Radio waves and height gain in fading zones

[jaumzaum]

Hello Guys!

I'm studying radio waves and I can't understand the following:

Radio waves propagates through Earths surface as a ground waves, line-of-sight waves or skywaves.
There is a phenomenon that occurs with line-of-sight waves (and maybe with ground waves?) known as height gain, in which the increase in the antennas height reduces the fading. Why that happens?

 

[berkeman]

the increase in the antennas height reduces the fading. Why that happens?

It helps to get the LOS RF signal propagation clear of ground obstacles (and even the curvature of the Earth for longer-range paths)...

https://www.gnswireless.com/info/point-to-point-ethernet-bridge

 

[jaumzaum]

Thanks @berkeman!Can you explain me why lower frequencies increase fading?

 

[berkeman]

Can you give a link? Lower RF frequencies tend to follow the curvature of the Earth better, so are less line-of-sight limited. For HAM radio, the 150MHz (2 meter) and 440MHz (70cm) bands tend to be pretty line of sight, while the lower frequency 40/20/10 meter bands tend to be pretty good at following the Earth's surface.

 

[jaumzaum]

Can you give a link? Lower RF frequencies tend to follow the curvature of the Earth better, so are less line-of-sight limited. For HAM radio, the 150MHz (2 meter) and 440MHz (70cm) bands tend to be pretty line of sight, while the lower frequency 40/20/10 meter bands tend to be pretty good at following the Earth's surface.

Thanks!

The book I am reading is called General Radio-operador Special, EROG, a Brazilian book that is written in Portuguese.
It says the following (translated):

"Fading Zones consist in zones where one receives a direct (line-of-sight) radio signal along with a sea-reflected radio signal, resulting in a weakening or complete cancelation of the resulting signal when they interfere destructively. The two main factors that influence fading zones are antenna height and frequency. The lower the antenna is installed, the wider the fading zones will be. For a given antenna height, the lower the frequency the higher the fading. "

Also, when you say a wave follow the Earth curvature, you are referring to ground waves right? This is one point I don't understand properly. I can understand a line-of-sight (direct wave) and I can understand a sky-wave (the ones that reflect in the ionosphere). But why would a wave follow the curvature of the Earth?
I know some line-of-sight waves can propagate 20-30% farther from the optic-horizon because of refraction effects, but VLF waves can travel all around the globe and come back, I don't think refraction in the air would explain this.

 

[berkeman]

"Fading Zones consist in zones where one receives a direct (line-of-sight) radio signal along with a sea-reflected radio signal, resulting in a weakening or complete cancelation of the resulting signal when they interfere destructively.

Oh, it may be a language/translation issue. I call that "Multipath" interference. To me "Fading" means attenuation.

Also, when you say a wave follow the Earth curvature, you are referring to ground waves right? This is one point I don't understand properly. I can understand a line-of-sight (direct wave) and I can understand a sky-wave (the ones that reflect in the ionosphere). But why would a wave follow the curvature of the Earth?

I believe it is similar to wave diffraction around obstacles and the dependence of diffraction on frequency. I'm not sure about that, however.

 

[Baluncore]

For a given antenna height, the lower the frequency the higher the fading.

The fresnel zones are measured in wavelength separation between the LOS from the ground surface. When measured in wavelengths, lower frequencies are closer to the ground, so have more path cancellation.
https://en.wikipedia.org/wiki/Fresnel_zone

Also, when you say a wave follow the Earth curvature, you are referring to ground waves right?

The atmosphere is more dense close to the ground, so the lower parts of a LOS wave-front travel slightly slower than the upper parts. That curves the "radio LOS" to partialy follow the curvature of the Earth, but not sufficiently to completely overcome the Earth's curvature.

I know some line-of-sight waves can propagate 20-30% farther from the optic-horizon because of refraction effects, but VLF waves can travel all around the globe and come back, I don't think refraction in the air would explain this.

VLF propagates in what is called the Earth-ionosphere waveguide. The bottom of that waveguide is the Earth's conductive surface, the top of the guide is the conductive ionosphere. Attenuation in the guide is significantly less than the inverse square. The VLF follows the guide around the Earth.

 

[tech99]

Thanks @berkeman!Can you explain me why lower frequencies increase fading?

For the line of sight case, the atmosphere normally causes a slight downward ray bending, which is favourable to propagation. However, it varies with weather conditions, so the ray can either bend down more or bend upwards. In the latter case, it is equivalent to the Earth having a smaller radius, so the ground can obstruct the path. It is usual in microwave link planning to increase the height of the antennas to avoid this happening. If we use a lower frequency, the required ground clearance is greater due to the longer wavelength, so if height is not increased there is more chance of ground obstruction fading.

 

[sophiecentaur]

Oh, it may be a language/translation issue. I call that "Multipath" interference. To me "Fading" means attenuation.

This is a massive topic and can't really be dealt with briefly but:

Some history: fading is a phenomenon which was observed long before it was understood. LF and MF signal propagation was found to work way beyond what was expected but long distance signals faded (varied) periodically but the absolute level couldn't be predicted accurately That wasn't explained until the modes propagation were understood. We know that's due to diffraction / interference / multiple paths and the presence of the Earth (curvature and ground resistance), Ionosphere and Troposphere. Multipath effects can be variable (shifting reflection levels) and cause fading or they can be stable and cause steady increases or decrease in received signal levels.

Height Gain is due to reflections from the ground which produce an image of the transmit antenna which causes a vertical interference pattern. For appropriate heights, the beam pattern gets sharper and sharper at the centre, which increases maximum received signal level (=gain) and also sidelobes. With horizontal polarisation, the reflected signal is in anti phase which cancels the horizontal part of the beam and tilts it upwards. Great for HF which is not used for local reception and the angle can be used to select an appropriate ionospheric level for a 'good' skywave path.

For MF transmission, the ground wave propagates over the curve of the Earth, which tilts the wave forward, rather than 'letting it go'. The wave in higher angles will hit the lowest 'D' layer which is relatively dense and is absorbed in the day, giving clean reception right out to where the signal level fails. When the Sun goes down, ions quickly recombine and the D layer disappears. The sky wave carries on up and bounces from the E or F layer (depending on time of day, sunspots etc.) and hits the ground again, producing terrible skywave interference (and also interference from other distant MF transmitters.

[Edit: Antifading MF aerials are made very tall with top loading. That 'pulls' the current maximum up the mast and tends to depress the main beam, making the ionospheric reflection further away and reducing fading in the near service area.]

Height gain at VHF and above is more a matter of getting the propagation path over the height of buildings and trees and extending the horizon.