[TowerTalk] Yagi designs?

[Ian White GM3SEK]

Peter Voelpel wrote:
>
>I wonder why he is not mentioning that in his book to take precaution 
>and avoid certain boom length?
>
>In opposite he writes:
>
>"The simplified Yagi model also neglects the conducting boom; this is 
>justified only if the real yagi is completley symmetric around the 
>boom. Symmetry guarantees zero electrical potential at the center of 
>each parasitic element and no mutual coupling to the boom. Hence no 
>current will be induced in the boom by the parasitic elements. If the 
>driven element is open at its center and driven from a balanced source, 
>then it will induce no urrent along the boom. Thus, for the symmetric 
>Yagi with balanced feed there are no currents in the boom, and 
>therefore no effect of the boom on antenna performance"
>
>Source: Yagi Antenna Design, First Edition
>
>So if that is true, any antenna nearby inducing current into the boom 
>by mutual coupling is not doing any harm and will not disturb as the 
>boom is not ccoupling to the symmetric elements of that antenna.

The above is all true, but it seems to be missing the point. 
Paraphrasing Lawson, the important point is:

The assumption of symmetry in a simplified model will guarantee that the 
modeler will NEVER see any effects due to boom currents and combined 
boom/element resonances. Likewise, they will NEVER see boom-coupled 
interactions between antennas that are modeled as perfectly symmetrical. 
In addition, the assumption of a 'floating feedpoint' at the driven 
element will guarantee that the modeler will NEVER see any effects due 
to currents on the outer surface of the coax feedline... because the 
antenna model doesn't even have a feedline.

Nothing is completely symmetric in real life, and we do have to use 
feedlines, so all of these effects will exist... to some extent. The 
question is whether they are big enough to be important, in terms of 
pattern distortion, unwanted coupling and risk of RFI.

The particular question about boom or boom/element resonances is related 
to the current magnification that will take place if the design has an 
accidental resonance in the same or a different amateur band. This could 
make the antenna much more vulnerable to a small asymmetry in the 
installation.

Careful antenna modelers will look for these unwanted effects, and do 
whatever is necessary to ensure they are NOT big enough to be important.



-- 

73 from Ian GM3SEK