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Radio Interference for R/C flyers

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Max Feil

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Sep 4, 1992, 2:28:48 PM9/4/92

I wonder if you folks on the net could help me out. I just joined a committee
in my club that is looking at updating the club's frequency rules. I wrote
a little discussion paper that I'm planning to circulate to the rest of
the committee, but I would appreciate some feedback first.

Thanks in advance,
Max

Radio Interference Problems for R/C Flyers By Max Feil
========================================== Sept 1992

Theory: Adjacent channel energy & IF; intermodulation, harmonics.
-------

First, some very general, oversimplified theory on radio interference
causes. Please correct me if I am wrong since I am not an RF engineer.

When we talk about the frequency that an R/C radio system operates on, we
really mean its "center frequency", since both the transmitter and receiver
operate within a band of frequencies that is several kilohertz (kHz) wide.
This means that the center frequency of the receiver can be slightly
different than the center frequency of the transmitter and things will still
seem to work ok, but since power decreases as you move out from the center
frequency, range will be reduced. Incidentally, this is why range checks are
important. A bad range check may indicate that either the transmitter or
receiver are out of tune and their center frequencies no longer line up. A
crystal change can produce the same effect. The radio must be fine tuned
afterwards to ensure that the transmitter and receiver are centered
correctly, both with respect to each other and with respect to other
radios.

The characteristics of this band of frequencies around the center frequency
also determine the effects of radio interference. If your receiver
encounters a second signal that is too close to its center frequency and
which falls within this band, then interference will likely result. The
closer the interfering signal is to the receivers center frequency, the less
power is needed to cause interference. In the extreme case, if somebody
turns on their transmitter and is on exactly the same frequency as you, you
will crash even if their signal is very weak, for example if their antenna
is down or if they are flying several kilometers away. Conversely, if
somebody is operating on a frequency that is quite far away from the center
frequency of your receiver, they can still interfere if their signal is
strong enough. I will come back to this point later.

There is a second source of interference for your receiver. Pretty well all
receivers convert the signal they receive to a lower frequency through the
use of one or more special internally generated frequencies called
"intermediate frequencies". The principle is called "super heterodyning"
and it involves mixing the received signal with intermediate frequencies in
one or more stages. Receivers with one stage are called "single conversion"
and almost always use an intermediate frequency (IF) of 455 kHz. Receivers
with two stages are called "dual conversion" and usually use a first IF of
10.7 MHz and a second IF of 455 kHz. Dual conversion receivers are generally
of better quality than single conversion receivers. They have a narrower
bandwidth, but are also more complicated, more expensive, and more critical
of component tolerances, definitely requiring retuning after a crystal
change. In general, a receiver will experience interference if it receives a
signal that is too close to its first intermediate frequency. For example,
in the case of a single conversion receiver, a signal of 455 kHz will cause
interference.

So far we have seen that radio interference affects a receiver if an
interfering signal is too strong and/or too close in frequency to either the
receiver's main frequency or to its intermediate frequency. For example, if
you fly on channel 44 (72.670 MHz) and you have a single conversion
receiver, then you will experience interference if a radio signal is present
that is too close to 72.670 MHz or too close to 455 kHz. For all intents and
purposes, this is the end of the story. All R/C radio interference can be
explained in one of these two ways, so keep them in mind. However, life is
not so simple because we have looked at interference only from the
receiver's point of view. We haven't looked at the possible ways that
offending frequencies can be generated. The two main concepts here are
"intermodulation" and "harmonics". I will explain these below.

When radio transmitters operate, they generate energy not only at their
desired frequency, but at multiples of this "fundamental" frequency as
well. These are called harmonics. The same is true when you hit a piano key
or pluck a guitar string. (Harmonics can be mathematically derived using
Fourier series and are present anytime you don't have a pure, sinusoidal
waveform, a fact that I cannot help mentioning since I am a mathematician.)
For example, if a radio frequency of 455 kHz is present, then a harmonic
exists at 910 kHz (2 x fundamental), 1365 kHz (3 x fundamental), etc.
Luckily, the power of each successive harmonic (2nd, 3rd, 4th, etc) is much
lower than the previous one.

Intermodulation is perhaps the most important concept of this article. It
takes place when more than one radio frequency is present, and is defined as
the production of sum and difference frequencies from the set of original
frequencies present. For example, if two frequencies f1 and f2 are present,
they will "intermodulate" and produce two additional frequencies f2 minus f1
and f1 plus f2. These are called the 2nd order intermodulation products
(2IM). Again, being a mathematician, I must point out that these frequencies
can be mathematically derived using high school trigonometry.
Intermodulation is also noticeable in everyday life. When two tuning forks
of almost the same frequency are struck at the same time, a slow pulsating
"beat frequency" is created which is quite audible. This is the difference
frequency you are hearing. Anybody who plays guitar will also recognize that
intermodulation plays a big part in being able to tune their instrument. Now
let's go further and note that the 2nd order intermodulation (2IM) products
combine further with the original frequencies to again create sum and
difference frequencies that are the 3rd order intermodulation products
(3IM). Luckily, with each successive order of intermodulation (2nd, 3rd,
4th, etc) the power of the signal decreases. As an example, consider two
people flying, one on channel 44 (72.670 MHz) the other on channel 40
(72.590 MHz). The sum and difference frequencies created are 145.260 MHz and
80 kHz respectively. These are the 2IM frequencies, of which 80 kHz is the
more important one. The 80 kHz signal recombines with the two original
frequencies to produce new signals with frequencies of 72.590 - 80 = 72.510
MHz and 72.670 + 80 = 72.750 MHz. These are 3IM products, and note that
they correspond to channels 36 and 48! They are usually not a big problem
since the power of third order products is quite low, especially if the two
transmitters involved are more than 15-20 feet apart.

So, now we have talked about the sources of interference for a receiver,
namely something being too close to either the main frequency or the
intermediate frequency, and we have also talked about how various (perhaps
unexpected) frequencies are generated by transmitters through
intermodulation and harmonics. If we take a complete picture of all the
radio frequencies in the air at any given time from R/C and non-R/C sources,
their intermodulation products, their harmonics, the harmonics of the
intermodulation products, the intermodulation products of the harmonics,
etc, etc, we get some idea of the mess that is out there and what a tough
job a receiver has to do.

