DX reciprocity?

Discussion in 'General Technical Questions and Answers' started by Pushraft, Nov 20, 2008.

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  1. W0IS

    W0IS Ham Member QRZ Page

    You've just proposed a hypothesis, and it sounds plausible. Now, you have to come up with some experimental data to prove this hypothesis.

    So far, hams with hundreds of years of collective experience have stated that they have never (or only very rarely) come up with any data that would support the hypothesis of one-way ducting.

    There might be giant RF flap valve in the sky that we haven't seen. I would say that the burden on you is to establish its existence. This will be a great discovery. Get your license, discover it, and you'll probably get a Nobel prize.
     
  2. Pushraft

    Pushraft Banned

    How do you know that? What proof do you have to back that statement up? Are you assuming that because both sides of the link hear each other that they are using the same path? I am still not convinced that is happening. It think it would be difficult to prove one way or the other so my assumption is valid if it cannot be disproven.

    By the way I sometimes "work" a few beverages when receiving. My latest experimental beverage "antenna" is Samuel Adams Winter Ale or something like that. Seasonal with a nice spicy flavor to it. There is also one called Pumpkinhead that has a nice pumpkin/spicy flavor to it. Try them.
     
  3. WB3BEL

    WB3BEL Ham Member QRZ Page

    Actually quite a lot is known about ionospheric propagation. Besides the investigations that amateurs have done, a lot of research has been done by performing ionospheric sounding experiments all around the world. In addition, a ton of investigation went into developments like HF over the horizon radar.

    So in reality a lot is known about the way radio signals travel through the ionosphere and the impact to propagation at HF.

    I think that it is fair to say that the working theory is good enough to match the majority of situations.

    Sure there are sometimes anomalies that are hard to explain. Some amateurs are fascinated by this aspect of the radio hobby. They chase after these strange and transient propagation phenomena. Maybe it is part of what makes the radio so much fun trying to figure out what is going on with limited data and primitive apparatus.

    -Harry WB3BEL
     
  4. G3TXQ

    G3TXQ Ham Member QRZ Page

    Not much different!

    At all elevation angles from 5 degrees thru 20 degrees the biggest difference was 2dB. In other words: no practical difference on ionospheric propagation paths.

    "Local DX work" - isn't that an oxymoron ?

    Would you like any more of my time before you accept that it's height that matters ?

    Push - please answer this simple question honestly: "Why do you think W8JI would bother to build a 180ft tower if he could achieve the same results at a lower height by tilting his beam?"

    73,
    Steve
     
  5. W0IS

    W0IS Ham Member QRZ Page

    Through long experience, I know that as propagation changes, it always changes more or less equally for both sides. If my signal were following a different path, then there would be situations where one signal started fading, but the other signal did not. That doesn't happen.

    Again, it's possible that we simply have not discovered the giant check valve in the sky. There's a Nobel prize with your name on it waiting for it to be discovered. :D
     
  6. KC2UGV

    KC2UGV Ham Member QRZ Page

    I encountered one with my 11M antenna hooked up to a scanner. All I had to do to find it was tilt the antenna which changed the takeoff angle enough to see an image resonance on my receiver that is connected to my spectrogram software! hi hi
     
  7. Pushraft

    Pushraft Banned

    I dont think it is an oxymoron. DX in this hobby means distant contacts which I interpret to mean contacts that can be made farther than with just terrestrial paths and using the atmosphere for assistance. If you skip as short as 500 miles, that would be considered by me as a local DX since most of the skips I get here are 1000+ miles. Maybe "local DX" would be considered NVIS instead but if the launch angle is say less than 45 degrees, that is not Near Vertical.

    I sometimes have a problem with terminology. For example, if someone asks "what is the distance you drive to work every day?". The term distance has nothing to do with my work being distant (very far). It could be down the 1/2 mile down the road for example. That is related to the DX "definition" above.

    I dont know why people put antennas so high up on towers for DX work. I would assume it is to try to keep them about 1/2WL above ground like they do with the WWV antennas in Colorado. For example, their 20 Mhz antenna is 7.5m above ground which isn't very high (about 25 feet). If mounting it much higher (like 180 ft) would be better, why didnt they do that? Note their 2.5 Mhz antenna is about 200 ft high. I think they are all 1/2 wave verticals with a 1/4 WL radiator and 1/4WL sloped radials.

