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WHICH HF BAND TO USE...
AND WHEN. DAY OR NIGHT?
Texto original de G4NSJ / BY G4NSJ (http://www.g4nsj.co.uk/which.shtml)
There seems to be some confusion over which HF amateur band to use and when to use it. Below, is a brief guide to the bands and when to expect results. 80 metres used to be great for inter-G working during the day, but that seems to have changed. Things are changing all the time so don't trash this guide just because you get nowhere on 80 metres one evening. One night, 80 will be brilliant and, the next, total crap. The bands are rather like women... Unpredictable, moody, prone to inexplicable swings... All due respect to women, of course! Auroral activity (weird stuff happening around the north pole) can affect the bands, as can your neighbour’s clapped out vacuum cleaner. And as for switched mode bloody power supplies!
Why is 160 metres better for inter-G working than 20 metres? See THE IONOSPHERE below the band listings for a clue as to propagation, skip distance, sky wave, ground wave, fading, and more... OK, the bands are listed below. Do bear in mind that this is a guide only.
160 Metres. Top Band. 1.8-2.0 MHz.
This is almost on top of the medium wave broadcast band. Old 1960s pirates familiar with medium wave and aerials for this band will have no problems with top band. 160 metres is great for local contacts during the day, using ground wave, with long distances obtainable at night when the band opens. During the summer months, the night time distances may be several hundred miles, or even thousands of miles. During the winter months, communications over distances of several thousand miles can be achieved regularly.
80 Metres. 3.5-3.8 MHz.
80 metres is very similar to 160 meters but with greater distances obtainable during the night. 80 is a fairly reliable band, but there are times when it just dies. It's used for regular nets, particularly inter-G working. The whole of Europe may be worked during the night if conditions are good. And, with a decent aerial, you can squirt a signal over the pond the the US.
40 Metres. 7.0-7.1 MHz.
This is usually open to somewhere or other. During the summer months, daytime distances between 300 and 500 miles can be achieved. A great band for inter-G working during the day. At night, distances of 1000 miles or more are not uncommon. With the right aerial, you can work the world on this band at night. Winter days with 500 miles or more are usual and night time brings in the intercontinental stuff. Not as affected by the sunspot cycle as 20 and 10 metres. There are times when you'll hear people say the the skip is long. When this is the case, inter-G working is pretty much nonexistent. However, during long skip conditions, the band should be open to Germany and other european countries.
30 Metres. 10.100-10.150 MHz.
A lot like 40 metres, but this band can only be used on CW and RTTY. (This doesn't seem to apply to the French!) The band has a longer range than 40 metres, with daytime distances of 1000 miles or more achievable.
20 Metres. 14.000-14.350 MHz.
This is a great band for DXing, even with a crappy aerial. The whole of Europe can be worked, usually twenty-fours hours a day. It's a dreadful band for inter-G working, unless you want to chat to the bloke down the road. Of course, if you do that, you'll probably be heard thousands of miles away. As we get higher in frequency, the aerials become smaller, making this an ideal band for the guys with small gardens. Check out my Coffee Break Vertical For 20 Metres.
17 Metres. 18.068-18.168 MHz.
Propagation on this band is virtually the same as 20 metres.
15 Metres. 21.000-21.450 MHz.
Very much like 20 metres but unpredictable. More affected by the sunspot cycle. When this band is open, working the US is fairly easy.
12 Metres. 24.890-24.990 MHz.
Very much influenced by the sunspot cycle. This band can be used for local communications, rather like citizens band. When the band is open during the day, the world can be worked with a pretty basic aerial. The band often remains open late into the night, which can be interesting.
10 Metres. 28.000-29.7000 MHz.
This band is affected most by the sunspot cycle. As with CB, this band is pretty good for local work. But, when it's open, it's really great. From the UK, the American FM repeaters can be worked with no trouble at all.
THE IONOSPHERE
THAT MYSTERIOUS STUFF UP IN THE SKY.
Forget about the D layer and the F layer and all that stuff for the moment. As far as we are concerned, the ionosphere is a kind of ceiling above the earth. OK, so we send out a radio signal and it hits the ionosphere. This signal is the SKY WAVE. Why? Because it goes up into the sky. Got that? I know it's difficult but do try to pay attention.
At certain frequencies, the ionosphere is like a mirror. It reflects our sky wave signal back to the Earth. Where the signal lands on the Earth depends on the angle at which the signal hits the ionosphere. Shine a torch into a mirror and move it about. The reflected beam moves depending on the angle of the torch.
This has nothing to do with the mirror on the ceiling above your bed. Having said that, the principal is exactly the same. Send something up to the mirror above your bed, and it will bounce off the mirror and land somewhere on the bed. The less said about that, the better!
When people talk about SKIP, they mean the distance between the transmitter and the nearest point from the transmitter where the sky way returns to the Earth and can be received. In other words, the land over which the radio wave jumps is the skip distance. No sky wave signal will be received in this skip zone. Or, at least, very little. Yes, I do realize that aerials send out radio waves in all directions and angles. But we're not going to complicate things just yet.