Radio systems: old canadian frequencies; recent pre-1991 radios;
-------------- 1991 radios: single conversion, dual conversion, JR's ABC&W.

To lead up to a discussion of specific problems that need to be addressed at
today's R/C flying field, I will start with a brief history of radios and
radio frequencies in use in Canada. I will concentrate on just the 72 MHz
band, and ignore the 27 MHz (CB) band, the 50/53 MHz ham frequencies, and the
75 MHz surface frequencies.

In the past, the R/C spectrum was not as crowded as it is today. Most R/C
activity was restricted to an original set of 6 frequencies which were
specified not using channel numbers, but by using a two-colour flag system.
Purple/white was 72.320 MHz, red/white was 72.240 MHz, etc. These channels
were no closer than 80 kHz together, and the original radios were designed
around this 80 kHz spacing and used single conversion receivers. In fact,
many of these radios are still in use today, which is why many clubs in
Canada, including the Stetson Flyers and the Ottawa Remote Control Club,
still follow 80 kHz spacing rules on their frequency boards through the use
of a 5-pin wide system. The next step was the establishment of 50 R/C
channels, all 20 kHz apart, starting at channel 11 (72.010 MHz) and running
to channel 60 (72.990 MHz). Note that the 6 old frequencies fall "in-between"
these channels, and therefore are sometimes referred to as "channel 26 and
a half" or "channel 22 and a half", etc.

At first only even channel numbers were available, and people in local clubs
could fly if they were at least 80 kHz (or 4 channels) apart. This was the
intent anyways, but due to non-linearities in the official MAAC frequency
board, the 5-pin system actually restricted flyers to 120 kHz spacing between
channels 32 and 46, and between channels 54 and 58. This was an unnecessary
restriction and led to unneeded congestion. In the several years before 1991,
radios were being sold that could handle a spacing of 40 kHz, and which were
equipped almost exclusively with single conversion receivers. Examples are the
Futaba Conquest AM series, and the Futaba 5 channel PCM. Then, in
preparation for 1991 and the introduction of the odd channels, these
so-called "wide band" radios were phased out in favor of "narrow band"
radios. Just what "narrow band" and "gold stickered" mean I will go into in a
minute. The new 1991 radios need to handle 20 kHz spacing, and many sport
state-of-the-art dual conversion receivers, but even in the strict 1991
environment, single conversion receivers are still being sold for some radios
(for example the Futaba Attack AM series).

So we have seen a progression of radio models, basically in three categories
based on their capabilities:

1) Old Canadian: 80 kHz spacing, single conversion rx, wide band tx.
2) Pre-1991: 40 kHz spacing, single conversion rx, wide band tx.
3) Post-1991: 20 kHz spacing, single/dbl conversion rx, narrow band tx.

When we talk about a "narrow band" radio, we mean one that can handle 20 kHz
spacing. Unfortunately not all 1991 radios come with true "narrow band"
receivers, just narrow band ("gold stickered") transmitters. The idea is that
the manufacturer attempts to ensure that you never shoot somebody else down.
However if your receiver is not narrow band (i.e. dual conversion or ABC&W),
somebody with wide band equipment can still shoot you down. This is rather
like the world of automobile insurance, where liability insurance is
mandatory but collision insurance is optional.

In Canada our situation is more complicated than in the U.S. We get 99% of
our radio equipment from the U.S. and follow most U.S. rules, but unlike in
the U.S. we have not officially obsoleted any old equipment. There are still
some radios in legal operation from category 1 (above), and many radios in
operation from category 2.

Problems in today's environment
-------------------------------

Our goal is to make available as many R/C channels as possible while doing
our best to ensure that no potential for interference exists. There will
always be unanticipated factors such as radios out of tune and interference
from external non-R/C signals, but we want to at least avoid known problems.
We also want to explore all possible options before making rash
hard-to-enforce decisions such as banning certain types of radio equipment or
disallowing certain channels.

The following problems must be handled:

1) Spacing

Two radios must not operate on frequencies closer than the spacing they
are capable of handling. 20 kHz spacing (i.e one channel apart, for
example one flier on channel 30 the other on 31) is only possible if both
fliers have narrow band transmitters AND receivers. If one of the fliers
has a wide band transmitter OR a wide band receiver, then the spacing must
be wider, for example 40 kHz or 80 kHz.

2) 2IM

No two radios should operate on frequencies such that their difference is
too close to the intermediate frequency (or the second harmonic of the
intermediate frequency) of any receivers being used. For example, if
somebody is on channel 60, and somebody else is on channel 14.5
(brown/white 72.080 MHz) the difference frequency is 910 kHz. This is the
first harmonic of 455 kHz. This will affect EVERY single conversion
receiver in the air. They will all crash no matter what channel they are
on. A similar problem is between any two people flying 23 channels apart.
This is a difference frequency of 460 kHz, which is very close to 455 kHz,
and which has the potential of affecting EVERY single conversion receiver.
Fortunately, not all single conversion receivers seem to be affected since
460 kHz is not right on 455 kHz. Also, receivers from radios in
categories 1 and 2 (above) will tend to be more affected. (I had one crash
and one near crash due to 23 channel 2IM with my Futaba 5 channel PCM that
I bought in 1988.) There is no telling which single conversion receivers
are more susceptible than others. The only sure way to avoid this problem
if we don't incorporate it into the frequency board is for EVERYBODY to
use only dual conversion receivers, but this would obsolete at least 50%
of the radios out there. (Note that the main reason a dual conversion
receiver is not affected by 2IM is that all direct sum and difference
frequencies that can be created in the 72 MHz band fall either way below
or way above 10.7 MHz.)

3) 3IM

While not a big problem, 3IM is still an issue, as it has always been.
The best protection for this problem is to ensure that people always
stand in their pilot's box when flying so that no two transmitters with
their antennas up come closer than 15-20 feet to each other. This is
because intermodulation products increase in power if the two sources
are close together. Another rule to keep in mind is not to walk too close
to somebody else if both your antennas are up.

4) Strong signal overpowers receiver.

