Towers & Such 2021

Viewing 15 posts - 16 through 30 (of 170 total)
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  • #49394
    LinleyG
    Participant

    On all the sets I’ve looked at, the signal level function is more of a S/N* meter rather than a signal amplitude meter. KNMT may be just the signal with the lowest multipath.

    A preamp may help just a bit. ATSC threshold with a clean signal is when the signal exceeds kTB noise (-106dBm) plus ATSC’s required S/N (15.5 dB) plus the set’s noise figure. Most sets have a noise figure of about 5dB or 6dB. This means that, with that clean signal, they will come above threshold with a -85dBm or so signal. Sets have enough gain to overcome low signal amplitude but not when the signal crashes into the noise floor of the world. S/N is all that matters, not signal amplitude alone.

    In situations where there is a long coax between the antenna and the set, the coax’s loss will move that threshold up 1dB for every dB of its loss. Preamps are mostly useful then to put at the antenna end of that coax to raise the signal’s amplitude enough that the coax’s loss doesn’t matter and the preamp’s noise figure dominates the situation. A preamp is also useful when the antenna’s signal is split to two or more sets, but is does the most good when it’s at the antenna.

    Preamps typically have quite low noise figures. That’s why it may be helpful in your situation. If the set’s noise figure is, say 6dB, and the preamp’s is, say 1dB or 2dB, you will gain perhaps 4dB or 5dB S/N. You will, in effect improve a weak signal’s S/N enough to bring it above threshold or give it more margin above threshold.

    All these numbers assume a clean signal. In real life, multipath tears up the signal and the receiver’s threshold increases as its equalizer works to fix the problems. There isn’t much known about how much the threshold is increased for a given amount of multipath.

    Remember that once one has enough S/N that the signal has lots of margin (say 10dB to 15dB above threshold), more signal doesn’t help. In fact it sometimes causes problems.

    Linley

    * Signal to Noise ratio. Needs to be added to the Jargon File. Noise Figure could be added also.

    #49397
    Andy Brown
    Participant

    S/N is listed.

    #49401
    LinleyG
    Participant

    Missed it.

    Sorry about that.

    L

    #49404
    Jeffrey Kopp
    Participant

    I got my amplifier today. Some channels come in a bit better, one a lot worse.

    #49410
    LinleyG
    Participant

    Jeffery,

    Which stations got better?

    Which one got worse?

    Just pondering if the preamp is overloaded. Does it have an FM trap? Overloading because of FM signals can clobber channels 8, 10, 11 and 12. Overloading on DTV or LTE signals can get others.

    Linley

    #49412
    Jeffrey Kopp
    Participant

    Hi, 10 got better, 6 got worse, 2, 8, 12 and 32 stayed the same, actually they all got a bit worse. I took it off. I think it has a trap.

    #49418
    chessyduck
    Participant

    Please consider adding COL, “Community of License” to the jargon file. For an FM signal a 70dBu signal must cover XX% (85% ?) to be considered as a COL candidate except for NCEs, where 50% must be covered by the 60dBu contour. There’s a similar metric for DTV signals.

    For FMs the alternate calculation approach uses the advanced Longley-Rice algorithm under the “Skytower” precedence.

    #49422
    LinleyG
    Participant

    Jeffery,

    As per usual with coverage problems, facts blow up fanciful theories and leave the situation as muddled as ever.

    The high VHF signals (channels 8, 10 and 12) were the same which destroys any overload from FM theory.

    Then, channel 2 got worse while channel 6 was essentially unchanged. Channel 2 is actually transmitted on channel 24 while channel 6 is on channel 25. The two are literally transmitted from the same antenna with (it is said) the same power. But, at my house in Aloha, channel 2 is consistently 3dB or 4dB louder than channel 6 on an outside antenna and a dB or two smaller on an inside antenna. Go figure.

    If it weren’t for the dratted Covid, I would loan you a ChannelMaster 600 MHz LTE filter to see if you are getting whacked that way. Since you say that your other (preamplified) antenna works well, that’s probably not the problem either. So it’s not worth the ~$35 cost for you to make the experiment.

    If you want to experiment and it’s easy, you might trade the locations of the two antennas just to see if the problem travels with the antenna or whether the location is pretty much the issue.

    Linley

    #49425
    semoochie
    Participant

    Perhaps, it’s an issue inside your receiver, with that particular channel.

    #49429
    boisebill
    Participant

    With UHF just moving the antenna a few feet (in any direction) will result in changes to signal strength between channels.
    Even VHF. Just like when you’re stopped in traffic and the FM station gets fuzzy, so you move up a foot or so make it come in better.

    #49430
    Jeffrey Kopp
    Participant

    The amplified Terk on the living room TV is on top of the cabinet, five feet off the ground, while my bedroom TV unamplified antenna is on a desk. That may be some of the difference. I am not going to swap them as the big TV works fine now and I don’t want to mess it up. I just fiddle with the bedroom antenna a bit when I change channels. The real test is when it rains.

    #49431
    Andy Brown
    Participant

    If you’re really a glutton for punishment, check out boundary conditions. In the study of electromagnetic waves and their propagation, reflection and refraction, whether or not boundary conditions are met is basically (from the perspective of a receiving antenna/tuner – the whole enchilada). The angle of incidence to the boundary is critical (see second link).

    Kid: Dad, why when we go through the tunnel do we lose the radio station?

    Dad: Because boundary conditions are not being met.

    Understanding what is a boundary (a change in medium, like from air to the siding on the house or from air to just about anything that will reflect or refract) and what boundary conditions are and when they are satisfied or not is just not easy to explain.

    https://fas.org/man/dod-101/navy/docs/es310/propagat/Propagat.htm

    https://www.ihe.kit.edu/img/studium/Wave_Propagation.pdf

    In lay terms, receiving any RF signal indoors becomes subject to all the OTH aspects of propagation anomalies which are easy to observe in nature and impossible to explain without a lot of time, patience and study.

    You have to be really good at math and physics.
    I barely understood it back in the day.

    Just sayin’

    #49432
    Jeffrey Kopp
    Participant

    When I was a radioman, my rule was: when the sun goes up, the frequencies go up, and when the sun goes down the frequencies go down, and the further you are the frequencies are higher. I was amazed how many radiomen did not grasp this basic principle. In Alaskan waters our comm sta was Kodiak though San Francisco would have been easier due to the higher frequencies, especially in winter.

    #49534
    Jeffrey Kopp
    Participant

    I was mistaken, my Terk antenna is not a Yagi but a log-periodic. Illustration: https://www.terk.com/indoor-antennas/?sku=LOGTVAZ

    I found clipping a wire between the elements at the base improves performance on UHF by 1 bar.

    #49586
    DarkStar
    Participant

    On January 31st (not sure of the time), Stadium on KATU 2.4 will be swapping with TBD on KUNP-LD 47.2 (KUNP 16.2).

    I have no idea if/when TBD will be leaving KRCW 32.4.

    The NOC will be making the routing changes around 5 AM, however the program guide data might be delayed.

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