Hi Bill,
Phasing center cannot be assumed with dissimilar yagis. What is usually
done, is to monkey with phasing lines until a null at low angles is
obtained. Then remove 180 degrees. The difficulty of this is often the
reason to spend the resources to get up two identical antennas. This has to
be done where multiband yagis are stacked.
Stacking 2 element yagis is a special case because ground can unequally
shift the two yagis. Controversially, the case can be made that a stacked
pair of "Shorty 40" yagis are little benefit over the higher one by itself.
This is because a two element does not have the natural null straight up
and down found in three or more element yagis. The 2 element stack
literally un-phases itself in the near field.
There are many things that simple models won't evaluate, like environment
or poorly isolated feedlines, etc. Ground reflection changing Z of a yagi
will get corrected on site by changing the size of a driven element or
matching device, which in turn can change its phase.
A model worrying about phase must have guy wires, etc, all other conductors
near the tower.
73, Guy K2AV
On Fri, Feb 3, 2017 at 12:58 PM, Bill Conwell <bill@conwellpdx.com> wrote:
> Is it a fair assumption that dissimilar yagis (e.g., Ed's 15m Telrex and
> M2)
> can be phased (e.g., in a vertical 2-stack) by mounting the yagis so their
> driven elements are vertically aligned? I have an impression that it isn't
> necessarily so - that the apparent phase center, if you will, for a yagi is
> influenced by the coupling between the DE and other yagi elements. Thus,
> for example, a point 10 wavelengths out horizontally from a yagi's DE won't
> necessarily be in phase with the DE feed point.
>
>
>
> /Bill, K2PO
>
>
>
>
>
> _______________________________________________
>
>
>
> _______________________________________________
> TowerTalk mailing list
> TowerTalk@contesting.com
> http://lists.contesting.com/mailman/listinfo/towertalk
>
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|