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Comparing vertical antenna signals

Discussion in 'CB Antennas' started by Marconi, Dec 8, 2010.

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At what point on the antenna in height do we use to compare antennas.

Poll closed Jan 7, 2011.
  1. Actual electircal feed point where the coax starts to radiate?

    48.3%
  2. At the antenna mount?

    24.1%
  3. Where the maximun current node is located?

    17.2%
  4. At the top of the antenna?

    10.3%
  1. Marconi

    Marconi Supporting Member

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    When we compare antennas how do we mount them so the results will be close to fair.

    If we have a 40' foot mast,

    do we just mount the antennas on the top of the mast and test,

    or

    should we set the tips of the antennas at equal heights and test,

    or

    should we try and determine the maximum current nodes for the antennas and set the center of the nodes at equal heights and test

    or



    should we determine what the actual electrical feed point is and set them up at the same height and test?
    #1
  2. Marconi

    Marconi Supporting Member

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    I voted.
    #2
  3. HomerBB

    HomerBB Well-Known Member

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    me, too.
    #3
  4. i voted feed point since i think most folks will put most antennas as high as they can get the feed point .
    #4
  5. Shockwave

    Shockwave Sr. Member

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    The practical way is the feed point height. Since most of us have a mount for our antenna, it's mast is usually not going to be adjusted a few feet either way to accommodate a new vertical. The fairest way to compare two antenna systems is to have the center of radiation at the same place above ground on both antennas to be tested. This is how the FCC determines the height of what they consider the radiation point on FM broadcast antennas. The center of radiation is easy to figure out on balanced antennas like the GM. On end fed verticals it becomes more complicated to pinpoint but it will be some place between the base and mid section of the radiator with most verticals.

    We often talk about the term TOA with respect to where the lobe containing maximum energy lines up with the horizon. It is the height of the feed point that has the biggest effect on the take off angle and the numbers of additional secondary lobes and nulls above the primary. This is true of every antenna I can think of from the vertical to the inverted V. That's probably another reason the center fed Gain-Master performs as well as it does. It has a higher center of radiation then other verticals. Your results would be more accurate when feed point heights remained constant rather then antenna tips.
    #5
  6. Henry HPSD

    Henry HPSD Well-Known Member

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    Hi all,

    Intresting!!!

    For DX situations i would go for feedpoint heigth.
    For local situations i would go for top heigth
    (as the concept of what the antenna cant see cant work theory)
    For overall average combination i would go for center.

    More importanly would be where is the RECEIVING antenna...
    If you have the possiblity to set the receiveing and testing atnenna several wavelengths apart (approx >10) youre in a good place.
    Both at central heigth from each other. In that way youll have no effect of TOA/groundwave/building reflection etc)
    and youll need at least a couple additional antennas at ground level (or more than one) to "suck up" the ground reflected signals.


    @shockwave,
    It is deffintily not that i have anything against you m8, i appriciate your honesty and way of thinking..but i sometimes have questions.. so perhaps you could "teach" me.

    Where did you get the knowledge that changing feedpoints heigth "has the biggest influence"..
    If we would take a look at a dipole and change the feedpoint...does changing feedpoint heigth changes TOA? no to my believe it does not.
    The gain within the TOA changes but the pattern remains equal.
    So...what does change....gain does,impedance does, bandwidth does...
    Take a look at another antenna the cubical quad. Now feed the top of the quad or the bottum, now does that feedpoint change has the biggest influence on TOA?

    You also could see it different..if what your saying is true in the elevetion pattern of the atennna for verticals, it also would be true in the azimuth pattern if we polorized the antenna horizontal.
    Now does a windom or a dipole have different patterns...? (ofcourese both half wave)

    Your refering to the higer center of radiation of the Sirio gainmaster as giving it its lower TOA (if im rigth). If that was the case....should we bring in tomorrow a top feed vertical dipole? that should outperform everything on the market...see where im getting at??

