RC RFI Scanning FAQ
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Home > Articles & Tips > RC RFI Scanning FAQ

[Courtesy of Joel Foner joel.foner "at" fonerassoc.com, December 2000]


Maintained by Joel Foner joel.foner "at" fonerassoc.com
Last update Dec 15, 2000


  1. What is RC (radio control) RFI (radio frequency interference)?
  2. Where can RFI (radio frequency interference) come from?
  3. What are symptoms of interference?
  4. What other things can cause interference-like symptoms?
  5. Is there anything I can put on-board to watch for interference?
  6. Why would I want to have a scanner?
  7. What really happens when a scanner scans - what happens when if "finds something"?
  8. If I only fly on one frequency, why would I want to have all the frequencies loaded?
  9. What are image frequencies, and why would I care about transmissions on a frequency that doesn't match my transmitter frequency?
  10. Do I need to load all the RC frequencies and image frequencies if I'm just checking my installation on one frequency?
  11. What is good and bad about the stubby "rubber duck" antenna that comes with most scanners?
  12. When would I want to build or buy another antenna for my scanner?
  13. If I check for RFI using a scanner in the pits, and I don't find any, am I guaranteed to not find interference in flight?
  14. How can I check out a flying site to see if it is clear of interference?

1) What is RC (radio control) RFI (radio frequency interference?

Anything that interferes with continuous, complete control of a remotely controlled system! The next few sections will detail more about interference, but suffice it to say that interference can come in many forms, and exhibit a variety of symptoms. Something to keep in mind is that there are other things that can generate similar symptoms to true radio frequency interference - so it is important to troubleshoot carefully before assuming you're really dealing with RFI.

2) Where can RFI (radio frequency interference) come from?

RFI can be generated by a surprising number of sources - some of them obvious, and some downright sneaky!

First there are interference sources that are generated by RC transmitters:

- Someone else's RC transmitter on your frequency, left on at the field (in the pits, sitting on the transmitter rack left on, sitting in someone's car left on - maybe never turned off when it left the house!)

- Another RC transmitter on your frequency, but off-field. Examples of this are:
* residents of houses around the field perimeter
* someone flying a park flyer near the flying field (ouch!)
* someone running a car or boat on an AIR RC frequency nearby (it happens...)
* any other location within three miles where an RC transmitter is turned on
Three miles is the spacing required by AMA between flying fields - mostly to prevent in-air problems where an aircraft is high and off-field towards the "other" field. You don't want the receiver to be able to get confused by a signal from the "other" field as its own...

- Someone operating a "pre-1991" wide band transmitter. There aren't many of these around, but they "splatter" onto frequencies other than the one marked, so if it's marked as a different frequency than yours, due to the wide transmit window, it may put out enough energy on your frequency to cause problems with your receiver.

Then there are some (maybe) surprising sources of interference:
- The aircraft itself!
* if electric motor-powered, motor brush noise, without appropriate spark suppression capacitors
* long servo wire runs, untwisted and/or missing chokes
* loose metal-to-metal joints, such as
metal clevis to metal control horn
metal control rod to metal clevis
metal to metal bell crank attachments and parts
tail boom supports (notorious RFI emitter on heli's if loose)
landing gear
dry or noisy ball bearings (also more prevalent on heli's due to the high bearing rotation speed, but also found in aircraft motors with worn bearings on occasion)
* non-metal joints, in particular carbon fiber airframe components
loose or marginally conductive carbon-fiber (CF) connections
any other spot where CF can rub against CF or metal
* scale aircraft hardware and features (especially machined, non-lubed, assemblies)
* rough-running engine, that generates lots of vibration and exaggerates any of the problems above
* on-board electronics
Bad spark plug wire shielding on a gas motor
Loose spark plug wire connection(s) on a gas motor
Turbine ECU's (electronic control unit) for a turbine powered jet - it's basically a computer, and if not thoroughly shielded by design, it may "leak" emissions in a frequency range that is problematic to RC receivers
On-board video downlink transmitters
* there's probably more - but this is enough to think about before you automatically blame something else, right? ;)

- OK, so it's not the aircraft - what other transmitters can cause interference?
* High power transmitter systems, such as pager transmitter towers
* Television stations
* Truckers with over-legal boosted transmitter rigs
* Any other non-RC transmitter with high power, and potentially misadjusted or miscalibrated internals

- And how about some transmitters that don't look like transmitters...?
* Old, cracked power line insulators (read about this in a magazine - wish I could remember which one - random interference problem at the field turned out to be a bad power line insulator on a pole near the field!)
* Farm machinery (the old unlubed metal to metal thing...)

3) What are symptoms of interference?

