Topband: 8 circle: DXE vs Hi-Z

[Tom W8JI]

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


----- Original Message ----- 
From: "Lee K7TJR" <k7tjr at msn.com>
To: "'Tom W8JI'" <w8ji at w8ji.com>; "'Bob Tabke'" <bob at onehorsecreek.com>; 
<topband at contesting.com>
Sent: Monday, December 15, 2014 11:09 PM
Subject: Re: Topband: 8 circle: DXE vs Hi-Z


> The primary difference between DXE and Hi-Z 8 circle arrays is the fact 
> that
> Hi-Z uses ALL 8 verticals actively at the same time where DXE uses only 4 
> at
> a direction.
> Using all 8 verticals allows the use of a smaller diameter and a
> performance edge on Directivity over the larger 4 active array.
> All 8 element arrays do NOT work the same way.
>   Lee  K7TJR
>
>
>
> -----Original Message-----
> From: Topband [mailto:topband-bounces at contesting.com] On Behalf Of Tom 
> W8JI
> Sent: Monday, December 15, 2014 7:24 PM
> To: Bob Tabke; topband at contesting.com
> Subject: Re: Topband: 8 circle: DXE vs Hi-Z
>
>> - DXE wants a 320' diameter and Hi-Z wants 200' for optimum performance.
>> It's hard to tell what DXE performance is because it does not disclose
>> RDF, beam width or F/B. And neither vendor supplies EZNEC files so I
>> can see the effect of varying the layout. So I'm not sure how to
>> decide what array size is best for me. It would be wonderful if
>> someone has a model for these two systems.
>
> Bob,
>
> The ideal spacing of arrays like this is entirely dependent on the 
> frequency
> range and goal you have for pattern or directivity. It is NOT dependent on
> the design or manufacturer, there are no magical space saving tricks.
>
> The circle diameter determines both endfire and broadside spacing, and
> spacing determines the beamwidth. Something in the 330-350 foot range 
> across
> the element pairs is near optimum for 160 directivity. You can use it down
> to spacings where the element-to-element spacing is about 35-40 feet on 
> 160,
> but it might as well be a four element vertical or some other array at 
> that
> spacing.  You can narrow the 160 pattern by going larger than 350 feet, 
> but
> the array can develop unwanted lobes. If element-to-element goes over 135
> feet or so, you will start to have F/R issues.
>
> This is the way every single eight element circle will work.
>
> The primary difference between the DXE and Hi-Z is construction quality, 
> and
> that determines cost. The DXE unit is a metal case that serves as a
> groundplane for the connectors, and a much better PC layout. How much that
> translates into better performance depends on how "pure" the rest of the
> installation is. If the installation is sloppy or compromised, that will 
> set
> the limit more than construction.
>
> The DXE is a nearly direct copy of what I use here, which is a very clean
> layout with PC mounted connectors and a super good groundplane between
> connectors to prevent ground loops that affect performance and minimize
> chances of lightning damage.
>
> One reason I especially worry about connector grounding is my eight
> verticals are spread in around a 350ft circle, and each has several 70 ft
> buried radials. The large physical size of a system like this sets the
> system up for large common mode currents in storms, it is actually a 
> yearly
> event here to melt the shields off at least one cable with a nearby 
> lighting
> hit (within a few thousand feet) because of ground loop currents, and yet 
> I
> almost never have box troubles.
>
> I use a 20ft vertical with a small loading coil and series load resistor 
> in
> my elements, and a three wire hat.  Mine is single band 160 (although I'm
> very slowly working on a 16-element circle for 160-80). People who operate
> here just love the 8 circle.
>
> I can send you an EZNEC file that would roughly approximate the array.
>
> 73 Tom
>
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