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Old 11-Oct-2010, 5:17 PM   #21
cardinalfanrc
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Thanks for the additional suggestions. I moved the antenna to the other side of the chimney this weekend and my signal on channel 7 increased from 60 to 90. With our toddlers terrorizing the house, I haven't had a chance to see what affect this has on the picture quality. I'll update soon.

kb2fzq, I've tried hard to notice a pattern to the pixelation but haven't come up with much. However, we just don't watch enough TV to have a great sampling. But it seems to occur at all times of the day/night in a random pattern. I've started a record of each time it occurs to see if I can figure out a pattern. As far as RFI goes, I did unplug everything in the house (except the TIVO and receiver) and it didn't help. I'll try again by turning everything off at the circuit breaker and also turning off cell phones.

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)?

I'm hoping the repositioning of the antenna with increased singly strength solves my problem. If not then, I guess it would seem to be an interference problem. I'll keep you updated.
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Old 12-Oct-2010, 5:07 AM   #22
cardinalfanrc
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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. Channel 2, I can understand since the signal strength has dropped from 90 to 55-60 now, but channels 5 and 9 have signal strength of approximately 90 and are getting frequent pixelation. I'm still not sure whether I have a signal problem or interference problem. Interestingly, 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?
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Old 12-Oct-2010, 9:23 AM   #23
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Quote:
Originally Posted by cardinalfanrc View Post
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)?
HLSJ=High (frequency VHF antenna) Low (frequency VHF antenna) Signal Joiner....HLSJ
The high side to the antenna, low side empty, the output (it will say "line") of the HLSJ to the TV...it's like a "T", the downward side of the T is the output..
This setup shouldn't bother anything else...
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Old 12-Oct-2010, 10:34 AM   #24
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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.



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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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Old 12-Oct-2010, 12:58 PM   #25
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Since the issue on VHF is solved, I don't feel the HLSJ or any attenuation is necessary. It is nearly impossible to overload without amplification. You would need to be less than 5 miles from transmitters (except FM) to experience overload in most instances.

The problem channels are all UHF now, so I suggest moving the antenna up/down a few inches at a time to find a sweet spot that works best for the remaining channels. It may take a bit of experimentation, but I find this quite effective time after time, especially when multipath is present.
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Old 12-Oct-2010, 4:53 PM   #26
mtownsend
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Ordinarily, I agree that overload is not likely in a situation like this, but to continue to process of elimination, I wanted to test a few other hypotheses.

Most importantly, the inverted VHF elements should be fixed. By having some of the elements flipped around, it is quite likely that it is changing the gain and radiation pattern for VHF channels. It may also be introducing a slight amount of gain in frequency bands we don't really want. It's hard to say what shape the radiation pattern is right now and whether or not there is any increased gain on non-TV frequencies.

It is also possible for nearby first responder radio equipment to be operating in or near the TV spectrum (e.g., EMS dispatch can be on 155 MHz, WMTS can operate in 608-614 MHz, older legacy equipment might operate on vacant TV channels). If there are any police / fire / ambulance / hospital base stations directly in the path of antenna and very close, it might still be enough to overload the front end of the receiver.

A check of FM Fool indicates that there probably aren't any FM transmitters close enough to cause overload problems, but I don't know how close any of the other possible transmitters (police/fire/ambulance/etc.) are. Anything closer than a mile or two has the potential to cause problems.



Here are some of the clues that seem interesting:

1) The pixellation seems to appear on multiple channels spread across a broad range of spectrum at the same time. This would seem to argue against any kind of co-channel or adjacent-channel interference problems.

2) Moving the antenna from one side of the chimney to the other made VHF perform better, but made multiple UHF channels go bad.

3) There seemed to be some pixellation on multiple channels yesterday afternoon, but then there was a period of time in the evening when none of the channels showed any pixellation. This seems to imply that whatever is causing the interference came and went at around the same time across all of the channels. Is this pattern in any way related to human operators using or not using their radio at certain times of the day?



It would really help if we could attach a spectrum analyzer to this setup to see what's going on.

Last edited by mtownsend; 12-Oct-2010 at 4:54 PM. Reason: Originally logged on at the wrong computer... Ooops
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Old 13-Oct-2010, 2:20 AM   #27
cardinalfanrc
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Before a quick update, I just would like to sincerely thank all of you for your thoughtful and helpful replies. I'm sure you all have other things to do than help troubleshoot my attempts to banish Comcast from my house.

