Conference vmszoo::rc

    How close is close? There is a TV4 station in Boston, which is
    probably less than 20 miles (closer to 10 in a straight line)
    from CMRCM and CRRC fields.

    How can we verify if this is causing us problems?

    --. .-.. .- -..   .. --   --- -. -.... -- . - . .-. ...
    Dave Hughes

    The RC02 / TV4 is whats going on here in St. Louis. Neat huh?
    
    Dan Eaton

    re: .108
    
    ..    .-- .. ... ....    ..    .-- .- ...
    
    .... --- .--. . ..-. ..- .-.. .-.. .     ... --- --- -. !!!!
    
    
    We better not let this get started.
    
    The TV04 transmitter in DENVER I am sure is within the 20 mile radius
    that you spoke of.   Well at least we know what is happening.  
    
    Hey is that channel 20 you are on???
    
    YES
    
    HEY LET GO OF MY TRANSMITTER.....OW.......HEY STOP....NO....I WONT
    UGH...TURN ......IT.....ON!!!....PLEASE MISTER!!!!
    

    I just read the radio columns in MA and RCM for this month and there
    was a reference to the interaction of TV4 and CH 20.  I was very
    disappointed that there was no explanation for this with frequency
    numbers and such.  The article also implied that dual conversion
    receivers would not be affected by this interference.  Mention was also
    made of separation between pilots of at least 20 feet - a rule that is
    never followed at CRRC.  I wish someone who understands this stuff
    would write a good clear article about it so we could have the
    knowledge necessary to make the right choices around what equipment to
    buy and what rules to follow at our fields.  ** Are there any radio
    engineers out there? **  I don't want hand waving I want facts.
    
    Charlie
    

Charlie, I'm afraid the previous reply is right; that the bottom
line is that its going to be a problem for everyone, and its very
complicated.  On the TV4 and ch20 business, I'll tell you that I
heard Steve say what caused it in technical terms, and for the
life of me I couldn't parrot it back -- and I make my living
writing about technical things like that.

I think its a similar thing to 3IM in that the signals from the
tv station and the ch20 radio mix and by this action slip in
above and below the local oscillator as false IF signals --
images as they're called.  Dual conversion works because the
first converter stage tracks 10.6 Mhz (incidentally, this IF
frequency is synonomous with FM modulation) above or below the
local oscillator, putting the potential images out of the RC
band and down far enough that you can positively tune them out
with the IF cans and other filtering.  I think also that the dual
conversion, by definition, greatly narrows down the number of
potential interferring stations. 

I have a series of articles by Fred Marks that explains how this
works -- I'll look them over for anything that I can steal for a
note on this.

The more I look into the subject the more I'm convinced that the
only real answer is PCM with failsafe.  And failsafe is something
that is more than an occasional insurance policy in the 1991 world.

I don't blame the AMA for not wanting to get into the dictation
of how the plan will be implemented.  Its up to us local clubbies
to find out the "bad" stations, and to buy and use equipment on
different frequencies -- and to have equipment that can be
changed in frequency because when a new pager pops up we gotta
move.

Each club in this brave new world will have a frequency
monitoring committee who will be responsible for staying on top
of the changes.

    The flier I recently received on the Scale Masters Qualifier held
    in Denver last weekend also warned that "Channel-20 is prohibited
    at this field."
    
    I don't know that I'm aware of all the potential problems but, as
    I understand it, if there's a TV channel-4 anywhere nearby, it somehow
    mixes/modulates with the ch-20 signal and creates another signal that
    hits radios some 22 or so channels away with alarming strength and
    regularity.

      |
      | |      00	 Adios,      Al
    |_|_|      ( >o
      |    Z__(O_\_	(The Desert Rat)

>    I don't know that I'm aware of all the potential problems but, as
>    I understand it, if there's a TV channel-4 anywhere nearby, it somehow
>    mixes/modulates with the ch-20 signal and creates another signal that
>    hits radios some 22 or so channels away with alarming strength and
>    regularity.

Experts will chime in and correct me if I am wrong but the problem is not
limited to being 22 channels away.  That problem of being 21 channels away
will not hit a narrow band receiver.  The channel 4 TV signal modulates
the channel 20 RC signal and shoots planes down on ALL the frequencies.
Narrow band doesn't help.  The only way to stay flying when channel 20
is on and you live near a TV channel 4 transmitter is to physically place
yourself significantly closer to your plane then the guy with the channel
20 transmitter.

