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Spektrum Radio Link
By Tom Cogswell

Optimizing Your Spektrum Transmitters Radio Link

- By John Adams, Technical Director for Spektrum

When flying basic foam, wood, or fiberglass, electric, gas, or glow powered models at typical distances, the signal-to-noise ratio (radio link) is very favorable providing a robust control link even in the noisiest of RF environments. With more complex aircraft like turbine jets, giant scale aircraft etc. that are equipped with systems (like turbine engines with metal tail pipes, large fuel tanks, large gas engines, retracts or anything else that is made of metal or carbon) or are flown at great distances, proper radio equipment selection, installation and testing can be critical to provided and verify adequate radio link performance. This is especially true for transmitters that are EU compliant (EU-328) with limited EIRP power output (<100mw)  

Spektrum systems offer features and functions that allow you to optimize and test your Radio Link performance prior to flight, on the ground and then in the air providing you with exact information as to how each component of your system is performing in actual use. Following are recommended tips that will allow you optimize your installation and to safely evaluate the RF link performance even before flying.

Spektrum Tips and Recommendations for High Value Models

SRXL2-Next Generation RF Link Performance

Spektrum DSM2 and DSMX was introduced to the market nearly 20 years ago and at the time the 2.4gHz radio chips used were state of the art. Since then, 2.4gHz technology and performance has advanced and several years ago Spektrum introduced transmitters and receivers with next generation radio chips. All NX and iX series transmitters and all currently available receivers feature SRXL2 technology. In testing using both SRXL2 receivers with NX or iX series transmitters in EU mode, a 18% increase in range is typical. Using SRXL receivers only with a Dx series (legacy radio) an increase of 10 to 12% is typically achieved. The gains in range are even greater in US mode. This is due to the new generation of radio chips being more sensitive, the output power can be adjusted more accurately so the transmitter can be tuned within .1 dBm of maximum legal output and the signal is simply cleaner. If you are flying a high value model aircraft, an airplane that contains significant conductive materials like metal or carbon or a model that is flown at great distances (like giant scale sailplanes), it’s highly recommended that you use Transmitters and receivers with the latest generation chipset (SRXL2) to achieve the best possible RF link. In addition, SRXL2 also provide improved telemetry connection with more reliable and faster data transfer.      

Remote Receivers are your Friend

2.4gHz radio signals can be attenuated (reduced or blocked) by metallic/ conductive materials like carbon, metal and even fuel. By mounting remote receivers strategically throughout your aircraft, you can prevent these metallic objects from coming between the transmitter and all the receivers. In complex aircraft that contain significant conductive materials it’s recommended to use as many remotes as possible (limited by the model of receiver used). Note with some aircraft it’s normal for one or more receivers to be blocked from the signal in certain attitudes especially at great distances, and the system relies on the remote receivers that are not being attenuated. Consider this when selecting a location for each remote RX.   


Flight Log Data

Spektrum offers the ability to precisely evaluate the RF link performance of each receiver on the ground, during and after flight. Furthermore, RF link performance (Flight Log Data) can be saved via telemetry and reviewed later via a PC application ( ) allowing the evaluation of Link performance literally ever second of flight.


Interpreting Flight Log Data

Fades- A B R L

A fade occurs when a receiver (internal or remote) has a single bit loss (smallest possible loss of RF) It’s typical to have numerous fades during a flight and the number of fades per flight can be highly variable. Flight duration, RF environment, flight distance from TX, etc. can have a significant effect on the number of recorded fades. It is recommended to compare fades of each receiver to evaluate the mounting location and if one remote consistently has significantly higher fades, you may wish to reevaluate its location. Note it’s typical that internal remote A to have significantly more fades than the remotes. This is because the main receiver is typically mounted in the center of the model surrounded by servo leads and other systems (fuel, esc, batteries, etc.)

Frame Losses- F

A Frame Loss occurs when all the receivers (internal and remote) simultaneously experience a Fade. In complex aircraft or aircraft flown at great distances, it’s desirable to have less than 100 Frame losses per minute of flight. Note that it takes 45 contiguous Frame losses to cause a hold.  

Holds- H

A Hold in flight is never acceptable. A hold occurs when all the receivers (internal and remotes) experience 45 simultaneous fades (loss of signal). This is a loss of control event. If you experience a hold in flight do not continue to fly. Evaluate the system per the steps below to determine what is causing the issues and correct them before the next flights. Note that entering the system setup screens will cause the RF output to turn off causing a hold and some customers mistakenly believe the hold occurred in flight. To reset the flight log you must power cycle the receiver.  

Verifying RF Link Performance

With critical and high value models, it’s recommended to verify RF link performance in stages; first on the ground, then with short duration close in flights and then by extending flight times and distances. This will allow you to identify any shortcomings in the system.


Advanced RF Ground Testing

Step 1

A helper is needed for this step.  With the system bound and operational, have the helper hold the model at chest high 30 meters from the controlling transmitter.

Step 2

With the transmitter held in the normal flying position and facing the model, access the range test screen. Note the screen displays real time Flight Log Data through telemetry.

Step 3

While viewing the screen, press and hold the range test button (low power mode, normally controlled by the I button) and have the helper rotate the model in all attitudes, always keeping the model between himself and the controlling transmitter. Do this for 2 minutes while observing the Fades, Frames and Holds noting what if any positions of the model cause RF losses.

Step 4

If everything looks good (fades less than 200, Minimum Frames and no holds) then do the same test while operating the servos, retracts and any other onboard system during this test. Note it’s common for the Flight Log Data to be slightly higher as these systems can cause some RF link loses.

Step 5

Now carefully do the same test with the motor running. A helpful tool for while the motor is running is a model test stand to help keep it in place. If at any time during these tests a Hold occurs or unacceptable Frame Losses occur, stop testing, and reevaluate the system.  For example, in Step 4 if a poor RF link is discovered it’s likely a noisy servo or feedback from a retract. Unhook individual servos and retest to determine which servo is causing the issue and replace. If its engine related, it’s likely feedback from the ignition or ESC/BEC. Disconnect or swap parts until the issues is discovered. While rare, it is possible that the receiver or transmitter is at fault. If possible, substitute transmitter and receivers and retest to determine if that’s the case.

Note: if the aircraft is too large to hold the helper can rotate the model on the ground making sure to capture every available attitude.

 Initial Flight testing   

Step 1

Only when the above test procedures are verified should you proceed to the next phase of testing. On the first flight fly for a short duration close in. This will limit the exposure (short duration) and provide a favorable signal to noise ratio (close in) ensuring the best possible RF link. After the flight, evaluate the flight log Data. Minimal fades, no Frame losses and no holds should be displayed.

Step 2

On the next flights extend the distance and flight times gradually observing the flight log data after every flight. It’s normal for Fades and frames to increase at these greater distances and remember to note that it takes 45 contiguous frame losses before a hold occurs.

Step 3

Continue extending the range and time until you are confident that the RF link is robust. If at any time the RF link is questionable, repeat the Advances Range Check procedures to determine the cause.


Auto Logging

The latest firmware versions for the NX and iX Transmitters have a feature called Auto Logging. Each time the transmitter is turned on the telemetry is automatically recorded, and a separate file is created with the date and model name. These files are then available using your PC and accessing the internal storage of your transmitter. The files can then be opened and viewed using the TLM viewer application . This can be extremely helpful is determining the cause of any issues that may occur (low battery, ESC brownout, Loss of RF Link, etc.  )

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