From: mkfeil@bcrki9.bnr.ca (Max Feil)
Subject: Radio Interference for R/C flyers
Organization: Bell-Northern Research
Date: Wed, 09 Sep 92 17:22:31 GMT

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.

  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 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.

  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.  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). 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.

  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, 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.  
    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 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

     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.

  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 (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.

  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.
  
  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: mkfeil@bnr.ca


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



To: mkfeil@bcrki9.bnr.ca
From: pfoss@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: sbixby@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 - sbixby@cup.portal.com)

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

From: seanm@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: cwatters@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: mckiou@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 <1992Aug12.111135.2223@inland.com> bloom@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: budd@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: gbastin@x102c.harris-atd.com's message of 15 Jan 91 02:53:44 GMT

In article <5242@trantor.harris-atd.com> gbastin@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
budd@elxsi.dfrf.nasa.gov
--
Max Feil  mkfeil@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
Newsgroups: rec.models.rc