The only question I have is what will happen in 1991 when we have channel
19 and 21 and paging systems with Megawatts on channel 19.5 and 20.5?

Good move by the 495th.  A letter to members notifying them about
the channel 20 ban is probably all you need to pack with your radio
when you send it back.

Bye          --+--
Kay R. Fisher  |
---------------O---------------
================================================================================

    The Quinapoxit Model Flying Club has also banned channel 20 use. 
    Currently, there are no members with a channel 20 radio so there was
    no impact on the members.

                       _____
                      |     \
                      |      \                          Silent POWER!
      _        ___________    _________   |            Happy Landings!
     | \      |           |  |         |  |
     |--------|-  SANYO  + ]-|  ASTRO  |--|              - Dan Miner
     |_/      |___________|  |_________|  |
                      |       /           |     " The Earth needs more OZONE,
                      |      /                       not Caster Oil!! "    
                      |_____/

    Channel 20 and TV-4 hits many RC channels. Not sure that I can say
    which. Channel 20 + TV-4 (20-25 miles west of TV-4) combined to
    hit my channel 16 Futaba PCM and drive it into failsafe every time.
    Both radios were narrow band xmitters, and receivers.
    
    
    							Dave
    

I tracked down the reason for the TV channel 4/ RC channel 20 interference
the other day. It turns out that the sound carrier on channel 4 is 450Khz
below the channel 20 transmitter carrier. The problem is that this is very
close to the 455Khz IF frequency of single conversion recievers. What appears
to happen is that the front end of a reciever is not very
selective. They rely on the narrow bandwidth of the IF stage to get their
selectivity. When the channel 4 sound carrier combines with the channel
20 signal in the mixer in the front end of a reciever, the mixer happily
generates the 450Khz difference frequency and feeds it to the IF amplifiers.
Since this is only 5 Khz off of the IF's center frequency it shouldn't have
a lot of trouble getting through and driving the servos crazy.

RE:.140

This shouldn't affect dual conversion recievers since their first IF is at
10.7 Mhz instead of 455 Khz and the 450Khz difference frequency should be 
rejected nicely by the first IF stage. Is your receiver dual conversion?

                        !
                      --+--
Glenn Schrader     o___<0>___o    CSS::SCHRADER (264-4170)
                     *  *  *

    the one in question was not. Another PCM I have is Dual Conv.
    
    
    						Dave
    

Wasn't the Ch20/TV4 interference 3IM at 460 onto the 455 stage? Won't either 
Ch19 or Ch21 be close enough when we get down to '91 narrowband?

One of my concerns is that I'll be buying another radio and I'd like to get a
number of features on it along with Dual conversion Rx. I'm not looking to spend
megabucks but I'd like to get the stuff I'll need for the next few seasons. I'm 
currently flying an Attack (cheap radio to get back into the air after 6 years)
so I'm leaning towards Futaba for compatability (J series) but I need to look
under $300.

Sorry for the digression but one of my concerns is immunity to 3IM once the odd
channels become available in 1991. We're already seeing some problems when we
have 2 radios 46 channels apart. Odd channels will give us ones 23 apart also.

I guess what I'm trying (badly) to ask is, is there a radio "feature" that uses
a decoding stage other than 455 to avoid some of this? Is this the "dual 
conversion" feature and what does Futaba mean by "triple tuned"?

>Wasn't the Ch20/TV4 interference 3IM at 460 onto the 455 stage? Won't either 
>Ch19 or Ch21 be close enough when we get down to '91 narrowband?

Probably - I read one article a couple of months ago that said a channel 22
radio emptied the sky.

>One of my concerns is that I'll be buying another radio and I'd like to get a
>number of features on it along with Dual conversion Rx. I'm not looking to spend
>megabucks but I'd like to get the stuff I'll need for the next few seasons. I'm 
>currently flying an Attack (cheap radio to get back into the air after 6 years)
>so I'm leaning towards Futaba for compatability (J series) but I need to look
>under $300.

If Futaba makes a dual conversion for under $300 you've got it made.

>Sorry for the digression but one of my concerns is immunity to 3IM once the odd
>channels become available in 1991. We're already seeing some problems when we
>have 2 radios 46 channels apart. Odd channels will give us ones 23 apart also.

