Following up on
a partial response to Pete’s thought provoking question as to whether space diversity gain could be a mechanism for alleviating
frequency selective fading, presumably originating as a result of multipath.
Quote:
Originally Posted by Pete Higgins
That’s a lot of separation between your long Yagi’s. Do you think that ~1.5 wavelengths of spatial diversity is mitigating your frequency selective fading?
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The writing below is an opinion, subject to revision.
There are several methods of combining two or more independent signals to achieve diversity gain. The list below represents some techniques, but is not a complete list.
Wikipedia was consulted for some of the
diversity combining names.
1. Maximal Ratio or Ratio Squared
2. Equal Gain
3. Switched
4. Selection or Scanning
5. Other
Maximal Ratio or Ratio Squared: The gain applied to each signal is determined by the S/N ratio of each signal. Higher S/N signals are increased in gain relative to low S/N signals, then combined.
Equal Gain: The signals are phase shifted into alignment and added without altering gain of either signal
Switched: The receiver uses only one signal until the signal becomes essentially unusable or drops below a fixed threshold, then switches to the other signal.
Selection: Signal strength of each separate path is evaluated; only the strongest signal is used.
Other: Other methods have been devised, depending on field of application. An example for TV might include post-detection combination of the video from each separate antenna or path. This method requires separate tuners with timing and delay hardware suitable to align two independent video streams; then combining based on some of the principles suggested by the above techniques.
Combining in a summation/additive device such as a splitter (reversed), hybrid combiner, or transmission line combiner, does not fit into any of the above categories. These directly additive combinations make no allowances for either phase or amplitude adjustment before combination. Having no means of modifying the signals before addition permits potential destructive interference between signals to continue. The main reason multipath is a problem in reception is destructive interference.
Destructive interference arises when signals add in such a way as to reduce signal strength. Electromagnetic (RF) signals are characterized by magnitude (amplitude) and phase; they add according to principles of vector addition.
Examples:
1. Two signals of near equal amplitude and nearly 180 degrees phase shifted relative each other, add to nearly zero amplitude.
2. Equal strength signals with less than 180 degrees relative phase shift can interfere with each other (depending on amount of phase shift) , but not complete cancellation.
3. Signals that are 180 degrees out of phase with unequal amplitudes interfere with each other, but do not completely null to zero.
There are many possibilities in between those of the preceding examples where two or more signals add to a sum (amplitude) that is lesser than the amplitude of either signal alone.
The summation of two antennas (in a stack arrangement) essentially increases the antenna aperture. Antenna aperture is the effective area in space over which signals are intercepted. In special cases such aperture increases could alleviate mutlipath effects by intercepting better quality signals. If so, this is a matter of aperture placement rather than diversity gain. If the multipath effects are random and equally distributed in space, then a larger aperture will not produce diversity gain.
An increase in aperture usually means a decrease in antenna beam width. Decreases in beam width may eliminate or reduce some potential multipath signals. (More on this later)
It appears, the additive combination of two antennas does not eliminate (through the mechanisms of diversity) the potential for the destructive signal combinations resulting from multipath signals.
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