Frequency vs Mode

[C W Morse]

I was made aware of a thead on another board (that I cannot access) by another ham. The gist of the thread is the use of FM for chasing DX (or "shootin' 'skip'" as is the CB term) as opposed to AM. Certain of the fellows flatly state, according to the ham that told me about it, that FM does NOT "skip". :drool: They sincerely believe this, and the thread apparently *almost* deteriorated into an argument. Actually, I've seen this bit of cludge on other CB forums. The truth IS................., it has NOTHING whatsoever to DO with FM, AM, CW, or SSTV. It is the FREQUENCY of operation that determines, in the main, the ability of a signal to travel thru the ether RATHER than the MODE. Frequency Modulation is simply a MODE of transporting intelligence (voice, data, etc)! Generally, and to keep it simple, the LOWER the FREQUENCY, the LONGER the distance. The HIGHER we go in FREQUENCY, the less likely our signal to "skip" (and we who use the higher bands generally do not use the work "skip" to describe the phenomena of long-distance radio signals). IF you use AM on 27 MHZ and the station you are talking to is 120 miles away, AND you switch to FM, you should be able to STILL converse and, actually, the QUALITY of the FM transmission should be better (generally).

Where the people become confused is, they associate FM with FM broadcast AND the police transmissions they hear on their scanners and it doesn't GO as far. The reason is NOT the "FM", but the HIGHER FREQUENCY of operation that is causing this! It is understandable since those who only operate ONE band, namely 11 Meters, have no experience with the behavior of other bands and frequencies. They only have that ONE realm of "expertise"! Of course, FM is ILLEGAL on CB!


For those who absolutely insist (like the MYTH that one MUST use 18 feet of coax cable ) that FM will not "skip", I invite you to listen to 29.620 FM (and others) on the 10 Meter band when conditions are favorable, and the repeaters are running. Today, I was listening (South Carolina) to the W1OJ repeater in Boston, Mass. There were stations in SC, VA, CT, TN, and GA---ALL, of course, using FM and very understandable. If FM doesn't "skip" and the MODE was the determining factor instead of the FREQUENCY, we couldn't hear Boston from SC!!!!!

CWM

 

[Justme]

H.F signals on the same frequency behave the same, beeing it in C.W., SSB, AM or FM

In Europe FM on CB is the mode to work, besides SSB, some sporadic E and you can hear the FM modulated carriers come in from all over Europe here, since most EEC counties adapted all mode 40 channels here.

One thing you learn when going for an license here.

Even better, worked all Europe here on 2/70 Fm, up to 1500 miles, SSB is preferred, but i had lots of fun when the bands opened and people were unaware of it, till i came in with an FM signal in an local qso from an 1000 miles away

 

[dxhound]

True!!!! FM can be used to make dx contacts. BUT, there are always exceptions!!! We all know that FM uses slight Frequency variations to tx the "intelligence". Well those variations (however so slight) may be just enough that the atmosphere only allow a few "segments" of the signal to reach the same destination. That would be just enough to obscure the "intelligence" and make it useless to the FM discriminator in a radio, which would sound like a garbled mess to the human ear. A prime example of this happening for AM would be when your working DX off the Northern Lights and here a wavering echo in the rxed signal. The reason is that the signal is scattered all over and your rxing the "intelligence" at different times. This works for AM because the frequency does not change. But in FM mode the frequency deviation and time of sequence is crucial to the FM discriminator being able to properly decode the information for a usable transmission.
Im sure this will spark much controversy but it should prove to be an interesting topic.

 

[Justme]

Working Aurora SSB (CW) is the only modulation possible.

Even then in SSB your modulation sounds very strange and the same for the other party, done that more then an few times.

The only DX i see the need for an rogerbeep (K) on 2/70.
The recieved sound is very distorted, if you hear it for the first time you really have no clue what the h*ll you hear.

It takes some experience to get an feeling for it, but i develloped that, and have an ball if it happens, putting my beam up north and work stations while the antenna is in an totally different direction as they are, pointing it to the northern light for the strongest signal possible.

The whispering sound is very distinct, when you heard it once, you'll never forget it.
Speak slow, very clear pronounciated, and have fun.

Normally working Sporadic E in FM or Ducting sometimes multiple hops, the audio can be an bit distorted, but not to the extend as Aurora.

Working contrails and bouncing off planes i worked an few stations too, HB (switserland, Italy, in short bursts of 20 seconds can be fun.

After 31 years i saw most of it, done it, and still have fun when people tell me there are no DX possebilletiess and i just worked an contact on 2 meter of an 1000 miles...

p.s.