    I am trying to learn here like many other people but I guess I have more trouble accepting certain things without scientific proof. Meanwhile, I will check my ARRL book (18th edition) and see what they say about identical DX paths. I am pretty sure it said there are anomalies where that isn't the case.

    Oh and about the tipping, some people upgrade their cables and/or antenna just to get a few dB gain so to me, a 2dB gain IS significant whether it be for ground wave, direct wave or sky wave. Remember, stacking 2 identical antennas only gives you about 2.5 dB more gain yet look at how many people do that. Whether it is significant or not was not the point. The point was I said it would make a difference in tipping the antenna and I was right. For HF that is not practical which is probably why many people dont bother, but technically I was correct.
     
  8. KJ4AUR

    KJ4AUR Ham Member QRZ Page

    Removed May 07, 2009
     
    Last edited: May 7, 2009
  9. KC2UGV

    KC2UGV Ham Member QRZ Page

    Nobody said that there aren't anomalies of one-way propagation paths. I'm positive there are sometimes where one end's signal doesn't take the same path as the other. Lends itself very well to the random walk theorem. It's just the further you get from the designated "path" the lower probability it happens (Electrodes in the water example).

    And when you think of path of least resistance, think of water pipes. A 100 Ohm resistor can be seen as a 1" water pipe. A copper wire as a 9" water main. And 2 Mega ohm resistor is a .025" water conduit. Sure, some water will go through the .025" conduit, but the majority will go through the 1" pipe.

    And then there, we go to ionosphere. The ionosphere has an infinite number of "water pipes" of various "sizes", so, if you were to follow a single electron, the probability (Hence signal strength) is that it will follow the best path is the same for both ends. Does that make a little more sense?
     
  10. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    >Pushraft;1402855]I dont think it is an oxymoron. DX in this hobby means distant contacts which I interpret to mean contacts that can be made farther than with just terrestrial paths and using the atmosphere for assistance. If you skip as short as 500 miles, that would be considered by me as a local DX since most of the skips I get here are 1000+ miles. Maybe "local DX" would be considered NVIS instead but if the launch angle is say less than 45 degrees, that is not Near Vertical.<

    ::NVIS only works at lower frequencies and would not apply, for example, to 20 meters at all -- ever. Frequency's too high. This has been found by 50 years of experimentation by military and research institutions. You can tilt a 20m antenna any way you wish and never achieve NVIS. Maybe that's part of the confusion.

    >I dont know why people put antennas so high up on towers for DX work. I would assume it is to try to keep them about 1/2WL above ground like they do with the WWV antennas in Colorado. For example, their 20 Mhz antenna is 7.5m above ground which isn't very high (about 25 feet). If mounting it much higher (like 180 ft) would be better, why didnt they do that? Note their 2.5 Mhz antenna is about 200 ft high. I think they are all 1/2 wave verticals with a 1/4 WL radiator and 1/4WL sloped radials.<

    ::Height above ground has more influence with horizontally polarized antennas. The ground reflection angle changes inverse proportionately with height above Earth, so to make it as low as possible, you'd want a beam to be very high. 1/2 WL isn't bad, but 1WL is better and 2WL is better than that, if the goal is to achieve "maximum DX," meaning using fewer ionospheric hops to reach a distant point.

    >Oh and about the tipping, some people upgrade their cables and/or antenna just to get a few dB gain so to me, a 2dB gain IS significant whether it be for ground wave, direct wave or sky wave. Remember, stacking 2 identical antennas only gives you about 2.5 dB more gain yet look at how many people do that. Whether it is significant or not was not the point. The point was I said it would make a difference in tipping the antenna and I was right. For HF that is not practical which is probably why many people dont bother, but technically I was correct.<

    ::Tipping a vertical antenna changes many things. Remember, the earth below is always a powerful influence. When you tilt a vertical, radiation angle changes higher on one side and lower on the other, ground reflection angles change, all sorts of stuff changes. Even the antenna feedpoint impedance likely changes. This can all be modeled pretty well if you know all the variables, but remember engineers have been working with electromagnetic effects including propagation for about 100 years now, and accumulating meaningful data for at least 60-70 years, so there is already an enormous knowledge pool that indicates "if this worked, everybody would be doing it" -- especially hams, who are hell bent on experimenting and squeezing the last dB out of everything.
     
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