A eureka moment. Understanding isn't knowing.

Discussion in 'VHF/UHF - 50Mhz and Beyond' started by KK4YWN, Nov 4, 2014.

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  1. KK4YWN

    KK4YWN Ham Member QRZ Page

    I've gotten tired if the Internet because people abuse the privilege and perpetuate incorrect information repeatedly. But today I learned something and I want to share it with the hope that hams everywhere will perpetuate it instead of some of the wonky unverified tomfoolery that gets passed around.

    By the way: I have passed around my fair share if tomfoolery. I'm trying to do better.

    And so, I give you: 2m simplex information that doesn't suck.

    I'm a couple hundred dollars into fixing a path problem that can't be fixed. It started with a weak copy on a real nice and very funny fellow 22 nautical miles from me. He hates repeaters and so do I. We get on great. Well, when we can talk.

    I knew my station was marginal. His equipment is top shelf. I'm at fault so I started upgrading. And the net result is abysmal so far. New low loss coax and gain antenna. I also have a rotator and a beam in my collection of parts but they aren't in the circuit yet.

    After the first round of mods I'm irritated. I don't like guess-work that costs $$. And so: I went googlin'.

    I learned to place dots on maps and drone lines between them. Then I learned to evaluate the terrain along that line (actual line of sight!)

    Holy crap no wonder we need so much power. We are working directly into the dirt (plus a stadium and several overpasses).

    It's a thought twenty miles right? Our height difference is only about 75 ft over rolling terrain but boy does it matter.

    This all made e curious for a fix. I read all kinds of vhf propagation writings before something finally occurred to me:

    I don't know where the horizon is and I'm not alone.

    According to google earth, the horizon is about three miles from my house (relative to my friend house).

    His horizon is close to his house (relative to my house).

    So you see: the horizon isn't some thing way out there on the horizon. It's some thing between you and your friends house. It might be several things.

    As it happens RCA figured out that horizontal polarization works better than vertical over-the-horizon.

    So someone wrote in a ham book/website/forum that horizontal is better for weak signal work over-the-horizon and a lot if people (including me) just really didn't grasp what that actually means.

    Nevermind the modulation. Forget it. Use horizontal polarization with fm all you want.

    Those crossed yagi beams are starting to look like the ticket.

    Anyway: find your horizon. The real one. It's kinda important.
     
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    Such things can always be overcome, and experimentation is good but a great deal of experience might be better.

    Horizontal polarization for "over the horizon" paths on VHF does work better, which is why since the beginning of the television era, TV antennas have always been horizontally polarized. They reduce loss over restricted paths, where signals have to skew over stuff. RCA did indeed find this, back in the 1940s. But of course it only works if stations on both ends of the path are horizontally polarized.

    Having operated VHF-UHF-SHF for 48 years now (up to 24 GHz), I've found "everybody" who knows anything about propagation on those bands uses horizontal polarization for obstructed paths. But in all those years I've also found that bouncing signals off natural reflectors for a known path can work well.

    Never forget operating the ARRL UHF contest in August 2010. I operated from my friend Wayne N6NB's home station in Orange County (low elevation, but 10-band station) and Wayne along with a dozen others were out "roving." When they got to Monrovia, CA, about 60-70 miles north of "me," we all wanted to work on all the bands through 10.3 GHz.

    Wayne told the group to all aim "north," which was in completely the wrong direction for me, but it's what worked. They aimed into Mt. Wilson, a 6300' high chunk of rock to their north, 180 degrees away from where I was, and they were all S9. When they swung their antennas around to aim "at" me, I couldn't hear most of them.

    But he'd been through this before, and just knew what to do. Experience counts, a lot.
     
  3. KK4YWN

    KK4YWN Ham Member QRZ Page

    Understanding the why is going to drive my up a wall.

    In my mind it seems the wavefront of a h-pol signal is likely to diffract a complete wave over a horizontal obstruction, whereas a vertical wave can only diffract a portion of its power because the obstruction consumes a portion of the wavefronts power. In a downtown area where obstructions are buildings, a vertical wave can diffract more power than a horizontal wave because the edge of the buildings are also vertical.

