[Courtesy of Joel Foner joel.foner "at" fonerassoc.com, December 2000]
RC RFI SCANNING FAQ
Maintained by Joel Foner joel.foner "at" fonerassoc.com
TABLE OF CONTENTS
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.
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
- 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:
- OK, so it's not the aircraft - what other transmitters can cause
- And how about some transmitters that don't look like transmitters...?
3) What are symptoms of interference?
There are several symptoms of interference:
These symptoms could occur in several situations:
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...
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:
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)
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.
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.
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
What's good about the "rubber duck" antenna that came with your
What's not so good about the "rubber duck" antenna that came with
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.
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.
Once you start scanning, you'll notice that the output from our transmitters
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
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
End of Document