[TowerTalk] Perfect Field Day beam?
Tue, 4 May 1999 09:28:18 -0400
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
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