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Discussion Starter · #1 ·
Ahh, Autumn - when the golden hues grace the trees in many regions of the country - Autumn, when rain or snow return as a nearly daily event - Autumn, when we are reminded of the coming winter winds by the leaves being blown from our neighbors trees into our yard so we have to rake them up - Autumn, when television reception, especially of UHF channels, changes daily. Okay, enough of the poetic.


This is a little long, but from the number of 'what happened to W/K???-DT' posts in the past week it may be helpful. Radio propagation IS affected by foliage and moisture. This is especially true of UHF reception, but even low band VHF reception is affected too. Moisture, either falling from the sky or trapped in the leaves of trees attenuates signals and causes the signal to bounce off things differently. Wind moving the trees around can change these affects dynamically.


In the Summer the affects on TV reception are pretty consistent. The leaves are green and laden with moisture. Getting them wet from a rain does not really change things much because the leaves are already full of moisture. Wind can have some affect as the reflections and attenuation of nearby trees can change unpredictably. But, overall, things are pretty consistent.


In Autumn the leaves begin to dry out before falling. They loose their moisture. They don't absorb or reflect signals the same as they did in the Summer. Rain has a greater affect as it clings to the leaves increasing significantly the moisture they hold, if only temporarily. As the leaves fall off the trees, the attenuation of the signals is decreased markedly. The large reflective surface of the trees also is reduced to the much smaller area of the tree itself. Rain, especially heavy rain, does have an affect on signal propagation. A storm from moving in between the transmitter and receiver sites will have an affect on reception. All of this changes on a daily, even hourly, basis.


In Winter when the leaves are gone propagation settles down again, but it is not the same as Summer. Absent the leaves on the trees the signals can bounce off of things that the trees blocked in the Summer. The multipath characteristics of the signal will be different with the leaves gone. Wind is less a factor too. Snow, however can become one. Snow accumulation on a roof will affect the reception of attic mounted antennas. Snow accumulation on a neighbors roof will affect the reflection of the signals off of his roof. Big, fat, wet, snowflakes falling gently to the ground will absorb signals. Ice accumulating on outdoor antennas will also cause a reduction in signal levels and antenna efficiency. Reception conditions can radically change every few days or week to week.


In Spring the leaves begin to appear again. Slowly, as the leaves grow to full size, the reception characteristics will change. As rain comes and goes it begins to have a lesser affect as the leaves reach full size. The wind, however, begins to play a bigger factor as it moves the leaves and changes their reflection and absorption of signals. These changes happen pretty slowly and the affects are noticed on a week to week or a month to month basis. Eventually the rains diminish, the trees are full of foliage and we are back to Summer again.


The cycle resumes. If you have relied on off-air reception of digital signals for less than a year, and from the sales numbers this may be pretty common, you have not yet experienced the full cycle. If you are a long time DTV viewer you may have forgotten about the reception changes a whole year before.


Remember that the 'signal level' reported by digital television receivers is a 'receivability' factor and not just 'signal strength'. The 'receivability' is displaying, on a relative scale, the number of errors in the digital signal that affect successful decoding of the signal. Signal strength is only one factor. Multipath reflections, static (such as off building and trees) or dynamic (such as off trucks and planes) are another. All of these can and do change seasonally.
 

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Good stuff Thanks :)


I guess I will LEAVE my antenna where it is.....:eek:
 

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Quote:
Originally posted by Lee Wood
Bumping up so the weekday readers see it too.
Seasonal variation is not a problem in that world where the other modulation system is being used. You don't hear of reception problems in Australia any season.
 

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It is starting here in Woodinville (Seattle) now... Oh Well, the leaves will be down soon and things will settle in.
 

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Discussion Starter · #6 ·
Quote:
Originally posted by Bob Miller



Seasonal variation is not a problem in that world where the other modulation system is being used. You don't hear of reception problems in Australia any season.
Oh, Please, Bob. Even COFDM is not immune to changes in propagation. And, maybe, the reason you don't hear anything from Australia about seasonal changes is that they have not yet gone through a full year with a large enough installed base to realize the changes.
 

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Lee, thanks for the great summary. I really enjoyed reading it. I was going going nuts last night with my TU-DST50. Man the wind was blowing and the leaves were flying and the rotor on the roof was being tweaked to no end. Now on the other hand my Mitsubishi SR-HD5 was still pulling a solid 100 signal strength with a rare dip which I imagine is multipath as you described so well. Second generation hardware is definitely moving HD reception forward.



Rick
 
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