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Author Topic: Antenna stacking article  (Read 19445 times)
W0BTU
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« on: January 10, 2011, 03:24:15 AM »


I would appreciate some input by all the antenna experts here on an article I just uploaded.

http://www.w0btu.com/files/antenna/vertical_stacking.html

Please let me know what you think, good or bad.

This is just a rough draft. Some wording needs to be changed, paragraphs need to be moved to a more logical order, and of course some graphics would help, but the thrust of my vertical stacking argument is there.

I would like to publish this in a ham magazine, but whether that happens or not is another story.
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K2DC
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« Reply #1 on: January 10, 2011, 12:36:59 PM »

Mike,

   Just a comment or two.  I've dealt with phased arrays at work for many years.  I agree that for a practical ham application, a vertical stack is more effective than a horizontal stack for the reasons you stated.  In either case, the gain is a function of the distance between the two, and certainly can be more than 3 dB.  Directive gain is a function of aperture size - up to a limit, depending on the nuber of elements and regardless of what the elements are.

   You mentioned stacking two 2M quagis only 8' apart, which is over 1 wavelength.  I believe that is too far apart.  There is a reference in the literature (could be Krause, could be Balanis, but I think it was Stutzman and Thiele, I can't remember) indicating that the maximum gain increase for a two element array occurs at a spacing of about .78 wavelengths, and drops from there.  And that's not affected by what the elements are - point sources, dipoles, yagis, quagis, or whatever. 

    By spacing them farther apart you have created at least a portion of a grating lobe.  The grating lobe portion may be supressed by the element (quagi) pattern and not radiated , but the power that would have been radiated in the grating lobe is simply reflected by the elements and impacts the efficiency of the system.  The literature indicates that the optimum spacing for the 2M quagis would be just over 5'.


GL & 73,

Don, K2DC
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W0BTU
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« Reply #2 on: January 10, 2011, 03:35:08 PM »

Hello Don,

I appreciate your comments, thank you!

However, IIRC, maximum gain occurs at a point where the apertures of the two antennas just touch. Since the aperture of a beam antenna is a function of its boom length, longer beams with more elements require more spacing to obtain the maximum theoretical 3 dB gain than shorter ones do.

I think the general rule of thumb is to stack two beams half the boom length apart. It's at that point where their apertures just touch and maximum gain occurs.

Of course, when you stack for maximum gain, the pattern is not as clean, as you mentioned. You get all those extra little lobes. In my way of thinking, antennas should seldom be stacked for maximum gain.

As for my Quagis, I thought that 8' apart was not the point of maximum gain, but the spacing that developed the cleanest pattern at the expense of a little gain.

Maybe someone else will come along here and correct me, though. :-)
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KB1GMX
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« Reply #3 on: January 10, 2011, 04:26:05 PM »

All you say is two Quagis...

If they are in the 6-8 element region with a length of 10-13ft  then 2.4M (8ft) is likely about right
to maybe on the close side.  Overbecks listing has the 10 element spaced 3.35M which seems far
apart for the boom length.

I've run two of Kent Britian's cheap yagies (6 element 70inch boom) at 2M
with a 2.1M spacing with good results and a fairly clean pattern but after losses
to splitter and cables  the gain increase was 2.65db gain, it's hard to get 3 or more db
without pattern getting dirty.  Did a lot of pattern testing for that one.

The best way is model the pair (or more) using a good modeling porgram.

Another source of info is   http://www.mydarc.de/dk7zb/start1.htm

His work is much more recent and has modern modeling software behind it.

Allison
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W0BTU
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« Reply #4 on: January 10, 2011, 09:18:21 PM »

All you say is two Quagis...

If they are in the 6-8 element region with a length of 10-13ft  then 2.4M (8ft) is likely about right
to maybe on the close side....

They are the 8 element 2 meter quagis mentioned in Wayne's article in the references, with 14 foot booms.

N6NB says the optimum stacking distance is 11', however, I don't remember whether that was for horizontal or vertical stacking. Horizontal stacking (boom-to-boom distance) should be greater than vertical.

The page about stacking looks interesting! I'll be studying it shortly. Thanks.
« Last Edit: January 10, 2011, 09:22:30 PM by W0BTU » Logged

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