Tropospheric ducting or scatter vertically polarized?

Discussion in 'VHF/UHF - 50Mhz and Beyond' started by KJ4RZZ, May 8, 2017.

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  1. KJ4RZZ

    KJ4RZZ Ham Member QRZ Page

    Hmm ok, but I'm not really here to discuss the slim jim.

    "Slim Jim produces a lower takeoff angle and better electrical performance than a 5/8 wavelength ground plane antenna. The approximate gain in the H-plane of the Slim Jim is from 1.5 to 2.6 dB"

    from:

    https://en.wikipedia.org/wiki/J-pole_antenna



    We we need more height for line of sight, that is basic knowledge. At 28 miles for line of sight we would need our antennas about 100'.

    But this is why I'm asking about propagation, not line of sight.

    How can we work scatter or tropospheric ducting?

    Is horizontal polarization BETTER for either of these and why? Don't say "because DXers all run horizontal for SSB", that's not a technical answer.

    Do we want a high takeoff angle to reach each other via scatter?

    Do we need a low takeoff angle to reach each other via ducting?

    Which is better for 28 miles? Scatter?
     
  2. K8JD

    K8JD Ham Member QRZ Page

    The VHF radio horizon is farther away than an OPTICAL horizon.
    I re-read Tom's original post and he says they don't have line-of-sight with 30 ft antennas over 28 miles, This is just not true, what they probably have is Urban Diffraction and absorbtion in the Tampa-St. Pete area. the terrain is pretty flat or partly over water. Lot of big buildings could be in the way and receiver desense from electrical noise from neighborhoods may also be a factor in the path problem there.
    Just need more signal to over-come all that.
    SCATTERING microwaves.
    I recall the microwave antenna we had at Cam Rahn Bay to send signals to Saigon by tropo scattering, agout 125 miles. It was called a "billboard reflector" the thing was 30 ft high and 50 ft wide and slightly curved to aim the signal from the waveguide emitter into a straight path.
     
    Last edited: May 8, 2017
  3. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    If all you can have is a vertical antenna, then consider getting one that has some real gain, and more reliable in performance than an unspecified and perhaps homebrew "Slim Jim" or Jpole with their inherent difficulties.

    My favorite suggestion would be A Hustler G-6 or G-7
    https://www.dxengineering.com/parts/hsr-g6-144b
    https://www.dxengineering.com/parts/hsr-g7-144
     
  4. KJ4RZZ

    KJ4RZZ Ham Member QRZ Page

    OK thanks this is the type of answer I was looking for
     
  5. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    If your buddy has room for a 1/4 wl ground plane he surely has room to place this 2 element quad yagi. It will give around 4-5 dB of directional gain, a tad more if adds a third element. Cheap and easy to build with scrap parts. Put it on a stick and it works great for hand helds. This may be just enough gain to make the path for you.

    NOV 1994 - QST (PG. 74)

    Repeater Eater, The

    Author: Murphy, George, VE3ERP
    Article: QST Archive [PDF]
    Keywords: CONSTRUCTION ANTENNA 2 METER QUAD HOMEBREW VHF
     
  6. KJ4RZZ

    KJ4RZZ Ham Member QRZ Page

    Thanks

    Yes there is water between us he's in Bradenton and I'm in Saint Petersburg. But I can easily hit repeaters 20 miles past him another 20 miles, but those are very tall and above my horizon and within line of sight.

    I'm getting conflicting information here.. very confusing!
     

    Attached Files:

  7. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Too close for scatter. You can't create ducting with any kind of antenna or transmitter power, it's a tropospheric phenomenon and only Mother Nature influences it. Ducts form on their own and are very unreliable.
    If you were both horizontally polarized, that should indeed work better, and this has nothing to do with the mode. It has to do with attenuation over non-LOS paths on VHF. RCA determined this in the earliest days of television broadcasting (1930s) and it's the reason why to this day television antennas are horizontally polarized.
    Tropo forward scatter is almost always available but varies a lot with weather (tropo) conditions and would not be needed for a 28-mile path. Ducting is way too unreliable to worry about. For the 28-mile path, since "beams" are evidently out of the question, one or both of you needs to raise his antenna higher above ground. You don't need LOS for this, but even small increases in antenna elevation make quite a difference.

    I have a telescoping (crank-up) tower and on 2m, if I crank it all the way down my 2m antenna rests at about 33' above ground; all the way up, it's about 66' above ground. Same coax, same location, only thing that changes is "height."

    With it "down," repeaters in San Diego (120 miles away) are pretty weak. With it "up," they're full quieting. Under no condition is this anything close to a line of sight path.
     
    K6CLS and KO4LZ like this.
  8. K8JD

    K8JD Ham Member QRZ Page

    Direct wave is going to be a stronger signal than any ducting and FAR greater than any scattering that could occur at yuor distance.
    To take adevantage of any ducting, a weather front has to be lined up between the points you are communitating between. look at a NOAA map to see where fronts are today, tomorrow , they move.
    Vertical antennas are more likely to receive local noise than a horizontal, since yours is non directional it also picks up noise from all directions. Noise will de-sensitize your receiver. That's just ONE REASON.
     
    Last edited: May 8, 2017
  9. KJ4RZZ

    KJ4RZZ Ham Member QRZ Page

    Thanks

    Ok so these repeaters are definitely beyond the horizon at 120 miles.. this confuses me.
     
    Last edited: May 8, 2017
  10. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    Simply untrue. Ducted signals can be as strong as direct path signals. It depends on the strength of the ducting.

    You need to be more specific. I'm sure you know there a numerous forms of ducting, each with different causes and characteristics, e.g. Temperature inversions, ducting aloft, surface ducts due to humidity/temp., frontal boundary, etc.
     
    Last edited: May 8, 2017

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