I don't have the math memorized but the concern is suppose to be 21 channels
apart not 23.

>I guess what I'm trying (badly) to ask is, is there a radio "feature" that uses
>a decoding stage other than 455 to avoid some of this? Is this the "dual 
>conversion" feature and what does Futaba mean by "triple tuned"?

I'm on thinner ice now so experts correct me if I'm wrong.
Dual conversion receivers actually have the moral equivalent of 2 IF
strips at two different frequencies.  One of them may well be at 455 KHz.

The "Triple tuned" receiver just means that it has 3 stages of IF (or two
stages with tuned RC networks on each side of each stage).  It is still
a single conversion receiver.  This is just double talk like the JR
ABC&W receivers.  Bottom line is they are not dual conversion.

Don Kraftt(sp) is partially right - banning channel 20 is not the answer.
But he is wrong in one respect - It isn't 1991 yet so (for now) banning
channel 20 protects (to some extent) flyers with legal, approved equipment.
It doesn't protect us from the beepers coming on line (NOW they are not 
obligated to wait for 1991).  Channel 20 should not be allowed until 1991
and for all the good one channel will do I think the AMA should not
make it legal ever - IMHO.

===============================================================================

Summary

There are 3 levels of 1991

1.  1991 legal
    This means your transmitter has a gold sticker and the FCC won't
    arrest you or take away your existing FCC license.

2.  1991 certified 
    All of level 1 plus
    This means your receiver meets the AMA specification.  This does
    not mean dual conversion.  Only some Airtronics and some JR equipment
    is certified.

3.  1991 safe
    All of level 1 and level 2 plus
    This means your receiver is dual conversion.  This protects you from
    the 21 channel interference and the channel 20 and TV channel 4 
    interference.  All AMA certified Airtronics and some AMA certified
    JR equipment makes this level.

Summary II
    The cheapest 1991 safe system is the Airtronics Vanguard FM dual conversion
    at approx $130.  The 6 channel version is closer to $160 and worth the
    difference in price (not so much for the channels as for the dual rates
    and mix functions you gain).

Summary III
    If you have an Airtronics radio (AM or FM) send it in for the ($79 or less)
    upgrade to dual conversion 1991 FM.
    
    If you have Futaba radios start shopping.
    
    If you have JR radios get one of their new dual conversion receivers
    and be prepared to pay $150 for it.
    
    If you want Futaba equipment (because you believe and trust them and
    have had good luck with their equipment in the past) then you'll
    just have to wait till they are certified as meeting the AMA specification.
    My guess is this will happen within 1 Futaba month (9 months).

Summary IV
    Don't worry about a thing except beepers and channel 20 until 1991.

Summary V
    There should be a lot of good bargains on servos and batteries at the
    upcoming auctions.

Bye          --+--
Kay R. Fisher  |
---------------O---------------
================================================================================

Ok, I'll try this one -- Go easy on me if I make a mistake:

< Note 105.144 by TEKTRM::REITH "Jim Reith DTN 235-8459 HANNAH::REITH" >
               -< Questions and feature clarification request. >-

>Wasn't the Ch20/TV4 interference 3IM at 460 onto the 455 stage? Won't either 
>Ch19 or Ch21 be close enough when we get down to '91 narrowband?

I think the TV interference is just slightly off the 455 IF;
something like 460.  CH 19 & 21 have not been mentioned as being
susceptible to this problem, and I think its because they are NOT
close enough.

>One of my concerns is that I'll be buying another radio and I'd
>like to get a number of features on it along with Dual conversion
>Rx. I'm not looking to spend megabucks but I'd like to get the
>stuff I'll need for the next few seasons. I'm  currently flying
>an Attack (cheap radio to get back into the air after 6 years) so
>I'm leaning towards Futaba for compatability (J series) but I
>need to look under $300.

As I've stated elsewhere, and will state again in this reply, I
think Futaba sucks.  But then again, I'm changing my mind about
K&B... Dual conversion FM/PCM is the only system that will
survive in a full 1991 environment.  Period.

>Sorry for the digression but one of my concerns is immunity to
>3IM once the odd channels become available in 1991. We're already
>seeing some problems when we have 2 radios 46 channels apart. Odd
>channels will give us ones 23 apart also.

