|term antenna - RF Cafe Forums|
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Post subject: term antenna
Posted: Tue Feb 03, 2009 10:04 pm
Joined: Fri Dec 14, 2007 11:23 am
i am new with design antenna, can anyone tell me what is isolation and diversity?
only that i understand isolation mean that interaction between an antenna and its surrounding
i am confuse what correlation between isolation and diversity if we have 2 antenna in one device?
Post subject: Re: term antenna
Posted: Wed Feb 04, 2009 4:41 am
Joined: Fri Feb 17, 2006 12:07 pm
Location: London UK
I suppose isolation is the reciprocal of coupling but I have not seen it mentioned as an antenna parameter. Coupling has 2 meanings: there is coupling between adjacent elements in an array which is either desired because it enhances either sidelobe levels or gain, and unwanted coupling that degrades performance or complicates the design process.
So far as isolation is concerned, there is a parameter called cross-polar isolation, which is the degree to which two antennas with nominally opposite polarisation still cross-couple due to imperfection. Thus if an antenna is nominally vertically polarised but with a small say -15dB of horizontal vector, then the isolation to an antenna that is hrizontally polarised will be -15dB.
Diversity takes several forms and is normally encountered when a path between antennas has a time-varying refractive index and a possibility of two or more paths: one direct and the others by reflection. Spacing two receive antennas maximises the probability that the paths to one of them will result in a vector sum that is a maximum, whilst at the other antenna the received signal has a vector sum that is a minimum, ie has faded due to out of phase vector addition. The antennas can be spaced apart vertically or horizontally (space diversity), or the polarisation can be opposite (polarisation diversity), or two different frequencies can be used (frequency diversity). The ITU-R publish recommendations how these can be implemented.
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