    You say this is "true' for every antenna, are you sure?

    kind regards, Henry

    11 meter Dx antenna systemx
    #6
  7. Robb

    Robb Yup

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    Nice point about the top load.
    Isn't a top load better for current distribution than a base load?
    We admire the SGM because it is center loaded with an even distribution of current.
    Right?
    #7
  8. bob85

    bob85 Supporting Member

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    that depends on what antenna,

    cebik suggested to me equal current maxima heights, but which current maxima?
    the sigma has two and one is at the bottom, choosing either is unfair to one or the other antenna in any test that makes current maximas equal in height,

    then theres the astroplane, an antenna specifically designed to operate at the same tip height as the 5/8waves of that era and provide a lower radiation angle from an elevated current maxima,
    testing it in any way other than what it was designed to do would imho be unfair,
    avanti should have supplied the 1/2wave mast as its not just a suppoort its part of the antenna whos length before isolation influences radiation angle, saving money has confused the whole astroplane issue,

    what about the gainmaster, its at a disadvantage if you take the real feedpoint height and place it at the same height as an endfed antenna, the oppoosite is true if you take it as where your coaxial plug connects,


    imho there is no one fair relative height,
    if i had to choose id go with equal feedpoint height but i don't think thats fair in all cases.
    #8
  9. Henry HPSD

    Henry HPSD Well-Known Member

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    @ BOb
    yes i guess you've got a good point...it is the situation wich brings in variables.

    @ Robb,
    Top Load or top Fed?
    Top loaded shortend verticals have a advantege above base loaded shortend verticals.
    Top Feeding a antenna is another thing.

    I admire the SGM due to its current distribution yes..It does not require a radial system that is what is the biggest asset......but it is not a new thing...As already mentioned int the SGM thread.

    PS..a terrible misconception is about the end fed halve wave vertical not needing radials..(i used to be one of those believers aswell!)...but thats another story for another time.

    73 H.
    #9
  10. jazzsinger

    jazzsinger Bullshit Buster

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    i think the best way to measure any antennas is with the same pole/coax. some of the choices sort of blur things as the feedpoint/mounting point for most practical cases is usually the same thing.I believe this is down to Eddies choice of wording the options though and a couple of antennas that disguise the true feedpoint. i'm hoping where the coax starts to radiate is a freudian slip by Eddie, lol.

    the just plunk them on the same mast does away with all other variables and just tests the design,good bad or indifferent, much the same as the olympics have a level start line and atheletes weight, stride, good looks, charisma or ego don't play a part, just their ability to perform with what they have. its a simple case of one design against another irrespective of wavelength, current nodes, real or imagined gain,ground plane kits, differing feedpoint locations etc.

    the current maxima does have much merit which i have discussed with bob many times, but as he and others point out that can be manipulated by inductive/capacitive loading and some antennas have multi current maxima points.





    On another note which i think equally applies here:

    in the past few hours i've been watching activity on just about every US cb site, the divisions in loyalty and the bullshit spread to defend that loyalty even to the point of twisting/defying physics, in the US cb scene is unbelievable.

    I know at this minute i'm the demon for having the audacity and balls to take Kale to task on claims made about the new predator antenna, the more i did the more bullshit arose, i realise he is very well respected but some of the things he is claiming and others around him are claiming are impossible, not to mention the reluctance to give prices begs the question just how level is that playing field?

    i also did the same with Shioda and his fierce brand loyalty on the ranger tft/usb radio thread and other previous ranger threads, in the past i have done likewise with Jay over claims about the i10k, and with Robb over his blind faith in magnum radios. i've also done it many times in the uk too.


    this isn't about scoring points or winning friends, although my approach has won me many technically minded friends and even more bullshitter enemies. its about getting the truth and hard facts out to the hundreds of newbs and non technically gifted people coming onto all cb forums looking for the best way to invest their hard earned dollars/pounds/euro's.

    as your all aware i'm not American so i don't have the pressure on me to fit in with the incrowd or face isolation on the US cb scene, quite frankly i don't give a fuck what people think of me good bad or indifferent, but i do care that people who genuinely seek answers should be given the truth and bullshit should be called out at every turn.

    Money is tight for many of us on forums, many have spouses they have to keep sweet, they can't afford to be pissing money up against a tree on bullshit claims that do nothing to enhance their station.

    The bottom line is opinion is all well and good, but when it is presented as hard fact then someone has to play devils advocate, it might as well be me. Because i ain't got fuck all to lose, i can walk away from any forum at any time and just not give a toss, cb was a huge thing in my life for many years but at this stage of life i prefer to give something back to a hobby that gave me so much, even though my present interest in it and all radio is waning.

