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Old 24-Aug-2013, 3:59 PM   #63
tripelo
Senior Member
 
Join Date: Jan 2010
Location: Dallas, TX
Posts: 123
Quote:
Originally Posted by Pete Higgins View Post

...What I have trouble rapping my head around is which mechanism is responsible for what I see as channels fade both short term (multi-path?) & for hours at a time (tropospheric absorption?)...
Multipath is difficult to counter because of the variety of sources that can cause it. Multipath is the main reason that mobile TV is not very practical for 8VSB (the US DTV standard).

It can be difficult to distinguish between multipath and direct signal fade.

There can be two classes of multipath:

1. Static Multipath (resulting from fixed objects)
2. Dynamic Multipath (resulting from moving objects)

Static or long term signal attenuation can result from multipath, when the multipath sources are stable (fixed). Examples could be reflections off a building or a fixed object that partially cancel the direct or desired signal. One prominent contributor is the surface the earth. As can be seen in the previous vertical antenna patterns, for specific angles-of-arrival, a ground reflection can cause a static fade.

Dynamic of fast fading (in minutes or seconds) is usually the result of multipath (Doppler effects). For Doppler to occur, something has to be moving such as antenna moving (Mobile TV).

For fixed antennas the something could be;

- Traffic in a city,
- An airplane flying over,
- Trees moving in the wind,
- Atmospheric motion (tropo scatter), or
- Something else.

For long distance reception, the atmosphere is the most likely ‘something’ that causes Doppler.

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Daytime Signal Fade (Independent of Multipath)

Gradual bending allows radio signals to somewhat follow the earth’s curvature for some distance beyond normal line-of-sight. This gradual bending is somewhat independent of phenomenon like ‘tropo scatter’. As you know, long distance TV signals can fade (reduce in strength) in daylight hours. This can be due to smaller temperature gradients that result when the sun uniformly heats the air in the lower atmosphere during much of daytime hours. Smaller temperature gradients result in less bending of the signals back towards the surface, thus the daytime fade.

Note: Temperature is not the end cause of the bending of RF signals. Temperature is a measure of one particular driving factor (thermal energy) that can decrease atmospheric density, thus affecting the density of everything contained in the atmosphere (water vapor, etc).

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Quote:
… RCA pre-amps...have ordered two of the last ones because when I went back to the site they no longer list them.
Maybe your good reports caused a run on the RCA preamplifiers. Well, it was/is petty good deal.

Quote:
... I just hope you’re not going to bill me for it later?
Thanks for your comments about the elevation beamwidth and multipath.

No charge.

.

Last edited by tripelo; 24-Aug-2013 at 8:45 PM. Reason: Add note & ground comment
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