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View Poll Results: At what point on the antenna in height do we use to compare antennas.
Actual electircal feed point where the coax starts to radiate? 15 50.00%
At the antenna mount? 7 23.33%
Where the maximun current node is located? 5 16.67%
At the top of the antenna? 3 10.00%
Voters: 30. You may not vote on this poll

Comparing vertical antenna signals

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  #1  
Old 12-07-2010, 10:20 PM
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Default Comparing vertical antenna signals


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?
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Old 12-07-2010, 10:36 PM
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I voted.
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Old 12-07-2010, 10:42 PM
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me, too.
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  #4  
Old 12-07-2010, 10:44 PM
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i voted feed point since i think most folks will put most antennas as high as they can get the feed point .

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Old 12-07-2010, 11:36 PM
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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.

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Old 12-08-2010, 12:38 AM
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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

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  #7  
Old 12-08-2010, 12:44 AM
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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?
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Old 12-08-2010, 01:10 AM
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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.
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