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Old 12-Oct-2010, 9:34 AM   #24
mtownsend
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Join Date: Dec 2009
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Quote:
Originally Posted by cardinalfanrc View Post
I saw no pixelation except for channel 7.1, which had several brief periods of pixelation. When I checked the signal strength on channel 7.1, it's about 60. This is a little odd because the signal strength from the other channels broadcasting from the same tower is about 90.
This might be an indication that the VHF portion of the antenna is not performing as well as the UHF portion of the antenna. Even though the signal strengths "in the air" are similar across channels, you are relying on different parts of the antenna to pick up VHF vs. UHF stations.



Quote:
I changed the direction of the antenna within a 45 degree arc and this didn't seem to make any significant difference in signal strength on any of the channels. So is a channel strength of 60 low enough to cause pixelation on my channel 7.1?
Signal meters on most receivers usually estimate signal quality as opposed to signal strength. In many cases, the value is related to the number of data errors being detected in the digital data stream. If too much data is lost or if certain critical portions of the data are lost, then the video stream cannot be properly reconstructed, and you end up with pixellation, macro-blocking, smearing, and/or dropouts. However, since there is no way of knowing exactly how your receiver computed the number 60 (there is no standard computation method), its impossible to tell how the signal meter reading relates to picture quality.

In most cases, there is no correlation between actual signal strength (power) and signal meter readings. That is, even a very weak signal might achieve a score of 100 if it is very clean. Conversely, a very strong signal might get a lower score (e.g., 60) if it is getting messed up by multipath interference, distortion, or noise.



Quote:
I'm wondering if the problem is related to interference from trees and my neighbor's 2 story house. Below is a link to a picture taken from the level of the antenna. Basically the antenna is about the same height as the neighbor's roof and points pretty much right at the neighbor's house. Could this be causing the pixelation?
Your neighbor's house and the surrounding trees do not appear to be that big of a problem. Trees and homes do create some multipath, but based on what is visible in your photo, your situation does not seem that bad.

BTW, it looks like some of the longer elements at the back of the antenna (the VHF elements) may be flipped the wrong way. Those click-in-place top/bottom elements are supposed to alternate. That is, if one element has the top piece going to the left and the bottom piece going to the right, the next element should have the top piece going to the right and the bottom piece going to the left. In your picture it looks like you have two of those elements in a row going in the same direction. You probably need to un-latch some of those pieces and flip them around to get them into the proper position. This is a common mistake since the installation instructions don't spell this out clearly enough.



Quote:
I currently have a 10 foot mast. Should I try a taller mast to clear the neighbor's roof?
No, that should not be necessary.



Quote:
Originally Posted by cardinalfanrc View Post
I moved the antenna to the other side of the chimney this weekend and my signal on channel 7 increased from 60 to 90.
It would be interesting to see if fixing the antenna element positions changes anything.



Quote:
Tower Guy, can I get an HLSJ at Fry's. I assume L is the low input (which I leave empty) and then run cable from the high input to my TV? Will this mess up any of the other channels (major networks are all I'm interested in)?
The HLSJ is a diplexer with its frequency split in between low-VHF and hi-VHF. The high input passes everything from high-VHF and up (including UHF), so this should not disturb your reception on other channels.

BTW, the HD7694P only covers high-VHF and UHF, so it already has pretty low gain on low-VHF (which includes FM) frequencies. Adding an HLSJ will take out even more of the low-VHF signal. I'm just not sure how important this is.



Quote:
Originally Posted by cardinalfanrc View Post
Awww crap. I think I'm making things worse. So, with moving the antenna four feet to the other side of the chimney, channel 7 now has 89-93 signal strength (up from 60) and I haven't seen any more pixelation. Unfortunately, I now have pixelation on channels 2, 5 and 9.
Quote:
there was a bit of pixelation this afternoon (channels 2,5,9) but I haven't really seen any yet tonight. I'm really confused as to why I can't get a reasonable signal based on my TV fool report.
My next step is to try putting the antenna at the peak of my roof (not the chimney) to see if this makes any difference. Any other suggestions?
If the interference comes and goes sporadically, I wonder if it is being caused by people talking on their key-up / key-down radios (e.g., ham radio operators, fire/police/ambulance, etc.). Do you have any police stations, fire stations, hospitals, or ham operators nearby that are in the path of your antenna?

I agree that your signals are very strong and should be very easy to receive cleanly. It is a mystery why you're seeing pixellation. Since nothing else seems to be fixing the problem, the next thing I would try is to attenuate the signal going into your receiver (with something like this). Supposing that there is a nearby transmitter that is causing interference intermittently, it might be introducing enough RF energy to overwhelm the front end of your receiver. If you attenuate the input signals, it might lower the RF energy enough to allow your receiver's front end to cope with the signals better (allowing the built-in AGC and adjacent channel rejection mechanisms to do their job more effectively).
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