Home |
Search |
Today's Posts |
#4
|
|||
|
|||
wave polarisation
On 9 mayo, 10:02, Szczepan Bia©©ek wrote:
U˘Żytkownik napisa©ř w ... On 8 mayo, 10:35, Szczepan Bia©řek wrote: I start reading about acoustic analogy. I found that: "Over long distances, the atmosphere can cause the polarization of a radio wave to fluctuate, so the distinction between horizontal and vertical becomes less significant." From:http://whatis.techtarget.com/definit...843762,00.html The my question a 1. What means "long distances" in km (or miles), 2. What is the best orientation of the antenna for long distances. S* Hello, Under normal circumstances, polarization change in line-off-site conditions (think of max 40 mile) is not that much, so antenna polarization does matter (unless you use at least circular polarization on one side). In a propagation path that is dominated by multi-path effects (reflection at buildings, hills, foliage, etc), you get almost random polarization and then the polarization is not that important. Your cell phone and indoor WIFI are examples. Extreme weather conditions can also lead to polarization changes or a random polarization component (ducting superrefraction). For sea water up to VHF, reflection depends on polarization. For ground-ground links (for example ship shore) mostly vertical polarization is used (as the sea water helps in this case). So if you want to receive these communication, you use a vertical polarized antenna. The largest change in polarization you will get when the waves have to travel through the ionosphere. At HF (ground-ground link via ionosphere), the polarization vector rotates many times. This is due to Faraday rotation. Also ground-satellite links suffer from this effect. The higher the frequency, the less the change in polarization. For example at 100 MHz you should think about 30 full rotations (that is more then 10k degrees), while at 10 GHz the change in polarization will be about 1 degree. Circular polarization may help to mitigate the influence of Faraday rotation. At HF sky wave (100....1000 mile via ionosphere) polarization of the antenna matters. This is not because of the polarization change of the waves due to Faraday rotation, but because of the reflection characteristics of mother earth. In HF antennas, reflection on mother earth is used (in combination with antenna height) to get the required elevation radiation pattern of the antenna. Reflection on earth depends on polarization. Hopefully this helps you a bit. You do not use the words "transversal" and "EM". The only evidence of polarization is antenna directional sensitivity. You talked about radiowaves, that are EM waves. In free space, only progapation mode is transversal (that means both E- and H-field are perpendicular to the direction of energy propagation. With regards to audio, in gas, only lossless propagation mode is longitudinal (molecule movement and pressure vectors are parallel to the direction of energy propagation) . In the acoustic analogy a radio waves are normal spherical electric waves emitted from the two sources (ends of the dipole). So the sources are polarised, not the waves. Waves interfere. Do you agree? Not agree, the waves are also polarized, that can be physically measured. Polarization is determined by the E-field vector. See my topic "frequency doubling" . I am only a science hobyist. The second question was: " What is the best orientation of the antenna for long distances? For old radio antennas. Very long horizontal wire. On UHF (for example 2450 MHz), long distance can be 20 km, but on HF 500 km is not called long distance. So the meaning of long distance depends on the frequency band. You should distinguish between the actual polarization of the antenna and the physical appearance. depending on how you feed it, a very long horizontal wire can be sensitive to vertical or horizontal polarized waves. Though the equations for acoustical waves look similar to those of EM waves, the orientation of the field components is completely different. When you require a more specific answer, you should make your question more specific. I tried to give you a general answer for the various forms of radio wave propagation. Best regards, S* Best regards, Wim PA3DJS www.tetech.nl |
Thread Tools | Search this Thread |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
Incoming radio wave polarisation | Antenna | |||
Newbie ?: I've Built A Simple 1/4 Wave Dipole for 2 Mtrs. Could IMake a1/2 Wave? | Antenna | |||
Newbie ?: I've Built A Simple 1/4 Wave Dipole for 2 Mtrs. Could IMake a1/2 Wave? | Homebrew | |||
fa- PHILCO STANDARD WAVE-SHORT WAVE SWITCH>OLD++ | Swap | |||
fa- PHILCO STANDARD WAVE-SHORT WAVE SWITCH>OLD++ | Equipment |