The best filters are in your ear, when you learn to llisten in difficult signal circumstances, it is DSP that does some good tricks, but trained ears beat the heck out of any DSP filtering, use headphones, an new world will open after you train your hearing.

 

[W5LZ]

I agree with most of what's been said here. Can't say that I agree with everything said, because I have no direct experience with some of the things said (very little opportunity to work any aurora 'skip' for a number of reasons, for example).
I'm also sort of 'leery' about 'absolutes'. Some times they just are not as 'absolute' as you/I might think. The only really absolute 'absolute' I can think of is that if you ain't got Kenwood, you got squat. (Don't bother disagreeing, that's one of them 'absolutes', you know.)
In general, I do agree with most of what's been said. Propagation is a frequency thing, mainly, not the type of mode used. The mode can certainly make a difference, and the simpler the mode the better chance of a successful contact, as far as propagation goes. Simple as in the 'complexity'/variations in the mode. The more 'complicated', the more variations in that signal, the greater the chance of missing part of it for whatever reason. Does that mean that a 'complicated' type mode can't be used for working 'skip'? Nope, just means that it doesn't really have the advantages of the simpler modes.
- 'Doc


(I'm amazed! Said all of that before my first cup of coffee.)

 

[SR385]

No FM skip??

I have a QSL on my desk somewhere from Cape Ann MA from central NJ. 2m simplex FM

That certainly wasn't ground wave. I have a decent station, but it isn't THAT good ;-).

 

[Captain Kilowatt]

A prime example of this happening for AM would be when your working DX off the Northern Lights and here a wavering echo in the rxed signal. The reason is that the signal is scattered all over and your rxing the "intelligence" at different times. This works for AM because the frequency does not change. But in FM mode the frequency deviation and time of sequence is crucial to the FM discriminator being able to properly decode the information for a usable transmission.

I believe you mean "aurora" when you say "working DX off the Northern Lights".Kind of the same but differant. AM does NOT work well for aurora DX as the bandwidth is too wide,not as wide as typical FM, but still too wide. In fact SSB is most often too wide for good aurora contacts. Even CW will sound raspy, very much like an old spark transmitter.I'm not saying AM will not work but it certainly has a very low sucess rate compared to the narrower bandwidth modes.The problem is due to phase distortion of the sideband components (not SSB but rather occupied bandwidth of any mode) arriving out of phase.I have worked lots of aurora DX on 6m and sometimes the SSB signals are barely intellegible even though the signals are very strong.

 

[Captain Kilowatt]

No FM skip??

I have a QSL on my desk somewhere from Cape Ann MA from central NJ. 2m simplex FM

That certainly wasn't ground wave. I have a decent station, but it isn't THAT good ;-).

VHF, and in particular the FM broadcast band, has DX possibilities due to several reasons. Sporadic E,tropospheric ducting, and rarely F layer propagation is all possible on the FM broadcast band. The best I have heard myself was many years ago and was a very strong signal from WOKI in Knoxville Tennessee. From here that is 1200 miles.I have heard recordings made in Ireland of the CBC FM transmitter in Sydney Nova Scotia at a distance of 2300 miles. Hams in California often work 2m into Hawaii from their mobiles when the Pacific Duct opens up.That is a good hop at nearly 2500 miles.Remember however that the MODE has NOTHING to do with it. It is the FREQUENCY that is the determining factor on whether DX will happen or not.

 

[dudmuck]

Sure its possible to chase DX using FM, but its not going to be effective as the more narrow-band modes. I would only try it if the band is really hot, like in a year or two when the sunspots start.

It has been said that narrow-band modes like CW or PSK31 have as much as 16dB gain over SSB when the appropriate filter is used for the mode. Receiving CW/PSK31 type of signal, its best to have a filter less the 100Hz wide, where with SSB about 2.7KHz-wide filter is usually best. With AM and FM, a 6KHz filter is needed which lets in more noise, and I mean FM with a modulation index of 1. On two meters, i think filters are double that maybe 15KHz wide.

The way skip fades signals has a worse impact on FM, because the gain & phase flatness is randomly impaired after bouncing off the ionosphere.

It all doesnt make shooting skip on FM impossible. It just requires better conditions compared to more narrow modes.

Direct ground-wave communication, AM & FM should perform equally well because there is no fading. FM has the advantage for audio quality because the deadkey power is 100% as much as when there is full modulation.

And as far as FM being illegal on CB, its the only legal mode in UK.
Britain outlaws AM CB on US channels. british CB is 27.6 to 27.99 using FM only.