DX reciprocity?

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

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  1. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    >Pushraft;1403260]Here is an interesting article. I think what they are saying here is that height above ground of a Yagi(-Uda) antenna is mostly the determining factor of launch angle if and only if it is already over about 3/4WL in height. Below that height, boom length also makes a significant difference with respect to take off angle.


    This part is probably the most significant to HF using horizontally polarized beams:<

    ::Actually, the comments made in that article don't apply to HF. The article is about 6 meters, which is 50 MHz, and VHF. It's very different.

    Remember, 50 MHz and above is VHF, and everything about that involves terrestrial work or the occasional sporadic-E layer (or even more rarely, F2-layer -- that occurs only at sunspot cycle peaks, which we are very far from, right now); but 99% terrestrial. HF is virtually all ionospheric, using a very high layer to reflect signals. There's no "ground wave" to speak of at HF (we can enjoy a little bit of it on 80m and 160m if we use vertical polarization, but that's about it), and "tropo" on HF works, but it's not what we are normally shooting for. We are all optimizing antennas for ionospheric work, and the comments about 6 meter antennas don't apply.
  2. W8JI

    W8JI Ham Member QRZ Page

    It actually is not frequency related or path related. The issue of downtilt or uptilt is related to the height of the antenna in wavelengths, the polarization, and the beamwidth of the antenna. It is NOT frequency or path related.

    For down tilt or up tilt to do anything, the following conditions have to be met:

    1.) The antenna has to be a significant number of wavelengths above earth or the reflecting media.

    2.) The antenna has to have a very narrow beamwidth when compared to the pattern width of a similarly polarized non-directional or poorly directional antenna at the same height above the reflecting media.

    You can see that a Cell antenna, with a very narrow elevation pattern and many wavelengths above earth, would have a large effect from downtilt or uptilt. It is hundreds or thousands of wavelengths above earth, and it has a very narrow beamwidth. A very high VHF antenna, like a 2 meter repeater antenna on a steep tall mountain or a very tall structure, would often have a narrow enough pattern and could be high enough.

    As we move down in frequency we are less and less likely to find the pattern narrow enough or the height tall enough. It's difficult to find antennas that big or that high in wavelengths.

    So we should be clear it isn't the frequency that matters or the application. It is the height of the antenna and the elevation beamwidth that allows tilt to have an effect.

    What we can really do at HF is move nulls around or fill nulls. We can have a big increase in very high angle radiation if we filled the high angle null(s) in.

    We can't force a lower angle than the earth wants at the height we are at with the antenna, all that would do is start to reduce the gain.

    Peak gain will always be at an angle where the height above earth causes the earth to re-enforce the primary signal. We can't change that without changing the height. Tilt won't do it.

    Of course on CB, almost anything can happen. For example:


    GAIN: 15.5 DB
    SIDE REJECTION: 45-50 DB True
    WEIGHT: 20 lbs.
    LENGTH: 12 feet
    SWR: 1.1

    it not only has 15.5 db gain, it has 45 dB F/B and a power multiplier of 50 (17dB) and an audio gain of 19 dB!!!

    Antennas on CB just work different than they do anywhere else. They can have different audio and carrier gain, they can have 45 dB F/B, they can get 17 dB power gain from 4 elements and 19 dB of modulation gain!

    Down tilts also work on CB. It's just different than the models and books up there.

    73 Tom
  3. Pushraft

    Pushraft Banned

    Good old 27 Mhz. It is where I got started. I still have a 27 Mhz "rig" too. Oh well, those days are over now. Interesting stuff though thanks.
  4. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    ::You're just jealous because you don't have one like that for 160 meters. The same reference might as well state they have a 15 dB 160m antenna and you can install it in a condo attic.
  5. EI8DRB

    EI8DRB Ham Member QRZ Page

    15dB on a 4 element beam... that's it... I'm getting one!
  6. PA5COR

    PA5COR Ham Member QRZ Page

    Must be 15 dB over a dummy load :D
  7. AC2BB

    AC2BB Ham Member QRZ Page

    Also it even has 19 dB of audio gain so you can just hook up a headset and amplify your voice and hearing almost 100 times!!! I need to get one of those
  8. EI8DRB

    EI8DRB Ham Member QRZ Page

    Hmmm... I wonder if it works the other way too? If I sat at the front of it, would I experience a little piece and quiet for once.
  9. W8JI

    W8JI Ham Member QRZ Page

    I don't care what else it has, if I can't tilt the boom to aim at the right angle I don't want it.
  10. G3TXQ

    G3TXQ Ham Member QRZ Page

    Push, I believe you are using a 20m dipole at the moment. I forgot to say in one of my earlier posts that I think there some better options for a 30ft x 50ft roof space.
    Last edited: Nov 22, 2008
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