Jim Brown wrote:
> On Mon, 23 Nov 2009 15:01:13 -0600, Stan Stockton wrote:
>
>
>> Am I missing something in the way I am modeling it or should I
>>
> expect to see the results shown by the computer. I am suspicious of
> the model since I can change the vertical portion of the reflector
> to #10 wire instead of #18 wire and the max gain moves up about 40
> khz. I would have thought it would have moved it down some?
>
> Stan,
>
> I have a lot to learn about Yagis (and this is sort of a Yagi, if I
> understand what you're doing), but I've been studying a bit in the
> ARRL Antenna Book and building some Yagis. One thing I think I've
> learned is that the thing that makes it work is the phase of the
> current in the passive elements, which is set by their physical
> length as a fraction of a wavelength. As I understand it, the
> resonance of the driven element does not matter (or does not matter
> much) in determining directivity, and can be varied to achieve a
> match (with the hairpin or gamma or whatever).
>
Granted and over simplification, but: Gain and F/B are determined by the
phase angle and coupling between the elements. IOW the phase between the
driven element and the reflector is such that the signal cancels (more
or less) in the direction of the reflector, but the re-radiated signal
from the reflector is in phase when it gets back to the driven element
which reinforces the signal strength in that direction. Technically
IIRC the radiator does not have to be resonant pe se, BUT the matching
network, spacing, and length determine the phase along the length of the
driven element do have some effect. For one, regardless of length there
is an infinite impedance and SWR at the end of an element which results
in a 180 degree phase reversal. What about the phase of that wave when
it reaches the driven point and how does that affect the performance?
Remember the driven element should be at a phase angle of zero as all
the other elements are referenced to it. Also, length does affect the
radiation pattern of the driven element. IOW, make it "long" and the
radiation would be more off the ends and besides the mechanical
problems, it's likely it would not make a good driven element for a
parasitic array. No, I have not modeled it.
73
Roger (K8RI)
> Others may know a lot more than I do about this, so take this as a
> clue, not a definite "I know this for sure" sort of reply.
>
> 73,
>
> Jim K9YC
>
>
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