From: mkfeil@bcrki9.bnr.ca (Max Feil)
Newsgroups: rec.models.rockets,rec.models.rc
Subject: Radio Interference for R/C flyers
Date: 3 Sep 92 21:07:21 GMT
Distribution: na
Organization: Bell-Northern Research

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 group, but I would appreciate some feedback first.

Thanks in advance,
Max




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


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 the 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. A receiver will experience interference if it receives a signal that
   is too close to its first intermediate frequency. 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 an
   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 from the receiver's
   point of view, be 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 mathematican, 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.

   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 excusively 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
     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.

  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: mkfeil@bnr.ca


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

<deleted>
--
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.rockets