[TowerTalk] CushCraft Query

[Jim Lux]

At 01:14 PM 6/22/2006, you wrote:
>Why would the impedance change with a different driven element.

I don't know that it would. Another poster said that the D9 is designed for 
a different Z than the A3's normal driven element (which seems reasonable).

I would expect, for instance, that the traps might need to be in different 
places in the context of a 3 el antenna instead of in free space, because 
the coupling among the various segments would be different.

One thing that I would look at (if trying to approach it analytically, as 
opposed to modeling it or just trying it) is whether the coupling from the 
reflector and director to the piece of antenna beyond the traps of the 
driven element has an effect.

In the standalone element case, you only have to deal with the coupling 
within the element, say, the 10 meter section close to the feed, to the 
parts beyond the trap.  The designer adjusts the lengths of the segments 
(positions of the traps) to allow for all this.

However, in the 3 element case, you have some coupling from, say, the 20 
meter tips of the reflector to the 40 meter tip of the new DE, beyond the 
trap.  It's not end on, but it's also not lined up exactly, (it's what's 
referred to as elements in echelon in the antenna books) so the coupling 
will be low, BUT, coupling is coupling, and you'd have to evaluate it to 
see if things will be screwed up or not.

[Just as a numerical example, if you have a parallel pair of dipoles, a 
quarter wavelength apart, the mutual Z is about 37-26j.  Stagger them so 
that the dipoles are a half wave apart (center to center, laterally) and 
the mutual Z is about 9.55-11j.  Make them collinear, and the mutual Z is 
21+6.6j.]


----o----
----o----

or
----o----
          ----o----
or
----o---- ----o----


The upshot is that putting pieces of metal (e.g. the 40 meter ends of that 
40,20,15,10 dipole) in the near field of an antenna that depends on mutual 
coupling among the elements to work, and somethings going to change (and 
not usually for the better).

here's what you're basically building...(in fabulous ASCII-art)

           ---o---
     ----||---0---||----
           ---0---


>   The
>reflector and director lengths and spacing are not being changed and I
>thought that is what sets the impedance.

The lengths and spacings also set the currents in the elements, which is 
what determines the overall pattern of the antenna.  The Z also changes as 
you say.  But, to a first order, the reason a Yagi has a low feedpoint 
impedance is that you're essentially driving three dipoles in parallel from 
one feedpoint.  Two of them are connected into the circuit by the mutual 
coupling, so changing the spacing and/or length changes the coupling which 
changes the Z if that element, as seen by the DE.


>   If I remember correctly the A3 is
>a 50 ohm antenna and swapping the dipole for the DE would probably only
>involve changing the lengths for the best SWR.

It might be that adjusting for best match might give you a functional 
antenna, it might also turn out that it completely screws it up. It's also 
not something I'd want to try and figure out by "cut and try", running it 
up and down the tower to check the pattern.

Jim