DTV Repack Is Causing ‘Harmful Interference

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KC0SAI

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I have a weird issue with this repack, and I have been trying to figure this out for months. It started ever since this repack too.

Whenever a semi or bigger truck drives in front of the path of my TV antenna, it causes TV channel 14 which is my channel 4 to have interference. I thought it was LMR, but it is all semis and big panel trucks not just a few semis or big trucks. Plus I would not think LMR is transmitting all the time too.

I thought it could be something bouncing off the trailers, but it does not matter if it is tanker, box, grain, car, and flat bed trailer or no trailer, it does the same thing. I have even seen a rollback and garage trucks do the same thing too.

So what else does all semis and big trucks have in common?
 

KC0SAI

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They are large metal objects that reflect RF.
That is what I thought too, but why would it be the same amount of interference between a long gas tanker and a small little rollback. The road is only 25MPH, and it is always two seconds of interference. The only ones that might cause longer interference is fully load grain semis.

From what I have seen, cars, pickups, pickups with any sized trailers, farm tracker, or school buses does not cause this interference.
 

prcguy

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You are describing a common problem of multipath distortion cause by reflections off big metal objects like trucks. Airplane flutter or multipath caused by aircraft flying between the transmitter antenna and receive antenna is probably more common and I see that all the time.

You have the direct path from the TV transmitter and the TV antenna on your house. If something big and reflective gets between the antennas it can reflect a good amount of signal that arrives at your TV antenna a few nanoseconds later and under perfect conditions the reflected signal is 180 degrees out of phase from the direct path canceling out the direct path and your TV signal fades out for a second or two.

I have a major flight path to LAX right between my house and most of our local TV transmitters. If I look at a TV signal with a spectrum analyzer and also look at the mountain 40mi away where the TV transmitters live, I can track an airplane going down the path 15mi away and can see a distortion on the spectrum analyzer at the same time as the signal reflected off the aircraft tries to cancel out the direct path.

If your local TV station moved from a VHF channel to UHF its probably more susceptible now to vehicles reflecting things and causing trouble.


I have a weird issue with this repack, and I have been trying to figure this out for months. It started ever since this repack too.

Whenever a semi or bigger truck drives in front of the path of my TV antenna, it causes TV channel 14 which is my channel 4 to have interference. I thought it was LMR, but it is all semis and big panel trucks not just a few semis or big trucks. Plus I would not think LMR is transmitting all the time too.

I thought it could be something bouncing off the trailers, but it does not matter if it is tanker, box, grain, car, and flat bed trailer or no trailer, it does the same thing. I have even seen a rollback and garage trucks do the same thing too.

So what else does all semis and big trucks have in common?
 

KC0SAI

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Mar 23, 2015
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You are describing a common problem of multipath distortion cause by reflections off big metal objects like trucks. Airplane flutter or multipath caused by aircraft flying between the transmitter antenna and receive antenna is probably more common and I see that all the time.

You have the direct path from the TV transmitter and the TV antenna on your house. If something big and reflective gets between the antennas it can reflect a good amount of signal that arrives at your TV antenna a few nanoseconds later and under perfect conditions the reflected signal is 180 degrees out of phase from the direct path canceling out the direct path and your TV signal fades out for a second or two.

I have a major flight path to LAX right between my house and most of our local TV transmitters. If I look at a TV signal with a spectrum analyzer and also look at the mountain 40mi away where the TV transmitters live, I can track an airplane going down the path 15mi away and can see a distortion on the spectrum analyzer at the same time as the signal reflected off the aircraft tries to cancel out the direct path.

If your local TV station moved from a VHF channel to UHF its probably more susceptible now to vehicles reflecting things and causing trouble.

If you want to know, it is only TV channel 14. They use to be on TV channel 41 before this repack, and I have never had any issues before this.

Plus none of my other channels have this issue from what I have seen. Plus I tried to move the antenna maybe a few degrees and different locations to try to get rid of the idea of multipath distortions, but this issue is still around.

The only thing that I have not tried is an LTE filter since I have small cell about a block from my house, but I would not think it would be that tho because that was enabled just a few months now, and this has been happening longer then that.
 

prcguy

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If I had the same problem I would use a spectrum analyzer and a friend with a big truck that could drive the path and stop if necessary as I take measurements. You need to prove what the source of the problem is before buying something that may not have any effect.


