Total Copper Content Corollary #2: OAOS

Discussion in 'Antennas, Feedlines, Towers & Rotors' started by KL7AJ, Jun 5, 2011.

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  1. KL7AJ

    KL7AJ XML Subscriber QRZ Page

    OAOS.......Occupied Area of Sky.....or size DOES matter!

    The concept of aperture as used in optics can be readily translated into antenna gain. For example, the gain of a loop antenna is directly related to the enclosed area of the loop. A big parabolic dish has more gain than a small parabolic dish. This is fairly intuitive.

    However, how does this translate to end-fire antennas like long yagis, which don't have particularly large cross sections to incoming signals. A thin wire dipole, for instance essentially has zero area. However, as a first order of COMPARATIVE approximation, the "area" of a dipole can be considered a square with the edge equal to the length of the dipole. So a ten meter 1/2 wave dipole would have an "aperture area" of about 5 meters by five meters or 25 square meters. Again, this is only for comparative purposes, not as an absolute factor.

    A cubical quad optimized for maximum gain will have almost identical gain as two square loops side by side (again spaced for optimum gain). Both configurations have nearly identical OAOS, as well.

    Figuring out the OAOS of a long single yagi is a little trickier, but if you were to take the total aperture area of each element, it will have close to the same gain as a loop with the same enclosed area.

    Stay tuned


    Eric
     
  2. K7FE

    K7FE QRZ Lifetime Member #1 Platinum Subscriber Life Member QRZ Page

    Eric,
    That is a good question for the gang. I am sure that you know, but for others; antenna gain is a function of "effective aperture." Effective aperture in many cases is larger than the physical aperture, however it can be smaller. A Yagi is a good example of that, small area - lots of gain. Gain is a function of effective aperture. In other words, the higher the gain, the higher the effective aperture of the antenna.

    73,
    Terry
     
  3. M3KXZ

    M3KXZ Ham Member QRZ Page

    Eric, how about this then. 0.5 wl dipole for 20m, in free space, average gain -0.06dB (would be -0.01dB with zero wire loss, but copper makes it -0.06dB). This antenna will have an aperture of 10x10= 100sqm.
    Double the length to 20m, aperture now 400sqm, but average gain is -0.04dB. An improvement of 0.02dB by increasing aperture fourfold.
    Now increase length to 30m, aperture now 900sqm, average gain is now -0.12dB.
    Now increase length to 40m, aperture now 1600sqm, average gain is -0.06dB.
    Now increase length to 50m, aperture now 2500sqm, average gain -0.15dB.
    Now 100m, aperture 10000sqm, average gain -0.14dB.
    Now 500m, aperture 25000sqm, average gain -0.47db.

    The only thing that seems to change significantly with the dipole, in free space, when increasing the aperture further and further, is the generation of lots of spikey lobes and nulls. But even the spikey lobes on the 500m one are only 5 or 6dB stronger than the lobes on the 10m long one, and this is at the cost of massive deep nulls. The average gain when increasing from 10m (aperture 100sqm) to 500 (aperture 25000sqm) gets worse, by about 0.4dB - not really significant. Is any of this significant - cost/benefit analysis....is it worth making a dipole longer to increase aperture when there is no increase in overall gain?
     
  4. EI4GMB

    EI4GMB Ham Member QRZ Page

    I agree. For example if you take a dipole which is significantly shorter than a half wave dipole you will find that the gain and thus the capture area is the same for both antennas. Indeed, current antenna modelling and theory support this fact.

    Kind Regards

    Fred EI4GMB
     
  5. KL7AJ

    KL7AJ XML Subscriber QRZ Page

    This is all under the proviso that any expansion of an antenna is accompanied by phasing adjustments to optimize gain. Indeed, stretching a dipole beyond about 5/8 wave merely creates more sidelobes....but if you configure it as a COLLINIEAR array, with phasing stubs, the gain increases properly. Hope this helps. :)

    Eric
     
  6. M3KXZ

    M3KXZ Ham Member QRZ Page

    But even then, there's no increase in the average gain of the array - only sharper focus of the lobes. Unless I don't get something??!?!??!? :)

    1 copper wire 0.5wl dipole... average gain -0.06dB
    Add second one, colinear, same, equal phasing... average gain -0.06dB for the array
    Add third one.... as above... -0.06dBi for the array
    Add fourth one.... the same
    Add fifth one.... the same
    Add sixth one... the same

    So all the time, this is increasing the aperture yet the average gain is unchanged. All it's doing is more strongly focussing the front and rear lobes, at the expense of gain elsewhere. And altering phasing doesn't do anything to the average gain either - it just alters the focussing of the array. And then you can stack them again and again to end up with a massive "aperture" but the average gain for the whole array still stays exactly the same.
     