    If I'm off the mark just tell me and I'll go read some more.
     
  4. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    RF "line of sight" is right at 1/3rd farther than the visual horizon. That is why a 4/3rds Earth curvature graph is used when plotting RF paths.

    Over the years, I have done many hundreds of "Earth intercept" RF path plots ranging from single hop (i.e. microwave stations) to doing a 360-degree base station / repeater coverage plot. One can certainly learn a few things about what can, and cannot, be covered from any particular location just by looking at the Earth profile.

    Even if the land "appears" flat, the situation can be quite different. Years ago, there was a person who decided to put in a community repeater (450 MHz segment) and installed it on a tower about 10-miles outside of a particular town in Oklahoma. The land between the tower and the town appeared perfectly flat. Unfortunately, after installing the equipment, it was found that from almost 90-percent of the town a mobile unit could not access the repeater. The distributor, that had sold the equipment, finally contacted me about solving the situation.

    A trip was made and all the equipment was "checked out". No problems with the equipment. Finally, I did an Earth intercept plot from the tower to the town. Although the land looked flat, there was a several hundred feet difference in the height above sea level between the tower and the town. Basically, line of sight was lost about a half-mile outside of the town. The town, itself, was basically in a hole! Now, the person who installed the tower should have taken note of the fact that the local cable television operator had to have an almost 600-foot tower to receive television stations from Oklahoma City which was right at 60-miles away!

    Sometimes, there is a building, water tower, etc., that is not in the direct line of sight which reflects the desired signal into just the "right" direction. By pointing a directional antenna towards that object a satisfactory signal level can be obtained.

    It is because of terrain that a repeater definitely comes in "handy"!

    Glen, K9STH
     
  5. K4ISR

    K4ISR Ham Member QRZ Page

    Just tossing out an idea here...
    With everything I have been reading and dealing with making my own antenna lately... One big thing is the radiation patterns of the various antennas. I was curious if you had a 1/4 wave antenna to use (even temporarily) which has a higher angle of radiation and may get you the distance you need. 1/2 and 5/8 tend to keep their strongest signals parallel to the ground, but with elevations changes between you and others, a 1/4 wave may get you the higher radiation pattern to overcome the elevation differences.
     
  6. KK4YWN

    KK4YWN Ham Member QRZ Page

    A higher angle of radiation only helps when the receiver is above the transmitter.

    For point-to-point links over typical terrain using average antenna heights lower angles of radiation are required. These links use diffraction as the mode if propagation. I'd venture to say that 99.9999% of amateur radio links depend on diffraction.

    So here are my rules for diffraction mode links:
    Put as much power into the horizon as you can.
    Wavefront polarization must match the physical attributed of the any obstructions.
    The horizon is always much much closer than expected. Always.

    With all that said: vertical polarization and high angles of radiation make little sense in nearly all cases.
     
  7. AI3V

    AI3V Ham Member QRZ Page

    Do a bit of reading about "tropospheric scatter"

    It is the mode 99.999% of vhf/uhf/shf over the horizon propagation uses.

    The traditiomal. 4/3 theory fails here.
    Rege
     
  8. KV6O

    KV6O Ham Member QRZ Page

    I have been using Radio Mobile to calculate coverage and path losses for a few years now. Valuable tool, but you need to go out and test in the real world as well.

    Steve
    KV6O
     
  9. K8MHZ

    K8MHZ Ham Member QRZ Page

    Do a little reading on 'knife edge' propagation. There isn't much out there, but basically things like mountains and buildings can create new waveforms that keep following the earth. AFAIK, this only occurs with horizontal polarization, hence the use of horizontal antennas for distance.
     
  10. KK4YWN

    KK4YWN Ham Member QRZ Page

    I don't think this is at all true. I think tropo scatter sounds sexy and hams want to use it to explain their links. I'm no pool shark but angles are easily verified.

    Again, if we evaluate the reality of "over the horizon", we find that a lot of people assume improbable propagation modes are at work.
     
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