Conference vmszoo::rc

    

>    (i)   What's the difference between AM and FM?
 
	AM and FM are two methods of altering the characteristics
	of the RF carrier with the signal you wish to transmit. The
	carrier is the frequency you tune too, in our case 35.something
	Mghz
	
	Amplitude Modulation (AM), you vary the amplitude, or peak
	voltage of the carrier proportional to the instantaneous voltage
	of the signal you wish to transmit.

	Frequency Modulation (FM) you vary the frequency of the
	carrier.


   (ii)  Which is best (I've heard it said AM is best but FM was the
          "bees knees" some years back)?
    
        AM is simpler to encode/decode.. It requires very little bandwidth
	(2 x the highest signal frequency).. It is rather more susceptable
	to noise (interference)...

	In general noise will ba additive to the carrier, ie modify the
	amplitude of the carrier. Since with AM the amplitude represents
	our original signal, the noise will be decoded along with the
	signal......

	With FM we are not interested in the amplitude of the carrier, only
	the change in frequency.. so (pratically) it doesn't matter how
	badly the noise affects the carrier amplitude..... There are a lot
	more harmonics involved with FM, ie it needs a very wide bandwidth...

	This is why FM radio sound a lot better than AM radio, and the stations
	are much farther apart... FMs the way to go. I doubt you'd find
	a 35mhz AM set in the UK, maybe Acoms make one..

	It's much easier to explain with pictures, very simple really
	
>	(iii) Do I really need dual rates?

 	I'm a great fan of dual rates... for example you can mechanically
	set the controls up to give lot's of movement, ie aerobatic, and
	use the rates to make it more docile. As you get used to the plane
	you can make it as aerobatic as you like...


>    (iv)  What's the advantage of "ball raced" servos?
 
	There's less play in the output shaft, and they las a lot longer.
	Of course as the shaft wears, the resolution goes...
	You could wear out a set of non-balraced servos in a couple of
	weeks in a helicopter.

>   (v)   What other things should I be considering (I've decided I
          need at least five channels and 3 servos to start with - I've
          got my "eye" on Futaba)?
    
 	The Challenger will suit you down to the ground (x'cuse the pun)
	I use one for my planes, and I've had no problems. I've got a 
	lot of confidence in this set, and that is important...   

The dollar/pound rate is pretty favourable at present and if I decided
    on a personal import, would that be sensible? We use 35MHz here in the
    UK and of course I'd need to be able to get it serviced in the UK too.
    Anyone got any experience in this area? 
    

	I doubt if that's feasible for a basic set like the Challenger...
	The more expensive pcm sets usually have a plug in RF section, so
	it would be an easy matter to convert to 37mhz..

	It sure makes your eyes water to se the prices of model stuff
	in the US.. I want a Schluter Champion helicopter.. over here
	they are 450 pounds, in the us they are 450 dollars, at 1.6/1
	pound that is cheap...

	I also want a 120 4 stroke. If I can't get the laser twin into
	the Jungmann's cowl I'll go for an enya 120R.. 250 pounds over
	here, 250ish dollars........  

	cheers

	bob

    Bob,
    
    Thanks for the input. Do I understand right when I say that 35MHz
    is not used "over the pond"? Thing is, I have a couple of firends
    in the 'States who've offered to ship the gear to me, not to mention
    the number of chaps here who visit quite regularly. I'm more than
    happy to pay DRAT on the excess of $35.00 it'll cost as at
    1.6-1.7/�1.00 it's still worth it provided it's the same as sells
    here.
    
    At least I understand "wobbled carriers" now and why one's better
    than the other.
    
    Keith.
    
    P.S. If you want me to arrange a personal import for you....maybe
    it'll reduce the beer I owe you!!

    Keith,
    In the US, 35Mhz is not available for RC use.  If you could contact
    the manufacturers reps in England for Futaba or Apex (JR in England)
    and find out if and how much it would cost to convert from 72Mhz,
    it might be an alternative, but it sounds like it would be more
    expensive than buying English.  
    
    Ken

    I know that Airtronics offers a free conversion to 53 MHz for their
    CS7P-AM radio if you send them a 72MHz radio that is brand new.
    They may very well do the same thing for other bands if they support
    those bands. If Airtronics radios are used in England on 35 MHz,
    you have a good bet they'll do the modification. You can call them
    in California, their number is listed in any of their ads in the
    magazines.
    
    Dave

    
    I disagree with the FM recommendation, mainly because they didn't
    do a very good job with the receiver design.  If you have to fly
    at a field that also has AM sets you might have problems.  I don't
    know what most 35MHz systems use, but I know there's a lot of
    discussion in an earlier note about AM/FM/PCM(FM).  I suggest you
    look back and try to find it.
    
    Bill

    I have seen several severe cases of interference with FM sets at
    our field.  Noone that I know would buy or use a single conversion
    FM set here.  The new dual conversion stuff is all FM and it is
    supposed to be much more selective.  Until all of that gets sorted
    out, I would recommend an AM set.
    

    Thanks for the input guys.
    
    The only small problem is that if no-one sells AM sets here in the
    UK as Bob indicated.......maybe that makes FM ok?
    
    I'll continue looking here and keep you posted.

    Yes, I've heard that even the old FM sets do very well in an FM-only
    environment.
    
    Bill
    

The problem with the FM is that it is not as resistant to a badly
tuned adjacent transmitter as AM is.  In an FM only environment,
or with 1991, the problem seems to go away.  As a plus, the FM is
more resistant to other types of interference, such as
atmospheric noise, or a distant adjacent transmitter. 

    There are six flyers in my club that are strictly running FM
    systems that meet the 1991 criteria. None of us have experienced
    any radio interference.
    
    						Tom

Tom, just offhand, what kinds of radios are they using?  Are
there any 1991 AM radios in the pack.

I was browsing through the new Tower flyer today over lunch.
Interesting that they don't mention 1991 anywhere...they seem to
carefully avoid the subject of the superiority of one type radio
over another.  Even in the PCM ads they don't mention the
advantages of that system.  

    
    	We are all running JR Century VII radios. JR has this et up
    they call ABC&W. I'll try and find an add and copy the description
    of what this ABC&W means. They say clearly in their adds that they
    are 1991.
    
    	I just sent for my chopper radio last night. One thing to note
    here is that I have found that it's rather difficult in the set
    of an airplane radio for use on a chopper yet not so difficult
    to set up a chopper radio for a plane.  If you think that you may
    be interested in chopper but want to start with planes then you
    may want to buy a chopper radio. Turn off all the bells,whistles
    and mixing and use the radio as a std. 4 channel airplane radio.
    
    
    						Tom

	re .6

		AM is so much better than FM that it's not
	generally available in the UK.... say no more......
        I think FM Rxs are all dual conversion over here...

	re .14

		On a Heli set two independent channels, ie
	throttle and collective are controlled by one stick. Even
	with all the magic mixers I doubt that a fixed wing set will
	do this and still leave the aileron and elevators to function
	normally...

	bob
		
	

    
    I didn't say AM was better than FM in an FM-only environment.
    You're fortunate that you have an FM-only environment in the UK.
    But be careful about using them near AM systems if the situation
    does happen to come up.  Sounds like your best bet is FM because
    you might give your FM buddies trouble with an AM set.
    
    Bill

    Thanks once again for all the info. Looks like I'll stick to FM
    - don't want a bunch of angry modellers on my tail, do I?!?!
    
    Keith.