seb146 From United States of America, joined Nov 1999, 11068 posts, RR: 16 Posted (3 years 6 months 1 week 5 days 13 hours ago) and read 1791 times:
The sun affects layers of Earth's atmosphere. That means AM signals travel farther at night with less resistance in the atmosphere. That is the basic way I understand it. How does it work at the poles? With the sun not shining for weeks at a time, are signals able to reach farther and farther during the winter? If there were a 5000 watt AM station in Barrow, Alaska, would it be heard easily in Norway later in the winter?
LAXintl From United States of America, joined May 2000, 24124 posts, RR: 50
Reply 1, posted (3 years 6 months 1 week 5 days 13 hours ago) and read 1761 times:
You might want to read up on FCC licencing rules. First there would be no 5,000 watt station authorized in a local small market like Barrow anyhow. Local markets are limited to a maximum transmitter power of 1000 watts with use of a non-directional antennas.
fca767 From United Kingdom, joined Nov 2006, 1724 posts, RR: 0
Reply 2, posted (3 years 6 months 1 week 5 days 13 hours ago) and read 1759 times:
Quoting LAXintl (Reply 1): You might want to read up on FCC licencing rules. First there would be no 5,000 watt station authorized in a local small market like Barrow anyhow. Local markets are limited to a maximum transmitter power of 1000 watts with use of a non-directional antennas.
I was reading up on how radio waves are made...It's pretty mad trying to understand from most sources.
I now understand that it's just like how we speak, but just goes straight to electrical waves with not too much messing around.
MD11Engineer From Germany, joined Oct 2003, 13723 posts, RR: 63
Reply 3, posted (3 years 6 months 1 week 5 days 11 hours ago) and read 1709 times:
Radio wave propogation dependeds on the wavelength and the conditions of the upper atmosphere, especially the level of ionisation of several layers, which is in trun depending on solar activty and time of day.
AM, FM or SSB etc. are only modes of modulation, this means how the "payload" signal is attached to the radio wave and have nothing to do with propogation (though in practical broadcasting AM is mostly used for lower frequencies up to about 30 MHz, while FM is used for higher frequencies).
Solar activity through hard radiation causes air molecules an d atoms in the higher atmosphere to become ionised, they lose electrons and turn into seperate positively charged rump ions and free electrons.
This makes these atmospheric layers (within limits) electrically conductive. The density of these charges determines the ability of the layers to either reflect or to swallow radio waves of a given frequency (wavelength).
Therefore there also exist differences between night and day.
At the lower end, with long and medium waves (up to about 3MHz), the radio waves largely follow the ground. During daytime they get attenuated quickly by the socalled D-layer , while at night or at times of low solar activity, due to less ionisation, they tend to have a longer range.
When we reach the HF region (3MHz to 30MHz), there exist large differences in how far the radio waves interact with the atmosphere. Up to about 10MHz the radio waves get easily swallowed by the D-layer, but also get reflected by the higher F-layer, so that they can bounce back to earth, covering larger distances. The best propogation for these waves is during nighttime, e.g. in the ham radio 40 meter (7Mhz) band, you can have intercontinental range at night, but only a few hundred km during daytime.
Once you get to higher frequencuies, the ability of the D-layer to absorb radio wave energy decreases, while the higher the F-layer is ionised, the better it reflects the radio waves. Often they get bounced back several times between ground and F-layer to cover large distances.
But there is a limit to this: Normally the higher the ion density in the F-layer the higher frequencies get reflected, but above about 30MHz even the maximum density isn´t high enough to reflect the radio waves. Instead they go straight into outer spaces. Therefore frequencies above 30MHz (VHF, UHF and higher) are usually line of sight only.
There is one exception: During summers on days with lots of sun a third ionised, regionally localised, layer can form below the D-layer, which can reflect high HF and higher frequencies. This situation is called "sporadic E layer reflection" and will only last from a few minutes to a few hours. Suddenly a FM in the VHF band station, which normally only has a range of 100 miles, can be heard thousand miles away.
seb146 From United States of America, joined Nov 1999, 11068 posts, RR: 16
Reply 4, posted (3 years 6 months 1 week 5 days 8 hours ago) and read 1656 times:
Quoting MD11Engineer (Reply 3): This situation is called "sporadic E layer reflection" and will only last from a few minutes to a few hours. Suddenly a FM in the VHF band station, which normally only has a range of 100 miles, can be heard thousand miles away.
I knew this sort of thing was possible. I have been in Pendleton Oregon during severe thunder storms and heard stations from Minnesota and Wisconsin for about 30 minutes on FM. Wild.
