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Antenna for channel 36

Discussion in 'Electronic Design' started by Peabody, Nov 28, 2012.

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  1. Guest

    One without a modifier. ;-)
    Bandwidth you can get by detuning the directors and reflectors, at
    least somewhat. If you need more, a Yagi isn't the right topology. A
    log-periodic is a better choice.

    I don't understand the 300/75 balun thing. Isn't a 1/2 wave dipole 75
    ohms directly? It's been a long time...
    ....and it'll yell at you, calling you autistic.
  2. tm

    tm Guest

    Oh, good info. Thanks. And I agree, the elements would be better if solid.

    The spacing from the rear reflector could also be a factor if not correct.

    Years ago I built up a ten foot yagi from 1/2 inch copper pipe on Ch-22 and
    put it in the attic. It really worked well. I used a pre-amp that went from
    300 ohms balanced to 75 coax to the basement distribution system. Used it
    for when the cable (often) went out (Comcast).

  3. Jasen Betts

    Jasen Betts Guest

    If you put up a huge mast you may get a usable signal,
    but it's probably beyond hope. do any of your neighbours
    get a good signal from that station.

    We get marginal analogue UHF here and no usable digital UHF.
    and all the digital is UHF here.

    I put up a satellite dish and tht works well except in high
    winds, I need to put some stay bars on it or move it to a less
    aesthetic, more sheltered location.
    A 7 element yagi isn't going to give buckets of signal strength,
    but there's no saying what it might do to the signal quality.
    Some designs use a loop for the driven element
    yes, the geometry of the antenna is critical
  4. Jasen Betts

    Jasen Betts Guest

    It looks like a bowtie phased array done wrong.

    The feedpoint should be in the centre, or the vertical wires should
    cross. I guestimate it to be about 150 ohms, so it may need a
    custom balun.

    That reflector may be too close too, or is that to change the
  5. Peabody

    Peabody Guest

    Here's the link to the analysis of my location:

    The channel I'm trying to get is real 36 (virtual 35) KRSC, which
    is 29.5 miles away on a magnetic heading of 35 degrees. And going
    out in the back yard with my Boy Scout compass, I find that the
    hill situation isn't what I thought. The hill is actually more to
    the North. So based on the TVfool analysis and what the station
    engineer told me, a rooftop antenna should definitely work. But,
    you know, it's only one channel, and I don't think I want to got to
    that expense for one channel. So I'm going to try the Yagi and see
    what happens.

    Oddly, there is one local channel that I can't get, and that's
    KTUL, real channel 10 (virtual 8). And that's despite the fact
    that real 11, within 10 degrees of 10, comes in fine. Very
  6. Joerg

    Joerg Guest

    Of course I could play such tricks. But we have TV signals coming from
    four locations and the respective channels are all over the map. So this
    would get old really fast.
  7. Peabody

    Peabody Guest

    Jeff Liebermann says...
    Yes, I'm sure. The Zenith picks up nothing as virtual
    channel 8 even on repeated auto-scans at different antenna
    positions, or when I try to go to virtual 8 on the remote.

    And it also has a menu function where you can go to a real
    channel number (i.e. - 10) manually. It shows something is
    there, about like it does on channel 36, but too low a
    signal strength to bring it in. Turning the antenna doesn't
    have much effect. Well, there must be something in the
    way, I guess, but it's still surprising since it's a VHF
    channel, which should come through pretty readily.
  8. miso

    miso Guest

    But with a yagi, you have many more dimensions to get right. Driven
    element, space to next element, first direction, space, second director,
    blah blah blah. That was my point. The "Loop yagi" doesn't have many
    things to cut, while a multi-element yagi does.
  9. miso

    miso Guest

    I prefer SPLAT!, though you need a bit of linux knowledge to run it.
    Radio Mobile is OK, but you need to set up some options at sufficient
    tolerance to get a decent profile. The advantage to Radio Mobile is it
    runs in windows, while SPLAT! needs to be compiled.

    Both programs are free.
    I've also done LOS analysis in GRASS, but that is really tough to use.
    There is a paper by some high school students doing wifi analysis with
    GRASS and I seriously doubt they did it without some adult help. Just
    loading GRASS itself is a bear since the order of compilation is critical.
  10. Mark Zenier

    Mark Zenier Guest

  11. Guest

    You might want to try a Fractal Antenna. Though they are not designed for a specific frequency, they seem easy to make.
  12. Incorporate a screw-driven hose-clamp perhaps?
    VK3BLN has an interesting and extremely effective DF antenna for 70cm.
    A DF antenna doesn't necessarily want maximum gain or even F/B ratio,
    what it wants is to minimize lobing, across the band. Difficult.
    It's a five-element Yagi that has two short-long pairs. We believe it
    works like a 3-element yagi but with the pairs coupling to broaden the
    response. It took a lot of optimising - using a custom build of 4NEC
    with a multi-dimensional slope optimiser.

    Clifford Heath.
  13. You can get hose-clamps with inches of adjustability. I'm certain you
    could adjust to within 1mm and have it stay there too. The ones I'm
    thinking of have a screw as a worm-drive on the ribbon.
    It does. The BLN foxhunt team has that all in-car with custom SDR
    with spectrum display (user-settable brick-wall filters to eliminate
    adjacent-channel interference) and automated lobe analysis that sends
    bearings to display on the GPS maps. Motorised antenna with motorised
    polarization adjustment too. They're very hard to beat!
    I don't think he's published either the antenna nor the custom mods to
    NEC, but contact me privately and I can at least send you the dimensions.

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