This problem is quite common. If two flyers are standing relatively far
apart, say at opposite ends of the flight line, and the first person flies
their plane too close to the second person, the second person's radio
signal will be so much stronger than the intended signal that the first
person's receiver may experience a short burst of interference. This is in
keeping with the discussion earlier which explained that an interfering
signal need not be close in frequency if it is very strong. The best
solution here is the same as in problem #3: stand in your pilot's box.
Pilot boxes should be far enough back from the edge of the runway so that
no plane will get too close in normal circumstances. Safe flying
practices, i.e. low passes only over the far side of the runway, also help
here.

The frequency board & club rules: Possible solutions.
---------------------------------

Well, I've said almost all I can. The next step is to design an improved
frequency board and/or modify club rules. I will now list some of the
solutions that I have heard other clubs put in effect (also see attachments):

i) Ban odd channels.
ii) Allow the use of dual conversion receivers only.
iii) Go to a special pin system that forces you to take all necessary pins,
for example the pin(s) for channel(s) that are 23 channels away. An
effective system is described in the AMA handbook (I have more
information on this if you want).
iv) Go to a special computerized frequency board where the computer decides
whether you can fly based on rules similar to those listed earlier.

In conclusion, there are some basic principles involved in radio
interference, and these result in about 4 main problems that a frequency
board and field layout must overcome. The first two listed above, namely
spacing and 2IM, are the most pressing, especially with the introduction of
the new odd channels in 1991.

Max Feil
Stetson Flyers & Ottawa Remote Control Club
Internet Email: mkf...@bnr.ca

---attached messages follow----->

To: mkf...@bcrki9.bnr.ca
From: pf...@gmr.com (Pete Foss PO/46)
Subject: Re: Anybody with experience in 2IM problems please contact me by email
Date: Fri, 7 Aug 92 11:42:15 EDT

While we have not had crashes due to 2IM, my club (Skymasters RC of Michigan)
has adopted a frequency plan where you put your membership card in the slot
behind your frequency and take at least two pins (your frequency and 23
channels away). In addition, some channels also take pins that are 45 and 46
channels away. On the frequency board beside each frequency there is a list
of "interference" pins that you must also take off the board to fly. If one
you need is not there, you don't fly.

Let me know and I can send you specifications for our board (and a picture) if
you like.

PS. As a scale nut, I stopped at the air museum in Hamilton, Ontario on my way
back from Maine. WOW!!!

------------------------------------------------------------------------------

From: sbi...@cup.portal.com (Steven W Bixby)
Newsgroups: rec.models.rc
Subject: Re: Anybody with experience in 2IM problems please contact me b
Date: Fri, 7 Aug 92 10:28:01 PDT

My club has an interesting 'device' set up for this very purpose. I had asked
about it, and supposedly, it was detailed in the AMA manual, and my club
did it exactly to that spec. Basically....

It's a frequency pin board, with paddles, each paddle having two (some three)
channels listed on it. THe channels listed on each paddle are 23 apart -
ie, channel 17 and 40 are on the same paddle. Also, the two channels are
listed such that the old wide-band frequencies (ch 32-56?) are listed all in
the same 'row' on the paddles (while sitting in the box), and the new channels
are listed in another row below the first. Here's a crude and simplified
listing of some of the paddles....

vv vv vv vv vv
40 41 42 43 44
17 18 19 20 21
^^ ^^ ^^ ^^ ^^
Each column is a representation of one paddle, with two 23-apart channel
numbers on it. So, you take a paddle for your frequency, and thus take the
paddle for the 23-rd apart channel also. Note, that the channels that have
more than one 23-apart, such as 12/35/58, are all on one paddle.

That's not all, however - also provided are 'blanking paddles'. These are
paddles that go into the slot your paddle came out, IF YOU ARE USING an old
even-numbered channel, such as 36, 38, 40, etc. This blanking paddle has
'ears' that cover up the adjoining channels on the two adjacent paddles
so that the wide band radios won't interfere/be interfered by adjacent channel
numbers. So my chart above, after channel 42 is taken:

vv vv vv vv vv
40 xxxxxxxxxxxxxx 44
17 18 xx 20 21
^^ ^^ ^^ ^^ ^^
The next part is that since channels 18 and 20 are still showing, and you wish
to use one of those (which are definitely narrow-band), you can just take
the paddle and fly.

For clarity, the paddles are labeled so that the upper row is printed in RED
for channels 32-56, and the lower row in blue (everything but 32-56). This
prevents some confusion, by indicating if you pick a red channel, then you
must use the blanking paddle - if blue, no blanking paddle.

This system is working VERY well for us (Peninsula Channel Commanders,
based in the SF bay area, and flying in Half Moon Bay, CA), since we did have
some problems with 2IM and 3IM. Most have been eliminated by going to
narrow band, but some people are still using the (one-step-before-narrowband)
radios, and they're still able to fly. The biggest single problem with
this method, is that even if adjacent channels such as 34 & 35 are both
narrow-band, by the rules only one of them can fly at a time. But unless
you allow more than four planes in the air as we don't, it isn't really a
problem at all.

I imagine this is confusing, but it's a little hard to explain in text,
but a drawing is pretty simple and makes sense - again, the maker of our
paddle board at our field told me he got the design directly from the AMA
manual - although I haven't seen the manual recently....

If you have questions, please let me know, I'll try to answer them.

-swb- (Steve Bixby - sbi...@cup.portal.com)

------------------------------------------------------------------------------

From: se...@terapin.com (Sean McCaskey)
Subject: Re: Anybody with experience in 2IM problems please contact me by email
Date: 14 Aug 92 14:31:21 EDT

About your 2IM problem..... Our field is a dual conversion only field, so I
suppose we would have no problems anyhow.

------------------------------------------------------------------------------

From: cwat...@acorn.co.uk (Colin Watters)
Subject: 2IM problems - also in the UK
Date: 14 Aug 92 12:31:02 GMT
Organization: Acorn Computers Ltd, Cambridge, England

I don't know if anyone outside the US replied but we have the same problem in
the UK on the 35MHz band. For a while you could not get crystals for channel
numbers above (our) no. 80 were available.

When numbers 80+ became available we had problems with people on low channel
numbers (60ish interfering with channels 80+). On closer examination these
turned out to be 23 channels apart.

Our channel spacing is 10 KHz so 23 channels is 230KHz = half the IF (455KHz).

I believe the recommendation from the BMFA (British Model Flight Association)
is to avoid channels 80+ or put two pegs on the board.