Quoting LAXintl (Reply 1): First there would be no 5,000 watt station authorized in a local small market like Barrow anyhow. Local markets are limited to a maximum transmitter power of 1000 watts with use of a non-directional antennas.
I am just wondering if it is possible. I knew there was a power limit, I just couldn't remember what the limit was. As I read the responses and think about my own experiences on the West Coast in winter, I would say long distance stations across the poles would be possible. I have heard "mid-level" stations from San Diego and Phoenix in Washington state early on winter mornings.
iakobos From Belgium, joined Aug 2003, 3312 posts, RR: 36
Reply 5, posted (3 years 6 months 1 week 5 days 7 hours ago) and read 1653 times:
Quoting seb146 (Thread starter): The sun affects layers of Earth's atmosphere. That means AM signals travel farther at night with less resistance in the atmosphere.
Electric fields propagation is a very intricate bunch of phenomena.
The part of the atmosphere which plays a major role is the ionosphere, thereby the name 'ionospheric layers'.
Layers in the plural because there are three of them, called D, E and F (which further splits into F1 and F2)
D, the lowest one (60-90km), only present during daytime, is the one that affects the spectrum of 'low frequencies' where we find MW medium wave AM broadcasts (500-1700KHz).
It is true to say (MD11) that those 'medium' waves tend to follow the curvature of the earth, the ones going upwards are erased by strong attenuation in the D layer.
D disappears at dusk and the MW are then, in addition to the "ground wave", reflected by E or F depending on the circumstances (the "sky waves"). Very often a receiver will simultaneously catch several waves, the resultant being either addition or substraction of the signals, which leads to the effect called fading.
Quoting seb146 (Thread starter): With the sun not shining for weeks at a time, are signals able to reach farther and farther during the winter? If there were a 5000 watt AM station in Barrow, Alaska, would it be heard easily in Norway later in the winter?
Not shining..., well we are talking about ionized layers at altitude (300 to 800km).
I am no specialist of high northern latitudes but my guess would be "it could be heard".
MD11Engineer From Germany, joined Oct 2003, 13723 posts, RR: 63
Reply 6, posted (3 years 6 months 1 week 5 days 7 hours ago) and read 1643 times:
You don´t even need big power for longrange HF contacts (human or natural interference excluded). I talked to a guy in Baltimore, Maryland using only 100 Watts and a mediocre antenna. Obviously around the poles the earth magnetic field distorts the normal conductive layers, but, since the charged particles from the solar wind get pulled towards the poles and concentrated there (see polar lights), the effect must even be stronger.
For us hams, who are limited in transmission power by law (e.g. for HF 2 KW in the US, 750 W in Germany or even lower in many other places) the biggest problem is human interference. E.g. I can hear hamsd from the US or Japan very well, but my 100 Watts simply get shouted over by the big guns with their power amplifiers (though I bought a 500W amplifier two weeks ago and am just fixing up my antenna so that I can use it. I will also get a commercial East German made HF transmitter as used for ATC or merchant marine ground and coastal stations, as well as HF broadcasters, within the next few weeks, which delivers a maximum of 2 kW, but output power can be lowered by turning a knob, so that I will stay legal. Together with a matching receiver, which I already own, I´ll then have the equipment of an equivalent of Shanwick Oceanic at home and should be able to talk to radio hams all over the world).
AM as a means of modulation is not very efficient. Most transmitter power (about 75% ) is being used to transmit the leftover carrier wave, which doesn´t carry any information. Since the signal also contains two side bands (which each contain the full information), the signal used a huge bandwidth. Single sideband transmission is much more efficient. Basically you´ll start with an AM signal, but in the transmitter, before you´ll get to the final power amplifier stages, you´ll filter out the carrier frequency and one sideband (historically hams use the lower sideband below 10MHz, and the upper one above 10MHz, while commercial operators almost exclusively use the upper sideband). Then only the remaining sideband gets amplified and transmitted.
On the receiver side the missing carrier gets added again and the signal then demodulated to get the audio signal.
This means that while AM equipment is cheap and easy to produce (to receive an AM signal a simple detector receiver consisting of an antenna, a coil, a variable capacitor a diode and an earphone) is enough, SSB equipment is much more expensive and complicated.
FM uses quite a bit of bandwidth, but has the advantage that it isn´t affected by interference like lightning (which change the signal amplitude). Due to the bandwidth requirements, it is more used in higher frequencies, like VHF and UHF, where more stations at given bandwidth, can fit into a frequency band. It gives better quality of sound.
Other modulation modes, like CW (Morse code through switching the carrier wave on and off), RTTY (Radioteletype, using a two tone audio signal transmitted via SSB) or various other digital modes like PSK (Phase shift keying) etc.
use less bandwidth and can be better received even if the received signal is weak.