    For years i've watched and heard people being fed bullshit from all directions, i'm so glad when i started one of glasgows most respected radio people gave me a huge insight into facts, many people don't have that luxury, it is them i ask these questions for so they can prosper on radio like i did.

    i hope events in the past few days on other threads will encourage dealers/promoters/reviewers to present honesty although i very much doubt that will be the case.

    i have no time for is it a bird,is it a plane, no its super antenna/radio/accessory claims. All i want to see is those genuinely seeking answers given the best available knowledge by those with the technical expertise to do so.

    on that score i tip my hat to guys like shockwave, bob 85, henry hpsd, and many others who despite all the bullshit around them strive to give that advice as honestly as they can.

    i also tip my hat to the moderation team here who have let me ask these questions (albeit occasionally censored or warning me to be careful),and have reasoned with me on the occasions i may have bent the rules slightly :tongue:,no doubt under a barrage of complaints about me, this is what gives wwrf the edge when it comes to sorting out fact from fiction.i don't set out to discredit anyone or destroy businesses, i gain nothing from that,but i do think if knowledge isn't shared honestly and openly there is not much point having it.

    The truth IS out there !!!!!
    #10
  11. HomerBB

    HomerBB Well-Known Member

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    I am one of those folks who are reading, and listening, so i carefully express my thoughts when in the company of folks with notably greater understanding of subject matter than I have. But jazzsinger has expressed perfectly in the quoted paragraphs why I chose theat the feedpoint option.
    To my way of thinking, considering that we all seem to agree that where one mounts a given antenna makes a difference in its performance from whether the same mobile antenna is mounted on the fender, trunk, bumper, or on the very top of the vehicle, to what kind of soil is beneath a base antenna, to height above ground and/or structures in the area, etc, then the only consistently reasonable way of coming to the conclusion of which antenna compares most favorably for use in a given spot is to see what happens on the mast your going to mount it on.

    On the other hand, there is always free space modeling to level the playing field to the hypothetical potential of antennas where a working environment is bent in favor of design in a more objective way. Now if I could just hook a coax to one of these models . . .
    #11
  12. Shockwave

    Shockwave Sr. Member

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    Henry, I'm not offended at all my friend. Your comments and input often make me think harder then usual and I enjoy this. You also make some very valid points that I sometimes overlook. For example changing the feed point on a quad from top to bottom does nearly nothing that I'm aware of. I believe the reasoning behind this example is that the quad is a symmetrical balanced antenna. Reversing the feed point on the quad creates no changes in the radiation current distribution on this antenna.

    Many of the modeling tests done in EZNEC with the sigma confirm drastic changes in the TOA when the height is altered. In my application I see primary lobe angles shift between 8 degrees close to the ground and nearly parallel to the horizon at typical broadcast tower heights. My ideas on feed point height come mostly from the inverted V on 75 meters. When my feed point is 35 feet off the ground it does good in short hop DX. Get it up around 70 feet and it does better in long hop.

    In the case of the Gain-Master I'm not so sure it's higher center of radiation is responsible for it's lower TOA. I think it's center feed is beneficial to the gain but I also suspect the lower TOA has something to do with keeping all radiation currents in a constructive phase. Of course that is made possible by center feeding. I think top feeding would raise the center of radiation but destroy benefits of current distribution on the center fed design.

    You have found some antennas I overlooked where you can change the feed point height without physically moving the antenna. I'm still fairly confident that changing the physical height of the antenna and feed point will change the elevation angle for verticals and azimuth angle for horizontals due to ground reflections. If your thoughts are different, please explain as I'm always eager to learn.
    #12
  13. Henry HPSD

    Henry HPSD Well-Known Member

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    Hi Shockwave,

    Totaly agreed on: heigth changing TOA...
    And good that we agreed that feedpoint heigth changes does not directly mean a
    change in TOA.

    What is intresting is that if you take a look at the SGM and the radiation pattern as given by sirio.
    It is low across the horizon. As one expects for a vertical over "perfect" ground.
    The 5/8 wave wich they compare it with would be to my believe be a pattern that can be found in"real world". Ofcourse that is less.
    If they placed a 5/8 wave on "perfect ground" it will have a similair TOA as the SGM.

    Dont forget either that you can actully test antennas in "freespace" although that will cost a fortune...

