George M. Myers
Radio Technique
70 Froehlich Farm Rd., Hicksville, NY 11801
Abstract
How to use the information obtained with a scanner.
What can you learn from a scanner? Present this month's column is a book:
Table of Contents
- Preface
- What a scanner can do for you—an overview
- Lesson #1: The sounds of RC systems
- Lesson #2: Signal strengths and squelch controls
- Lesson #3: The sounds of adjacent channels
- Lesson #4: Third-order intermodulation
- Lesson #5: Other sounds of intermodulation
- a) First order—direct interference
- b) 2IM—The 23-channel problem
- c) Image—The 45.5-channel problem
- Lesson #6: How much of this stuff is really interference?
- Interference
- Commencement address
Preface
In my AMA position as frequency coordinator for District II, I receive requests for loan of the scanner that the AMA has assigned the district. I usually interrogate the requester to find out what kind of troubles are being investigated. It usually turns out that most of them can be solved mathematically. For the rest, I usually think how futile the monitoring effort will be, because the person who will do the monitoring doesn't know what to look for—or how to identify it.
"I usually think how futile the monitoring effort will be, because the person who will do the monitoring doesn't know what to look for—or how to identify it."
In a flurry of inspiration one morning, I put this together. Then I passed it around to a lot of people for comments, to make it as complete and accurate as possible within the scope of the simple handbook that I wanted it to be. I hope you will find it helpful.
What a scanner can do for you — an overview
A scanner (a radio receiver that automatically and sequentially tunes a list of preset frequencies) lets you listen to radio transmissions. But if you don't understand the significance of what you hear, you don't learn anything.
When you get your scanner from your district frequency coordinator, the tendency is to turn it on and go looking for voice transmissions (because you can understand them). You can find voices on some frequencies, but you shouldn't find voices on 72 MHz RC channels, because we have those channels exclusively. Does that mean you are free from interference? Not really.
The key to finding interference is knowing what to look for, and where to look for it.
There are many kinds of radio transmissions. You are familiar with the sound of the 550–1600 kHz AM and 88–108 MHz FM broadcast radio, because you have that in your automobile.
You know about the 26.995–27.255 MHz Citizens Band, where the RC channels are supposed to be free of voice interference.
When you monitor the 50–54 MHz Amateur frequencies (there are others), it is possible to hear voice, Morse code, slow-scan TV, packet radio bursts (bursts of digital data), teletype, and digital bit-streams (which may be AM, FM, or PM [phase-modulated] systems), single-sideband transmitters, etc.
You think you are familiar with the sound of 54–890 MHz TV, because you have a set in your living room. But the sound of TV is more than just the soundtrack for the movie (which is FM sound, by the way). The vertical oscillator pulses at 60 Hz, the horizontal oscillator squeals at 15.75 kHz, and the picture signals are supersonic.
There are things in the 72–73 MHz MRS/PRS/RC band that carry voice, such as mobile telephones. There are also pocket pagers, which sometimes sound like fast telephone push-button tones and sometimes sound more like sleigh bells. All of them are on the even-frequency channels (and not on the RC channels).
Depending on how the scanner is designed, you may hear some things from the 108–135 MHz aircraft radio band, such as a VOR (aircraft radio navigation aid), which is a low BRRR—BRRR—BRRR accompanied by Morse code.
Then there are the incidental transmitters: computers, welding machines, lightning strikes, high-voltage transmission lines which sometimes corona, and on and on.
"I strongly recommend the use of a tape recorder as a tool . . . like a notebook. Record the sounds from the scanner, and talk to the recorder while you are setting up tests and adjusting the scanner's controls."
The point I'm making is that there is a lot to hear. Identification and interpretation of what you hear is as important as hearing it. This article will help you to teach yourself how to interpret some of what you hear.
Since this is an aural experience, I strongly recommend the use of a tape recorder as a tool for learning. Use it like a notebook. Record the sounds from the scanner, and talk to the recorder while you are setting up tests and adjusting the scanner's controls. We could make such a tape for you, but if we do all the work you won't do any of the thinking, so you won't learn anything.
Lesson #1: The sounds of RC systems
When all else fails—READ THE BOOK. It will tell you how to turn on the scanner and make it do its tricks. Bill Hershberger has written a manual which interprets Japanese techno-speak into simple English, with sketches of the control locations added, and that manual should be available with the AMA's scanners by the time this is published.
(Recess to read Bill's manual.)
Welcome back. Now, let's discover the difference between the sound of AM, FM, and FM/PCM by turning on various RC sets, then listening to them on the scanner. You will quickly learn to tell the difference between a simple two- or three-channel AM set and a complex, multi-channel FM/PCM RC system just by listening.
