GeorgiaBoy

I have been around RC awhile and need to know the "plus's & minus's " regarding radio equipment that are "FM and AM". Can you have a radio on channel 48 AM and a radio on channel 48 FM? Can you operate both of these radio's at the same time? Is there an advantage of FM vs AM or AM us FM?
Thanks for your help....GeorgiaBoy

00hex

You cannot have two radios transmitting on the same frequency. It doesn't matter what modulation they are using. I suspect that the FM receiver will be a little more tolerant of the interference from the AM signal than the other way around, but it still won't work.

Unfortunately there are no spread spectrum R/C radios yet [img]i/expressions/face-icon-small-wink.gif[/img]

FHHuber

FM, maintains a fairly constant wave amplitude (power of the signal), slightly varying the wavelength, centered on the designated frequency. The constant high transmitted output power helps reduce interference from corrupting the signal sensed by the reciever.

AM varies the signal strength, maintaining the same wavelength.

The signals can easilly interfere with each other, If the transmitters are in close enough proximity to each other. The FCC regs on power output for R/C systems is the same for AM and FM on the frequencies we use. (except the "toy" bands... with FAR lower allowed power) so the max amplitude of the AM is the same as for the FM.

FM is less suceptible to "3IM" interference than AM. (2 transmitters on separate frequencies, interfereing with a 3rd frequency) Modern "narrow band" systems are all good at 3IM rejection.

Generally, modern radios will be FM, or VERY low end units. I don't know of any 4 channel or more systems currently made that are AM.

Note in your car radio the difference in clarity of sound with the AM and FM stations, especially at night in some areas. (I can't listen to AM at night around here... too much interference. AM R/C systems seem to have low effective range around here.)

GeorgiaBoy

Thanks all for the info.....life is good. [img]i/expressions/face-icon-small-smile.gif[/img]

Ladyflyer

My Feedback: (2)

Your servos don't "hear" the information the same way your ears hear the audio. If you look at the demodulated signals going to the servos from both AM and FM radios in a thuderstorm you will have a hard time telling them apart,if at all.
It is often assumed FM is less prone to interference but does not always show true. FM receivers do tend to lock on to the strongest signal which is usually a "ggod thing" . At times the strongest signal is actually a reflected signal . It may not seem logical but the directional qualities of the antennae we use will have the plane fly thru a null or virtual hole in our direct signal path. As we fly thru the null a reflected signal will get the attention of the receiver. The receiver may miss a reset pulse or two in that time which causes an apparent "hit".
An FM receiver will take a bit longer to recover from the hit while the receiver is capturing the signal again ,AM receivers recover nearly immediately. You may also notice the FM receiver in your car drops out when you stop the car in a certain spot,then when you move the car a foot or so for or aft the station returns strong and clear. That is multipath where a pair of signals slightly out of phase make the reciver innefective.

There are a lot of people flying AM gear today that are very content and confident in the reliability of the equipment. There is no need to sell it short .The differences are very overstated and they are really on equal footing .

Whatever you wind up with enjoy it.

blueangel

I know FM is supposed to be less likely interfered with. AM isn't supposed to be as reliable. It was told to me whatever you do, do not get an AM radio! I have only had FM radios, and they have never been interfered with.

Ladyflyer

My Feedback: (2)

Blue , you were given poor advice. There are many people still using AM gear with excellent results,myself included. I have Kraft and Proline AM gear that have been trouble free for over 30 years. They all get regular maintenance and are every bit as solid and reliable as the new stuff.

http://www.rcgroups.com/forums/showt...threadid=70768

HGU 34

Your best advice is to ask around and see if there are particular frequncies in your area that are prone to interference. I would not worry too much about AM versus FM. I have both, use both and have no real preference for one or the other.

FHHuber

If AM is so good...

Why bother with FM for ANY R/C radio? (the RX is harder to make, and heavier.)
Why aren't there a bunch of modern AM 6 ch computer radios?
Why would FM command a higher price?
Why are Futaba and HiTech moving toward making even thier 3 channel systems FM now? (and rumors of the 2 channel systems going to FM too...)

blueangel

There must be a reason all the companies are switching to FM. Maybe they found a problem with the AM. Or they make more money from FM so they want it all to be FM. Just my 0.02 cents.

Ladyflyer

My Feedback: (2)

Rest assured there is no functional problem with AM that made the companys switch. There are however integrated circuits designed for FM broadcast radios that also find application in RC receivers . The shared components actually makes FM receivers CHEAPER to build than AM receivers. FM receivers are nothing exotic or difficult to build. FM doesn't command a higher price. A good AM RX will cost the same as or more than the FM counterpart. The manufacturers run by the "Economy of Scale "principle which basically says the more units you make,the less each unit costs. If they make only FM receivers and transmitters you cannot opt for AM ,therefore you buy all FM and the cost per unit is diminished. Simple economics. Not that FM is an overall superior system. We all know the Beta vs VHS saga. The best system is not always the system that prevails.
Too much adoo over too little. Both modes work equally well .

Rodney

Congradulations to Lady Flyer, she is right on in her comments. Incidently, the reason the old AM units suffered from 3IM (third order intermodulation distortion ) was that they mostly used single conversion. Those made with double conversion did not have that problem.

3dd

My Feedback: (6)

am is not legal in the us anymore

Ladyflyer

My Feedback: (2)

Says who ?

big al

Hi Ladyflyer, As far as I know AM is legal and I have sent an email to Futuba.com requesting information regarding this matter. I will let you all know the results. I have an old FP-T4NBL that I intend on reusing in the near future. It is gold stickered and probably a single conversion receiver (FP-R 114 H). If anyone knows please let me know. Thanks - Big Al

hpapilot

My Feedback: (1)

As has been said in many threads before, AM IS CERTAINLY LEGAL. Many companies do still make AM radios, and trust me, they wouldm't if they were illegal.

I am using an Futaba AM FG 7 channel, narrowbanded. It works fine for me.

copilot2

My Feedback: (74)

www.futaba.com. commonly asked questions about narrow band etc. Make a print out of this information found at thier web site. All narrow band recievers made by Futaba are also listed. This will remove any doubt as to what questions you have in this regard. Do it right now.