------------------------------------------------------------------------------

From: mck...@cbnewse.cb.att.com (kevin.w.mckiou)
Subject: Re: Anybody with experience in 2IM problems please contact me by email
Date: Mon, 17 Aug 1992 20:05:52 GMT
Organization: AT&T

In article <1992Aug12....@inland.com> bl...@inland.com writes:
>receivers. At my field in the suburbs of Chicago, single conversion AM
>receivers are pretty useless. I have witnessed 3 crashes as a result of
>some kind of radio interference hitting Futaba Attack AM receivers (Futaba
>claims these are narrow band, but they are not dual conversion). I don't

I live in the western suburbs of Chicago and have been flying at least
3 to 5 days a week for the past 2 years using Futaba Attack and
Cannon super-micro receivers. Both receivers are AM single conversion.
I also fly with another guy who also uses the same two receivers. He
uses the Cannon gear in world competition. I have NEVER seen radio
interference except when flying directly over another transmitter
at low altitude (~50 ft - coming in for a landing). Even then, the glitch
is minor. I even fly with transmitters on both adjacent channels -
No Problem! I rate the Futaba Attack receiver EXCELLENT (the Cannon Rx
is the only one that I have experienced any interference with).

So...a different opinion.

Kevin McKiou
NAR 51581
AMA 380751
AT&T Bell Labs, Naperville, Ill

------------------------------------------------------------------------------

From: bu...@drynix.dfrf.nasa.gov (Gerry Budd)
Subject: Re: ABC & W (was Re: PCM vs. FM)
Date: 15 Jan 91 17:01:53 GMT
Organization: NASA Dryden, Edwards, Cal.
In-reply-to: gba...@x102c.harris-atd.com's message of 15 Jan 91 02:53:44 GMT

In article <52...@trantor.harris-atd.com> gba...@x102c.harris-atd.com (Gary Bast
in 60293) writes:

> Can someone please tell me what is meant by ABC & W. I have seen this
> for some time now, and no where has it been explained. Is it some type
> of digital encoding, or error correction coding, or is it something much
> simpler? Is it just marketing hype :-) ???

> I fully understand single conversion versus dual conversion, and
> intercept points/noise figures. I just haven't seen a definition of
> "ABC & W". Thanks in advance!

ABC & W stands for "Automatic Blocking Circuit with Window" which is
descriptive of the circuit logic of the receiver. ABC & W is standard
on all current JR receivers, both single-conversion and
dual-conversion. It apparently uses a very narrow-band active
filtering technique to eliminate some of the problems inherent in
dual-conversion designs while retaining the advantages. It is a
proprietary design that JR has patented. As I understand it Futaba
has made several attempts at "cloning" the ABC & W design but hasn't
been able to alter the design enough to avoid legal action if the
receivers were put into production.

My experience has shown that ABC & W isn't marketing hype at all. It
really is a lot better.

Jerry Budd
bu...@elxsi.dfrf.nasa.gov
--
Max Feil mkf...@bnr.ca | Disclaimer:
Bell-Northern Research | What do I know? I'm just a Nerd on the Big Ranch.
P.O Box 3511 Station C, |
Ottawa, Ontario, Canada.| "Enrich The Soil, Not EveryBody's Goal" Peter Gabriel

Max Feil

unread,

Sep 9, 1992, 1:22:31 PM9/9/92

Here is the final version of this article. I have made some changes.
Thanks to everyone who provided feedback!

Max

Radio Interference Problems for R/C Flyers By Max Feil

========================================== Sept 9 1992

Introduction
------------

In the increasingly popular hobby of radio controlled model airplane flying,
frequency congestion has prompted a series of changes over the years to allow
more flyers to use the same frequency band. Today's increasingly dense
frequency environment demands that extra precautions be taken to avoid
interference problems, which in R/C flying can result not only in the loss of
prized aircraft but personal injury or property damage as well.

I will attempt to explain in my own words the issues involved in trying to
minimize both congestion and interference problems. I will start with some
simple theory, and then apply this to the four main interference problems
that can result with radio equipment that is in use today. The goal of this
article is to stimulate discussion and increase understanding to allow the
members of R/C clubs to update and improve their frequency rules to help
provide a safe and enjoyable flying site.

Theory: Adjacent channel energy & IF; intermodulation, harmonics.
-------

First, some very general, oversimplified theory on radio interference causes.

Keep in mind that I am not an RF engineer.

transmitter and is on exactly the same frequency as you, you may crash even

pluck a guitar string. For example, if a radio frequency of 455 kHz is

present, then a harmonic exists at 910 kHz (2 x fundamental), 1365 kHz (3 x
fundamental), etc. Luckily, the power of each successive harmonic (2nd, 3rd,
4th, etc) is much lower than the previous one.

(2IM). To help illustrate this, I will point out an effect similar to
intermodulation that is noticeable in everyday life. When two tuning forks of

almost the same frequency are struck at the same time, a slow pulsating "beat
frequency" is created which is quite audible. This is the difference
frequency you are hearing. Anybody who plays guitar will also recognize that

difference frequencies play a big part in being able to tune their

instrument. Now let's go further and note that the 2nd order intermodulation
(2IM) products combine further with the original frequencies to again create
sum and difference frequencies that are the 3rd order intermodulation
products (3IM). Luckily, with each successive order of intermodulation (2nd,
3rd, 4th, etc) the power of the signal decreases. As an example, consider two
people flying, one on channel 44 (72.670 MHz) the other on channel 40 (72.590
MHz). The sum and difference frequencies created are 145.260 MHz and 80 kHz
respectively. These are the 2IM frequencies, of which 80 kHz is the more
important one. The 80 kHz signal recombines with the two original
frequencies to produce new signals with frequencies of 72.590 - 80 = 72.510
MHz and 72.670 + 80 = 72.750 MHz. These are 3IM products, and note that they
correspond to channels 36 and 48! They are usually not a big problem since

the power of third order products is quite low. Also note that
intermodulation products are actually created WITHIN the transmitter or the
receiver. Newer receivers are quite good at keeping intermodulation products
generated within themselves to a minimum, and transmitters will only generate
significant levels of intermodulation if they are closer than about 20 feet
together.

