jim Jarvis wrote:
> Sorry for extending this, but another thought came up.
>
> (Doug...aren't you glad you got this one started??)
>
> (Gene... the tilted log post was mine, at first... )
>
> So here's the thought, with respect to tilted logs:
>
> The full length of the log isn't really active at any given frequency.
> Only a short portion of the boom, and maybe 2 or 3 elements have real
> current in them. Therefore, is the tilt of the boom all that appreciable,
> in terms
> of takeoff angle?
>
> A tangible example: With a 16' boom log (is the t8 16 or 18', I've
> forgotten? no matter.)
> ... the booms would need to slope at 30 degrees. If the last 2 elements
> were primary in forming
> the pattern at 14 MHz, and they were around 4' apart..... the slope would
> result in the forward elements
> being only 2' lower (or higher, depending on if top or bottom log) than the
> rear element.
>
> Is this really significant in the takeoff angle? Intuitively, it wouldn't
> seem to be the same thing as
> a yagi with the director 16' in front of the reflector.
>
> What am I missing?
>
I'm working a model on that now. And I agree with you. The vertical
pattern is so wide (90 deg 3dB beamwidth, for instance) that the tilt
doesn't seem to be a big deal. The height above ground, though, is a
big deal.
When it comes to increasing forward gain at low angles, I'm not sure
that the V arrangement (as opposed to strictly parallel) makes a whole
lot of difference. The gain in the direction of the main lobe of a pair
of antennas a sufficient distance apart doesn't change very much with
spacing. Yes, there are changes in the sidelobe structure, but with
directional antennas being the elements, not much power gets to the side
lobes (using straight pattern multiplication).
I'm starting to think that the reason for the V configuration (where the
apexes almost join) might be mechanical convenience.
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