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Re: mounting crossed yagis



on 8/7/00 6:06 PM, Franklin Antonio at antonio@qualcomm.com wrote:

> Finally, someone commented on a paper that claims the boom doesn't
> matter.  The theory behind that paper was that along the Yagi's boom (in
> the near field) the E field is not constant, so if you put a crossboom at a
> place where the E field is small, then not much current will flow in the
> crossboom, so it will have little effect.  Sounds good, but unless someone
> can tell me where those magic places are for my particular Yagi, I won't
> know how to take advantage of this magic.

The other problem is that you ideally want to mount your antenna at the
balance point (with coax ON and hanging from the antenna).  This way, there
is minimal strain on your rotor.  What if the balance point is a peak in the
e-field.  

You are also correct in that the minimum for one of the Yagis might not be
the minimum for the other as they are spaced 1/4 wavelength apart.

I would stay away from the metal boom.

I agree with your analysis as well of the interaction effects.  You are
correct, you decrease the coupling for one antenna, but increase it for the
other.  So do you win anything?  Who knows.  I mounted mine in the X because
it interfered the least with my guy wires.


> On a related subject, I think antenna vendors should tell people how far to
> keep other antennas away from a long Yagi.  I suspect that many hams who
> mount multiple long Yagis to the same crossboom put 'em too close
> together.  It would be great to see simulation data on gain loss due to
> nearby antennas.

My KLM manual did.  However, I think a good rule is a minimum of a half
wavelength and preferably about a full wavelength.  I think that for my 2M
Yagis the recommended 48 inches which is about 3/4 wavelength on 2M.

73,

Jon
NA9D 


-------------------------------------
Jon Ogden
NA9D (ex: KE9NA)

Member:  ARRL, AMSAT, DXCC, NRA

http://www.qsl.net/ke9na

"A life lived in fear is a life half lived."

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