Topband: 8 circle: DXE vs Hi-Z

[JC]

Hi guys

I would like to commented on the subject of RX comparison

Tom when you introduced the RDF methodology to measure directivity, you
really hit the nail in the head. I'm working on RX antennas only since 2005,
after hundreds of tests, I am sure that just 1db RDF matters a lot.

When you compare RX antennas you really want to know how much you can
improve from your TX antenna Signal to Noise Ratio. Better RDF means better
SNR, similar RX antennas performance have similar RDF. 1 RDF does help a lot
when the signal is just  2 db above noise and you can't pull it out, adding
just 1 db you can change from 339 to 449 and log a QSO, or new country.

3db SNR is just what you need on cw.

The implementation of the RX is different from EZNEC , you need to consider
all elements neat resonance that will be part of the RX system and
deteriorate RDF, it means deteriorating SNR. Common mode noise is not well
understood for most of DXer's including grounding, these are factors to
consider as well.

My recommendation is to kook in the space you have and select the best RDF
Rx antenna for your available space.

Nothing beats the 13.8db RDF from 8 circle array, but you need 300ft radius
to achieve that directivity. If you are able to broadside some good RX
antennas and get over 14 dB RDF you shall expect better SNR than the 8
cycle/300ft. 

Remember to detune your TX antenna during RX,  It is hard to measure that
and sometimes the only way is to compare the SNR from the TX antenna with
the RX antenna, is you are using a 11db RDF system you should see more than
10db SNR over the TX antenna. It means you can hear Q5 signals not even
detected by the TX antenna, it is not about move comfortable e copy , it is
about to hear what is not there in the RX antenna.

Detuning he tower won't fix other common mode noise, like cables not
grounded, bad grounding, rotor cable 120 ft long working like a vertical,
etc, It is necessary  detune them all.

Regards
JC





-----Original Message-----
From: Topband [mailto:topband-bounces at contesting.com] On Behalf Of Tom W8JI
Sent: Tuesday, December 16, 2014 7:52 AM
To: Lee K7TJR; 'Bob Tabke'; topband at 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


----- 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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