If you want to know, it is only TV channel 14. They use to be on TV channel 41 before this repack, and I have never had any issues before this.

Plus none of my other channels have this issue from what I have seen. Plus I tried to move the antenna maybe a few degrees and different locations to try to get rid of the idea of multipath distortions, but this issue is still around.

The only thing that I have not tried is an LTE filter since I have small cell about a block from my house, but I would not think it would be that tho because that was enabled just a few months now, and this has been happening longer then that.
 

GTR8000

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And here is another very clear reminder that public safety takes a back seat to television stations when it comes to the protection of the T-Band spectrum, not the other way around.



1613708344181.png

 

RFI-EMI-GUY

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Is there a decent TV station database any longer? I was trying to hook up an over air antenna to my TV and TVFOOL and others seem to be inaccurate as to what is constructed.
 

MtnBiker2005

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Try Antenna Point app for smartphones.


 

AK9R

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All kinds of TV station data available at RabbitEars.tv


Click the link, find your market, and get a list of TV stations in that market. The site has also other ways to slice and dice the data.
 

gmclam

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Is there a decent TV station database any longer? I was trying to hook up an over air antenna to my TV and TVFOOL and others seem to be inaccurate as to what is constructed.
I noticed some of the originals haven't updated their databases since the first digital switch, or at the latest 2017. If the site wasn't updated in 2020 after the repack, it is obsolete. I found a few, here's one: Antennas Direct | TV Transmitter Locator and Mapping Tool and here's another: TV Broadcast Towers (OTA DTv)
 

W1KNE

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And here is another very clear reminder that public safety takes a back seat to television stations when it comes to the protection of the T-Band spectrum, not the other way around.
That list from the unfortunately FCC has some errors in it, and I contacted the WTB to hopefully get those cleared up.
 

W1KNE

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That list from the unfortunately FCC has some errors in it, and I contacted the WTB to hopefully get those cleared up.
Wow that was some good English, Mike. :rolleyes: I meant That list from the FCC unfortunately has some errors in it.
 

RFI-EMI-GUY

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I noticed some of the originals haven't updated their databases since the first digital switch, or at the latest 2017. If the site wasn't updated in 2020 after the repack, it is obsolete. I found a few, here's one: Antennas Direct | TV Transmitter Locator and Mapping Tool and here's another: TV Broadcast Towers (OTA DTv)
I will try those. I got really weird results when scanning. I moved a TV into a bedroom and hooked up external antenna, first time I scanned it was stuck in CABLE mode instead of ANTENNA and only one channel (plus many virtual channels) showed up. After rectifying the problem, I get more channels but the one found in CABLE mode is gone. Weird
 

RFI-EMI-GUY

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gmclam

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Yeah, those two disagree. I wonder if anyone has updated after repack?
I went to several sites. No one site was 100% correct. Plus I get a lot of "out-of-market" channels, so I have to be creative to learn the information. Another issue I ran into was not all signals being on the air 24/7. I found some low power signals only on the air during the day. And of course, distant signals come in better at night.

Physical channels 2 to 13 are the same for cable or antenna. The frequencies for channels above 14 are different. Over-the-air channels are identified by their logical channel by most tuners/TVs, and that can make the scanning elusive. If your receiver has an option to scan and UPDATE, that's what I try to use (otherwise it replaces everything it previously discovered).
 

RFI-EMI-GUY

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I went to several sites. No one site was 100% correct. Plus I get a lot of "out-of-market" channels, so I have to be creative to learn the information. Another issue I ran into was not all signals being on the air 24/7. I found some low power signals only on the air during the day. And of course, distant signals come in better at night.

Physical channels 2 to 13 are the same for cable or antenna. The frequencies for channels above 14 are different. Over-the-air channels are identified by their logical channel by most tuners/TVs, and that can make the scanning elusive. If your receiver has an option to scan and UPDATE, that's what I try to use (otherwise it replaces everything it previously discovered).

Yeah that's part of the problem. This older Sony Bravia seems to scan and replace the whole list and my antenna is subpar meaning i have to move it around as signals are coming from all over. It is a mystery where the channel 55.xx religious station that scanned on cable mode went to when I scanned properly on antenna mode. Weird.

I need to buy that "4000 FREE Digital Channels" Antenna sold in the airline magazines! Still waiting for my wooden whole house space heater hand crafted by Amish artisans.
 
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