avoid loud sound


SWR a video transmitter with a badly matched antenna and/or badly grounded will have high frequency currents going on its signal and power cables

When having range or interference problems, try to isolate different items, like turn it off, or change their position dramatically
or bypass its function until you find a change in your problem, then you have found the nasty item.


Radio Technical stuff

Any wireless radio system contain of a transmitter side antenna and a receiver side antenna.

Both sides must have same polarization to perform most optimal, the most common polarizations are horizontal, vertical, circular left or right.

If a horizontal is “talking” to a vertical, the link loss will have an added extra loss of 26dB

If a circular left is “talking” to a circular right, the link loss will have an added extra loss of 26dB

If a circular left is “talking” to a vertical or horizontal, the link loss will have an added extra loss of 3dB

This is why it is smart to combine a horizontal or vertical often mounted in a plane, with a circular receiver on the ground,

then the 26dB drop can most likely be avoided.

Pointing a whip style antenna to a plane is the worst thing you can do, imagine looking into the end of the whip,

It is almost impossible to see from a distance, radio waves work this way too, make antenna most visible and right polarization.

It is normal that radio links have a 26dB extra margin in its link budget / range calculation so you don’t loose contact -

when one antenna is rotated unlucky angle, a diversity system can take full advantage of its link budget,

so the resulting useable range is almost 10 times as much as a non diversity system, if no other parameters are changed.


A downlink diversity system also solves one other problem, fading and nulling, the most perfect diversity system would have 3 antennas

to handle signals from any angle perfect, but the gain from doing this is often minimal and cost and complexity is big, a 2 antenna diversity is the most common.

All cellular systems use diversity on the receiver side and brute transmitter power on the tx side to perfect the link, a cell side tx is over 26dB more power full

Over the handset transmitter to obtain an equal quality link, also handset side have a cheaper receiver with less sensitivity,

we do not have space for a diversity antenna system on a cell phone J


wavelength and frequency, a double frequency will have halve the wavelength. Long wavelength can not pass thru small holes,

like take a 27MHz walkie-talkie and try to use it inside two cars, the useable range is really bad, now go out of the cars and see you get 10 times the range,

at higher frequencies you don’t have the car problem, but air attenuation is higher with higher frequencies, so a longer range is easier to get using lower frequencies


Gain, adding more antenna gain will only make the beam more narrow, point the antenna right and you get more range, point the antenna wrong and you loose signal,

Any gain over 8-10dB will be hard to point to a moving target like a plane, you need a tracker system or a cheap friend that will work for free to point the antenna.

An 18dB gain yagi antenna on the receiver side and 500mW 2.4GHz video transmitter with 0dB antenna, have a proven LOS of 51km when both polarizations are right.


Range and dB, improving a systems link budget with 10dB will increase the range by a 3 times factor, 10dB power is the same as 10 times the power.

20dB more power gives 6 times the range, 100 times more power, combining more power with more gain is often the way to get longer range,

and also improving receiver side sensitivity is a good way to go.


Bandwidth and range, video transmitters have also audio links, stereo and mono exist, the video signal is 5MHz wide, and the audio is 5KHz wide

A factor 1000 in bandwidth, so in theory the same range will be achieved on the audio as the video with only 1/1000 of the power,

that is why those systems have a much lower power in the audio, often see -20dBc to -30dBc, dBc means dB under the main Carrier -30dBc is the same as 1/1000


Thomas Scherrer - OZ2CPU - 2011