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Tropospheric ducting or scatter vertically polarized?

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

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  1. W1VT

    W1VT Ham Member QRZ Page

    http://www.navy-radio.com/manuals/0101-1xx/0101_112-02.pdf
    According to this reference, the 4/3 factor is the result of the dielectric constant decreasing with increasing altitude. If the atmosphere were constant with altitude, then the radio path would be the same as the optical path.
     
  2. W6KCS

    W6KCS Ham Member QRZ Page

    That's true, we depend on it for a few 6 GHz paths that are not line of site. It seems to be a pretty dependable effect. I've seen diffraction work over ridges many times too, in one area in particular where in a valley I can't hit a repeater about 140 miles away, but as soon as I go behind a ridge I suddenly have a decent path. I don't think that's troposcatter. Rege, you have a lot of good input on this forum, but that last post was pretty screwy.
     
    Last edited: May 15, 2017
  3. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    I don't think it is, either.

    The "radio horizon" stuff is all based on strong-signal work that's 100% reliable. Hams don't need that. I work stations 300-400 miles away on 2m SSB all the time under "flat" (normal) conditions. My optical LOS to those areas is maybe ten miles, so even 50% beyond that would be 15 miles. Anybody within 30 miles of me in any direction is very strong on 144/222/432/902/1296 MHz if we just aim antennas at each other...and at 100 miles, most are still fairly strong and easily workable using modest beams.

    Guys operating near big steep hills often catch a bounce off the hills and aim right at them to work the opposite direction. I operated the 2010 ARRL UHF contest from DM13 in Orange County (from N6NB's house) and when the SCCC "rovers" were all up in Pasadena about 50 miles to the north, when we aimed "at each other" on 3.4, 5.7 and 10.3 GHz there was no signal. They all aimed due north, away from me but towards Mt. Wilson, and became literally "59" copy. Simple 180 degree rotation, 50 mile path, rock solid through 10.3 GHz.
     
  4. W6RZ

    W6RZ Ham Member QRZ Page

    I've worked the Hawaii duct on 2 meters from the flat lands here in Silicon Valley. My path profile in that direction would suggest knife edge diffraction was in play.

    [​IMG]
     
  5. WA9SVD

    WA9SVD Ham Member QRZ Page

    What time of year? What year? What time of day? Chances are you caught the "duct."
     
  6. WB2WIK

    WB2WIK Premium Subscriber QRZ Page

    He caught The Pineapple Express for sure, the question is only "how"?

    I live at 850' asl in the San Fernando Valley and in all these years have only been able to work that duct exactly once from home, and I can't even explain how that was possible.

    My "guess" is the duct always has a floor, ceiling and walls but the floor might be flexible and actually roll over the hills at times. I do not have an ocean view from home, although it's pretty close -- the ridge at the south end of Topanga blocks me.

    I've been much more successful working the duct from Saddle Peak at 2800' asl (and pretty close to me, so I can be up there in 20 mins or so) where there's a completely clear view of the ocean -- worked it 5-6 times from up there, including one time on 2m using only a 5/8-wave whip as an antenna. But even that hill, although it's not so high, is often "too high" for the duct, and guys down in Santa Monica are hearing it better.
     
  7. W0AAT

    W0AAT XML Subscriber QRZ Page

    Just had a ragchew with a friend on 2m SSB... 136 miles away. S9+10 signals both ways and band conditions are flat. Another was only running 25 watts and not pointed up my way, he s 125 miles away and was S3... if he had pointed an antenna up this way he would have been pretty strong.
     
  8. AH7I

    AH7I Ham Member QRZ Page

    Horizontal polarization and some gain with both directed at the mountains may be your best bet.

    I do not have experience with ducting over land. I have sent telemetry on ~130MHz over land ~ 16 miles over a ridge with 5W to a 5 element Yagi at both ends and aimed at the ridge. It worked well enough for low bandwidth data over FM.

    When I was working as a navigator (TRANSIT was new, no GPS yet) we regularly used 400MHz range finders out to 100km with only an hour or so of dead time around sunrise and sunset. Signals were horizontal with loop Yagi at both ends. Best height for antennas was in 30' to 60' range.

    You guys might give 10m FM a try. Good luck and have fun.

    73, -Bob ah7i/w4
     
    Last edited: May 21, 2017
  9. AH7I

    AH7I Ham Member QRZ Page

    duplication error
     
  10. KA0HCP

    KA0HCP XML Subscriber QRZ Page

    Sure, knife edge diffraction may be helping your signal get out of the valley, but it is ducting that is carrying it the distance.

    Over water (sea) Surface Ducts or Ducts Aloft can form either from temperature inversions or from layers of high humidity. While we hams are interested in using them for talking, they also have military importance for Signals/Electronics Intelligence. ;) Intercept units will maneuver to place their antennas into such ducts.

    Under optimal conditions ducts take on the properties of virtual tuned Wave Guides that become very low loss paths and allow the propagation of very weak signals over hundreds or thousands of miles.

    [adding]
    Ducts vary in thickness, and thereby the frequency bands they will conduct. For VHF and UHF these ducts may be relatively thick, they also may be sometimes inches or feet in depth which is optimum for higher frequencies.

    Hams looking for long range contacts should put effort into atmospheric profiling, i.e. Temperature, Humidity and Pressure, over the desired path and finding ways to elevate antennas into the duct. The area just above wave height should not be neglected.
     
    Last edited: May 21, 2017

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