Today is Thursday, 2019-11-21
Some Icons by Dryicons
Today is Thursday, 2019-11-21
Some Icons by Dryicons
VHF radio waves (above 50 MHz) travel more or less in a straight line, and so cannot go much beyond the horizon. To increase the distance that an antenna can “see”, we raise our antennas as high as possible. The radio horizon is given roughly by: where d is in kilometre and h is in meters.1)
For example, VE7HZF's antenna is 20m above the ground, at an elevation of 100m overlooking the water. It means that his antenna can see about 45 km in that direction.
For direct waves to occur, the height of the antenna needs to be many times greater than the wavelength of the radio wave so that the signal doesn't interact with the ground. In this example, the antenna (120m above sea level) is 60 wavelengths high on the 2 meter band, and 170 wavelengths on the 70 cm band.
Another similar station could be reached at about 90 km.
Ground waves occur when the signal curves with the Earth until it becomes too weak to be detected. This phenomena happens when the frequency is below 3 MHz, partly because even antennas that are high up, are still relatively close to the ground compared to the wavelength of the radio wave. For example, even at the top of a 20m-tall tree, an 80m HF antenna is only a quarter of the wavelength high.
Because it is so low, the radio wave interacts with the ground where it loses some of its energy but also curves toward it. The good news, though, is that even though the radio waves lose energy in the ground, they can go beyond the horizon out to about 200 km.
Depending on the frequency and atmospheric conditions, it's possible for radio waves going up to reflect back down to Earth. From our location in British Columbia, we can very easily talk to people in Japan using Skywaves.
This process uses Ionospheric Refraction, which we'll see next.2)
The region of our atmosphere between 50km and 400km altitude is called the ionosphere3), and to radio waves, it can act like:
The reason for this complex behaviour is that the ionosphere is composed of electrons and electrically charged atoms and molecules (called ions) caused by the Sun's ultraviolet radiation (solar flux). Gas at higher altitude is more ionized because it is less dense, which makes recombination into neutral molecules more difficult. Depending on the frequency, radio waves travelling into the ionized gas can see an index of refraction that is less than that of the air below, which means that they can refract and reflect the way light does through water. Also, because ionization depends primarily on the Sun's activity, three main cycles dictate the characteristics of the ionosphere:
Depending on the time of day, the ionosphere separates into 3 or 4 layers (of different gas composition):
The distance radio waves can propagate via ionospheric refraction depends on many factors:
A radio signal will reach further when:4)
The above animation is a gross oversimplification to illustrate the point that, all else being equal, signals sent near the horizon using the F layer will go further. For example:
In reality, the ionosphere is a medium with a continuously varying index of refraction rather than a series of discrete “mirrors”. As such, how much signals “curve” also depends on the takeoff angle, and just like what we saw in the previous section, there's a critical angle that must be met for Total Internal Reflection to occur. So the real picture is more like this one:5)
Note the following important terms on the above image:
And finally, real antennas do NOT transmit their signal at a single take off angle but over a range of them, which can vary depending on the antenna type and how high it is over the ground. So in reality, many of these paths are used at the same time and even reflect off the ground and go back for a second or third hop. Communications exceeding 5000 km uses multihop propagation, which looks like this:6)
Whether a layer lets a radio signal through, reflects it back, or absorbs it depends on the frequency. In general:
If the goal is to get our signal to travel as far as possible, there's a kind of Goldilocks range of frequencies that we can use. If the frequency is too high, the signal will travel straight through the ionosphere as if it was a clear window. If the frequency is too low, the signal will get absorbed by the ionosphere as if it was a tinted window. The game is to find the frequency that will get reflected by the ionosphere as if it were a mirror, and not only that but we also would like to use the F layer because the higher the point of reflection, the further the signal will travel.
In the previous animation, frequencies below 6 MHz don't get through the D layer, and frequencies above 44 Mhz escape into space. Only frequencies between the LUF and MUF get reflected back down to Earth by different layers.
Sometimes, depending on the atmospheric and/or solar conditions, the LUF, is greater than the MUF. In that case, no reflection is possible. Signals either get absorbed by the D layer, or get through all of them and escape into space:
In general, during the day:
So normally, higher frequencies are better during the day (up to about 6m), and lower frequencies are better at night.
Here's a map of MUF that's updated regularly. You can think of this as a “weather map” for ham radio.
Finally, because the D-Layer disappears before the F-Layer recombines, and reappears after the F-Layer splits, the propagation can be interesting around sunrise and sunset. This is called the gray zone.
On the previous page, we discussed how the Ionosphere (the region of our atmosphere between 50km and 400km altitude) can, reflect and refract radio waves, let them pass straight through, or absorbed them completely mostly due to the sun's ionization of the gas in these layers.
Here we discuss how the troposphere (the lowest region of our atmosphere below 20km altitude) can also affect radio waves.