NTNUJAVA Virtual Physics Laboratory
Enjoy the fun of physics with simulations!
Backup site http://enjoy.phy.ntnu.edu.tw/ntnujava/

JDK1.0.2 simulations (1996-2001) => Electromagnetics => Topic started by: audioviking on October 28, 2009, 06:13:41 am



Title: Satcom Cross-polarization of V / H
Post by: audioviking on October 28, 2009, 06:13:41 am
I am a technical instructor for satellite technologies. One of the key concepts is also the hardest to explain.

-aiming a satellite dish to something you cannot see.
-fine-tuning that alignment.
-adjusting the skew of the LNA/B to align with correct polarization of intended signal
-show what happens if you look at opposite polarity.

Anyone here know of such a simulation?

regards
Travis


Title: Re: Satcom Cross-polarization of V / H
Post by: Fu-Kwun Hwang on October 28, 2009, 03:55:25 pm
Are you talking about something related to the following?

Quote
from http://en.wikipedia.org/wiki/Satellite_dish
The parabolic shape of a dish reflects the signal to the dish’s focal point. Mounted on brackets at the dish's focal point is a device called a feedhorn. This feedhorn is essentially the front-end of a waveguide that gathers the signals at or near the focal point and 'conducts' them to a low-noise block downconverter or LNB. The LNB converts the signals from electromagnetic or radio waves to electrical signals and shifts the signals from the downlinked C-band and/or Ku-band to the L-band range. Direct broadcast satellite dishes use an LNBF, which integrates the feedhorn with the LNB. (A new form of omnidirectional satellite antenna, which does not use a directed parabolic dish and can be used on a mobile platform such as a vehicle was announced by the University of Waterloo in 2004.[1]

The theoretical gain (directive gain) of a dish increases as the frequency increases. The actual gain depends on many factors including surface finish, accuracy of shape, feedhorn matching. A typical value for a consumer type 60 cm satellite dish at 11.75 GHz is 37.50 dB.

With lower frequencies, C-band for example, dish designers have a wider choice of materials. The large size of dish required for lower frequencies led to the dishes being constructed from metal mesh on a metal framework. At higher frequencies, mesh type designs are rarer though some designs have used a solid dish with perforations.

A common misconception is that the LNBF (low-noise block/feedhorn), the device at the front of the dish, receives the signal directly from the atmosphere. For instance, one BBC News countdown shows a "red data stream" being received by the LNBF directly instead of being beamed to the dish, which because of its parabolic shape will collect the signal into a smaller area and deliver it to the LNBF.[2]

Modern dishes intended for home television use are generally 43 cm (18 in) to 80 cm (31 in) in diameter, and are fixed in one position, for Ku-band reception from one orbital position. Prior to the existence of direct broadcast satellite services, home users would generally have a motorised C-band dish of up to 3 metres in diameter for reception of channels from different satellites. Overly small dishes can still cause problems, however, including rain fade and interference from adjacent satellites.

 I do not have a satellite dish. I need more information to be able to create a simulation.