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Re: Metal vs. Fiberglass Boom



> A crossed yagi antenna connected for circular polarization
> has two components at 90 deg out of phase wich resultant is
> a vector that rotates in the space 2 x 3,14 x f  every second
> or 360 degrees x frequency in Hz  every second.
>
> Now if you have  the 2 meter antenna in + configuration and
> the 70 cm antenna in X configuration or both in X configuration
> or both in + cofiguration the isolation between the antennas is
> the same because the above 2 meters and 70 cm vectors rotates.
> like a two bladed propeller.
>
> The same is true if you use a metal boom,no matter if you put the
> elements in X configuration or + configuration because in any
> case the EM rotating vector of circular polarizazion sees the metallic
> boom or better is obscurated by it,or cross it one time for every
> cycle of the exiciting frequency.
>
> To demostrate that X is better than + configuration is necessary
> to demostrate first for what physical law the resultant EM rotating
> vector on X configuration should not see the metallic boom crossing
>  it every cycle of the exciting frequency.
>
> 73" de i8CVS Domenico

This is true outside the near field, but within the near field near the 
actual elements, there may well be different interaction between antenna 
and boom in X and + configurations.  Modeling the near field is far more 
complex and usually not necessary, but in this case, you definitely have 
to look at least at the portion immediately surrounding the boom.  You 
can mount a Yagi perpendicular to a metal boom because the plane 
perpendicular to a Yagi's elements lies completely within a deep null in 
the E-field -- this has been experimentally proven and a number of long 
boom Yagi manufacturers exploit this by adding diagonal braces within 
this plane.  The nulls in the near field of a crossed Yagi CP antenna 
are much more complex and the effects of a metal boom in the near field 
would be very difficult to predict and strongly dependent on the exact 
positioning of both.

Only way to really settle this question is to test this on a range ..

Heard from a flight instructor:
"The only dumb question is the one you DID NOT ask, resulting in my 
going out and having to identify your bits and pieces in the midst of 
torn and twisted metal."

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