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The Myth Surrounding Antenna Take Off Angles

Discussion in 'Amateur Radio Antennas' started by Captain Kilowatt, Jun 24, 2012.

  1. Captain Kilowatt

    Captain Kilowatt
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    Everyone talks about as if it was the most important thing to consider when attempting to work DX stations or local stations and as if it was a cast in stone property of antennas that any deviation from the specs will surely mean poor contacts. I have been saying for years to forget about trying to optimize your TOA and simply get your antenna up as high as you can and don't worry about what height is the best for working into a specific area of the world. I found this article on Eham written by Tom Rauch, W8JI, that sums things up nicely.










    The Myth of Takeoff Angle

    from Tom Rauch - W8JI on April 26, 2010
    Website: www.w8ji.com
    View comments about this article!

    The Myth of Takeoff Angle

    Antenna discussions and articles often emphasize take off angle. We commonly read or hear that a low takeoff angle is good for DX, and that a high takeoff angle is good for local or short-range work. This is exactly what a good friend of mine, the author of one of the most popular antenna modeling programs, often showed angst over. He often expressed great reservation in including TOA in his software, but felt he had to include take off angle because people expected it. In fact, if we go to the help menu in most versions of his software and search for instructions on using TOA (or takeoff angle), help is not there!
    So what is takeoff angle? How can we use it? Is it useful for anything? My answer to that, surprisingly to the great believers in TOA, is TOA is not worth anything by itself. Let us look at some examples, and see how little value TOA really has.
    Vertical vs. Dipole
    We all know a vertical antenna is great for DX because it has a "low takeoff angle", and a modest height dipole is better for modest distances because it has a "high takeoff angle". Let us see how true this is.
    [​IMG]
    Figure 1: Dipole at 1/3 wavelength
    This dipole is 1/3 wavelength high (figure 1). The peak gain is 6.66 dBi, and it has a takeoff angle of 54 degrees. At 15 degrees, dipole field intensity is .42 dBi. Few who focus on takeoff angle would consider this antenna "DX worthy", or even think it would make DX contacts.
    [​IMG]
    Figure 2: Vertical with 16 radials
    We see now that a vertical with 16 radials (figure 2) has peak gain at 19 degrees, where gain is 2.21 dBi. Gain at 15 degrees, an angle considered very useful for long haul DX on lower HF bands, is 2.1 dBi. This vertical has only 1.7 dB more signal level than the useless cloud warmer dipole at the very useful low band DX angle of 15 degrees! At 19 degrees, the peak radiation angle of the vertical, the dipole and vertical are essentially equal in field strength even though the vertical has a TOA of 19 degrees and the dipole has a TOA of 54 degrees.
    This does not mean the vertical is useless for closer contacts, but it certainly is at a huge disadvantage compared to the dipole. It also does not mean the dipole will not work DX, or will not at times be stronger than the vertical over very long distances. Small differences in location, propagation, and installation could easily make the dipole the better antenna for long distances. The only sure bet is the vertical will not be good for very high angle propagation.
    Obviously, TOA by itself tells us little about an antenna's ability to be a good DX antenna. What then is important? With what should we really be concerned?
    The answer is almost painfully logical; we want maximum available field strength over the entire useful range of angles and directions. Peaks and nulls outside of the desired range of angles and directions are of no concern when our goal is developing the maximum most consistent signal into a specific desired location. We should not care about or even bother discussing TOA; we only want maximum field strength in our target area!
    Nulls, a Major Problem We Ignore