So, now we have talked about the sources of interference for a receiver,
namely something being too close to either the main frequency or the
intermediate frequency, and we have also talked about how various (perhaps
unexpected) frequencies are generated by transmitters through intermodulation
and harmonics. If we take a complete picture of all the radio frequencies in
the air at any given time from R/C and non-R/C sources, their intermodulation
products, their harmonics, the harmonics of the intermodulation products, the
intermodulation products of the harmonics, etc, etc, we get some idea of the
mess that is out there and what a tough job a receiver has to do.

Radio systems: old frequencies; recent pre-1991 radios;

-------------- 1991 radios: single conversion, dual conversion, JR's ABC&W.

To lead up to a discussion of specific problems that need to be addressed at
today's R/C flying field, I will start with a brief history of radios and

radio frequencies in use in Canada and the U.S. I will concentrate on just

the 72 MHz band, and ignore the 27 MHz (CB) band, the 50/53 MHz ham
frequencies, and the 75 MHz surface frequencies.

around this 80 kHz spacing and used single conversion receivers. In fact, in
Canada many of these radios are still in use today, which is why many
Canadian R/C clubs, including the Stetson Flyers and the Ottawa Remote

Control Club, still follow 80 kHz spacing rules on their frequency boards

through the use of a 5-pin wide system. The next step, which took effect in
1988, was the establishment of 50 R/C channels, all 20 kHz apart, starting at

channel 11 (72.010 MHz) and running to channel 60 (72.990 MHz). Note that the
6 old frequencies fall "in-between" these channels, and therefore are
sometimes referred to as "channel 26 and a half" or "channel 22 and a half",

etc. At first only even channel numbers were available, with odd channels
slated for introduction in 1991. This meant a minimum possible spacing of 40
kHz.

Most flying fields still kept to the old 80 kHz spacing, especially in Canada
where the original 6 frequencies were still in use. This meant that two
people could fly only if they were at least 4 channels apart. This was the
intent anyways, but due to non-linearities in the official MAAC (Model
Aeronautics Association of Canada) frequency board, the 5-pin system actually

restricted flyers to 120 kHz spacing between channels 32 and 46, and between
channels 54 and 58. This was an unnecessary restriction and led to unneeded

congestion which continues to this day.

In the several years between 1988 and 1991, radios were being sold that could
handle a spacing of 40 kHz, and which were equipped mostly with single

conversion receivers. Examples are the Futaba Conquest AM series, and the
Futaba 5 channel PCM. Then, in preparation for 1991 and the introduction of
the odd channels, these so-called "wide band" radios were phased out in favor

of "narrow band" radios. The new 1991 radios need to handle 20 kHz spacing,
and most have state-of-the-art dual conversion receivers, but even in the
strict 1991 environment single conversion receivers are still being sold for
some radios (for example the Futaba Attack AM series, and some JR receivers
which have special circuitry called ABC&W - "Automatic Blocking Circuit with
Window").

So we have seen a progression of radio models, basically in three categories
based on their capabilities:

1) Old: 80 kHz spacing, single conversion rx, wide band tx.
2) 1988-1991: 40 kHz spacing, single conversion rx, wide band tx.

1) Spacing

2) 2IM

receiver in the air. They could all crash no matter what channel they are

on. A similar problem is between any two people flying 23 channels apart.
This is a difference frequency of 460 kHz, which is very close to 455 kHz,
and which has the potential of affecting EVERY single conversion
receiver. Fortunately, not all single conversion receivers seem to be
affected since 460 kHz is not right on 455 kHz. Also, receivers from
radios in categories 1 and 2 (above) will tend to be more affected. (I had
one crash and one near crash due to 23 channel 2IM with my Futaba 5

channel PCM that I bought in 1988.) There is no easy way of telling which

single conversion receivers are more susceptible than others. The only
sure way to avoid this problem if we don't incorporate it into the
frequency board is for EVERYBODY to use only dual conversion receivers,

but this would obsolete at least 50% of the radios out there, at least in
Canada. (Note that the main reason a dual conversion receiver is not

affected by 2IM is that all direct sum and difference frequencies that can
be created in the 72 MHz band fall either way below or way above 10.7
MHz.)

3) 3IM

those intermodulation products that are created within transmitters

effective system is described in the AMA handbook (see attachments
for more info).

iv) Go to a special computerized frequency board where the computer decides
whether you can fly based on rules similar to those listed earlier.

I have not dealt specifically with interference from non-RC sources, for
example pagers, 2IM from audio of TV channel 4, etc. This type of
interference will follow the same basic principles as I stated in the body of
the article, but will be unique to a particular flying site and will require
local rules.

Max Feil
Stetson Flyers & Ottawa Remote Control Club
Internet Email: mkf...@bnr.ca

---attached messages follow-----> (See also RCM May '89, Sep '90, Oct '90)

Max Feil

unread,

Sep 14, 1992, 5:00:33 PM9/14/92

>Here is the final version of this article. I have made some changes.
>Thanks to everyone who provided feedback!

Oh no, not again! Well, believe it or not I like to be accurate, so I am
reposting a revised version of this article. PLEASE IGNORE THE LAST ONE
dated Sep 9. IT CONTAINED MANY INACCURACIES. Many thanks to Paul D. (can't
figure out his last name from his Email) for setting me straight on my
technical issues. That's exactly the kind of feedback I was looking for
from the beginning to put some reality into my feeble-minded ramblings.

If anybody wants me to forward them Paul's EXCELLENT in-depth technical
explanation, I'll be happy to. Or if enough people want it I could post.
In fact I think I will post it since Paul hasn't already. I hope you don't
mind, Paul.

Max

Radio Interference Problems for R/C Flyers By Max Feil

========================================== Sept 14 1992

Introduction
------------

I will attempt to explain in my own words the issues involved in trying to
minimize both congestion and interference problems. I will start with some

simple theory, and then apply this to the five main interference problems

that can result with radio equipment that is in use today. The goal of this
article is to stimulate discussion and increase understanding to allow the
members of R/C clubs to update and improve their frequency rules to help
provide a safe and enjoyable flying site.

Theory: Adjacent channel energy & IF; intermodulation.
-------

First, some very general, oversimplified theory on radio interference causes.

Keep in mind that I am not an RF engineer. I am also trying to keep things
as simple as possible for the average R/C modeler. If anybody wants some
more detailed, technical information, I have a very good article sent to me
by somebody who works in the radio industry that I can pass on to you.

