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Old 2-Apr-2011, 9:07 PM   #12
mtownsend
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Join Date: Dec 2009
Posts: 632
Quote:
Originally Posted by retired17 View Post
if two colocated antennas are not in a uniform field than you would not get the expected 3 dB gain when combined. Some of the stronger signal from one antenna will be reradiated out of the other.
A non-uniform field might be a valid explanation for one of your channels, but it's generally unlikely to be the case for multiple channels at the same time. The "hot" and "cold" spots described in Ken's discussion about non-uniform fields usually only occur when you have two multipath signals that are almost equal in strength. These equal, yet out-of-phase, signals can cancel each other out almost perfectly, creating the "cold" spots spaced several feet apart.

In most multipath situations, the dominant paths are not equal strength. It's even less likely that multiple channels will have this effect at the same time. The reflection/absorption of RF energy varies by frequency, so the behavior of multipath is often different from one channel to the next.



Quote:
The second source of loss is that a larger antenna with its narrower beamwidth will only see one multipath signal thereby losing the hot spot advantage.
It is generally good for reception to be able to isolate one multipath component out of many.

If there equal-strength multipath components creating the "hot" and "cold" spots, then yes, it's possible for a larger antenna to not-quite hit the "hot" spot, but as stated above, equal-strength multipath components are generally not that common.

Even if you had a hypothetical situation where you had say the line-of-sight path (to many co-located transmitters) plus a giant perfectly reflecting wall off to one side creating near equal-strength reflections across all of your channels. In such a situation, the "hot" and "cold" spots would occur for each channel at a slightly different location (as your hypothesis was suggesting). In that situation, you could use the higher gain antenna (with narrower beam width) to point at just one of the signal paths (i.e., the line-of-sight path), thus reducing the cancellation effect of the other signal path. This would make reception more uniform across all of the channels. Some of the channels will no longer perfectly capture the "hot" spots (possibly 3 dB worse than before), but by the converse analogy, none of the channels will perfectly hit the "cold" spots either.

In the majority of cases, it's better to have a tight antenna pattern to focus on the strongest clean signal path. By doing so, the system becomes less sensitive to the channel-by-channel multipath-induced fading variations that can occur.

This doesn't work if you have transmitter clusters in two different directions and need a wide antenna pattern to capture signals spread in multiple directions.



Quote:
I was using a CM4228 8-bay antenna to get the UHF channels because I believed it would help my reception plagued by multipath.
Do you know what is causing multipath in your area (tall buildings, trees, hills)?

Were you using the new 4228 or the old one?

Do you have transmitters spread in multiple directions?
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