    We know we want maximum signal over the useful and most desired angles and directions of radiation, but we should never confuse that with maximum peak gain in that area. Peak gain, by itself, is as useless and meaningless as TOA. Let's look at a few examples of high useless gain.
    Let's assume we are in the center of the USA, and we want to work most of the USA with our multiband antenna. We all know a low band loop, operated up near the high end of HF, has considerable gain. That is a good thing, isn't it? After all, everyone wants a high gain antenna.
    [​IMG]
    Figure 3: 80-meter loop on 15-meter band
    Our eighty-meter full wave loop antenna (figure 3) has 14.37 dBi gain, about 6 dB more gain than a dipole, when used on fifteen meters. While this may sound attractive, this 6 dBd of additional field strength divides between four major lobes and four minor lobes. The average gain in the main lobes is 12.46 dBi or about 4 dB over a dipole. Worse yet, that gain is in an azimuth beamwidth of only 21 degrees for each of the lobes, and next to those peaks are holes or nulls up to 30 dB deep!
    While we may have bragging rights of high isotropic referenced gain, our loop is very narrow area gain. Gain, at best, averages 4 dB over a dipole in four very specific peak directions of 44, 133, 224, and 315 degrees. The -4db points of the main lobes, at which point gain comes down to the peak gain of a dipole, is about 11 degrees.
    It is statistically difficult to find contacts inside the main lobes, where the signal level would exceed a dipole!
    [​IMG]
    Figure 4: Loop and dipole superimposed
    A comparison to simple dipole antenna (figure 4) shows the loop is better by as much as eight db in four very specific narrow directions. The total azimuth area where the loop is equal to or better than the dipole is 125 degrees. On the other hand, the dipole is better than the loop over 235 degrees of azimuth! When located almost anywhere in the mainland USA, we would have far better average signal levels in the USA with a small half-wave dipole antenna.
    This leads to a rule we often ignore or forget. If we cannot move the nulls, it is usually better to have a smooth pattern with a bit less gain. The last things we want are multiple nulls in the useful azimuth.
    Elevation patterns

    What is good for azimuth is also good for elevation. Once again, the last things we want are a plurality of deep nulls scattered throughout useful wave angles. A broader elevation pattern (with no change in efficiency or azimuth coverage) results in slightly less gain, with twice the elevation beamwidth typically reducing gain by 3 dB. This will never be the difference between someone hearing us S9 with one antenna and not hearing us at all with the other. Similarly, a difference in TOA from 30 degrees to 5 degrees will not make that difference either.
    The sole exception to this is if we have a pattern with a deep null that happens to fall right at the target angle or direction. The nulls are what cause the problems of significant loss of signal, not gain or TOA!
    [​IMG]
    Figure 5: Six-meter stack at W8JI with traditional phasing of high antennas
    With this pattern (figure 5), propagation would appear very spotty. A change in wave angle from just 2 degrees to 4 degrees could result in a 20 dB signal reduction. This is not a TOA problem. The problem is rooted in narrow main lobes with deep nulls between sharp peaks. If the signal arrival varied just 1-2 degrees, signal level could go from excellent readability to unreadable.
    [​IMG]
    Figure 6:Stack at W8JI, same mean height, with progressive phase lag
    One method of correcting the deep nulls is to add antennas and use a progressive phase shift between antennas. Since antennas normally fire in the direction of lagging phase, and since we want to fill nulls above the main lobe, the logical phase shift would be a progressive lagging phase shift with increasingly higher antennas.
    In Figure 6, the addition of two more antennas with a lagging 30-degree progressive shift results in an array much less sensitive to wave angle. It has virtually the same field strength, within 6 dB, from 0.9 degrees up to 11 degrees. It still has nearly the same peak gain at very low angles to open the band, or to use when MUF just crosses the operating frequency, yet does not have the problem with 20-30 dB deep nulls if the wave angle shifts as little as 2 to 4 degrees from a lobe peak. Such a pattern would provide much more consistent signals over time.
    Other Stacking Mistakes

