View Single Post
Old 15-Sep-2015, 1:33 AM   #6
rabbit73
Retired A/V Tech
 
Join Date: Aug 2012
Location: S.E. VA
Posts: 2,288
Your question is off-topic for this thread which is for database updates. Better to have posted it here:
Special Topics
http://forum.tvfool.com/forumdisplay.php?f=9

or here
Help With Reception
http://forum.tvfool.com/forumdisplay.php?f=7

But I will try to answer it now.

Quote:
But, what part of the antenna actually determines the overall gain.

1) the number of driven elements can certain be a factor but right now I am referring to 8 bays only.
2) the design of the driven element, Wire bowtie, flattened bowtie or flattened loop.
3) all the spacing involved where the driven element sits with respect to the reflector and distance between elements.
1) The number of elements is certainly the most important, but since you have limited your question to 8-bay collinear arrays (the proper name for the category), how you combine the two 4-bay arrays can affect the gain. If you combine the two 4-bays with a splitter in reverse, the maximum gain will be 2.5 dB; 3 dB for two 4-bays combined minus 0.5 dB combiner loss. There are other ways of combining two 4-bays that have lower loss if you are concerned about the last 0.1 dB.

Maximum combining gain will only be realized if the wavefront presented to the antenna is uniform and the plane of the antenna is perpendicular to the wavefront.



2) All those shapes will give the same gain at the design frequency if they are resonant at that frequency, but as you move away from the design frequency the gain curve will be different. The array will have less gain at frequencies below the design frequency and more gain above the design frequency, but "thin" elements will have a narrower bandwidth and "fat" elements will have wider bandwidth.

A collinear array has greater bandwidth than a yagi because it has a lower "Q," as defined by the max allowable SWR, and the fatter elements make the Q even lower.

Each bay consists of a collinear pair of half-wave dipoles that form a full-wave dipole. Each end of a half-wave dipole is a high impedance point, so where it connects to the vertical phasing line, the support point must be well insulated for low loss. Screwing the inner ends of the "whiskers" to wood can cost you 2 or 3 dB.

3) The spacing between the elements and the reflector is usually 0.2 to 0.25 wavelength at the design frequeny for max gain. A larger reflector can increase gain and so can a non-flat angled reflector.

The vertical spacing between the bays is usually 1/2 wave at the design frequency. When the antenna is operated at frequencies above the design frequency, the gain will increase because the vertical spacing and the elements become longer in terms of wavelength, with the max gain at 5/8 wave. As you move even higher in frequency the gain will decrease because the vertical spacing is more than when the capture areas of each bay just touch, and the element lengths are so long that the main lobe splits into two parts.

So, you can see that it is possible to construct an antenna for a specific frequency that puts the peak of the gain curve where you want it, which is not the same as the design frequency of an antenna that is for a whole band with the design frequency at the center of the band.

Have you looked at mclapp's website?
http://m4antenna.eastmasonvilleweather.com/index.html

Have you looked at the files by holl_ands?
http://imageevent.com/holl_ands/multibay/8bayrefl
sample gain curves
http://imageevent.com/holl_ands/mult...yrefl/cm4228hd
http://imageevent.com/holl_ands/mult...=0&w=1&s=0&z=4
Attached Images
File Type: jpg 4BayCurrentFlow.jpg (42.8 KB, 1918 views)
__________________
If you can not measure it, you can not improve it.
Lord Kelvin, 1883
http://www.megalithia.com/elect/aeri...ttpoorman.html

Last edited by rabbit73; 15-Sep-2015 at 9:57 PM.
rabbit73 is offline   Reply With Quote