Yes, the 3IM problem is only just beginning.  Please understand
though, we are not expected to operate in the full environment of
public service stations and odd channels, etc.  We are expected
to select alternate channels when we find interference; in other
words, to go somewhere else where the radio is quiet.  That's why
we have all those channels.   We will be seeing synthesized
channels on our radios in a very short time because of this fact.

>I guess what I'm trying (badly) to ask is, is there a radio
>"feature" that uses a decoding stage other than 455 to avoid some
>of this? Is this the "dual  conversion" feature and what does
>Futaba mean by "triple tuned"?

Dual conversion is the feature that allows us to use an alternate
to 455 IF -- but it does use 455.  The first converter is 10.7
mhz; this puts the image frequencies well outside of our band,
and allows us to build circuits that are frequency selective
enough to reject those signals (well, maybe if you stand under
their transmitter...).  After the 10.7 mhz stage, the signal is
again converted to 455 khz; thus rejecting further any signals
that sneak through the 10.7 mhz converter.

Its a double whammy on interference, which along with the filter
stages that Airtronics (for instance) is using actually allows
performance far exceeding AMA guidelines. This is the good news:
that the best receivers now available far exceed our hopes for
performance--they'll do the job better than what was planned.

Here is the operant principle that Futaba is using: The 1991
environment as seen by the FCC is transmitter only; the FCC wants
to make sure that we don't interfere with adjacent pagers.  This
is all the FCC cares about.  Lets call this the transmitter spec.
As is well known, this spec is very easy to implement, and is in
fact a no contest in all radios being made today.  Futaba must
meet this spec if it is to legally sell radios in the US.

The other half of the spec is in the receiver.  This one has
stated the AMAs guidelines to allow bomb-proof operation in 1991.
See, 1991 is legally ruled by the transmitter spec, but is
implemented in the *receiver* spec; it is here that the real
engineering for  1991 is taking place.  The FCC does not care a
fig about receivers -- please get this: its the crux of the whole
matter -- the law says transmitter, the implementation says
receiver.  Now, since this receiver spec is not legal, only an
AMA guideline, there is no enforcement power for it. Outside of
the AMA -- but its very very difficult to test a receiver under
the circumstances of an AMA event.  There's been some talk of
type accepting a receiver like we are doing the TXs, with the
gold and silver stickers, again its difficult to test a receiver
outside of the lab, and each receiver must be tested, type
acceptance will not guarantee it; 1991 is that tough. The posting
that AMA is doing in MA is of course the implementation of the
type acceptance, and so far its working.

Thus Futaba can resort to any marketing ploy it desires to get
around the AMA guidelines and sell radios -- and that's what
"triple-tuned" is; a marketing ploy.  I'm not sure what is
exactly being "triple tuned"; I think its the first RF amp, the
converter input stage and the first IF; but they may just be
trying to say that they have 3 IF amps; the standard. In any
event get this right: its still just a single conversion 455 khz
stage.  Something like an auto manufacturer telling us that a V8
has 16 valves -- it better have, or it won't work!

They are in effect  telling us and the AMA to put it where the
sun don't shine.  Now, I must also state that the new 1024
Futabas are dual conversion, but as far as I've read (between the
lines in this strange world of RC magazine publishing), they do
not perform absolutely bomb-proof up to AMA specs like Airtronics
and JR.  I don't like this kind of arrogance; if they're doing
this, I can imagine what else they're pulling in
non-contraversial areas.

I mentioned earlier that FM/PCM is the only system that will
survive in 1991 environment.  AM won't hack it, even dual
conversion.  Here is the reason.  We occupy the space between two
Public Service stations.  Now, this space is called a guard band.
This means that occasionally the PS stations will splash over in
their modulation into this band; that's what its there for.  I
have maintained from the start that they will splash over, very
frequently, because their business is coverage, and that gets you
coverage.  Please see my previous diatribes on my legal
speculations for this.

AM, because it detects the peaks and valleys will get this
splash, by definition, and will be blown away.  FM/PCM will not
because it doesn't look at peaks and valleys in the modulation. 
I dearly love ACE radios, but sorry about this, the 1991 Ace is
great right now, but it will not work in 1991 with adjacent PS
channels, dual conversion or not.

The best radios of today (those that have been blessed by the
AMA) will work under all imaginable environments of 1991.