Jim,
It's not the modulation scheme that causes "skip". That's propagation off of ionosphere which is caused by what frequency band you're in. Depending on whether it's day or night, skip will be different. HF is great at bouncing, of skipping, off of the ionosphere to give it distance and make it a good long-haul band. The higher you go in frequency, the more likely it is to go through the ionoshphere and not bounce back to earth. Typically, FM broadcasts are done in the VHF band, which does not skip well and is good for a more line-of-sight type reception only (typically out to about 40-50 miles). Also, AM broadcasts are typically done in the MF band and although not as good as HF for skip, will skip more at night (when the e and f layer of the ionosphere merge), but typically follow the curve of the earth better. Higher line of sight frequencies will multi-path or bounce off of buildings, causing delays in multiple signals to be recieved by the receiver. Sometimes you'll get what seems like skip out of VHF and UHF, but what that is is called "ducting". This is a phenomenum in the atmoshere that creates a virtual waveguide that the signal will travel in and causes it to travel extraordinary distances. Down off the coast of Florida and in the Middle East ducting is fairly common and it it's not uncommon for you to hear a UHF channel travel 150 miles, when typically UHF is no more than, based on antenna height, 15-20 miles.
cheers [smiley=xyxthumbs.gif],
Red
I know all of that, Red. However, you're missing the point - why is there no
27MHz FM skip? I promise you, it is different! You are giving all the correct info for the difference between the normal uses of AM in the HF band, as opposed FM in the VHF band, but you're missing the big picture - we are talking about using FM in the 11m band, NOT in VHF or UHF. When did you last encounter skip from an FM CB transmission? You didn't! Ducting, very very occasionally, yes, but not skip, ever! I'm sure back in the day you heard lots of skip from Europe, when all was AM. Now it's all FM, and on the exact same channel frequencies as the US - do you hear any? Nope. And that's not just because of lows in susnspot activity. FM in the HF band does not skip, and along with that, it does not perform like AM either. It is much more line of sight than AM at the same frequency, and is thus more affected by obstacles and terrain than AM. I worked with both 27MHz AM and FM for years, honest!
Jim
Jim
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Topic: CB question (Read 2886 times)