DX reciprocity?

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

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  1. G3TXQ

    G3TXQ Ham Member QRZ Page

    I agree - so why do you then seem to challenge everyone who tells you what to expect?

    Steve
     
  2. Pushraft

    Pushraft Banned

    How about the numbers for the same antenna with a 5 degree downtilt?

    Also if downtilt doesnt do "squat", then why do I see some sector antennas with adjustable downtilt for as much as 20 degrees? Are you answering the question about downtilt only for DX situations? What about people that want to do local stuff can they benefit from mechanical downtilt?

    Also one observation is your antenna in the model is more than 1/2WL high it is more like 1WL high (40m is about 133 feet). I wonder if this model was 1/2WL height and 5 degrees downtilt if that would be beneficial at certain takeoff angles.

    Also, if you dont know the optimum launch angle for DX, then how can you say a "flat" antenna (0 degrees tilt) is best or at least other tilt configurations are not much better? I guess you are dismissing a few dB as insignificant when in fact people stack antennas just for that reason, to get a few more dB. Sometimes as little as 1 or 2dB can make a difference. 1dB is like 26% more power on both ends of the link and 2dB is almost a 60% increase in power on both ends of the link.

    So assuming your 1500W amp is running thru a "flat" antenna and then you adjust tilt for maximum signal (say a 2dB boost), you just effectively picked up 1000 watts! Then you would have 2500 watts effective. I think that IS significant. Might not show up on the receiver's meter much but it may be enough to make a difference in a borderline link. Many times when I listen people are just below the noise level and I cannot hear their callsigns. If they had maybe double the power (3dB) I could probably make them out over the noise.
     
  3. Pushraft

    Pushraft Banned

    2dB would be close to 900 watts in the previous example, not 1000 watts. Slight error there. Also, for non DX stuff, downtilt should make a beneficial difference. If someone lived up high on a hill for example and wanted to beam a Wifi (2.45 Ghz) signal down to a relative at a much lower elevation (above sea level), then pointing the antenna downward may help because how would the antenna "know" where ground even is if those two links are using direct wave? That is, the signal will be captured at the lower elevation even before the wavefront in boresight has time to hit the ground.

    I think cellular phones use downtilt on the towers.

    I think for DX work it would be useful because you do not know the exact optimal launch angle for skips so having a way to adjust in manually seems like it would be beneficial.

    I am anxious to see your model for the 3 element at 1/2WL height and 5 degree downtilt.

    Also those gain figures for a 3 element beam seemed a little high to me. I thought 3 elements were around 6 dBd or about 8 dBi peak gain.
     
  4. G4ALA

    G4ALA Ham Member QRZ Page

    Attention Seeking

    He or she obviously seeks attention.

    Just as with small children and dogs, fail to respond and the sad behaviour will cease.

    Imagine if he or she posted a thread and no one replied. After a few times, do you not think the idiocy would cease? Respond, and the behavoural profile is re-inforced. Do not do it! A collective effort is required.

    Dont encourage him or her. You are wasting your time.

    G4ALA
     
  5. W0LPQ

    W0LPQ Ham Member QRZ Page

    ALA ... I think that the majority of us agree ... ignore him and he will eventually go away.

    However, he is so full of misconceptions .. it is downright ... funny.\

    We are supposed to increase OUR power so HE can hear us ... baloney.

    Raft, why do you think cell phone towers utilize downtilt ... think about it ... who are they serving ... airplanes ... nope..!

    Sheesh...
     
  6. N9VO

    N9VO XML Subscriber QRZ Page

    Although I think pushrod is just jerking you guys around, there is such a beast as the "variable take off angle" (VTOA) antenna. Navy used them for years. They do exist but won't get into the mixing of apples vs oranges in this arguement!!

    Check this out: http://www.antenna.be/art5.html
     
  7. Pushraft

    Pushraft Banned

    I located the paragraph in the ARRL book that is relevant to DX here. This is from ARRL Antenna Book 18th Edition Chapter 2 page 14 lower right. They are referring to "long-distance transmission and reception" which is basically the definition of DX.