Your transmitter will transmit strongest at frequencies very close to its
center frequency, with a decrease in signal strength as you move away from
the center frequency. Similarly the receiver will be most sensitive to
frequencies very close to its center frequency, with a decrease in
sensitivity as you move away from the center frequency. Note that the center

frequency of the receiver can be slightly different than the center frequency
of the transmitter and things will still seem to work ok, but since power
decreases as you move out from the center frequency, range will be reduced.
Incidentally, this is why range checks are important. A bad range check may
indicate that either the transmitter or receiver are out of tune and their
center frequencies no longer line up. A crystal change can produce the same
effect. The radio must be fine tuned afterwards to ensure that the
transmitter and receiver are centered correctly, both with respect to each
other and with respect to other radios.

The width of this band of frequencies around the center frequency is a major
factor in determining the effects of radio interference. If your receiver
encounters a second signal that is too close to its center frequency and the
the two bandwidths end up overlapping too much, then interference will
result. The closer the interfering signal is to the receiver's center

frequency, the less power is needed to cause interference. In the extreme
case, if somebody turns on their transmitter and is on exactly the same
frequency as you, you may crash even if their signal is very weak, for
example if their antenna is down or if they are flying several kilometers
away. Conversely, if somebody is operating on a frequency that is quite far
away from the center frequency of your receiver, they can still interfere if
their signal is strong enough. I will come back to this point later.

If this was the only way that interference could result, life would be
simple. However there are several other RF interference mechanisms and they
are much less obvious.

Pretty well all receivers convert the signals they receive to lower
"intermediate" frequencies through the use of one or more special internally
generated frequencies. The principle is called "heterodyning" and it
involves mixing the received signal with locally generated frequencies in one

or more stages. Receivers with one stage are called "single conversion" and
almost always use an intermediate frequency (IF) of 455 kHz. Receivers with
two stages are called "dual conversion" and usually use a first IF of 10.7

MHz and a second IF of 455 kHz. It is in the mixing process that several
problems may be introduced which can result in unwanted signals showing up
after conversion to the intermediate frequency. There are two main concepts
here: "image frequency" and "distortion".

Each conversion stage in a receiver will have an image frequency. It will
convert not only the desired signal down to the intermediate frequency, but
also any signal that is twice the IF either above or below the desired
signal, depending on the type of conversion being used (high side or low
side). For example, if you are using a single conversion receiver, the image
frequency will be 910 kHz (45.5 channels) away, either up or down (but not
both). If another transmitter in the R/C band is operating at this frequency,
you may experience interference. Note that image frequencies are not a
problem for dual conversion receivers since at each stage they are far away
from the desired signal and therefore easily filtered out beforehand.

The signal mixers that are used to perform frequency conversion in the
receiver also introduce a certain amount of distortion. This results in the
creation of extra frequencies called "harmonics" and "intermodulation
products". Harmonics are simply signals at multiples of the desired or
"fundamental" frequency. This is similar to what happens when you hit a

piano key or pluck a guitar string. For example, if a radio frequency of

72.030 MHz is present, then distortion will create harmonics at 144.060 MHz
(2 x fundamental), 216.090 MHz (3 x fundamental), etc. The power of each
successive harmonic (2nd, 3rd, 4th, etc) is generally lower than the previous
one. Luckily, harmonics are so far away from desired signals that they are
easy to filter out. Intermodulation, on the other hand, is perhaps the most

important concept of this article. It takes place when more than one radio
frequency is present, and is defined as the production of sum and difference
frequencies from the set of original frequencies present. For example, if two
frequencies f1 and f2 are present, they will "intermodulate" and produce two
additional frequencies f2 minus f1 and f1 plus f2. These are called the 2nd
order intermodulation products (2IM). To help illustrate this, I will point
out an effect similar to intermodulation that is noticeable in everyday life.
When two tuning forks of almost the same frequency are struck at the same
time, a slow pulsating "beat frequency" is created which is quite audible.
This is the difference frequency you are hearing. Anybody who plays guitar
will also recognize that difference frequencies play a big part in being able
to tune their instrument. Now let's go further and note that the 2nd order
intermodulation (2IM) products combine further with the original frequencies
to again create sum and difference frequencies that are the 3rd order
intermodulation products (3IM). Luckily, with each successive order of
intermodulation (2nd, 3rd, 4th, etc) the power of the signal decreases. As an
example, consider two people flying, one on channel 44 (72.670 MHz) the other
on channel 40 (72.590 MHz). The sum and difference frequencies created are
145.260 MHz and 80 kHz respectively. These are the 2IM frequencies, of which
80 kHz is the more important one. The 80 kHz signal recombines with the two
original frequencies to produce new signals with frequencies of 72.590 - 80 =
72.510 MHz and 72.670 + 80 = 72.750 MHz. These are 3IM products, and note
that they correspond to channels 36 and 48! They are usually not a big

problem since the power of third order products is quite low. Also, newer

receivers are quite good at keeping intermodulation products generated within

themselves to a minimum.

Note that not all intermodulation products are created inside the receiver.
Some intermodulation products are actually created within transmitters that
are operated too close together. Transmitters will generate significant

levels of intermodulation if they are closer than about 20 feet together.

So, now we have talked about the sources of interference for a receiver,

namely a signal being too close to either the main frequency or the image

frequency, and we have also talked about how various (perhaps unexpected)

frequencies are generated both by transmitters and within the receiver
through intermodulation distortion.

of "narrow band" radios. The new 1991 radios being sold today need to handle

20 kHz spacing, and most have state-of-the-art dual conversion receivers, but
even in the strict 1991 environment single conversion receivers are still
being sold for some radios (for example the Futaba Attack AM series, and some
JR receivers which have special circuitry called ABC&W - "Automatic Blocking
Circuit with Window").

So we have seen a progression of radio models, basically in three categories
based on their capabilities:

1) Old: 80 kHz spacing, single conversion rx, wide band tx.
2) 1988-1991: 40 kHz spacing, single conversion rx, wide band tx.
3) Post-1991: 20 kHz spacing, single/dbl conversion rx, narrow band tx.

