[TowerTalk] Perfect Field Day beam?
Guy Olinger, K2AV
Tue, 04 May 1999 15:54:40 GMT
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Kind of beating a dead horse here, I think. Just having two or three
radiators in the air certainly doesn't guarantee behaviour like a
yagi, even if it looks like one. If it has traps and all that can
happen to them, there certainly are any number of problems that can
render a "beam" very unyagi-like, without the need to melt down and
set fire to the traps.
I heard a story about a site that got some of those super-cheap Gotham
two element beams (remember those?), one each for 20, 15 & 10. Whoever
was responsible for putting the beams together got pieces mixed up.
Well, they *looked* like yagis...
My take on a beam that seems little better than a dipole is that it
probably has problems, and I would reverse your statement, that any
beam with only 1.3 db forward gain can't have 13 db F/B.
As you correctly state, those things don't go together in even a
barely well-designed (an incorrect assumption?) "yagi". But there is
nothing that inherently defines which measurement must be in error.
Anecdotally, I have been at a field day site where the tribander
seemed to hear better off the back of the "yagi". In my experience a
wimpy 1.3 db beam is entirely in the realm of possibility.
On Tue, 4 May 1999 09:28:18 -0400, you wrote:
>Hi Guy and Pete,
>> If the 1.3 db referred to db is over a dipole at similar height, we
>> are not talking about a lot of loss. What we have is front to side
>> like a dipole, and some very minimal FB or FB that is rather narrow if
>> looked at in a 3D pattern such as EZNEC-2.
>If a close spaced antenna has F/B, even as little as 13 dB F/B, it
>must increase the forward power by a minimum of 3 dB. That's true
>even if there is almost no pattern compression in the front or sides.
>I believe a pattern that meets the criteria of high backfire null and
>low overall directivity is only available with an array that occupies a
>large distance in terms of wavelength. In that case the wide
>spacing allows creation of a sharp null, without much cancellation
>at other angles.
>> The interpretation is that it is only slightly better than a dipole
>> with nothing approaching a broad, decent F/B and without any huge
>> forward gain. The point made was that the aggravation of beam,
>> rotator, mast, etc hardly seem worth the effort just to get 1.3 db
>> more, 1.3 db being imperceptible to most people.
>Then the question begs asking, "Where does the power go?"
>With a "decent" F/B in a close spaced array, power that was
>radiated out the back obviously has to go somewhere.
>It can't go straight up, the element spacing will not allow producing
>that type of pattern in conjunction with a back-fire null.
>There are two possible explanations.......
>1.) Power that would have been radiated out back is dissipated as
>2.) The power simply doesn't radiate as a surface wave signal at
>zero degree takeoff, and so it wasn't measured at the
>measurement point on groundwave a mile or so away.
>> Other tribanders show 4.5 db over a dipole at similar height. That can be
>At what wave angle?
>If the radiation doesn't go out the back or sides, it either turns to
>heat in the antenna or radiates at some angle above ground level.
>Anyone know where it goes?
>Isn't it reasonable to expect, if we are going to say (with no
>reservation) an antenna has no advantage over a dipole, we would
>need to verify it indeed has no advantage at a useful angle and not
>along the ground?
>On 160 meters, I set my antenna pattern nulls at 10-25 degrees
>above ground. I give a "rat's behind" what the pattern at zero
>degrees is. Best sky-wave performance and gain occurs when the
>null along the ground directly off the back is NOT at its maximum.
>Could it be that something like this is happening on tribanders? Do
>we know for sure?
>If the antenna (with loading concentrated in a small area, and no
>ground losses like a vertical would have) isn't melting and has a
>reasonable F/B, it must have a reasonable amount of gain
>somewhere. Where does the power go?
>73, Tom W8JI
Guy Olinger, K2AV
Apex, NC, USA
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