DX reciprocity?

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

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  1. Pushraft

    Pushraft Banned

    EZNEC gripe

    I dont know how to use EZNEC 5.0 demo properly. I modeled tipped UHF antennas using it and it told me 20 degree downtip is better for low angle radiation than vertical but when I tried it, it seemed worse. Probably my model was wrong.

    I dont understand what all those segments are too. Why does it frequenty tell me segments are too long? Too long for what?

    A better interface for newbies would be for an antenna program to just ask you a lot of question like is this antenna for a base station, mobile, outdoor, indoor... Then ask you will it be vertical, horizontal, dipole, monopole... giving examples, pictures, and descriptions of each. Other useful questions would be is it for DX work, local work, high angle work... explaining the pros and cons of each one.

    It would also be good if it gave you suggestions like how to optimize the setup for a particular task.

    This stuff about segments and cryptic abbreviations might be ok for techies, but newbies dont know what that stuff is.

    It would also be good if they showed you step by step how to model a few simple antennas starting from a blank screen. That is, walk you thru each entry that needs to be made. Teaching by example is very effective. Just showing someone completed templates doesn't explain what each entry in them is for and why they are needed.
  2. KJ4AUR

    KJ4AUR Ham Member QRZ Page

    Removed May 07, 2009
    Last edited: May 7, 2009
  3. Pushraft

    Pushraft Banned

    multiple height beams?

    It seems to me to get a better chance at making various distance DX contacts, one would need multiple antennas at different heights above ground to better control the take off angle. If someone just uses a fixed height antenna (100 ft on 20m for example), it seems that would favor certain skip distances but miss others. It this a correct assumption?

    Would it be better to mount a pair of identical beams at different heights and then just use an A/B switch to select them and would that make a significant difference if the antennas were at a 2:1 ratio height?

    Is there an optimal skip angle for each band such as 20m or can a single band such as 20m have 2 significantly different skip angles that work or is it a narrow range of skip angles for a particular band?

    I wonder if it is possible for say a 20m antenna at 1/2WL height (33 ft) to actually outperform the same antenna at 1WL height (66 ft) for certain distance DX contacts a little closer in.

    It seems that a flatter elevation pattern is great for terrestrial antennas but for DX, the correct launch angle seems important and if it is wrong, performance may suffer.

    WWV in Colordao for example. Why do they have all of their antennas at 1/2WL above ground? Is there something magical about that height? If they had their 20 Mhz antenna at the same height as their 2.5 Mhz antenna, it seems it would be 8 times 1/2WL high or 4 WL high. Would that produce a takeoff angle that is too low and actually miss some of the coverage area? Is that less than the critical angle to reflect off the ionospheric layers?

    My 20m attic dipole is exactly 1/2 WL above ground so I am wondering if that is a good thing or not in my case.
  4. G3TXQ

    G3TXQ Ham Member QRZ Page

    Push - sorry to say this will be my last posting ever to try to help you. You have simply tried my patience beyond endurance because you never ever seem to read up the references you are given.

    Way back in this thread I gave you a link to my web site which discusses various arrival angles for different length paths and different bands, and how those angles relate to the performance of antennas at different heights. It would have answered almost all of your questions, but it's obvious you just can't be bothered to follow up on the recomended reading and just want to be "spoon-fed" the answers.

    Just in case you decide to change your ways, here's the reference again:

  5. Pushraft

    Pushraft Banned

    Well that is pretty much what I expected. Maybe I missed your link I dont remember, sorry. Too bad VHF and UHF dont reflect off the ionosphere well. If they did, then you could just mount an antenna reasonably low and use a tilt adjuster to get some control over the launch angle which would be nice and fun to adjust while you have a contact.

    Most of my DX "half" contacts (Rx only here) are right around 1000 terrestrial miles. It would be fun sometime to calculate the angle to make that happen and calculate the actual distance the signal travels. It would be a guesstimate of course, not exact. I am thinking maybe 1500-1700 actual miles for a 1000 mile "straight line" (arc) distance. Does that sound about right?

    Considering all the possible problems that could be encountered along the way of a DX path, it is pretty amazing to me it actually works as well as it does. One scenario is if a single hop and a double hop are both making it for a particular link. They would be somewhat out of phase because of the different total distances. Also conditions are contactly changing which makes antenna testing and comparisons more challenging.

    Thanks G3TXQ for reposting the link it was informative and reassuring.

    (and at least my assumptions about antennas are getting better)
  6. W0IS

    W0IS Ham Member QRZ Page

    No, that sounds too high. I forget the exact height of "the ionosphere", but it's on the order of magnitude of a hundred miles. I'm sure a Google search (or one of those books people keep talking about) will give you a more accurate number. Simple trigonometry can then tell you the total path distance. And the best angle of radiation for a given distance can also be calculated relatively easily (e.g., making it reflect at the midpoint).

    When you do this calculation, you will see that there is only a modest increase in the distance the signal actually travels.

    I'm not going to do the calculations, because I'm too busy talking to people on the radio, and the calculation is unnecessary for most purposes.
  7. Pushraft

    Pushraft Banned

    After thinking about it more, that does sound high. If there is a low launch angle, even if the reflective layer is 200 miles up, the extra distance up to that height is much less than 200 miles. You can easily verify this with trig or just take a pole (mast) about the same height as your ceiling when vertical. Now start tipping it 10 degrees, then 20 degrees. The top of the pole will only be a little below the ceiling. In other words, a low angle (above the horizon) does not add much distance even for a reflective layer several hundred miles above the Earth's surface. The analogy is if you just a little bit of extra length to the pole, it will touch your same ceiling even tipped 20 degrees

    Another way to help verify this is check on a map some point that is say 10 miles North of you and maybe 3 miles East of you. The straight line distance is not even close to 13 miles. It is 10.44 miles. Since the ratio is 3 to 10 of the opposite to adjacent side of the angle between the 10 mile side and the diagonal (hypotenuse), Tangent (0.3) = 16.7 degrees. This is a reasonable launch angle so this example is relevant. Note that it seems you do not just add the height of the reflective layer to the total distance for each trip up and down of the signal. My approximation is using 2D "flat" trig so someone else can make it even more accurate accounting for the curvature of the Earth.

    That actual distance would be useful to know when calculating losses and/or just for bragging rights. For example, 3000 miles "sounds better" than 2750 miles.

    That Mr. Phythagorous was pretty smart for his time.
  8. Pushraft

    Pushraft Banned

    Merrily, merrily, merrily, merrily, time to raise my beam.
  9. KJ4AUR

    KJ4AUR Ham Member QRZ Page

    Removed May 07, 2009
    Last edited: May 7, 2009
  10. W0IS

    W0IS Ham Member QRZ Page

    Again, I don't know the exact figure, which can probably be found in a book. But it's much less than 200 miles.

    The space shuttle orbits at an altitude of about 150 miles. If they kept running into ionized particles, this would probably be a bad thing. From this, we can conclude that the height of the ionosphere is less than 150 miles.
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