...I recall a white paper that Motorola did many years ago that suggested a narrower lobe in the elevation pattern may diminish picket fencing as the 2nd signal many times arrived from a higher angle (local building reflection etc)...
There are lots of practical aspects to mobile antennas that don't always show up in
models. For example, since maximum radiation from a 5/8 wave antenna is from the
upper section, it is more prone to polarization shift at higher speeds than a 1/4 wave
whip as the antenna tilts backwards. Some studies on police cars at high speeds
showed that the quarter wave whip was often just as good as a 5/8 wave whip
in the 150 - 170 MHz range because of this. (It probably is applicable to ham
antennas at lower speeds using more flexible antennas.)
The angle of arrival will depend on the environment: on UHF in a downtown city with
lots of tall building that might be the case, but in rolling hills I would think not.
Along the same line:
And in order for the second 5/8 wave to be completely in phase with the first 5/8, the loading coil must be 3/8 wave long.
Actually it would be 1/4 wavelength, so that combined with the top 1/8 wave of
the lower element and the lower 1/8 wave of the upper element it makes 1/2 wavelength.
But this doesn't always work the way a designer would hope: being at the high impedance
part of the antenna, it often ends up with a self-resonant inductor rather than just a
"loading coil", and achieving the require phase shift without a ground reference is more
difficult than one would imagine by the number of designs that just use a stub and
assume it works. They tend to work much better when feed in the center rather than at
one end with some sort of phase-shift network. It would be interesting to see what the
actual vertical radiation pattern is for many antennas
I think of picket fencing when I pull up to a stop light here in the mountans and move forward a bit to get out of what is likely a multipath null on the NPR FM station I listen to.
I run into this at a few intersections on the way to work each day: I stop well back of
the car ahead of me, then creep forwards to get a good signal. One quirk, however, is
that because a repeater uses different frequencies for uplink and downlink, sometimes
the peak for received signal puts me in a null for the uplink, so I have to split the
difference.
Multi-path also causes the signal strength to vary with modulation on an FM signal, due
to the different deviation frequencies arriving with varying relative phase shifts over the
two paths. That distorts the audio, and increasing transmitter power doesn't help.
These are all good exercises for the reader following along at home to see how well they
understand why things happen this way.