Lesson #2: Signal strengths and squelch controls
We have to go outdoors for this lesson.
A good procedure is to have someone walk down a road with an operating RC transmitter while you listen on the scanner. You will learn a lot about how the squelch control affects what you hear. You will also learn about dropouts.
"You will also learn about dropouts. When the transmitter and scanner whip antennas are held vertically . . . every 13 ft., as the transmitter moves away from the scanner, the signal will drop to a minimum."
It is best to do this once on a road that doesn't have electric wires or poles alongside, and a second time on a road that does. On the ground, without suspended electric wires overhead, a continuous metal fence nearby, or buried wires or pipes below you to join the signal from the transmitter to your scanner's antenna, you probably won't hear anything after the transmitter is more than one-half mile away from the scanner. Keep track of the signal strength ("S" meter reading) as a function of distance from the transmitter. Plot signal strength versus distance.
You will also learn about dropouts. When the transmitter and scanner whip antennas are held vertically, you will observe that every 13 ft., as the transmitter moves away from the scanner, the signal will drop to a minimum. In between the nulls the signal will rise and fall to a maximum. Observe this variation on the "S" meter.
Point the scanner antenna horizontal and then vertical, and note the change in signal strength at various distances. Have the person with the transmitter hold it vertical and as high over his head as possible. Observe the effect on the "S" meter, write down a few data points, and plot yourself a curve. You have learned something about the effects of antenna orientations and height.
Overall, you will develop some understanding of the dynamic range of signals.
If you could fly over a road at about 20 ft., you would hear dropouts caused by phase cancellation, which relates to the fact that radio signals travel faster in air than in the ground. The exact speed difference is affected by moisture in the soil, so phase cancellation effects from this cause are variable. Ordinarily, they will show up as changes in ground range test distances. The difference between ground range as measured over a freshly watered golf course compared to over dry sand can be significant.
Sensitive airplanes may glitch (i.e., lose control momentarily) as a result of dropouts from any of these causes, but airplanes usually fly through dropouts so quickly that they go unnoticed. Helicopters, on the other hand, might be so unlucky as to try to hover in a dropout region. This should tell you something about hovering along at altitude some distance from the transmitter. PCM helps overcome dropouts.
"... or any pair (of transmitters) 23 channels apart might affect any or every wideband SC455 in the air, without regard to what RC channels they are on."
Lesson #3: The sounds of adjacent channels
Set the scanner to 72.680 MHz and set up a transmitter on RC44. You shouldn't hear anything unless the RC transmitter is too close. Move the RC transmitter slowly from 3 ft. away to 100 ft. away. Note how the signal changes. If the scanner has a wideband/narrow-band switch, do this once with the switch in each position, and note the results. What you are hearing is similar to what your receiver hears from an adjacent MRS/PRS channel—which are those transmitters located between and 10 kHz away from our RC channels. Note how the "S" meter reacts.
Scan from 72.00 through 73.00 MHz. You may observe that a strong signal can be heard on three or more channels in sequence; e.g., PRS37.5 (72.540 MHz), RC38 (72.550 MHz), and PRS38.5 (72.560 MHz). That is a wideband transmitter leaking onto adjacent channels 10 kHz away—adjacent-channel interference by definition.
Lesson #4: Third-order intermodulation
This one is easy, since 3IM signals generated by RC transmitters are always exactly centered on RC channels and can always be heard on a scanner. Put the scanner on 72.590 MHz (RC40) and leave it there. Set up an operating RC44 transmitter for 100 ft. away from the scanner. Have another club member stand about 3 ft. from the RC44 transmitter and turn on an RC42 transmitter. You should be hearing 3IM on RC40.
Scan slowly from 72.00 to 73.00 MHz looking for other signals. You should find 3IM on 72.650 and on 72.710 MHz. Note how loud each one is, and record the signal strength in "S" units. Stop the scanner on each one, and see if the 3IM sound goes away when one of the transmitters is turned off.
If you have a transmitter available for the channel with the 3IM on it, put that transmitter in operation, and see how far away it can be taken before the 3IM can be heard. You are learning about relative signal strengths.
Turn on several more transmitters—three, four, five, and six or more—at the same time, in sequence. Space them 30 ft. apart on a line perpendicular to the line from the scanner. Scan slowly from the first transmitter to see how the 3IM changes as the distance between transmitters increases. Observe how the "S" meter changes.
Scan from 72 to 73 MHz in the smallest available steps, with the squelch control turned all the way down (CCW). It helps if you do this with all AM transmitters, all FM transmitters, all PCM transmitters, then with a mix. Each one sounds different, doesn't it? By now you should be able to tell what transmitter types are making up the intermodulation package just by listening. You are getting an education.
