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Author Topic: 2 element 75m Ham Stick Beam  (Read 513 times)

KL7CW

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Re: 2 element 75m Ham Stick Beam
« Reply #15 on: March 25, 2023, 11:42:02 AM »

It has been shown repeatedly through the decades that a long thin loading coil is not as efficient as a larger diameter coil, closer to being square.  Also short loaded antennas, are more efficient if the loading coil is placed roughly half way out the element.  The more efficient low band mobile antennas have large roughly square (L approximately D) which look like they may have a diameter of perhaps 2 to 5 inches.  They may use slightly slimmer coils, for aesthetic or wind resistance considerations.  Look at some loaded Yagi designs in publications like "Low Band DX'ing" etc.  If my memory is correct the coils for one 80 meter Yagi were something like perhaps 5 inches in diameter, and the Yagi was at least half size, and the loading coils were part way out each element.
  About 20 years ago I was considering two 40 meter antennas.  One possible option was a loaded 2 element Yagi about half size with element space of about 1/8 wavelength. The other option was a full size dipole with no loading coils.  Physical constraints required the Yagi loading coils be at the center of each element.  I spent a lot of time modeling my options.  In my situation, I opted for the much simpler dipole, with nearly equal gain, since I have other options for my RX antenna if I need RX directivity.
    Helically wound whips are an alternative as CB operators, and 10 meter mobile operation I did back in the 50's with home brew verticals perhaps only 3 or 4 feet long wound on fishing rods.  Efficiency was not great, but it must have been much more than the few percent quoted for the very short hamsticks on 80 and probably 40 meters.  Once again I think our ham "rule of thumb" that antennas less than about half size are a significant compromise, but as you get closer to full size the efficiency may approach a full size antenna. Linear loading works, but in most situations is not equal to a properly designed coil.  One good way to load a short antenna is a top hat at the end of the element, or even a horizontal wire like 160 meter folks use for their successful inverted L vertical antennas.  These two examples may approach the performance of a full size (not loaded) antenna.
           Rick  KL7CW
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W1VT

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Re: 2 element 75m Ham Stick Beam
« Reply #16 on: March 27, 2023, 06:09:33 AM »

Why does the ARRL handbook still have a 3 element 40m beam with helically wound elements?   
I went looking and the closest match seems to be the 16 and 17th editions of the ARRL Antenna Book, a different publication from the ARRL Handbook. 
On page 6-14 to 6-14 "A Yagi Antenna With Continuously Loaded Elements"  A 2 element helically wound 40M Yagi.
On pages 6-16 to 6-18 "A Small Yagi for 7-MHz."  A 3 element Yagi with loading coils and capacitance hats.  The 16th edition was copyright 1991.

It appears in the 1983 copyright 14th edition on pages 10-10 and 10-11.

The full text search of the new digital books should make it easy to find by searching on "helically" unless you are looking in the wrong place.
« Last Edit: March 27, 2023, 06:15:16 AM by W1VT »
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