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TV reception cuts out when car passes by

  1. Dec 25, 2015 #1
    My wife's mom lives less than a mile from the tv tower any usually gets great reception. However any time a car passé the house it disrupts the reception. Any ideas why that happens and how we can fix it? The tv and had antenna are quite new.
     
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  3. Dec 25, 2015 #2

    Borg

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  4. Dec 25, 2015 #3

    nsaspook

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    DTV can be very sensitive to multipath signal reception. (The better RF chipsets have very complex multipath rejection circuits but still are unable to receive a stable signal while the antenna is moving even slowly)
    http://www.scientificamerican.com/article/the-multipath-to-clarity/
    http://www.eetimes.com/document.asp?doc_id=1139977
    I suspect the car acts as a signal reflector causing a out of phase additional signal at the antenna. If you have a highly directional antenna you can try to rotate it so the focus of the antenna is away for the reflection. Sometime an attenuator helps with strong signal multipath near TV broadcast antennas.
    https://www.antennasdirect.com/store/Variable-Attenuator.html
     
    Last edited: Dec 25, 2015
  5. Dec 25, 2015 #4

    anorlunda

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    Maybe the car is registered to the NSA :wink: just kidding.

    I think nsaspook is right and that the signals she gets are marginal. Digital does not degrade gradually. It can be flawless, then flip to garbage with only a tiny change in the signal.
     
  6. Dec 25, 2015 #5

    jtbell

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    A more directional antenna might help. In the absence of those reflected signals from cars, she'd probably be OK with the simplest non-directional antenna: VHF rabbit ears and UHF loop, like this. (OK, that one actually has some directionality perpendicular to the loop and "ears", but it's about the simplest TV antenna you can get.) One type that has gotten good reviews in the past is this one. This particular one is discontinued, but there are probably similar ones by other manufacturers. Unfortunately, no names float to the top of my head right now. You'd have to experiment with aiming it in different directions so as to minimize the effect of the reflected signals. The best direction might not be an "obvious" one.

    Whatever you do, don't get an antenna with an amplifier in it! It will surely overload and make things worse. As I recall, the second antenna I linked to (or some similar ones) came in both amplified and unamplified versions.
     
  7. Dec 25, 2015 #6

    CWatters

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    +1

    DTV doesn't degrade in the same way that analogue TV would. You can have a great DTV picture even though your reception is really marginal. Some TV can display signal strength and signal quality levels, perhaps check that, particularly the signal quality as cars go past. Check any connectors between antenna and TV.

    In some cases its possible to have too strong a signal. This can be an issue if they are still broadcasting analogue TV. If you have a distribution amplifier a very strong analogue signal can drive the distribution amp into overload. That in turn can cause problems for the digital signal.

    If the antenna is new perhaps get the installer back to take a look.
     
  8. Dec 25, 2015 #7

    CWatters

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    Does she have the problem on all multiplexes (groups of channels)? Is it possible she is only having problems with one or two multiplexes and she is getting these from another transmitter, not the one just a mile away?
     
  9. Dec 25, 2015 #8

    jtbell

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    Try moving the antenna to different locations in the room. If the cable on the antenna isn't very long, you can extend it with a separate RG-59 or RG-6 cable and a barrel (inline) coupler.
     
  10. Dec 26, 2015 #9

    meBigGuy

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    The culprit is probably multipath. You are receiving two signals that cancel, as opposed to the car blocking the signal. It's possible that the station's antenna is aimed away from you or you are in its shadow for some other reason, making the likelihood of sufficient reflection higher. You can possibly aim the antenna so it rejects the car.
     
  11. Dec 26, 2015 #10

    sophiecentaur

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    It is interesting that most of the above replies are assuming a receiving antenna that's in the room. Reception can be cleaned up considerably with a rooftop / loft mounted antenna. The service is planned on that basis in most countries, although people don't always bother, when close to the mast.
    A low gain, directional antenna with a good front to back ration (say a log periodic) can deal very well with multipath. If the TV tower is very tall, feeding a large service area, the pattern can have a vertical beamwidth of only a few degrees with a bunch of bananas, off centre which is very frequency dependent. (1km away at a vertical separation of a few hundred metres is quite possible near a big mains station). A good DTV receiver could be dealing well with that but reflections from a passing car could be the last straw (and it's varying - which may not help the clever adaptive receiver to cope).
    Signal strength should not be a problem so it could be worth tilting the antenna upwards at a funny angle to eliminate the worst multipath signals.
     
  12. Dec 26, 2015 #11

    jim hardy

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    What does the antenna look like? Can you post a snapshot?

    Sophie's suggestion of an antenna with a front/back ratio is mighty logical.

    Here's the page i used to design my yagi
    http://www.k7mem.com/Electronic_Notebook/antennas/yagi_vhf.html#Design_View
    per this guy a simple 3 element yagi has a decent front/back ratio
    http://www.antennex.com/w4rnl/col0300/amod25.htm

    upload_2015-12-26_4-54-37.png

    a simple straight piece of wire behind the existing antenna, called reflector
    and another in front of it, called director

    will turn it into a yagi of sorts
    My buddy made one from pieces of lampcord taped to his ceiling,

    That first link will calculate the lengths and spacing for you. A not very difficult experiment....

    There are plenty of hams here way more knowledgeable than i...


    see also https://www.physicsforums.com/threads/can-i-parallel-yagis.806936/
     
  13. Dec 26, 2015 #12

    sophiecentaur

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    Very easy to make (which is a huge plus point) but the log periodic is very well behaved over the whole operating bandwidth, with an excellent FBR and only a modest gain (no surprise back lobes at some frequencies). They are frequently recommended for areas of bad multipath (a pesky block of flats just behind).
    I have to declare an interest in that an excellent log periodic was developed at my department and it was used to solve a number of difficult reception problems.
     