Now turn off all the transmitters except two. Put them butt-to-butt. Scan from 72.00 to 73.00 MHz. Notice anything familiar? Two transmitters butt-to-butt sound like a bunch of transmitters that are physically separated, don't they? That's why we call that arrangement our Flight Line Simulation test set-up.
The rule for 3IM is: Any two transmitters will generate 3IM on a frequency midway between their operating frequencies, and as far above and below their frequencies as they are separated in frequency.
Somebody who has access to a spectrum analyzer will tell you that 3IM signals are about 30 dB below the transmitter signal—which is true. Other people who are familiar with the properties of RC receivers will tell you that interference occurs when a spurious signal is received 15 to 22 dB below the received control transmitter signal. Note that this means that a weak signal interferes with control by a strong signal. The difference in signal strengths, 8 to 15 dB, is your margin of safety from 3IM interference—all things being equal.
Your studies of signal strengths and dropouts will have convinced you that things never are equal. Signal strength drops rapidly as distance from the control transmitter increases, and dropouts that lower your transmitter's usual signal more than 15 dB are easily demonstrated by changes in relative antenna positions—as you have seen for yourself. You should be able to have an opinion about what people tell you. Here are some 3IM examples:
- RC30 and RC40 will generate 3IM on RC20, RC35, and RC50.
- The TV4 sound carrier (71.75 MHz) and RC13 can produce 3IM on RC28.
There are other TV4 combinations like this—RC18 and RC38, RC23 and RC48, and RC28, which dumps 3IM on both RC13 and RC58. The strength of the TV4 signal determines whether or not the resulting 3IM will bother anybody. If you have access to a Kraft synthesizer transmitter, you can easily check this situation out. Remember to write down the signal strength in "S" units. We will use the information in Lesson #6.
The suspect channels are RC13, RC18, RC20, RC21, RC23, RC28, RC38, RC48, and RC58, where you are in the presence of a strong (S = +) TV4 signal measured at the flight line. Note that only RC13 and RC21 will be new considerations in 1991.
If you don't have a strong TV4 signal in your area, then you may have something else, like TV5 or microwave beams. Microwave beams are very narrow and are likely to affect your plane only in particular spots in the sky. You may not be able to hear them on the scanner, but the towers are easy to spot with their tiny dishes pointing parallel to the ground.
You can take any list of frequencies and work out combinations of your own, based on what you find. Then, you have to evaluate whether or not the 3IM signals are strong enough to be considered interference (Lesson #6). You are somewhat limited in this assessment by the fact that the scanner on the ground doesn't hear as much as does the airplane receiver in the air.
Lesson #5: Other sources of intermodulation
a) First-order (direct interference)
- Turn on two RC40 transmitters on the same channel at the same time. Set the scanner on that channel and listen, first to one transmitter, then to both. You should hear a superheterodyne whistle (among other things) come from the second transmitter. That's First Order Intermodulation. Pretty loud, isn't it? You can hear this on your scanner as it happens.
b) Second-order intermodulation (2IM) — The 23-channel problem
- Now for a difficult problem: interferences which are generated inside receivers. You can't hear them, because the scanner receiver is different from the RC receiver. You have to find something, then apply some reasoning and some mathematics. This is where you earn your pay as DPC.
For this lesson, you must have a pencil, paper, and page 10 from your Membership Manual in addition to the scanner and tape recorder.
This is 2IM. The rule for 2IM is: take the operating frequency of a transmitter, then add and subtract the receiver's intermediate frequency to find the potential interferer. The most common intermediate frequency was 455 kHz (455 kc).
Narrow-band SC455s are less likely to be affected by 2IM than are wideband SC455s (SC = single-conversion). That's because most of the possible 2IM interferers are 5 kHz away from an RC channel, so the narrow-band receiver can ignore them. The narrow-band SC455s are becoming available.
Let's say you find something at 72.500 MHz. Add .455 and get 72.955 MHz. The closest RC channel is RC58 (72.950 MHz), which is 5 kHz away. That might produce interference in an RC58 receiver, if the signal at 72.500 has a strength of S = 5+ and the SC455 receiver is wideband.
Something worse: that pair of transmitters, or any pair 23 channels apart, might affect any or every wideband SC455 in the air, without regard to what RC channels they are on. It's all a matter of relative signal strengths. In general, be wary of any signal over -5 dBm.
The easiest way to evaluate this is to test it in the field with your airplane. Let your airplane (on the ground) tell you if it is disturbed by doing a ground run and noticing whether or not the distance is shorter than usual.
You aren't finished yet. Now look in the opposite direction (72.500 − .455 = 72.045). The closest RC channel is RC13 (72.050 MHz). Again, it might suffer interference if RC58 didn't. The best test is a field test.
