Over the air...... does it help to use two antennas instead of just one?

Discussion in 'TiVo Help Center' started by True Colors, Jul 3, 2011.

  1. Jul 3, 2011 #1 of 27
    True Colors

    True Colors Member

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    I have a Tivo premiere. I use over the air signals only.

    I have an antenna mounted inside my attic...... (FYI, it is not possible for me to put the antenna on top of my roof). Also, I do have a pre-amp hooked up.

    When I point my antenna south i get good reception on a few channels but not as good with some others.

    When I turn the antenna a slightly different direction I lose some reception with some channels but I gain with others.

    So my question....... could I put two antennas up there pointed in different directions and merge the signals together into one feed using a coax joiner and then send that into my Premiere? Would that help?

    Thanks,

    TC
     
  2. Jul 3, 2011 #2 of 27
    stahta01

    stahta01 Simple Member

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    It will help get weak signals; but, it can cause ghosting.

    No, idea if ghosting is a problem with Tivo premiere.

    Tim S.
     
  3. Jul 3, 2011 #3 of 27
    mr.unnatural

    mr.unnatural Well-Known Member

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    Ghosting only occurs with analog signals. Digital TV has a completely different set of problems. The main problem you'll run into with digital TV and multiple antennas is multi-path interference. This should not be much of a problem is the broadcast towers are in different directions and you use directional antennas. You can combine the outputs of two or more antennas into a single coax cable using a standard VHF/UHF splitter/combiner available at any RatShack or Home Depot.
     
  4. Jul 3, 2011 #4 of 27
    mec1991

    mec1991 Cranky old coot

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    Can you use a rotator on your antenna?
     
  5. Jul 3, 2011 #5 of 27
    Aero 1

    Aero 1 Active Member

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    I use two antennas, but they are pointed in the same direction. The reason for two is that some channels are in UHF and others are in the hi VHF band. The VHF antenna came with a combiner to easily combine both antennas into 1 coax.

    I have a feeling that's what you need. When you slightly move the antenna, it's probably becoming easier for the antenna element to pick up the lower frequencies.

    Post a tvfool report of your location for better understanding your situation.
     
  6. Jul 4, 2011 #6 of 27
    unitron

    unitron Well-Known Member

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    Assuming that nothing is/was wrong with your analog receiver, ghosting is/was caused by multi-path. Signal arrives at antenna, gets on screen. Bounce of signal off of something arrives at antenna, but delayed by extra distance of not traveling in a straight line, but in straight line to reflecting object and straight line from reflecting object to antenna. (other two sides of triangle add up to greater length than hypotenuse) Gets put on screen slightly to the right of original image due to the way CRT scanning works.
     
  7. Jul 4, 2011 #7 of 27
    Resist

    Resist Well-Known Member

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    That's probably only an option if he can get power in his attic to run the rotator.
     
  8. Jul 4, 2011 #8 of 27
    JimboG

    JimboG New Member

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    If he can run a coax cable down from the attic to the TV he should be able to run a three or four conductor rotor control and power line up to the attic.

    You do not need a power outlet in the attic to run an antenna rotator.
     
  9. Jul 4, 2011 #9 of 27
    gastrof

    gastrof Hubcaps r in fashion

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    It might be better to go with the "two or more" antenna solution.

    That way, the Tivo is always getting good signals for all channels, and nobody has to be there to adjust a rotator.

    As for combining a feed from more than one antenna, I seem to recall you needing more than just a splitter used in reverse.

    Might not apply with digital, but I'd poke about a bit on the internet and see what sort of combiners there might be for OTA antennas.
     
  10. orangeboy

    orangeboy yes, I AM orangeboy!

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    Diplexer may be what is wanted.
     
  11. a68oliver

    a68oliver Member

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    Crawfordsvil...
    Two or more antennas can be combined to provide better reception. However, as has been mentioned, antennas pointed in different directions can create multipath issues.

