Fm vs Am

[weatherman49]

Well call me stupid if ya want but I have a Question or 2 about radios with FM modes!

Correct me if I'm wrong! (FM) Frequency Modulated signal (AM) Amplitude Modulated signal

Now for the Questions!

1 all the 10m radios have fm modes but you never hear any one mention using them Why not?

2 from what I understand you have to have a higher power level to broadcast an fm signal the same distance as a lower power am signal ?

3 will a am signal be heard on fm if on the same freq?

now with those out of the way my thought was this if you gave the 40ch 11m radios fm mode that would almost be like giving them an extra 40 channels and that would be cool as ice!

Or is there an allocation already in exsistance for 11m fm I looked but could not find anything?

 

[Moleculo]

1. Because in the U.S., everyone just uses those radios for CB anyway, which is SSB and AM

2. Not accurate at all. However, FM is much less efficient (uses more bandwidth) so it tends to propogate less effectively than AM.

3. Yes, but garbled. It will sound like interference

In Europe, CB is FM in most countries, but you can't have two different operating modes going on the same frequency....it would just be too much interference to each other.

 

[Beetle]

Correct! FM=Frequency Modulation; AM=Amplitude Modulation.

1. At the high end of the ten meter ham band, there's an area on the bandplan specifically earmarked for FM. The reason FM isn't bantered around more regarding these "radios" is that most of the folks who use them are CBers, and FM is illegal on 11 Meter CB. Period. AM and SSB only.

2. Not true. Under the same conditions (same power, same antenna, same frequency and so forth), a radio signal will propagate the same way.

3. You might hear an FM signal with an AM receiver, or an AM signal with an FM receiver. Whether you'd be able to UNDERSTAND the transmission would depend on a few things.

4. It would NOT be a good thing to use FM on the 40 channels that CBers have available to them. FM signals are wider (because it's the FREQUENCY that's changing (and when a female is transmitting, her higher voice makes the signal even wider).

 

[weatherman49]

2. Not true. Under the same conditions (same power, same antenna, same frequency and so forth), a radio signal will propagate the same way..


Then why don't you here fm skip from a broadcasting station with 100,000 watts Say out of Washington but on the AM side sometimes youll pull a distant station is this because of the freq fm broadcast runs on?

3. You might hear an FM signal with an AM receiver, or an AM signal with an FM receiver. Whether you'd be able to UNDERSTAND the transmission would depend on a few things

LIKE?


not trying to be a bhole just trying to learn!

 

[FL Native]

ask someone near with FM to switch to FM on the same channel/frequancy your on and you will then understand.

or even have them switch to SSB. different sound but the same idea.

 

[bob85]

yes you can receive perfectly readable fm transmissions on most am receivers by slope detecting in other words offset your tuning so the fm station is on the slope of your filter which will give the detector the changing amplitude it needs to resolve audio as the fm station deviates across the slope of your filter,

 

[C W Morse]

2. Not true. Under the same conditions (same power, same antenna, same frequency and so forth), a radio signal will propagate the same way..


Then why don't you here fm skip from a broadcasting station with 100,000 watts Say out of Washington but on the AM side sometimes youll pull a distant station is this because of the freq fm broadcast runs on? (quote).

The reason for that is a function of the frequency , not the mode of operation. The AM stations operate on the 500 to 1600 KHZ BANDS, the FM stations are on 88-108 MHZ bands. The lower frequency 'skips' as CB folks say and the higher frequency generally does not (usually). In fact, this is one of the reasons that FM is generally kept up on the higher ends of the spectrum; to keep the larger bandwidth from creating unacceptable interference to other stations. 8)




not trying to be a bhole just trying to learn!

CWM[/i]

 

[Beetle]

Weatherman - I think your questions have been answered - I just now got in from dinner and a couple of meetings so I didn't get a chance to reply sooner.

As far as slope detection of FM on an AM receiver, it does work with some receiver circuitry, but not with all, at least in my experience. The higher the frequency the better the results.

 

[weatherman49]

Hey guys thanks for all the replies I am just looking into different ways to keep the peace between the hams and the cb folks maybe someday I,ll come up with somthin! Until then if ya look south and see alot of smoke thats probably my brain or my radio one or the other!

Then again it may be both at the same time!!! :shock: :shock:

 

[Moleculo]

Under the same conditions (same power, same antenna, same frequency and so forth), a radio signal will propagate the same way.

I didn't say it didn't propogate the same way...I said it's not as effective. There's a big difference. The less bandwidth you require, the better you chances for having a "readable" transmission, especially under poor conditions.

 

[weatherman49]

.
...I said it's not as effective.

Wait a minute! Is that not what the FCC wants cb to be SHORT RANGE? if they went to fm would that not cut back on some of the crap from 900 miles away? IE I'm an Arkansas and I turn on my radio to talk to a friend maybe 15 miles away and all I can here is 509 in the sandpile calling for Mr. big radio in Washington State!

my whole idea was that if they made an fm mode regular 40 ch radio that if skip was real bad on the am ch 20 then switch to fm viola no crap from skip I could talk to my buds and not take up the ham bands and that would make alot of people happy! or not!

 

[Beetle]

Moleculo - my comment wasn't directed at you, but at Weatherman's initial post. You posted while I was still typing.

 

[Moleculo]

haha...yeah i know how that happens Beetle!

 

[N8YX]

As far as slope detection of FM on an AM receiver, it does work with some receiver circuitry, but not with all, at least in my experience. The higher the frequency the better the results.

In actuality, the wider the receiver's IF bandwidth the better this technique works.

If you try to slope-tune an FM signal with an AM rig whose IF chain has a filter narrower than ~6khz, you're not going to effectively recover the full audio spectrum.

One of my R7s has the following filter complement: 8, 6, 2.3, .5, .3 khz. I can successfully slope-tune an FM signal with either of the two wide filters engaged.

Contrast that with the other R7, whose filter complement is as follows: 4, 2.3, 1.8, .5, .3 khz. Even with the widest filter engaged, that receiver won't recover enough audio bandwidth to make the slope-tuned FM signal bearable for listening purposes...one can pick out bits of the conversation but you would not want to listen to it for a protracted period of time.

 

[KD7UGY]

I forgot how it works but FM isn't supposed to be used below somewhere around 29mhz because of it's wide bandwidth. Maybe someone else will know the exact frequency.