    We make many other stacking mistakes. I almost made one here at my house when I was considering stacking four 20-meter antennas. Fortunately, I put the myths about peak gain and takeoff angle aside and looked carefully at the nulls and beamwidth.
    [​IMG]
    Figure 7: Proposed 20-meter four-antenna stack
    My proposed four-antenna stack (figure 7) would have a 23.5 dB null at 9 degrees, which would be right in the hot spot of 20-meter signal a large portion of the time. It also would have less than dipole gain, sometimes significantly less, at any wave angle above 8 degrees. While it is mechanically easier and cheaper to stack multiple antennas one above the other, it is a huge waste of hardware because the pattern is just not that good.
    [​IMG]
    Figure 8: 2X2 H-stack for 20 meters
    The antenna in figure 8 is an H arrangement of four antennas. This system uses half of the four-stack tower height, but requires either two towers side-by-side or one tower with two cross supports a full wavelength long. Since the antennas are small (four elements on a 26-foot long boom), I have decided that either arrangement would work.
    Notice the peak gain is 20.05 dBi, which is about 11.5 dB over a dipole at optimum height. This system gives under 0.5 dB of peak gain, and gain is useful from well under 2 degrees up to 8 degrees. I have almost doubled the vertical beamwidth, making the antenna far less sensitive to wave angle variations.
    [​IMG]
    Figure 9: H-stack with upper lagging phase
    Lagging phase on upper antennas (figure 9) fills the deep pattern nulls while shifting the main lobe higher. It costs very little gain to do this, peak gain only decreasing 0.3 dB. Field strength is greater than a dipole at optimum height at any angle up to 20 degrees. By switching between these two patterns, this simple system covers most conditions of long distance propagation.
    Summary

    I hope the article shows why constructive conversation and debate, and looking at things from a different perspective, can lead to better understanding of systems and better antennas. If readers follow my thought processes as I progressed through these various antennas, they should notice I never considered take off angle at all. I looked at gain over the desired angles, focusing especially on nulls and null depth. My goal was maximum signal at all angles within the useful range of angles and directions. Nulls are especially poisonous to our signals. Few will notice a ten or more degree change in peak radiation angle, unless the lobe is very narrow and accompanied by deep nulls a few degrees off peak. If we focus on only one aspect, we can hurt ourselves in the end. We can obtain gain through a very narrow pattern, but if that pattern is full of nulls, we might lose a majority of contacts over time.
    Sure, it is nice to brag and say we have 10 dBd gain. It is nice to say we have a low take off angle antenna, or a high angle for local work. Factually, take off angle by itself means nothing. Neither does peak gain. We need to focus on how well a planned antenna works over the most useful angles and directions. We need to pay particular attention to nulls, because the deep nulls, in particular multiple deep nulls, are really what prevents us from hearing and working what would otherwise be spectacular signals.
     
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  2. pro151

    pro151
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    Oh boy. There you go again, tilting at our 11M windmills.

    You said it yourself in another post, something like "Normal laws of physics don't apply in the 11M band" :laugh:
     
  3. BJ radionut

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    05-25-2012, 07:39 PM
    [​IMG]BJ radionut [​IMG]
    W9WDX Amateur Radio Club Member
    [​IMG]


    [​IMG]
    Antennas for Receiving and Transmitting

    Blows a lot of gain factors out the window....
    "The super antennas" against the lowly dipole....:D
    I found it interesting...
    All the Best
    BJ


    http://www.eham.net/articles/23758 ....another I found interesting....:pop:
    "Factually, take off angle by itself means nothing. Neither does peak gain".....

    .....I love that quote!

    CK: There you go again....boiling that pot of beans....You know the masses believe we are full of it and they will continue to "spread" their dung until it falls on more of the uninformed...
    Have a great weekend OM
    All the Best
    BJ
     
  4. Captain Kilowatt

    Captain Kilowatt
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    Thanks BJ. I also like Tom's quote "Factually, take off angle by itself means nothing. Neither does peak gain".....

    In essence it means that the 8 dB of absolute gain an antenna may have at the take off angle means nothing compared to the 20 or 30dB deep nulls it may have nor does it even reference the height of the vertical pattern with regards to the distribution of gain over the elevation angle. Ever hear someone say that their antenna worked DX better into a target area when it was lower to the ground? I suspect it was not because they optimized the TOA but rather that they minimized the nulls in the pattern at the angle of which the incoming signals arrive at. I could be wrong but I have never had an antenna that worked better at a lower height than it did higher up for general contacts over varying distances.
     
  5. HomerBB

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    I am using an 11 meter horizontal dipole today at 32' height. I am talking to Station 39, Bundy, AUS, and 5936, Nam, HA, and 369, Stan in HA. Sometimes the lowly dipole works in out just fine.
     