When we talk about a "narrow band" radio, we mean one that can handle 20 kHz

spacing with multiple frequencies in use at the same time. Unfortunately not

all 1991 radios come with true "narrow band" receivers, just narrow band
("gold stickered") transmitters. The idea is that the manufacturer attempts
to ensure that you never shoot somebody else down. However if your receiver
is not narrow band (i.e. dual conversion or ABC&W), somebody with wide band
equipment can still shoot you down. This is rather like the world of
automobile insurance, where liability insurance is mandatory but collision
insurance is optional.

operation from category 2. In the U.S. some of these radios may still also be
in operation, but since they are illegal there is less chance of encountering
one at a sanctioned flying site.

Problems in today's environment
-------------------------------

The following problems must be handled:

1) Spacing

We would like to use as narrow a spacing as possible, however two radios

must not operate on frequencies closer than the spacing they are capable
of handling. 20 kHz spacing (i.e one channel apart, for example one flier
on channel 30 the other on 31) is only possible if both fliers have narrow
band transmitters AND receivers. If one of the fliers has a wide band
transmitter OR a wide band receiver, then the spacing must be wider, for
example 40 kHz or 80 kHz.

2) Image frequency

Anybody using a single conversion receiver should ensure that no other
transmitter is operating 910 kHz away. Luckily this affects only a few
people since 910 kHz spans almost the whole 72 MHz band and since one
transmitter would have to be on an old half frequency. The only radios
likely to be affected (in order from most likely to least likely) are
those on channels 14.5 (brown/white), 58.5 (yellow/white), 60, and 13.

3) 2IM

No two radios should operate on frequencies such that their difference is

too close to the intermediate frequency of any single conversion receivers
being used. For example, if somebody is on channel 20, and somebody else
is on channel 43 the difference frequency is 460 kHz, which is very close
to 455 kHz. This could affect EVERY single conversion receiver in the

air. They could all crash no matter what channel they are on.

Fortunately, not all single conversion receivers seem to be affected since

460 kHz is not right on 455 kHz, and since they have varying abilities to
suppress unwanted distortion. Receivers from radios in categories 1 and 2

(above) will tend to be more affected. (I had one crash and one near crash
due to 23 channel 2IM with my Futaba 5 channel PCM that I bought in 1988.)
There is no easy way of telling which single conversion receivers are more
susceptible than others. The only sure way to avoid this problem if we
don't incorporate it into the frequency board is for EVERYBODY to use only
dual conversion receivers, but this would obsolete at least 50% of the
radios out there, at least in Canada. (Note that the main reason a dual
conversion receiver is not affected by 2IM is that all direct sum and
difference frequencies that can be created in the 72 MHz band fall either
way below or way above 10.7 MHz.)

4) 3IM

While not a big problem, 3IM is still an issue, as it has always been.
The best protection for this problem is to ensure that people always stand
in their pilot's box when flying so that no two transmitters with their
antennas up come closer than 15-20 feet to each other. This is because
those intermodulation products that are created within transmitters
increase in power if the two sources are close together. Another rule to
keep in mind is not to walk too close to somebody else if both your
antennas are up.

5) Strong signal overpowers receiver.

The frequency board & club rules: Possible solutions.
---------------------------------

below for more info).

iv) Go to a special computerized frequency board where the computer decides
whether you can fly based on rules similar to those listed earlier.

In conclusion, there are some basic principles involved in radio

interference, and these result in about 5 main problems that a frequency
board and field layout must overcome. The first and third listed above,

------------------------------------------------------------------------------

So...a different opinion.

------------------------------------------------------------------------------

Max Feil

unread,

Sep 14, 1992, 5:04:50 PM9/14/92

Here is a great technical description of the issues being discussed in a
related thread. It was not written by me.

From: 'galaxy.nsc.com!p...@gatech.uucp'
Subject: Radio interference

Max,

I have read your posting and appreciate your efforts to explain the problems
people have with R/C gear, however I feel that while some of the conclusions
are correct some are not and the explanations are not really accurate. I know
that you are not an RF engineer so please do not take this as a flame - I work
as an RF engineer and have attempted to explain my view of what is going on
and I hope you will find the following constructive. If this email generates
questions on your behalf please feel free to email me.

RADIO COMPONENTS

To understand what is happening in a radio you need to understand what the
following components do;

filters; These are devices that only allow certain frequencies to pass through.
In a radio they are almost always bandpass filters and can roughly be
specified as having a centre frequency and a bandwidth. The ideal filter
(which can be proven not to be realisable) would allow the frequencies within
the bandpass to pass through the filter with no attenuation and stop all
other frequencies from passing through at all. Real filters pass most of the
signal at the centre frequency and gradually reduce the amplitude of the
signal as the frequency moves further away from the centre frequency. Filters
can be made to approach the ideal filter but the closer you get the heavier
bigger and more expensive they become. As filters approach this ideal they are
said to have a higher order. It turns out that for a given order, a filter will
have a narrower bandwidth if its centre frequency is lower (remember this as it
will explain why we have frequecy conversions in radios later). So if a filter
that is small light and cheap has bandwidth of 350khz and centre frequency
of 72MHz an equivalent order filter at 10.7MHz will have a bandwidth of 53Khz
and at 455KHz a bandwidth of 2KHz. So *filters are easier to make narrowband
at low frequencies*

mixers; An ideal mixer takes two input signals and multiplies them to give its
output. This is all they do. Real mixers will introduce gain or loss and more
importantly for this discussion introduce distortion. This distortion
characteristic can be described as a power series so that for an input x the
output will contain (a0 + a1.x + a2.x^2 + a3.x^3 + ....) a0 is the dc offset
at the output a1 is the linear gain a2 is the coefficient for 2nd order
distortion which will produce 2IM a3 is the coefficient for 3IM. The co-
efficients usually decrease very rapidly but the higher power terms increase
faster with increasing x (amplitude) so that (if the gain does not compress)
the 2nd and 3rd order terms eventually exceed the linear term. The smaller the
coefficients of the higher order terms are the more linear the radio is - this
is good for preventing distortion but often bad for increasing noise so mixer
designers try to compromise.

amplifiers; Ideal amplifiers just amplify signals - real ones introduce
distortion - see above

Radio Receivers

The big problem that your airplane has is to know which transmitter to
listen to - at the aerial *every* radio transmission is present - other R/C
channels, pagers, local radio stations, t.v., C.B. etc etc. The receiver tries
to achieve this by by using filters to remove everything except the frequency
band that you are transmitting on. The simplest way to do this would be to
put a filter directly after the aerial that stopped everything but your
transmission band. Your signal could then be amplified and demodulated with no
further ado. This type of radio is called a tuned radio frequency reciever
or TRF for short. This is such a simple idea - why don't more people use it?
Well for the fllowing reasons;

i) The filter would have to be 20KHz wide at 72MHz which requires a very
expensive high order filter.

ii)All amplification and demodulation needs to be done at high frequency which
requires high power consumption circuitry.