    IF you have multipath issues, the solution is to use band pass/blocking filters on the two lines to only allow the strong signals from the desired channels to pass through and block the weaker (interfering) signals from each antenna. It can get quite complicated.

    The AVS Forum may have more info regarding antennas and reception issues which may help you. http://www.avsforum.com/ There are a couple of sections relating to HDTV
     
  12. HomeUser

    HomeUser Active Member

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    You could try a pre-amp to over-come the loss of the coax and splitters there are Amps that are powered through the coax. A quick search found this one Winegard AP-8700 Preamplifier (Amazon.com) with a little research you should be able to find some in the $20 range.

    To compute the signal loss add 3 db for each device (TiVo, TV ...) that is connected to the coax and 6 db for every 100' of coax the total is the gain of amp that you should look for.

    Using 2 antennas could work as stated in the messages above there may be problems with multi-path interference if the interference can not be overcome by repositioning the antennas it would require the use of cut channel antennas and traps increasing the cost. Then again there is some signal loss using the combiner and a pre-amp may be needed IAC.
     
  13. aaronwt

    aaronwt UHD Addict

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    it helped with my HD TiVos seven years ago. I set up a square shooter antenna but could not get one of the main stations since it was at a diferent tower. So I used a signal combiner with another square Shooter and Antenna to point in the direction of the other tower. All my stations came in fine after that. I have had zero issues with ghosting or multipath. It works fine with my Premieres as well. Although now my Antennas are only used as backup to my FiOS connection.
     
  14. unitron

    unitron Well-Known Member

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    Perhaps you could be a little more specific about brand and model number of that "signal combiner".
     
  15. lrhorer

    lrhorer Active Member

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    This can definitely work. Indeed, it is how most CATV systems and and all MATV systems used to obtain their OTA signals for inclusion in the signal line-up. Some small CATV systems probably still do, and probably a lot of MATV systems. It's often not quite as simple as one might think, though.

    No digital system ever will. Ghosting is strictly an analog phenomenon.

    How do you know? You may very well be experiencing mutipath issues, but at a level below which it would be problematical for your receiver. That's the beauty of digital. Signal issues that would produce a completely unacceptable picture in an analog system may be far below the level where they ever even make any difference at all to a digital signal. OTOH, it is also a problem with digital. The issue may be at a level just barely below where a problem is noticed, and if it gets just a tiny bit worse, suddenly the system fails completely. You may have no noticeable problem, at all, but your next door neighbor may get completely unacceptable results with an identical setup.

    I don't intend that as discouragement for anyone, just a heads-up not to be shocked if their multi-element antenna array gives them fits.
     
  16. lrhorer

    lrhorer Active Member

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    San...
    If it is not a diplexer (different frequency bands served by each drop / add port), then there is no difference between a splitter and a combiner, other than the direction the user sends the signals through the device. Any good quality 50-1000MHz splitter will work.
     
  17. unitron

    unitron Well-Known Member

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    I know that a splitter *can* be used backwards, or even as an injector, with varying results, but he didn't say "backwards splitter", he said "signal combiner", so I was wondering if it was anything more than a splitter turned around.
     
  18. lrhorer

    lrhorer Active Member

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    San...
    'Careful. If there are two towers (or clusters of towers) separated by a significant angle, then getting a more directional antenna will increase the signal from one tower while decreasing the signal from the other. The signal gain enjoyed by the line-of-sight tower may be over-come by the insertion loss of the combiner - typically about 4 dB. Adding a second directional antenna may greatly increase the signal from the second tower.

    Not on a digital signal, it won't. It can cause a sharp rise in un-recoverable bit errors, though.
     
