Re antenna radiator length:
I do not agree with the statement that most VHF Marine Band antennas are a quarter-wavelength with ground plane. At 156.8-MHz the wavelength in free space is about 75-inches. A quarter wavelength would be just under 19-inches.
The most common VHF Marine Band antenna in my experience is a half-wavelength vertical monopole with end feed. A half-wavelength in free space would be 37.6-inches, and in most of these antennas the whip length is slightly shorter, about 35.5-inches. The impedance at the feed point is high and must be matched to 50-Ohms with some sort of transformer.
A half-wavelength vertical may not need a ground plane but it does required some counterpoise
. The counterpoise
provides a path for the RF current to be a complete circuit.
There are several half wavelength antennas with metal whip radiators and a base transformer. A common element of the base transformer is to be enclosed in a metal cylinder. You can see this on the Metz antenna and also on the very similar GAM SS-2 antennas.
In most of these antennas, inside them you will find the metal cylinder encloses a single
winding coil connected to the antenna base and the coaxial connector outer conductor, with the coaxial feed line center conductor tapped onto the coil with a turns ratio of about 3:10. The impedance ratio is the square of the turn ratio, so you'd expect about 1:11 impedance ratio. That sounds too low, implying the antenna impedance would be about 580-Ohm, which is below what would be predicted for an end-fed half-wavelength. The pitch
of the windings on the coil are not uniform, so that may also be affecting the matching characteristics.
The whip length is made slightly shorter than half-wavelength due to having some effective length of the antenna reside inside the matching transformer enclosure, and also perhaps for tuning of the matching network.
I also suspect that the metal cylinder enclosure is important and provides part of the counterpoise. I suspect the design was reached by trial and error rather than by theoretical analysis. And these antennas seem to work well.
Re isolating the antenna coaxial connector shield from the mast:
Decoupling an antenna from common-mode noise currents helps to reduce noise induced into the antenna and sent down the feed line, and if isolating the antenna shield from the mast helps, then that is great. However, there will be some antenna currents flowing on any conductive surface near the antenna.
line types and connectors: I would avoid coaxial cables
made with foam dielectric for two reasons: the center conductor tends to migrate into the foam at cable bends, and the foam tends to melt when soldering to the cable.
A proper high-quality PL-259 connector properly soldered to Mil-Spec RG-213/U cable and properly wrapped with real Scotch 3M 35 Vinyl electrical
tape will be very environmentally resistant.
Avoid installations in which the PL-259 connector-to-cable joint becomes a load bearing connection and must support the weight of a long length of transmission
line hanging from the connector.
I do not see much advantage of RG-214/U cable. It is just a double-shielded version of RG-213/U. It is more expensive, weighs more, but has the same loss characteristics. Generally there are not other transmission lines run along side the VHF Marine Band radio transmission line, so signal ingress into the cable is not particularly a problem. Double-shielded flexible coaxial cable is usually used in environments where there are many other cables
with many other signals around. Also in a sailboat the cable is usually run inside a metal mast, so additional shielding is not really needed. There is one advantage to RG-214/U: generally it is made only in very high quality and will usually exceed the quality of any run-of-the-mill off-brand RG-8 cable, and its shield conductors are tinned copper, not bare copper.
Re advantage of height over antenna gain: many studies have shown that increase in height produces better signals at distant receivers than would be predicted by simple analysis. I discuss four models that predict this in more detail at
VHF Radio Propagation Over Water
continuousWave: Whaler: Reference: VHF Radio Propagation Over Water
See the section under the subheading "Effect of Antenna Height".
Re dielectric-grease: this is a grease that resists conducting electricity. Its principal purpose is to be a grease. As a grease it is often used to enhance the effects of seals
to keep out contaminants. It does not IMPROVE electrical
conductivity but rather prevents it.