    "When more than one ionospheric layer is involved in the wave travel (see Chapter 23), it is sometimes possible for the reception to be good in one direction and poor in the other, over the same path."

    So this would imply that the path is not equivalent in both directions as far as how good or bad it is. How would someone explain this phenomenon if the path is identical in both directions? That is, the exact same path is being taken in both directions for a link?

    I am also disappointed that the ARRL book (18th Edition), doesn't seem to have an entry in the index for beam downtilt as it applies to DX. The do talk about an antenna high up on a hill being downtitled 3 degrees to better cover a valley but they dont talk about up or downtilting a Yagi-Uda beam for th purposes of changing DX angle.

    This is one reason why I feel that reading alone wont answer all my antenna related questions. I could read that book for cover to cover taking several months and I would still not have the answer to this particular question.
     
  8. WB3BEL

    WB3BEL Ham Member QRZ Page

    The downtilt you are talking about I think is for cellular base station antennas.

    This represents a different condition where the signal from the cell base station is line of sight. In a case where the tower is high relative to some close in coverage region they sometimes downtilt the antenna to provide better coverage. This antenna might be a hundred wavelengths high. This is also done sometimes to restrict the cell range to prevent interference on the network where the frequency is reused in a neighboring cell.

    But this is totally different from HF where the propagation is beyond line of sight and the earth is in the path and the signal reflects off the earth on its way to the ionosphere for all low angles.

    For HF the elevation pattern is dominated by the height of the antenna above the lossy earth as others have already mentioned.

    -Harry WB3BEL
     
  9. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    ::Actually, Eric KL7AJ who is employed as a professional in this specific field, already answered your first question, as follows:

    "H.F. propagation is DEFINITELY non-recoprocal, unless you happen to live at the precise magnetic equator....about as likely as a politician uttering a truth. I go into great detail on non-reciprocal propagation in Chapter !7 of the Opus, "Bent Radio."

    As for the second issue, there is virtually nothing published about tilting antennas for working DX because it simply isn't relevant. I have just about every antenna textbook there is, and they all agree that tilting antennas to adjust takeoff angle for ionospheric DX work (not "atmospheric"!) doesn't do anything -- which is a good reason to simply omit the discussion.

    Now, HF ionospheric propagation and VHF-UHF-SHF-EHF+ propagation are not related at all. Tilting antennas to adjust for the target at VHF and above definitely works, and it's done all the time.

    Just that has nothing to do with HF ionospheric propagation.

    Mountaintop VHF+ antennas are regularly adjusted for downtilt, to cover the areas "down below" that would be in an "RF shadow" if not for downtilting.

    WB2WIK/6
     
  10. Pushraft

    Pushraft Banned

    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.

    http://amfone.net/ECSound/K1JJ24.htm

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

    As it turns out, when lengthening a boom, it DOES make a lower take-off
    angle, but ONLY when the Yagi is less than roughly 1/2- 3/4 wavelength above ground, depending upon starting boom length. The lower the Yagi is below 3/4 wavelength, the more dramatic the longer boom has on the lowest lobe coming down lower. This is very significant on HF freqs where 130' is 1/2 wavelength on 75M, etc.


    So for example, if you had a 3 element 20m Yagi(-Uda) at 33 ft (1/2 WL) high, 2 ways to significantly lower the takeoff angle would be:

    1) Raise the 3 element beam to 66 ft (1WL high)
    2) Increase the boom length but keep the same 33 ft antenna height.

    So if this is true, antenna height is NOT the only factor that effects launch angle unless you are already over 1/2-3/4 WL. Since on the lower bands raising a large antenna is not easy, this may be very useful.

    I am not saying that doubling the boom length at HF is easy. I am saying there appears to be more than 1 way to lower elevation angles of beams at a "low" height.

    Some people may find it easier/safer/more acceptable... to erect a longer boom HF antenna than to put a shorter boom HF antenna up higher.

    Can someone comment on if this article is correct or not?
     
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