When I moved the antenna to the other side of the chimney a few days ago, I aimed it to maximize channel 7 (the previous problem channel) but I didn't pay too much attention to the other channels. Today I changed the direction of the antenna (by about 20 degrees) so that all channels now get between 88-100 signal strength. I was getting pixelation frequently on channels 2,5, and 9 immediately before re-directing the antenna. After re-directing I saw NO pixelation on any channel for approximately 15 minutes. Unfortunately I didn't have time to watch longer and I'll be working tonight so I won't be able to check again tomorrow. I'm cautiously optimistic that I may be getting closer to a fix.

I still need to check the position of the antenna elements (as suggested by mtownsend) to make sure that they are flipped the correct way. If I'm still having trouble I'll try moving up/down as suggested by No Static At All and buying an attenuator. Interestingly there is a fire station directly in the line of the antenna approximately 1 mile away. Thanks again and I'll report back after more viewing time.
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Old 13-Oct-2010, 9:41 AM   #28
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I'd be interested in what state, county and city that fire station is located in, and it's official title (such as "Smallville Fire and Rescue")...I will look up what frequencies they are using...that station may be your issue....
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Old 13-Oct-2010, 6:31 PM   #29
cardinalfanrc
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Menlo Park Fire Station
Station #1
300 Middlefield Road
Menlo Park, CA, 94025 (San Mateo County)

To test this theory, I could always start a small fire in the back yard and watch some TV as the fire department arrived
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Old 13-Oct-2010, 6:43 PM   #30
cardinalfanrc
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I just mapped the fire station to determine the distance from our house. It's 0.8 miles. It however is not directly in the line of the antenna. It appears to be a little less than 90 degrees off. I assume this is as bad?
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Old 13-Oct-2010, 7:03 PM   #31
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Originally Posted by cardinalfanrc View Post
OK, I'll try lowering the antenna a bit.
Did you unfold all of the elements of the antenna before you erected it?
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Old 14-Oct-2010, 11:23 AM   #32
kb2fzq
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Originally Posted by cardinalfanrc View Post
I just mapped the fire station to determine the distance from our house. It's 0.8 miles. It however is not directly in the line of the antenna. It appears to be a little less than 90 degrees off. I assume this is as bad?
Menlo Park falls under the San Mateo county fire frequencies, which are 151 to 156 megahertz, channel 7 freq is 177 mhz...public services usually run some high power transmissions, and IF they are indeed interfering with you, you don't have to be in line of site to the culprit at 0.8 miles away, they will easily cover your reception at that distance, again "IF" it is them...
Other then a spectrum analyzer, which I'm pretty sure you probably don't have access to, a scanner with the fire frequencies for Menlo plugged in correlating the pixing with fire transmissions is the only way to prove it, that I know of....
I'll PM you the list...
And from looking at your area map, you seem to be in a metro area, there are probably more transmitters then you can count around your area, more bad news, freqencies can mix creating a different frequency, virtually impossible to find the frequency without a spec analyzer, I honestly hope the result of your issues is an easy answer...
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Old 14-Oct-2010, 10:22 PM   #33
cardinalfanrc
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So things are looking up - mostly. I haven't seen any pixelation since moving the antenna to the other side of the chimney and adjusting the direction. I've only had limited viewing time of about 15 minutes per day but haven't seen any picture problems on any channels during the day or evening.

Now for the bad news. Mtownsend was correct in noticing that the last set of elements (largest at the back) on the antenna were pointed the opposite direction so that they were not alternating with the other element sets. I fixed this. But while I turned one of the elements around to point right instead of left, the top metal bracket for the element fell off. The head of the pin keeping the bracket on top popped off. My wife can attest to the loud cursing coming from the roof. I reattached the bracket by wrapping both ends with bare metal wire to attach the top bracket to the bottom bracket (which the element in between). I though about using electric tape but didn't want anything with insulating properties. I really have no idea how the signal conduction from the antenna works. Is this likely to cause any significant problem? The signal strength seems OK on all channels except for channel 7, where it's about 82 (I think it was about 88 before). On the bright side, the picture currently seems OK in my limited viewing.

kb2fzq, thanks for the fire station info. I'm not sure what to do about it, except hope that it's not my real problem.
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Old 19-Oct-2010, 4:29 AM   #34
cardinalfanrc
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Miraculously, my picture quality problems seem to have been fixed. It appears that the combination of replacing all of the cable with new cable from monoprice and moving the antenna to the other side of the chimney did the trick. A big thank you to everyone who commented and offered advice. If you're ever in the San Francisco Bay Area, let me know so that I can buy you a beer
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