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Author Topic: Moxon, Yagi, Tak antenna  (Read 6390 times)

Posts: 227

« on: December 09, 2012, 06:30:41 PM »

I was checking out Moxon antenna design which is sort of like a horizontally bent yagi and wondered if anyone has tried bending the yagi into a quad configuration?? I could then build one with wire using the same structural design as the TAK Antenna? I drew a picture showing the driven elements and the reflector.

Posts: 17483

« Reply #1 on: December 09, 2012, 09:33:29 PM »

You CAN make a yagi using elements like a TAK antenna, though it won't
work very well.  You can shorten Yagi elements down to about 50 to 60% of full
size before the reduction in efficiency overcomes the expected gain of the
antenna.  (While the TAK antenna looks like an end-loaded dipole, it really is
an end-fed random wire because if you follow the instructions you get more
radiation from your coax than from the antenna structure itself.)

By the time you fold a dipole into a square 1/8 wave on a side the radiation
resistance has dropped significantly.  I have seen antennas designed where
there was a diagonal across the center of the diamond that was fed, and
the perimeter wires provided heavy end-loading to lower the resonant
frequency.  Because the wires come back relatively close to the feedpoint
(at least in one plane) the radiation from the ends of the wires tends to
cancel some of the radiation from the center, lowering the radiation
resistance and raising losses.  (This approach has been used with somewhat
larger elements to make a reduced-size quad.)

The Moxon approach of folding the ends of the two elements back towards
each other has a further advantage that the relative current in each element
can be adjusted by varying the end spacing, allowing an excellent front-to-
back ratio.

Posts: 227

« Reply #2 on: December 10, 2012, 08:41:02 PM »

If I understand you correctly?? because the two driven elements start getting close at the ends that cancels out the radiation in the middle of the elements??  Just like everyone else I am trying to get a directional antenna that has massive gain and fits in a shoebox<GRIN> Any Suggestions??

Posts: 17483

« Reply #3 on: December 10, 2012, 10:42:06 PM »

Do you need gain or directivity?

You can get directivity in a small package much more easily than gain.  That's because shrinking
the elements reduces efficiency, which reduces gain but not directivity.  So the beam still has
a maximum in one direction and a minimum in the other (sometimes one of those is more
important than the other) but the transmitted signal may be weaker than from a single
element antenna with little or no directivity.

W4RNL had a series of articles where he studied different ways to shrink a yagi, and came
to the conclusion that making the elements shorter than about 60% of normal length was
a practical lower limit where the efficiency started dropping faster than the gain increased.
But this required the  the boom length be left as is without being shortened also.  Similarly
I know people using quads with elements that are about 60% of full size - tuning is critical
and the SWR bandwidth is narrow, but they do work.  Again, you can make antennas smaller
than that, but efficiency suffers.  You are often better off making a single element antenna
as efficiently as possible than trying to make a beam out of shortened elements (at least
in terms of total radiated power in the peak direction.)

For a small beam, a Moxon works pretty well.  There are some other antennas such as
elongated quads that will reduce the turning radius at expense of a 3-dimensional structure.

Actually, a directional antenna with massive gain will fit in a shoebox... if it is designed for
a high enough frequency.  (And depending what size shoes you wear.)  I have a design for
an 8-element yagi with 10dB of free-space gain over a dipole that is about 13" long and 4"
wide, but it operates at 1296 MHz.  You can get a lot more gain than that at optical

It may help if you explain your specific requirements:  there are some practical differences
between building a short yagi for 80m than one for 440, and knowing what band(s) you want
to operate and what space you have available will help us better tailor our suggestions to
something that may be practical to try.

Posts: 9930

« Reply #4 on: December 11, 2012, 02:06:12 PM »

Moxons do work fairly well, basically a bent  yagi,  but all the rest of the "mini" antennas are like the Isotron line. bassically a capacity hat on the end of your radiating coax. about the smallest antenna I have used for  HF is the MA5B, which actually works pretty well if you get it up over 30 feet above the ground.
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