There are several symptoms of interference:
* Reduced operating range
* "Glitches" - causing one or more servos to jump to positions that were not directed by the transmitter
* Blackouts/Lockouts - complete non-responsiveness of the control system to transmitter commands

These symptoms could occur in several situations:
* on the ground, with engine/motor off
* on the ground, but only with engine/motor on
* on the ground, but only while "exercising" the control system
* in the air "all around"
* in the air, but only in specific "spots" in the sky or around the field
* on ranges of frequencies
* on one specific frequency

When troubleshooting a suspected RFI problem, it is important to figure out as best as possible what situation triggers the RFI - otherwise the search may take much longer than it needs to...

4) What other things can cause interference-like symptoms?

There are other problems that can cause symptoms that many folks assume is interference (Ever notice how there are some folks who are always yelling "I've been hit" and having radio problems, on channels that other folks have no problem on? Interesting, eh? Sometimes it's quality of receiver. Sometimes it's not...)

Here are some other things that can cause the symptoms of interference:
* Loose connector(s) in the on-board power system
* Old wiring in the on-board power system (enough broken strands in a wire run can cause "power starvation" to the component that "looks" like it's firmly connected!)
* Defective, corroded, or "about to fail" power switch in the on-board system
* Vibration-sensitive component, such as a vibration-sensitive switch
* Bad solder joint(s) on an on-board circuit board (in the receiver, on the power/control board in a servo)
If your receiver battery pack momentarily drops below the voltage where your receiver operates correctly, you will get "hit", and it will look like a blackout/lockout. If the pack is on the low side, and you "stir the sticks", you can draw a surprising amount of power from the pack, and the voltage will sag very noticeably. If it's near the bottom of the operating range of your receiver, the receiver can "go dark", but it's NOT RFI!
If one or more linkages have lots of drag, the power drawn to move that linkage will be _very_ high, and can pull the receiver voltage down to problematic levels (as in the last one)
Tip: Checking power draw with an ammeter on the bench while running each control through its full range can help to spot this stuff before it's in the air.

5) Is there anything I can put on-board to watch for interference?

There are probably others, but the unit I know of is the BC-6 by YNT Electronics (http://www.yntdesign.com/). The BC-6 plugs into your receiver, and counts missing bits in the signal as you fly, and then can report how many "hits" it saw in the air, as well as the lowest battery voltage during the flight.

6) Why would I want to have a scanner?

A scanner can be useful in two main situations:

- to check to be sure "your channel" is clear before powering up your radio and getting ready to launch (I do this before EVERY flight, as I've found powered-up transmitters on the rack, or in the pits, or in a transmitter impound a distressing number of times...)

- to research a suspected interference problem, both to identify the affected RC frequencies and locate the interference source

In short, you may want to have a scanner as an everyday tool to help prevent loosing your plane due to another transmitter at or around the field, and as a troubleshooting tool to figure out what's going on when you suspect interference (or when a BC-6 reports an unusual number of hits during a flight)

7) What really happens when a scanner scans - what happens when if "finds something"?

Usually, scanners work pretty much like the radio in your car. The radio in your car probably has a "station seek" mode, where you can push a button, and it will slowly move up and down until it finds a station broadcasting. Then it stops and plays that station for a few seconds, and if you don't stop it, then it starts scanning again until it finds another broadcast.

Radio scanners work the same way, although often you can control how long they pause, how "much" of a signal will cause them to stop, and a variety of other additional features.

Most scanners also support both a "frequency scan" and a "memory scan" mode. In frequency scan mode they work like the car radio - all frequencies between the end points are scanned, and anything it finds will cause it to stop and play. In memory scan mode, a scanner "jumps" from one stored frequency to the next, avoiding anything that isn't "on" a stored frequency. Both modes can be useful for locating and troubleshooting RC RFI issues.

8) If I only fly on one frequency, why would I want to have all the RC frequencies loaded into my scanner?

You would only NEED to have ALL the RC frequencies loaded into your scanner if you want to have a bunch of RC frequencies pre-programmed!

At the same time, saving AT LEAST the main frequencies of any transmitters you use can be handy, since then you don't have to dial around to the transmitter frequency each time you want to check things out. This also prevents "memory fades" where you punch in the wrong frequency for the channel you're about to fly on. The scanner stays on the last frequency you used when you shut it off, and "wakes up" to that last setting when you turn it on - but if you twiddle around with it, it is nice to be able to just 'recall' the frequency your transmitter is on.

9) What are "image frequencies", and why would I care about transmissions on a frequency that doesn't match my transmitter frequency?

Basically our receivers "listen" mainly on the printed receive frequency, but due to their internal design they can be susceptible to interference on the "image frequencies" if it's strong enough. Internally, RC receivers typically convert the high frequency signal in multiple steps, or stages, and these stages operate at a different frequency than the main transmit frequency. This means that parts of the receiver electronics (internally) are actually "listening" at these other frequencies - and if some outside signal is strong on these other frequencies it can get fooled.

These other frequencies are called image frequencies - and there is one upper and one lower image frequency for each receiver. The location of the image frequencies depends on whether the receiver is single or dual-conversion.

