Dennis, I'm also going to post this this reply on Antennaware
reflector, with only the question quoted as it would seem to be of
general interest and (unfortunately?) has a complex answer.
On Sun, Mar 7, 2010 at 9:39 PM, <dennisberry@att.net> wrote:
> Guy, you wrote:
>
> "I discovered that the simple act of winding wire around an end
> insulator changed the apparent electrical length of the wire and could
> not be handled by some programs, requiring freehand SWAG "adjustments"
> to wire lengths for end insulators."
...snip...
> Did it make it longer, shorter, x%?
>
> Thanks,
>
> dennis
Short version:
Winding around the end insulator made it electrically longer, if you
are measuring the same physical length from tip of wire to tip of
wire. The dielectric constant and configuration of the insulator would
vary the difference beyond adding the loop and wrap. HOWEVER, the
experience did not indicate any simple fixed correction that would be
useful for wire parasitic elements.
Long version:
The insulator threading and wrap had the effect of a FIXED CAPACITOR.
It did very little on 160 but by the time one was trying to construct
wire yagis on twenty it was a small capacitor blocking very high RF
voltage and having a considerable effect on the electrical length of
elements. The correction is not constant in length across bands
because frequency change in turn changed the amount of electrons that
could be forced into the end in a fixed very small percentage of RF
cycle time. The reactance of the end-effect capacitor was much lower
at 20 meters *times* the fact that a reactance change caused by a
fixed wire length change pulled resonance 8 times as much on 20 as
160. These two had the effect of making end-effect 64 times more
critical on 20 meters as on 160.
If you think about it, who do you know that has killer results with
wire yagis on 20 and up. Successful wire beams on 20 meters with a
mass of anecdotal positives are mostly quad designs, which have no end
effect and which the models show to be more forgiving of errors.
At some point we were subtracting three inches from each end on twenty
meters (but don't use that because we have not revisited it with
current equipment. The real horror of it at the time was trying to
work this with equipment like Autek RF1, where you had to break the
wire and insert a connector, which of course changed the resonance.
OK for dipoles, but immediately a problem for parasitic elements which
would be hung with no break in the center.
Measuring was further problematic at that time, because of slight
detuning from the coax from the center and certifying the length of
coax as being non-transforming.
At NY4A (in 1996?) we put up an 8 element 20m "long boom" wire yagi
suspended along with 80m and 40m elements on a 220 foot catenary
running NE/SW. The 7 20m parasitic elements were created with our
"SWAG" of the day based upon what we hoped were correct extensions of
the experience with the DE as a dipole at height. We have always
found that antenna problematic, even more so due to mechanical
considerations since the model shows it to be terribly sensitive to
having all the elements on a horizontal line. We intend to replace it
sometime with a 5 or 6 element quad without any of the pesky dipole
ends.
On the other hand, the five element 40m quad on the same catenary has
been a killer antenna sometimes producing the high USA 40m QSO count
in DX tests. Of anything serious we have put up the 40m quad came
closest to "'pute, cut, hang and play". In this instance, the only
"global" adjustments we made were related to the effect of ground
elevation on resonance points of elements. This involved changing the
model ground constants to give the same resonance on a single quad
element in the model as measured in the real situation.
We also have now the AIM 4170 analyzer which can calibrate to the FAR
END of a feedline complex, and we know how to lightly couple a wire
element without breaking it. Would have been oh, so very handy back
then (sigh). The AIM is only one of a new breed of complex
computer-based analyzers that will allow us to deal with these issues.
As an aside, since the 40m quad was done before the AIM techniques
were available, technically we don't know if we hit it on design. We
only have anecdotal results : >)
Never as simple as it looks.
73, Guy.
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