  6. BJ radionut

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    CK:
    I have never been fortunate enough to have an "Over and Under" configuration in regards to switchable Yagi(s)...
    However I have heard a few "Bug Guns" on 6m "Do the Switch"....
    And have heard some huge changes in signal strength....And not always the top antenna provided the greatest signal...
    I think you are correct in regards to the change being due to "moving" or "Better filling" of a null by the adjustment of the height....
    I do give the theory some thought...
    Example....The Lazy H (in it's original configuration)....Has 2 horizontal dipoles driven out of phase...with(I think) .12w vertical spacing....One would think even though your "splitting" your power at the feed point, with the larger overall lobe from the combined pattern radiated by the 2 elements....is this not a similar "effect"
    Or am I over thinking this?....or nuts as always!:pop:
    All the Best
    BJ
     
  7. Highlander_821

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    I remember on Copper's forum a few years back some guy was getting ready to put his beam up at around 50 feet agl. That idiot Lon 808 told him it would "perform better" at 36 feet because that was a wavelength for 11 meters.

    I tried to post a reply suggesting that that was nonsense (in a polite way, of course) and 808 would not let my post through. The poor slob probably took the shitty advice and cheated himself out of 15 more feet of height for his beam.

    {Cry_river}
     
  8. pro151

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    Height is might.
     
  9. KingCobra_CDX882

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    Height is likely the Most Important thing you can do in mounting your antenna (and it does not have to be in equal wavelengths to see observable results)

    Height Rules !!
     
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  10. BOOTY MONSTER

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    how did his dipole have 6.66 dBi of gain ?
     
  11. Captain Kilowatt

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    As for having stacked over/under beams, yes it is possible that each antenna may be set up to have no meaningfull nulls and each will have a different optimum TOA and that switching between them one may indeed be able to select the proper wave angle to suit the path at the time. TOA does play a role in that but for a single antenna it is not nearly as important as the nulls or the distribution of power over that major lobe.With a single antenna one has to be prepared to work signals over a wide range of incoming wave angles which is why I say that TOA is not the important factor in selecting height of the antenna.



    Horizontal antennas have what is called "ground gain" which is increased gain caused by the incident wave and a wave reflected from the ground combining in phase many, many wavelengths away from the antenna. This can add up to 5 or 6 dB gain to the "real" antenna gain. This is one of the reason you see such high gain figures on manufacturers antennas as they add the ground gain to their real gain and do not bother to tell you that. Unfortunately they also add it to vertical antennas which have no real ground gain and then there is the just plain bullshit and lie factor thrown in as well. This is why manufacturer's stated gains and program derived gain figures rarely if ever agree.
     
  12. BJ radionut

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    Total agreement....When your designing the antenna for maximum gain, most generally you have a severe trade off....Nulls....
    Your directive pattern can have the tendency to look like a saw blade with several "teeth missing"...
    Thus when propagation "falls-in" at different elevations than your peak(s)...you may or may not have acceptable signal levels to work the station...
    I feel it is better to have a little less gain and have larger,smoother pattern when only having the one antenna to do all jobs...

    I do like to have, in the case of most of the 'wire" antennas I use presently to have a second configuration to switch to...
    ie....A Delta loop for 75m at about 25ft average height(450 ohm feedline)
    vs:
    My "Backfire" Array(End fired/ 75m Lazy H/horizontal/shallow inverted V) average height 40+ft....(450 ohm feedline)
    Both have their strong and weak points depending on propagation and frequency operated at a given time of the day...
    Have a good day OM
    All the Best
    Gary
     
  13. BOOTY MONSTER

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    i guess i'm missing why the horizontal dipole which is directional is being compared to omnis . i also don't know what he is referring to about stacking antennas . i do know he is not a fan of sigma 4 type antennas . LOL

    i do know when i went from a 1/4wgp to a 5/8 (the 5/8 had a 8 ft feed-point , the 1/4 had a 17 ft FP) i had much more signals coming in and i could talk significantly farther .
     
  14. W5LZ

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    Booty M',
    If you'll change your description of that dipole from 'directional' to 'may be directional', it might make more sense. A dipole is not always directional, or maybe 'very ' directional. They can be as omnidirectional as atypical vertical antenna. And vertical antennas can also be directional...
    - 'Doc
     
  15. BOOTY MONSTER

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    how do you make a horizontal dipole produce a omni signal ?
     
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