jim Jarvis wrote:
> Whilst Jim Lux and Jim Brown joust over whether it's phasing, polarity, or
> time delay,
> I would assert that a time delay which is equal to a 180 degree phase
> transition at the
> frequency in question IS a phase delay.
>
> And that brings me back to where I THINK this particular nit began to be
> picked....
>
> If you have two LPDA's, and they're on the tower with the upper one tilted
> down, and
> the lower one tilted up, and you're able to produce .5 lambda spacing over
> the full octave
> range of the antenna by virtue of that tilt.....
>
> AND, if the feed system to the dipoles is a parallel transmission line.
>
> THEN I would assert that simply reversing the phase (ok, polarity if you
> like) of one of
> the antennas AT that feedpoint will result in the two either being IN or OUT
> of phase.
>
> At the risk of invoking the voice of the dead, Cebik felt that was correct.
>
> Am I missing something?
>
> N2EA/Jim
>
>
I'm not sure polarity reversing (BIP/BOP) is really what want in this
situation. Because they're 1/2 wavelength apart
Let's take the "fed the same" case first. At any given frequency, it
looks like two dipoles fed in phase, so the resulting pattern is
narrower than that of the dipole and the beam is horizontal (i.e. along
the line perpendicular to the line between the antennas).
What happens when we flip the polarity(!).. now it's like two dipoles
1/2 wave apart fed out of phase, which has a pattern that points along
the line between the two antennas (i.e. straight up and down). And, it
actually has a null at the horizon)
A real LPDA has a pattern that's already directive instead of the
dipole, so the "same polarity" (BIP) would squirt more in the horizontal
plane, and the "different polarity" (BOP) would more reduce the gain at
low angles (making it more like a dipole).
That's for LPDAs in free space (or a long ways up)
Another possibly bigger effect would be the change in effective height
of the antenna above the ground. Here it's even trickier, depending on
how high the lower antenna is AND because you've tilted the antennas, so
their boresights are pointing at an angle to the horizontal. The longer
the boom of the LPDA, the better off you are here, I suspect (the angle
is smaller, so they're both closer to horizontal)
Yes, indeed, it's a tricky modeling problem.
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