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Thread: Hard problem in distance RX OTA

  1. #1
    jvvh5897 Guest

    Default Hard problem in distance RX OTA

    I volunteered to help the local translator association with a problem. For 50 years they picked up a set of TV channels from a 120 mile away major city with little real problems from a hill near town, but with the digital transition (are we still in transit?) things have not been so good. With a few new antennas things got better, but there are still problems and I thought I might post and see what folks say and think.

    The problem is bad on signal from ch 7--about 175MHz digital--every day or two someone has to drive up the hill and unplug the convertor box to reset it. I/We think the problem is a software glitch that shows up when the signal get corrupted either by ducting from other digital TV source on the same channel and/or by "static" from arcing or other powerfull interferance.

    I've done a site inspection at the site and a few other hills to see what I could see. The most interesting things I've seen are a couple of really beat-up C band mesh dishes (ice falling from the towers seems to go right through or punch out panels) and I received a digital channel on the same channel as one of the translator's TX channels---that really argues that the digital signal is pretty robust if it can be received with a strong signal on the same ch. Now, conditions are not what they were 50 years ago at the site--there is another city not that far away that has lots of signals on the air, and the town the translators serve has grown and sprawled--not right around the TX site, but not that far either.

    Any thoughts? It is not going to be fixed with a new antenna---the other signals do OK--not great, but good enough though they too seem to need reset every now and then. It could just be the brand of convertor box--they are not standard consumer ones, but that does not mean that they are not closed source and with unknown testing standard (think windows98 blue screen) before release. (Don't ask about my thoughts on convertor boxes and TVs that do both analog and digital and the lack of info on how they work--I'm tempted to tear into my TV or take apart the convertor box I have much like I've reversed engineered STBs for sat use).

  2. #2
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    OK the best thing to do is get a good antenna array set for just that channel, you can do what we called a staggered array, this is where the antennas are staggered from front to back (if you have the room), or a stacked array.(quad works the best)

    You then phase match them into an amplified combiner/filter setup, this will maximize the gain on that channel only, and filters it for a clean signal to the translator.

    If you are using a digital converter to get the channel to the translator then its time for some upgrades, for some equipment ideas and pricing on commercial stuff that will do the job right I would take a look at some of the site's below.

    Code:
    http://www.dawnbroadband.com/site_html/***Show***_Catalog=DAWNbroadband_Category=All_SubCat=All_.html
    Code:
    http://www.trcorp.com/tvdigital.html
    Code:
    http://www.ptibroadcast.com/digital-translators.html
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  3. #3
    jvvh5897 Guest

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    They have the quad stacked array allready. Like I said, an antenna is not the solution--what they have does the job of getting what signal is there, the problem is that the signal is not always there to be picked up--ducting of distant stations gets stronger than the fringe signal there.

    The software seems to be the problem, if just the right things get corrupted at just the wrong time the box freezes (think about the old Magnum boxes and their freeze issue). I set up my consumer RCA convertor box last night with a poor antenna and got it adjusted on the only local digital station so that I could take the box' signal from good to poor to just at the point that it lost it all and the box did OK--I did find one way to get somethig like what they see at the translator site with their boxes, if I had the signal at the failure point and I went into the menu screen when I came out the box would hold the "Can't tune this station" message even if I brought the signal up to good levels--I had to change channel and then back to get the box working on the channel that I had on before going to menu. Not exactly the same software bug, but somewhat similar.
    The boxes on the hill seem to loose it when signal is poor, and that can happen when a plane flies by in the signal path (multipath interferance), or when tropospheric ducting brings in distant signals (wrong channel info), or maybe when strong noise source generates interferance (signal corruption). I did have one case where a scan on the RCA at one time failed to bring in the channel at another time and I had to rescan to get the channel--I know I had good signal, but I think the info stored what not really correct for the station even though it looked OK (at the time I did the scan I was looking for signal in a different direction so I did not really care if I got that one--this is one of the things I don't like about these boxes and TVs--they don't seem to allow for the fact that some situations require scans on different antennas and that you don't really need to clear out all the stations with a scan--I know some boxes do have a re-scan feature for specific channels, but most do not. My TV does not even give me a signal strength meter--don't get me started).

    I've suggested to the guys that they try my RCA box for a while and see what happens and to take the replaced box from the hill and run it on the bench much like I did with the RCA and a marginal antenna to see if the problem happens where it is easy to test.
    Last edited by jvvh5897; 07-29-2011 at 04:48 PM.