    Last edited: Jun 5, 2011
  7. KL7AJ

    KL7AJ XML Subscriber QRZ Page

    We generally aren't too concerned with the average gain of the array, except with regard to efficiency. The point of large arrays is the gain in the main lobe....increasing the ERP in a desired direction.

    Eric
     
  8. M3KXZ

    M3KXZ Ham Member QRZ Page

    So, really, then, what you're saying is that the increased aperture size merely acts to focus the lobes - like a lens? It doesn't collect more RF or emit RF.
     
  9. G0GQK

    G0GQK Ham Member QRZ Page

    If the bands were going mad with DX coming from every direction night and day nobody would have the time for QRZ. They would be logging DX for hours, not talking about how to do it.
     
  10. N3OX

    N3OX Ham Member QRZ Page

    I would put this more strongly: the average gain IS the efficiency, if we're talking about the average gain number as calculated in 4nec2, EZNEC, etc. If you take the power radiated into a little chunk in azimuth and elevation and add it all up in every direction, you get the total power radiated in all directions. That number divided by the applied power is simply the efficiency, and that's what the average gain reports.

    I assume these are "average gain" numbers from modeling software?

    The systematic shift should be looked at carefully by comparing the numbers with the wire loss turned off and on. Negative average gain numbers can mean different things. With wire loss turned on, it should tell you something useful: the efficiency of that particular configuration of wire. This is especially useful if you bring dirt into the equation later... you can just see what the predicted ground losses are with a single number.

    BUT it is also possible to get negative average gain numbers because of numerical problems (difficult wire junctions, close-spaced wires, improper segmentation, etc.) in the models, and this can lead to improper calculations of this number.

    If your model is working well, you should see an average gain of 0dB for your whole list of lengths with wire loss turned off. If you see that, then the numbers with wire loss turned on just tell you the efficiency. Your numbers wouldn't surprise me for the actual efficiency of super long antennas with a source in the center... but it's worth checking to make sure it isn't because you don't have enough segments for a 500m long antenna.

    If you don't get 0dB or darn close with zero wire loss, something is wrong with your model. If you get small negative numbers like you got, it probably makes sense because the longer antennas have more loss when the waves are exciting the far-away outer reaches of the antenna. But check it with zero loss and make sure you don't have model problems.

    Well it doesn't EMIT more, because it always emits what you feed it minus some that gets turned to heat. But a bigger lens does COLLECT more light. If you add a lens to a lamp, though, all you do is change the direction the light is going and absorb some power, for any real lens.

    Consider that I can burn a piece of wood using sunlight and a lens by collecting the power from a large ACTUAL area and focusing it onto a small one. But it also casts a shadow over the rest of the wood and surroundings since it's collecting light from a large area and concentrating it on a tiny smoking patch:

    http://youtu.be/ogJPwkpvWoE

    The lens, with its bigger aperture (bigger than the smoking spot itself) extracts more power from the incident field. So our "receiver" then gets hot enough to catch fire... maybe the gain is a bit too high :D

    I can also use the same lens to substantially improve the directivity of a light source, but I might not collect it all with my lens, and I still get some light that spills around the edge of the lens, like an antenna with a multi-lobed pattern but a higher gain main beam:

    http://youtu.be/8ez8QfWBHwA

    If I calculate the "average gain" of my lamp and lamp+lens by calculating the total optical power emitted in ALL directions with and without the lens, the only difference will be if the lens itself is absorbing some light (reducing the efficiency through losses). That's what the average gain test does: calculates the total power radiated in ALL directions and normalizes it by the applied power to give the dB efficiency. It adds up the main beam plus the edge spillover, plus the weak light leaking through the shadows (nulls).

    If my lens were lossless, the "average gain" of my LED lamp and lens would be exactly the same with and without the lens. But with the lens, I send more energy into the middle of the pattern, and on "reception", the lens greatly would increase the solar energy incident on my poor little IKEA LED (compared to the LED's parabolic reflector and small-aperture lens) if I were to stick it outside.


    73,
    Dan
     
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