    Back to that SGM.
    The way i look at it, not saying im rigth:
    Since we esthablished that the feedpoint in a vertical monopole isnt a big issue regarding TOA...well you see what i mean?
    The advantedge of the SGM lies within its capability to produce a signal without needing radials..(but so will any other center fed dipole.)

    I can not imagine the SGM would have a big advantege over a center fed dipole.
    Ofcoures it has some due to the larger capture area but thats about it i guess.

    Now, your thougths about the current distribution...
    Take a vertical dipole, imagine the current.
    As we know, it is high at the center and low at the bottum.
    Now feed that antenna at the bottum...it becoms a end-fed vertical.
    However, the current remains equal!
    I dont see how things would be different with the SGM, therefor i dont see how that would positive influence gain. If there is something im missing, well were all the same...all willing to learn.

    What it comes down to is large capture area, center feed no need for radials...
    And thats about it..

    Just noticed ...already to much said about the SGM...wrong thread....sorry marconi :)

    Kind regards,

    Henry
    11 meter Dx antenna systemx
    #13
  14. Shockwave

    Shockwave Sr. Member

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    I think the diagrams posted on the Sirio website are not radiation plots indicating the TOA. I see them as simply showing the currents on the radiator itself much like the "antenna view" window on EZNEC just with more detail. Not how the radiation propagates once it leaves the antenna.

    I still think the advantage of the GM is the center feed. There are many other antennas that produce a signal in the absence of radials however, none seem as effective as the GM. There are serious gain advantages when compared to a vertical untity gain dipole.

    I'm surprised we don't agree that when you move the feed from the center to the base, you have also shifted the point of maximum current to the base. This becomes significant in the case of the end fed 5/8 wave radiator because when the RF travels down the single radiator there is enough length that the base radiates out of phase with the rest.

    The capture area on the Imax 2000 is the same as the GM yet they are worlds apart in performance. This is one of those cases where the GM looks good in the models and when it's mounted on the mast.
    #14
  15. Henry HPSD

    Henry HPSD Well-Known Member

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    1st alinea:
    No, obviously im not talking about "current plots".
    Im talking about the plots where u see the gainmaster "producing" a lower and futher signal than a "conventional" 5/8 wave....the one where the signal is close to the horizon.
    Not along the axel of the antenna.
    You have spoken before about them "the almost "0degree" radiating angle.
    Im sure youll have to agree on me that they seem not fair...
    If they are please explain me how should i see them.

    2nd Alinea:
    When you say, i STILL think the advantege is center feed?
    Then im getting the impression you think im saying it is not..
    I think i did mention quite often that it is.
    The argument about other verticals produce signals without radials, that looks to me a bit like the ford/toyota comparisment...without radials does not mean they are effective.

    Non as effiencieny as the SGM?
    There are many antennas who rediate as effiencient as the SGM let us start with the center fed dipole again, sleeve dipole, slappe arnold etc...but we had that conversation already. Im not saying they show equal gain or anything were talking efficiency now.

    3rd Alina:
    I have should it said it differently. Yes the currents changes along a 5/8 wave single radiator with changing the feedpoint. Dont know what happend each time i think of the SGM i think of a center fed dipole..thats where it came from...but yes we agree and your rigth.

    4rd Alinea
    If you say the capture area of the Imax is the same as the SGM, are you realising you are saying the gain/wavelength ratio is the same.....
    If you put capture area in a physical way of comparing it with a the SGM....first that isnt really capture area and secondly the two mentioned antennas are totally different antennas.
    You can not compare a Dodge to a Ford etc.
    Capture area does often have a direct "link" with physical size, but they are different.

    Besides, i am sure there will be situations where the imax will outperform the SGM and vica versa......ill put it differently...im quite sure there are situations where a 1/4 GP will outperform a 3el Yagi...But you can not bring in a statement like that if you are refering to the same "physical" dimension (wich capture area is not).

    For the moderators and marconi,
    I appologise, for mixing up your thread.
    It is oke to move this to the SGM thread as it isnt going about the original statement anymore.

    Kind regards,

    Henry
    11 meter Dx antenna systemx


    Ps, shockwave, shall we continu in the sgm thread? just copy past and ill folow.
    Last edited: Dec 9, 2010
    #15


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