  4. #4
    slugworth Guest

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    I miss the signal beep function

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    Boy you do have some problems, as far as the ducting, are the stations that are causing the interference in the same general direction as the one your trying to receive?

    If so then there is not much you can do without some major changes to the site.

    One would be to go higher, this would improve the shot at the desired station and cut down on the ducting.

    Two would be a different location for the site, something at a greater right angle to the interfering stations, this would cut down on the incoming signal to the antenna from them.

    Three would be some very high directional antennas, I know you don't have a problem with the current ones but some with a tighter vertical beam-width would cut down on any atmospherics.

    Four would be a diversity receiver system with auto switching. (you know what that is)



    Without knowing the directions that the interference is coming from I couldn't tell you what to do on the directionality issues.

    The type of antennas you have now and the make model of the box that is dropping out would help, can you do a block diagram of the whole system with model numbers for me to look over?
    ♫♫♫ I知 a lumberjack and I知 OK ♫♫♫ I drink all night and sleep all day. ♫♫♫



    Coming soon to a crop circle near you.....

    There is a 66 and third % chance that I'm on the right planet...

    "I'm happier then a Jackalope in a balloon factory"

    "First rule of testing satellites"
    "If its working fine now, then don't mess with it"

    "Second rule of testing satellites"
    "If you did mess with it, and now it doesn't work, can you blame someone else?"

    "Third rule of testing satellites"
    "If you did mess with it, and it doesn't work now, and you can't blame someone else"
    "Can you HIDE it"?

  6. #6
    jvvh5897 Guest

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    Saddly, if we were still using analog, then the answers might be easy, but with digital there is no way to know where the distant stations are. Most of the station's assigned channels are different too, so what I know of what stations were involved in analog days, pretty much is not worth anything anymore. This morning I took a look at the translators and three of the four active right now were in the locked up mode and frozen--one just happened to have the signal meter showing and it showed 22, so at that point the signal was very poor. Incomming channels are around the ch30 range for most of them only the one mid-band VHF at ch7 and yesterday a front did pass over so ducting is likely the cause (Summer is the worse for the RX and fall improves things, winter is usually not bad at all--another reason to think ducting)--but the way the front was alligned I'm thinking the distant channels that caused error was not in direction of the antenna system. I've checked the FCC database and it looks like the possible directions that ducting could be a problem are many, specially given the range that ducting can get one. The hill is not far above the valley of town of service, but the radio horizon is huge there (think Everest).
    I'll see about getting box details--haven't been in the receiver building yet. Figured that solution is not one that others have tried already.
    BTW, we don't care that the signal drops out, what is the issue is that the boxes don't recover on their own. So, not looking at new antenna systems, or trying to get the signal from dropping out--no heroic measures needed. One suggestion is that we add a system to cut power a couple times a day, but if the box's programming is the issue (and that is not an issue that others thought of) that seems to be a problem that the makers should be addressing--We are not the only ones that get ducting.

  7. #7
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    Bumping this back up JVH,

    Then with what your describing equipment inside looks to be the issue, I have never seen a translator setup lock up due to signal loss, they would always go to the RX mute condition, this would then mute the transmitter,(TX power off) but it should recover, (after a small timeout period) if the equipment is in good working order and is not some type of consumer gear there should be no problems like your talking about.

    Type of equipment installed with model numbers will help greatly.


    P. S. if you want some sticker shock here is a list of some good DTV transmitters.
    Code:
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    ♫♫♫ I知 a lumberjack and I知 OK ♫♫♫ I drink all night and sleep all day. ♫♫♫



    Coming soon to a crop circle near you.....

    There is a 66 and third % chance that I'm on the right planet...

    "I'm happier then a Jackalope in a balloon factory"

    "First rule of testing satellites"
    "If its working fine now, then don't mess with it"

    "Second rule of testing satellites"
    "If you did mess with it, and now it doesn't work, can you blame someone else?"

    "Third rule of testing satellites"
    "If you did mess with it, and it doesn't work now, and you can't blame someone else"
    "Can you HIDE it"?

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    One suggestion is that we add a system to cut power a couple times a day
    Aren't there web-based remote control units you can use to manually reset the boxes (and control other stuff) if they freeze up? That'll save someone from driving up there to reset the things.

    Did it get worse over the summer? Maybe there's a heat issue?
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