  14. Dec 26, 2015 #13
  15. Dec 26, 2015 #14

    sophiecentaur

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    I would say that you probably get what you pay for, here. It may work fine but when conditions are difficult you could be needing a better installation. Mounting at a reasonable height would mean that the antenna may work as designed (if it was). In a room with wiring and people walking about, the pattern you get could be anyone's guess. It would depend upon the relative positions of the room in the house, the transmitter and the traffic. Many of these 'set-top' type designs are no better than the wire coathangers you get from the Dry Cleaning service. (Some of which have done excellent service for students all over the world). It could well be worth while using the full 10ft of cable and going near the ceiling to take it away from nearby objects (but then there can be beds etc upstairs!)
    I'm afraid your mistake was to ask on PF Electrical Engineering Forum. You have created a Frankenstein's monster here which can only give you Professional answers. :biggrin: No bodging allowed.
     
  16. Dec 26, 2015 #15

    jim hardy

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    What are the channels of interest in your area?


    Go to this site
    http://transition.fcc.gov/mb/engineering/dtvmaps/ [Broken]
    and enter Mom's address
    tell them your antenna is 30 feet in the air
    it'll show you all the stations you're apt to receive
    Clicking on one's call letters shows direction to tower and probable strength, more negative means harder to receive

    i get two stations just fine that show -50
    but not the ones below -70

    it also shows the RF channel actually used by the station.
    Analog TV was logical but digital TV is not . The channel number identifying a digital station is now just part of its name, not the number assigned to its RF frequency.
    When the transition happened many stations kept their call letters and analog channel number but shifted to a new frequency
    for example in my neighborhood channel 8, KAIT, is still on RF channel 8
    but channel 19, KTEJ, moved to a new frequency, RF channel 20 , when it went digital
    and channel 6, KEMV, moved to RF channel 13 and reduced power so much i no longer get it.

    The actual frequencies are still in the same old TV bands so there's no need for new antennas but marketeers made a fortune selling "digital antennas" - an absolute hoax.


    Here are the frequencies that go with RF channel numbers
    http://www.csgnetwork.com/tvfreqtable.html

    So to recap
    i'd use that FCC link to plot out what stations you want to receive
    and get an old fashioned antenna that somebody has discarded in favor of a "digital antenna"
    and find a direction to point it that works.

    Mom's local station should boom in on just a foot of bare wire
    so if you make an antenna tune its length for one of your weaker stations at a very different frequency.
    I marked up a map to show transmitter locations and wavelengths - half wavelength is good length for a dipole
    Since a folded dipole works over nearly an octave , one of them should cover all UHF channels from 14 to 83
    Old fashioned classic antennas are log periodic and have great directional behavior
    and if all your stations are UHF you could cut the UHF section off an old "analog" antenna for Mom
    I've made Yagis for daughter in North Carolina and sister in Oklahoma, tuned them precisely for a weak but desirable station letting the stronger ones just boom in. My sister's simple folded dipole, tuned precisely for Tulsa's PBS, got fifteen stations .

    Have some fun at this - and beat the system.!

    old jim
     
    Last edited by a moderator: May 7, 2017
  17. Dec 26, 2015 #16

    anorlunda

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    I hope that you all see the irony here. Remove the word digital and most of what is said here could have been a thread from the 1950s or 60s or 70s or 80s almost verbatim. I remember a skit of The Honeymooners with Ralph Cramden telling Ed Norton to stand in crazy poses to improve TV reception. It does't seem to have changed significantly other than that digital degrades differently than analog. If this was April 1, I would guess that Greg had set us up. :smile:
     
  18. Dec 26, 2015 #17

    nsaspook

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    2447447797_1dd11f22cd_z_d.jpg
    Old fashioned log periodics really work great in DTV multipath hell. I'm up river under the flight-path for the local airport and had picture dropout with almost every plane near the house until I installed my TV antenna system several years ago.
    It's a old Radio Shack vu-190xr log periodic (I needed VHF low) with a Channel Master 7777 pre-amp (gain for several signal splitters in the house). The motor was needed to tune the stable sweet spot with early 2nd gen receivers. The multipath equalization range in modern receivers is over 100 microseconds (Advanced tuner technology does active EQ to find and suppress echoes within the +- timing delay window) so the rotator is not used much now.
     
    Last edited: Dec 26, 2015
  19. Dec 26, 2015 #18

    nsaspook

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    My 2c about OTA DTV.

    We picked the high performance DTV solution of ATSC 8-VSB instead of COFDM (DVB-T). It promised high picture data rates (~20 mbps from a 6 Mhz RF channel) with lower transmitter power. It came with a price, the digital signal decoding can be fragile out of the narrow phase stable range of the receiver . Electronics can only do so much with an RF signal that's strong but distorted (like two people saying the exact same thing to you but one delayed a second and not be able to understand either of them). Most indoor antennas simply don't have the directional capability needed to remove the RF distortion before the receiver.
    https://transition.fcc.gov/Bureaus/Engineering_Technology/Documents/reports/dtvreprt.pdf
     
    Last edited: Dec 26, 2015
  20. Dec 26, 2015 #19

    sophiecentaur

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    The above few posts demonstrate to me the different broadcasting environments in the UK and US. In the UK, the few channels that are available are brought to most of the population without them having to make much of an effort. In the US there are many more channels available but it's up to the ingenuity and money of the individual. The 'plan' in the UK makes it nigh on impossible to watch stuff that you are not intended to see.
     
  21. Dec 27, 2015 #20

    dlgoff

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    Exactly. Now days they call them High Definition VHF/UHF Antenna. Think of all those "analog" antenna that went to landfills with the digital change thing.
     
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