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View Full Version : How far out to sea do FM and MW radio signals travel?



clacton0
2012-Mar-23, 03:46 PM
Hi
I have only ever been on a boat around the East Anglia Coast and got get plenty of signals here even when you cannot see any land.
I was getting stations from London a good 80 miles away very clear.
On the East coast you can always seem to pick up Holland and France to.
Am just wondering how far into the Atlantic Ocean and over places would these signals travel if you had a good decent set up on a boat?
From both the USA and UK?
Would they be any place in the Atlantic Ocean where you cannot pick up MW signal to?
Where I am the most powerful signals seem to be Radio 1 on 98.8 which can be picked up a good 100 miles away and French INFO 105.2 which I have heard in many places.

Thanks

Argos
2012-Mar-23, 04:00 PM
FM waves, unlike AM, can penetrate the ionosphere and get lost in space. Theyīre basically limited to line-of-sight range.

noncryptic
2012-Mar-24, 03:21 AM
Would they be any place in the Atlantic Ocean where you cannot pick up MW signal to?

Probably not. Medium Wave AM broadcast transmissions can routinely reach many thousands of km. Signals on Medium Wave frequency (around 1MHz) tend to follow the curvature of the Earth.


Where I am the most powerful signals seem to be Radio 1 on 98.8 which can be picked up a good 100 miles away and French INFO 105.2 which I have heard in many places.

Signals in that frequency range (around 100MHz) tend to go mostly straight over the horizon, reaching a few hundred km.
Atmospheric conditions can however be such that signals at those frequencies bounce between the Earth and ionized layers in the atmosphere and so cover larger distances, although that's much more typical for Short Wave frequencies (which that way can reach all the way around the globe).

danscope
2012-Mar-24, 04:09 AM
Some lower fequencies " Skip " off the ionosphere, offering and extended transmission . Think of it as a reflected wave
which continues to propogate within that channel . Of course height of a transmitter helps .. as well as transmitted power
. Many of us remember the old days of CB radio ( Around 27 Mhz ) . Late evening, we could work skip at far more distances than you'd expect with 6 watts transmitted power.

HenrikOlsen
2012-Mar-24, 08:06 AM
FM waves, unlike AM, can penetrate the ionosphere and get lost in space. Theyīre basically limited to line-of-sight range.
Erhm no.
It's not AM vs FM, it's the frequencies used for the two which makes the difference.
FM is always high frequency (short wave in old terms) which is why it isn't reflected well by the ionosphere.
AM can be long and medium wave (low to mid frequency) and can therefore be reflected.

Jeff Root
2012-Mar-24, 10:59 AM
It was clear to me that by "AM", Argos meant the band
from 535 kHz to 1605 kHz, and by "FM" he meant the
band from 88 MHz to 108 MHz, which appear to fit the
examples given in the original post.

-- Jeff, in Minneapolis

profloater
2012-Mar-24, 11:30 AM
The old line of sight thing is D in miles is approx sqrt (2h) h in feet so a 20 foot masthead antenna (sees) an horizon of about six miles, 30 feet about 8 miles so VHF is quite limited for a small boat. Therefore bouncing off the ionosphere is significantly useful.

Argos
2012-Mar-24, 03:56 PM
Erhm no.
It's not AM vs FM, it's the frequencies used for the two which makes the difference.
FM is always high frequency (short wave in old terms) which is why it isn't reflected well by the ionosphere.
AM can be long and medium wave (low to mid frequency) and can therefore be reflected.

[bold mine] Well thatīs what I was saying. Jeff Root got it right, fortunately. It seemed pretty obvious that people would understand I was talking about frequencies. It looks like you hold my intelligence in low regard. But thatīs alright. I still like you. :)

noncryptic
2012-Mar-24, 07:08 PM
Erhm no.
FM is always high frequency (short wave in old terms)

While we're getting down to details:

"Short Wave" = 3MHz to 30MHz

The ~100MHz band where FM is typically used for broadcast is well outside of that. 100MHz is within the "Very High Frequency" (VHF) band (30 to 300MHz).

There is no principal reason why Frequency Modulation can not be used at lower frequencies. In fact it is used in ~28MHz Citizens Band radio.
That's rather lower than commonly know FM broadcast - and those frequencies (~28MHz) do skip off of the ionosphere regardless of being Frequency Modulated.

Hornblower
2012-Mar-24, 08:48 PM
In analog TV broadcasting, the video was AM while the audio was FM. The "static" from lightning would disturb the picture without bothering the sound.

HenrikOlsen
2012-Mar-24, 10:09 PM
[bold mine] Well thatīs what I was saying. Jeff Root got it right, fortunately. It seemed pretty obvious that people would understand I was talking about frequencies. It looks like you hold my intelligence in low regard. But thatīs alright. I still like you. :)
I like you too and I'm not holding your intelligence in low regard, I'm sorry if my reply made it seem that way.
I was mainly thinking of the people who read the thread with no prior knowledge of radio, for whom your post could be read to mean it's the difference between AM and FM that causes the difference in reflection, rather than the difference in frequencies commonly used for the two.

ngc3314
2012-Mar-26, 12:33 PM
Relevant tidbit I picked up in a radio-astronomy project review - FM transmissions can occasionally reflect from transient ionized trails produced by meteors, and you have to be ~700 km or more from the transmitter to completely avoid this source of interference. Turned around, that suggests that one might very occasionally hear quick snippets of FM transmissions that far away. Low-frequency radio astronomers are developing an interest in certain small but quite isolated islands.

Extravoice
2012-Mar-26, 03:22 PM
Relevant tidbit I picked up in a radio-astronomy project review - FM transmissions can occasionally reflect from transient ionized trails produced by meteors...

There have been systems that use(d) meteor scatter to communicate bursty information. One is SNOTEL (http://en.wikipedia.org/wiki/SNOTEL).

IIRC, communication via any particular ionized trail severely limits the location of the transmit & receive stations. But for something like snowpack telmetry, you can sit around and wait for a properly aligned trail to appear.

Hornblower
2012-Mar-26, 06:09 PM
Intermediate range reception of ionospheric sky waves at night can be tricky. When I was in college, we could hear Philadelphia's WCAU-AM loud and clear in Charlottesville, about 240 miles from the transmitter. I had trouble receiving it in Washington, 100 miles closer. It was as if the reflection from the ionosphere did not come down at the steeper angle for the latter. Is this the meaning of the term skip in radio jargon?

Extravoice
2012-Mar-26, 09:55 PM
Yes. Years ago, I was involved in a program to model HF (3 to 30 MHz) radio propagation for a major military organization. It has been a long time, but as I recall, the ionosphere is quite variable, with several layers. Different frequencies will reflect off it at different angles at different times depending on several factors (that I can't remember).

Extravoice
2012-Mar-27, 03:16 PM
By the way, you can get multiple skips, where the radio waves bounce off the ionosphere and Earth's surface more than once. Of course the signal gets weaker with each reflection.
Using this technique, our local MARS (http://en.wikipedia.org/wiki/Military_Auxiliary_Radio_System) station in New Jersey was able to communicate with McMurdo Station...but only when conditions were correct.

thoth II
2012-Mar-27, 04:36 PM
I know people in the Florida Keys who had big TV antennaes on top of houses to get Miami TV stations about 150 miles line of sight; but the TV signals back then in the 1970s were very fuzzy, still they watched it for years until all the Keys got cable about 1982.