Final analysis
Thanks for making me cut the Maldol open to reveal a coil inside. This leads to a better understanding of what's going on.
To cut to the chase, the AL-500H provides the most benefit to low-end scanners with inferior front-ends, while it may not make an eye-popping difference to higher-quality handhelds as compared to a standard 1/4 whip.
Propeller-head analysis:
The primary function of the coil is to decouple the upper half of the whip while monitoring uhf milair. On vhf airband, the coil merely presents a small amount of inductance which shortens the overall length. Maldol engineered it to be resonant precisely in the middle of both bands. The introduction of the decoupling coil makes the LC circuit higher-q than a standard whip. Having ultra-thin elements also help in this regard, almost to the detriment of milair which needs wide elements to cover the extreme bandwidth, but a 2-inch diameter whip is not practical on a handheld for milair.
The MH-610 tribander works the same way - the decoupling coil cuts off the top half of the whip for 220mhz amateur use, but on vhf, the coil shortens the overall length so that the 2m amateur whip also serves as a 3/4 whip on the 70cm band. To do this right, element lengths and coil inductance are tricky, and I felt that the Diamond 320A did this trick a little bit better when measured on an antenna analyzer. Being hi-q, it was not a great substitute for airband monitoring and on 70cm the directional pattern of a 3/4 wave is not usually desired, but directionality may not be so critical for portable handheld.
Back to hi-q resonant AL-500H. The main benefit is that it provides a level of bandpass filtering that is usually not present in low-end scanners. While it doesn't replace proper internal filtering, it does help in areas that are susceptible to FM broadcast or NOAA overload for example. It is also helpful for dual-conversion scanners, especially those that use a 10.7mhz first IF, and being resonant certainly helps especially for low-sensitivity models, yet it won't stop the flamethrower out of band signals from getting through, nor replace a proper external bandpass filter.
Triple-conversion is no guarantee either if the manufacturer doesn't use adequate bandpass filtering. For units that have FM broadcast coverage, a way to reduce costs is to use only a single bandpass filter that covers from 88 - 138 mhz, rather than split these bands up separately with two bandpass filters. Worse yet, a single bandpass filter that covers the entire vhf band from 88 to 175 mhz. Yikes!
In the end, while the AL-500H is admirable, it is not necessarily needed on higher-quality receivers, although it is good engineering practice.
Compared to a standard 1/4 whip, my dual-conversion low-end RS Pro-404 loves it. On my Uniden 396XT, it doesn't seem absolutely necessary.
