From: mkfeil@bcrki9.bnr.ca (Max Feil)
Newsgroups: rec.models.rc
Subject: Radio Interference for R/C flyers
Date: 14 Sep 92 21:00:33 GMT
Organization: Bell-Northern Research

>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
------------

  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 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.

  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.
  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.


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 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.

  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. 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
-------------------------------

  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

     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.

     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
       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,
  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
Bastin 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