To get around this problem radios use either one intermediate frequency ("if")
and are called heterodyne receivers or more than one (usually two) in which
case they are called superheterodyne receivers (superhet for short).

Heterodyne receivers (single conversion receivers)

Heterodyne recevers work like this; All signals go into the aerial and a low
order filter selects frequencies +/- about 600khz either side of the centre
of the R.C. band. This filter is called the image filter and you should note
that it allows *all* the R/C channels through (yes I know! read on...).
They then go to a mixer which multiplies all the incoming signals by the
crystal frequency of the receiver (called the local oscillator with frequency
"flo"). For every input frequency "fs" to the mixer the output contains ;

fout = fs*flo

As you said it can be shown by trigonometry theory that;

cos(2*pi*flo*t)*cos(2*pi*fs*t)=1/2*( cos(2*pi*(flo-fs)*t)+cos(2*pi*(flo+fs)*t)

or the output of the mixer contains frequencies at flo-fs and flo+fs. Now the
output of the mixer is put through a filter with a centre frequency at 455KHz
and a bandwidth of 20KHz (this is called the channel select filter). This
definitely gets rid of the flo+fs terms as they are up at about 144MHz but
what gets through? well anything that satisfies the relationship;

flo-fs=+455KHz **or** flo-fs=-455KHz

you may well say what does -455KHz mean? - It is called the image frequency
and is actually a positive frequency the same as +455KHz but phase inverted
by 180 degrees (or multiplied by -1 if you prefer).

to select your transmission frequency the receiver crystal is designed so
that flo-fs=+455KHz so flo=fs+455KHz however if the input of the mixer
has a frequency at flo+455KHz (which is fs+910KHz) then you will get an
interference output frequency at 455KHz. You rely on the image filter
(see above) to reject this frequency *before* it gets to the mixer
(once it gets into the mixer there is nothing you can do about it)
however this filter has to be at least as wide as the R/C spectrum
which is channel spacing*number of channels (I think there are
60 channels now? so the image filter is then 1200kHz wide) so a single
conversion receiver can let frequencies 45.5 channels away interfere. whether
the interfering channel is 45.5 channels above or below your channel will
depend whether your receiver uses high side or low side flo injection (this
just means whether flo=fs+455Khz or flo=fs-455KHz respectively). If all
receivers used high side injection then channels 45.5 above you would interfere
with you but you would not interfere with them. For high side injection
receivers you want to be one of the high frequency channels, for low side
injection receivers you want to be one of the low frequency ones.

This effect has nothing to do with 2nd order intermodulation it is due to
a lack of image rejection in single conversion receivers.

2IM

As we have seen - any signals at 455KHz coming out of the mixer get through
the channel select filter. If the mixer circuitry has 2nd order distortion
(a2*x^2) then an input of a + b will be distorted to a^2 + a*b + b^2
the square terms of this quadratic can be ignored (they are easy to filter)
so that the effect of 2IM is to multiply input signals together which are
then multiplied by the local oscillator. the effect of this is that large
signals 455KHz apart at the input to the mixer generate 455KHz at the *input*
to the mixer -whether these get to the output and cause interference depends
on the mixer type - a good balanced mixer will attenuate these signals before
to the output.

In summary a single conversion R/C radio will always have poor image rejection
and if the image frequency is inside the band of the image filter 910KHz
away from your channel you will get interference. 2IM may or may not be a
problem if the mixer has either low 2nd order distortion or is well balanced
or both then 2IM will be less of a problem. 2IM becomes a problem when the
two interfering signals are strong and your signal is weak.

Superhet receivers (dual conversion)

These have 2 intermediate frequencies. The first mixer now has an output
image frequency 21.4MHz away (2*10.7MHz) these are easily filtered by the
image filter between the aerial and the first mixer. 2IM products need to
be 10.7MHz apart and these are similarly easily filtered by the image filter.
So now all we do is filter at 10.7MHz and detect our signal right? well if
the filter you used was 20KHz wide then yes you could do this but such filters
are expensive and the circuitry at 10.7MHz still requires fairly high power.
So a superhet filters at 10.7MHz with a cheap filter that is about 100khz
wide. After this filter you simply mix the signal with a local oscillator
whose frequency is fs+455KHz (sound familiar?) the output is filtered at 455KHz
with a bandwidth of 20khz to select your signal the image frequency at this
if is 910KHz but the filter at 10.7MHz has removed it - so no interference
from images or 2IM.

Can anything interfere with superhets then? Yes ;

If a transmitter transmits on your frequency it will fairly obviously
interfere if its strong enough. The interfering signal must be substantially
higher to interfere with an fm system than an am system (this applies to any
of the interferers listed above or below which is why fm systems are less
prone to interference - all else being equal). This situation will arise if:

i) Someone transmits on the same frequency as you

ii) Someone transmits on a frequency close to you with a wideband transmitter
(if his transmitter bandwith is 60KHz some of his signal will spill into
your channel if enough energy spills ....)

iii) 3IM ; the mechanism for this is identical to 2IM but now the cube of
the two input signals generate 2f1-f2 and 2f2-f1 ( simply look up the
trig for (cos a + cos b)^3 ) if two channels are spaced xHz and 2*xHz
away from you 3IM in the first mixer will produce an interferer at
the same frequency as you - it cannot be filtered once produced.
the image filter will not be able to remove these as they can be
any of other RC channels with the correct spacing (note the image
filter must allow all the channel through or you would not be able
to use all the channels (crystals) available in any set which would
be a production nightmare.

A final mechanism for interference is jamming whereby the radio is simply
overloaded by a *very* strong signal that gets through the image filter-
it simply overloads the circuitry.

I hope you find this of some value - I may have made some errors (after all
I'm only human!) but I think its mostly correct and if you disagree with
any of it or have any questions I will be happy to answer them.

Cheers - Paul

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