The "main frequency" (the one stamped on the transmit module or crystal of your transmitter) is used to check if someone else is on your frequency. The image frequencies are important frequencies to check in cases where you suspect on-board interference, or if there are high powered external transmitters on the image frequencies.

Interference and glitch problems involve checking both the main frequency and the image frequencies, since non-transmitter transmissions on any of the three frequencies could cause either range problems or flat-out glitches or control loss.

10) Do I need to load all the RC frequencies and image frequencies if I'm just checking my installation on one frequency?

As above, you may want to load only the three frequencies (main, and image+/image- for your receiver type) into memories, so that you can easily recall them from the R2 rather than having to remember the numbers.

Another reason to load the three frequencies into memory slots, is that then the scanner will check each of the slots and not stop on transmissions on other frequencies that you don't care about.

Do you have to? No. Once you start using a scanner at the field, you may find that everyone at the field will want your assistance once you show up with one, and having them all pre-loaded makes things easier. You can find a frequency chart, showing both main and image frequencies, along with setup information and configuration files for the ICOM IC-R2 compact receiver, at

11) What is good and not so good about the stubby "rubber duck" antenna that comes with most scanners?

What's good about the "rubber duck" antenna that came with your scanner?
* It came with it! (no extra cost)
* It works well across a pretty wide range of frequencies
* It's short, so it's easy to carry around, and doesn't get in the way very much
* For basic RFI tests, and especially for checking for other local transmitters turned on, it's great.

What's not so good about the "rubber duck" antenna that came with your scanner?
* It was designed with several tradeoffs
Small antennas are less efficient than big ones, so it can't pick up weak signals very well
It was not optimized for the RC channel frequency range, so in addition to being small, it's even less efficient and sensitive than it could be
It is non-directional, so you don't get much feedback on where an off-field interference source is.

12) When would I want to build or buy another antenna for my scanner?

If you are trying to locate weak, distant or off-field interference sources, a larger antenna that is specifically tuned for the 70-75 MHz frequency range can make a big difference. This will help you to triangulate on the source of the interference more easily, and detect interference that you might miss with the "rubber duck" antenna.

Note that a 41" whip antenna (for the mid 72 MHz range) is the right length to improve efficiency dramatically in the 70-75 MHz range compared to the "rubber ducky" built into most wide band receivers.

13) If I check for RFI using a scanner in the pits, and I don't find any, am I guaranteed to not find interference in flight?

There are no guarantees, but...

If the interference originates from somewhere on or around the flying field, the chances are very high that you will find it from a "pit check" scan.

If the interference source is off-field, you might pick it up from the pits, but you MAY NOT pick it up from the ground in the pits. How can this be?

What if the interference source is separated from the field by some structure, like a building, that shields the transmission from you at ground level? Once your aircraft gets high enough in the sky, or off-field enough to clear the building that is shielding you, the aircraft is now "lit up" by the remote transmitter!

Another case where this can happen is when you are flying off-field towards a remote RFI source (unknowingly of course!). You may be a part of a mile off-field, where the RFI signal is much stronger than on the field at ground level. This is a case where a higher efficiency directional antenna can help to track things down. Using a more efficient antenna, pointed towards the part of the sky where RFI symptoms area observed, can help to find the source.

14) How can I check out a flying site to see if it is clear of interference?
[Courtesy of Rich Fong <rxf "at" pacbell.net>] - with some edits...

Once you start scanning, you'll notice that the output from our transmitters is quite
strong. This what keeps our planes flying. But don't stop here. Hop in your
car, and start driving away from the pit area in the general direction of
the flight path, and evaluate the distance to signal strength ratio (using the signal level meter on your scanner if you don't have more advanced tools).

Keep driving until you are at least one mile, preferably two miles from the flying site. Now you are out at the fringes of where you fly, and the fringes of the range of the control system. Check the other signals (non-RC) out there, and compare their signal strength to that of the
RC transmitters. If these others signals are stronger than your transmitter's
radiated signal, or even at comparable levels on the ground, they can pose a problem. If they are not, then you're probably OK.

All this is a multi-phase process, as the other users may not be broadcasting at the time you take your measurements.

Taking measurements from the pit area has limited value in tracking off-field interference, as our transmitter radiated output within approximately 1000' is so strong, there "shouldn't" be any off-field interference that will cause any problems in the pits (no fair someone turning on a transmitter on your freq.). This varies a bit on your transmitters output too. If your RF module is weak or mis-tuned, and you're not outputting the power you should be, then this places you a greater risk too. At the far reaches of your flying pattern where the plane is farthest from the transmitter is where we are most susceptible to interference.

One last comment. Just because a frequency is clear at one moment DOES NOT mean that a broadcast transmitter in the area cannot come on the air while your flying. Hopefully if the frequency has been sweep'ed repeatedly week after week you will have been able to identify most of these other transmitters in the area. Still our high signal strength due to close proximity of our
transmitters, and receiver is our most powerful tool.

End of Document

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