  19. lrhorer

    lrhorer Active Member

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    San...
    Any splitter shares the incoming signal between two output feeds. In the simplest configuration, half the incoming signal goes to one leg and half to the other. This is known as a "2-way splitter" or "2-way Directional Coupler". Furthermore, because of the way passive RF devices are constructed, signal will pass equally well in both directions. (This is not the case with optical devices.) Thus, if one is dealing with a DC-8, which drops about 1dB on the low-loss leg and 8 dB on the high loss leg when passing from the input to the outputs, then the loss in the reverse direction is, respectively, the same. There are, however, devices which split the incoming signal not into two attenuated but otherwise identical copies of the incoming signal, but rather split the incoming spectrum into two or more frequency ranges, with relatively low losses between the input and outputs. These also work symmetrically between input and outputs, but while the in-band loss in both directions is low on each port, the out-of band loss is quite high. This is called a diplexer, and it can be used to split a broadband input into two output ranges - say VHF and UHF - or to combine separate sets of signals in each respective range into one wide spectrum. Unlike the directional coupler, which suffers a significant loss between its input and its outputs, diplexers may lose less than 0.5 db when signal passing in either direction.
     
  20. lrhorer

    lrhorer Active Member

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    San...
    This is correct. One must be careful, however. Simply boosting a signal is not the panacea many people think it should be. Indeed, many people dealing with low signal levels are quite disappointed to find an amplifier makes little difference, or even may make the situation worse.

    First of all, every amplifier contributes its own noise and distortion to the signal. If the signal is already noisy or distorted, the amp is only going to make the situation worse. Now, amplifier distortion is not usually a big problem for OTA amplifiers, but noise can be. Any amp attached to the end of a transmission line is going to amplify the noise precisely the same amount as it does the signal, plus adding its own noise. If the Signal/Noise ratio is 10 dB at the end of the piece of coax, then no matter how powerful the amp, the output is never going to have a S/N any better than 10 dB, and it is the S/N ratio that determines PQ. Note the name "pre-amp". The trick is not to try to get the amp to overcome a low signal, but to amplify the signal before it gets low, in order to overcome the attenuation it is about to encounter. Typically, the amp should be mounted as close to the source - in this case the antenna - as possible. Many pre-amps can be mast mounted. There are some cases where an amplifier is required even though the signal at the source is too high to feed directly into the amp. In this case, the amp should be inserted a little further down the line.

    No. Add 3.5 - 4 dB (3 dB is a little too conservative) each time the number of TVs doubles:

    UHF:
    2 TVs = 4 dB
    4 TVs = 8 dB
    8 TVs = 12 dB
    16 TVs = 16 dB

    VHF:
    2 TVs = 3.5 dB
    4 TVs = 7.0 dB
    8 TVs = 10.5 dB
    16 TVs = 14 dB

    It also depends on the topology. Ordinarily, 6 TVs (anything more than 4) would also equal 12 dB, but it is possible to obtain a 6-way splitter that only loses about 9 dB. In any case, 32 TVs would only require a 20 dB amplifier, not a 96 dB amplifier.

    Yeah, not that, either. First of all, it depends on the type and grade of coax used. The top of the OTA VHF band is 216 MHz. Assuming RG-6 drops, the loss at that frequency is about 2.9 dB / 100 ft, depending on grade. The bottom of the VHF spectrum is 55 MHz, where the loss is about 1.5dB / 100 ft. The bottom of the UHF band is 470MHz, extending up to 890 MHz. RG-6 cable attenuates typically between 4.25 dB (low end) and 6.5 dB (high end) at those frequencies.

    Note also it is not the total amount of coax in use or total number of ports, but the amount of coax and number of splits between the antenna and the receiver that counts. Thus, while most 3-way splitters have two 7 dB legs and one 3.5 dB leg, resulting in potentially much different signal levels to two of the sets, putting 50 ft of coax on both 7.5 dB legs and 120 ft of coax on the 3.5 dB leg results in all three TVs receiving fairly close to the same signal levels. No amount of cable loss, however, is exactly equal to any amount of flat loss. The tilt is going to be different on the 120 ft run to that on the 50 ft runs.
     

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