Hi Guys,
Excellent point Tom as we will have to disagree on this. I have built both
types of arrays here and like the smaller circle with active elements. I
believe it has a "cleaner" pattern.
You can see a 3D picture comparison of several arrays at
"http://www.kkn.net/dayton2014/dayton-2014-antenna-forum.html"; starting at
page 11 There are some real differences between 8 element passive BSEF and
the Hi-Z 8A npatterns. Receiving antennas are all about hearing what you
want and getting rid of what you don't want. I think the 3D pictures show
this pretty well. We almost never design RX antennas for gain, only how it
receives which is mostly about pattern or directivity.
The observations by Joel will indeed be interesting, These two antennas
are VERY close in performance and I think the differences will be
interesting, who knows, it may spur the development of something new.
Go Joel!
Lee K7TJR
-----Original Message-----
From: Topband [mailto:topband-bounces@contesting.com] On Behalf Of Tom W8JI
Sent: Tuesday, December 16, 2014 4:52 AM
To: Lee K7TJR; 'Bob Tabke'; topband@contesting.com
Subject: Re: Topband: 8 circle: DXE vs Hi-Z
Lee,
We probably will just have to disagree about this.
>From my viewpoint, the behavior isn't too much different than a big yagi
stack or other antennas we are used to.
The size of the array generally sets the directivity limits. We can add more
elements that are closer-in than optimum, and that can certainly help if the
size is smaller than optimum, but the trade is gain or pattern cleanliness
and sharpness for size.
The forward two elements and back two elements are too close to contribute
broadside pattern, which is what provides the clean pattern absent major
side lobes in the full size 8 circle. As a matter of fact, adding them in
destroys some of the broadside directivity.
If, however, we make the array so small that it loses broadside pattern
multiplication, then we can see an increase in directivity through the small
endfire length increase.
A .327wl radius array gives about .25 wl endfire spacing in the primary
cells (the center elements), and is not improved in pattern quality by
adding the forward and rearward cells. The two forward pairs and rearward
pairs are not only too close to have broadside pattern contribution, they
are closer endfire. They are about 75% of the endfire spacing in the central
quad, and nearly 40% of the broadside width. They certainly can contribute
endfire, but they actually remove broadside directivity in the process!
In an optimum size array the amplitude ratio from the primary quad has to be
4:1 or 5:1 or more to prevent some pretty significant pattern null area
deterioration when the additional 4 elements are added, because they
deteriorate broadside pattern multiplication faster than they contribute
endfire gain (at ~.187 spacing when the primary endfire cell has .25 wl
spacing).
If the array is made so small that there is little broadside contribution
from array width, then the addition of the four will improve things. There
isn't any broadside pattern to hurt. That isn't the same as a broad general
statement that using more of the elements allows the array to be made
smaller, unless we want to compromise pattern to have the same directivity.
I go through similar things with Yagi arrays. All of my Hygain 5 element
Yagis have been changed to four elements, and my KLM six elements have
become 5's. :) It isn't so much they work better, they just work different
in a way that is a better compromise for pattern, bandwidth, complexity, and
gain.
Everything is a compromise. If the target is maximum directivity and a clean
pattern (more like a flashlight), the array has to be large. It can never
be the same if small, or we all be running multi-element short boom antennas
in close-spaced stacks.
I do agree, however, if space is so limited the array can't use broadside
multiplication (which is the same as stacking gain in a Yagi array) then all
active elements with more elements is better.
73 Tom
_________________
Topband Reflector Archives - http://www.contesting.com/_topband
|