Here's a 2IM example:
- You find FM sound from TV4 at 71.750 MHz, and it has a signal strength of -4 dBm.
- Looking up-scale: 71.750 + .455 = 72.205 MHz, which is between RC20 and RC21, but closer to RC21. Because it is FM sound, the frequency wobbles a bit. It isn't exactly 71.750 MHz at all times. So you might get hit on either channel.
Of course, the manufacturers all know this, so expect that they will put the local oscillator on these channels above and below the transmitter, because of the way that local oscillators are defined in paragraph 4.10 of the "AMA Radio Control Utilization Plan" (page 11, column 1 of the 1988 Membership Manual). But if they do that, you have to avoid simultaneous flying with RC43 and RC44. And the high-band channels will have to check out TV5.
If paragraph 4.10 of the "AMA Radio Control Utilization Plan" (page 11 of your 1988 Membership Manual) is changed to put local oscillators outside the 72 MHz band (thereby minimizing 2IM between models), then intermodulation with TV4 and TV5 can be a problem. The best solution might be to stay off RC20 and RC21 altogether, if you insist on flying with an SC455 receiver in the presence of a strong (S = 5+) TV4 signal.
However, 2IM isn't confined to SC455s. Any receiver can be affected by 2IM. In the case of dual-conversion receivers, the first conversion usually has a local oscillator at 11.55 MHz away from the control transmitter. With that much offset, the front end usually can attenuate a potential 2IM signal to insignificance. So we doubt that owners of dual-conversion RC receivers have any reason to worry about 2IM.
Yes, 2IM with modern receivers won't become a problem until 1991—and then only if all 50 channels are sanctioned as planned. The odd-numbered RC channels create the problem, because many pairs of transmitters 23 channels apart can cause the difficulty. Using only even-numbered channels—as we are doing right now—there is no problem.
Or is there one? We already have MRS/PRS transmitters which are separated from our RC channels by an amount which can be expressed as (22 RC channels plus up to 10 kHz). For example, relative to RC16: 16 + 22 = RC38, plus 10 kHz, or RC36 plus 10 kHz (depending on numbering). They are just as likely to generate 2IM within existing SC455 receivers as are RC channels 23 numbers away, and multiple complaints received at various locations around the country indicate that they are doing it. It all depends on the signal strength on the flying field. So you have to find those MRS/PRS transmitters and consider what they're capable of doing.
Image interference: Any receiver is subject to image interference. From a practical standpoint, only SC455s on RC11, RC12, RC13, RC14, RC57, RC58, RC59, and RC60 have anything to worry about. They must be wary of transmitters 45.5 channels away. Such transmitters are MRS/PRS transmitters.
If you find strong signals (S = 5+) on PRS 10.5, 11.5, 12.5, 13.5, 14.5, 55.5, 56.5, 57.5, 58.5, 59.5, or 60.5, then you have to consider them to be potential interferences. (xx.5 is my shorthand code for a PRS channel 10 kHz above the RC channel number given. Channel 10.5 = 72.000 MHz.)
In summary, you find something with the scanner, note its frequency, then do some mathematics to see if what you found might be a problem.
If you are looking at a signal at 72.xxx MHz which has a strength of S = 5+ on the scanner, then you can expect that signal to be an adjacent-channel/intermod problem for wideband receivers in the area.
An S = 5+ simply says that every S = 5+ signal should be investigated. Try to find out what is causing it. Wet insulators on a high-tension line might be the cause, and that condition often clears up after the sun burns off. An S = 5+ signal should always be answered with a ground range check before flight.
Radio Technique / Myers
Continued from page 131
AT A CONTEST: The ground range should exceed 300 ft. When the transmitter has one section (about 8 in.) of its antenna exposed, an S = 9+ signal should be grounds for refusing to fly on any channel that might be affected in any way (image, 2IM, 3IM, or adjacent channel).
If you are looking at a signal outside the 72–73 MHz band and it has a strength of S = 5+ on the ground, then you should do the mathematics to find out if it can interact with an RC transmitter to produce image or 2IM interference with wideband SC455 receivers in the field. If it has a strength of S = 9+, then 3IM possibilities should be considered.
Narrow-band, dual-conversion receivers should be able to resist all problems from signals S = 8 and below.
Best defense: Do ground range checks before flight.
Commencement address
A scanner without a trained human brain to interpret its findings is just a noisemaker. Now that you have finished this short course, you are trained to the level of beginner. Go now, and learn how to use your scanner to find and identify interference sources. If you discover something that you think should be added to the lessons, let me know about it.
Transcribed from original scans by AI. Minor OCR errors may remain.
