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Author Topic: Which antenna measurements?  (Read 732 times)
KA4NMA
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Posts: 518




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« on: December 06, 2017, 10:36:03 PM »

I have seen several designs of this antenna with different measurements.  For example check out the 2 sets at  https://www.george-smart.co.uk/antennas/dual_band_satellite_yagi/

Which is correct? Can a yagi style director or reflector for 2m be added to increase gain on 145 MHz?

Randy Ka4nma
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WB6BYU
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Posts: 17179




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« Reply #1 on: December 07, 2017, 07:49:54 AM »

Yagi dimensions will depend on the diameter of the elements, the boom material, and how
the elements are attached to the boom if it is conductive.  Unless a design specified that
information, you can't necessarily copy it directly.

Or the original builder might have decided the SWR curve was "good enough" and not
bothered optimizing any further.  I certainly find myself doing that when modeling: 
adjusting dimensions to reduce the SWR from 1.12 to 1.05 : 1 because I can do so in
the model, when in practice I would have difficulty building it that accurately.


There have been various designs that add extra elements to a Moxon, but the traditional
Moxon is optimized for a 50 ohm feedpoint, and adding elements usually disrupts that.
Once you go to 3 elements you can make them full size and have a wide range of designs
to choose from depending on your trade-offs among gain, pattern, bandwidth, or matching
method.  But you'll have to experiment to find the right method that provides the same
feed method on 440.

Also, adding a 2m director can mess up the pattern on 440, where the director is 3/2 waves.
That's one advantage of this particular design - it keeps the 2m elements out of the 440
beam.  (That's one reason why you often see such antennas with the elements for each band
mounted at right angles to each other on the same boom.)
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