You might hear someone saying that the skip is long on 40 metres. Or short, come to that. They mean that, during long skip conditions, inter-G working is poor or impossible, whereas the continental stations are romping in. During short skip conditions, inter-G working is great. But the continental stations have all but faded away. See the above diagram for long and short skip..
Take a look at the diagram below and you'll see the radio signal leaving the transmitter, bouncing off the ionosphere, and returning to the Earth. The ground wave is something we'll come to in a minute.
Arranging your aerial so that it fires the signal almost straight up to the ionosphere is called NVIS (near vertical incidence sky wave). The idea being that a station pretty close to the transmitter will be able to receive a good signal even if there's a mountain in the way. Get it? The signal goes almost straight up, and down again, skipping over the mountain. This is all very well at certain frequencies such as five megs. However, at other frequencies, the sky wave will drive straight through the ionosphere and go boldly out into space where no wave has gone before.
MUF (maximum useable frequency)
The maximum useable frequency is the highest frequency (at any time) at which radio waves bounce off the ionosphere. There are MUF charts (don't be rude) available showing the maximum useable frequency over a period of time. These are useful if you have to keep in constant contact with a fixed station. However, we needn't worry too much about the MUF at the moment.
GROUND WAVES
OK, we'll come back to sky waves in a minute. The ground wave leaves the transmitting aerial and... Yes, that's right. It travels along the ground. OK, not on the ground, exactly, but very close to it. I do wish we didn't have to be so fastidious. This ground wave, which is slightly above the ground, gradually peters out with distance. No, I have no idea why it peters out and not pauls out. The signal is absorbed by buildings such as houses, pubs, bars, nightclubs, drinking dens... Sorry, I got carried away. The distance over which this pauling out occurs depends on the power of the transmitter, the aerial, the terrain... And loads of other stuff which is of no interest.
Virtually all contacts made on top band, 160 metres, during the day are by ground wave. The same applies to medium wave stations heard during daylight hours. At night, things change. In the dark of the night when the sun has gone and monsters lurk and things go bump, usually around chucking out time in the pubs, signals on top band are able to bounce off the ionosphere and return to the Earth. This returning to the Earth is sometimes thousands of miles away from the transmitter site. The reason for this is that the ionosphere changes its characteristics between the hours of sunlight and darkness. Rather like a friend of mine. More on that later. The ionosphere changing at night, not my friend.
This is a good time to quash a myth. People talk about the sky wave and the ground wave. OK, let's clear this up. These aren't separate waves. There aren't separate ground and sky waves leaving the transmitting aerial. The aerial chucks out waves in all directions. The ones following the ground are called... And the ones going skyward are called... It's all clever stuff.
Right, so you're sitting in your shack one evening listening to a station on top band and, one minute, the signal is very strong and, the next, it's all but disappeared. This could be due to several reasons but, very often, the sky wave is reaching your aerial along with the ground wave and the two either cancel each other out or combine to make a strong signal. This depends on the phase relationship between the two signals and stuff like that. More on fading or QSB later.
Back to the sky wave for a moment. We fire a signal up to the ionosphere and it's reflected back to Earth. So far, so good. But, when it hits the Earth's surface, it may bounce off and return to the ionosphere. Up down, up down... (I'm not going to make a lewd comment about a prostitute's knickers) Up down, up down, all the way around the world until it dies a death. One bounce off the ionosphere is great for getting from the UK to, say, Italy. But when working Australia, multiple hops are necessary. Take a look at the diagram below.
You may hear echoes when listening to a distant station on 10 metres. This is the signal going around the world more than once, hitting your receiving aerial at intervals, the time delay between each hit causing the echo. As I said earlier, the TX aerial sends out radio waves in all directions and at all angles. So it's not just a simple case of one wave going up down, up down. Also, depending on the angle at which waves hit the ionosphere, some will go straight though, some will be reflected, and some refracted.
Just a quick word about the height of aerials, particularly aerials for 160, 80 and 40 metres. We've talked about NVIS, firing the signal straight up, or as near as damn it. In fact, the majority of end fed wires and dipoles are NVIS aerials. Why? Because they are so low in comparison to the wavelengths concerned that they fire the signal skyward. The higher the aerial above ground, the lower the radiation angle.
Radiation angle? This is the angle at which the signal leaves the aerial. OK, the signal is fired in all directions and at all angles, but there's a concentration in certain directions and at certain angles. This concentration, or lobe, depends on many factors. One of the main factors is the aerial's height above the ground. The higher the aerial, the lower the radiation angle. This is great for DX as the signal travels at a low angle for many miles before glancing off the ionosphere and returning at a low angle to the Earth.
For inter-G working, usually, the lower the aerial the better. This is because the signal is fired straight up and most of it rains down and splashes all over the UK. In fact, for NVIS work, aerials are sometimes laid on the ground. Yes, they still work!
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