Glomex antennas

Today i was installing an antenna on a rib when i was asked if i could help a boat owner in the yard with question he had about the same antenne he was installing on his yacht, he had found that if he put an ohm meter between the whip antenna and the thread used to secure the antenna there was continuity which meant once the coaxial cable was screwed in there would be a short between the sheilding and the core.
We tested the same antenna i was installing which gave the same result, we both thought that the shielding was always insulated from the core?

Can anybody explain why these antennas are different.

 

It really depends on the design of the Antenna. Some Antennas are designed with a DC Path between the Radiator and Ground, and some Antennas have no DC Path to Ground. Most Marine VHF Antennas are the Grounded DC Path Type simply because the OEM's don't want Static Buildup on the Radiating Elements, and with this type of design any static goes directly to ground. RF is a totally different realm than either DC, or even AC, so some things that would never seem to be true in Power Systems, are very True in the RF realm. You can NOT test an Antenna with an Ohm Meter, unless you know how it is built, and usually even then, any definitive test is very subjective. Antennas need different Test Equipment. Like a Wattmeter, RF Impedance Meter, or the like. This is why Marine Radiomen get paid so much money for their expertise.

 

The DC short is present in all antennas designed for a specific frequency band. It is a strip or coil which resonates in that band and has a low impedance below and above that.
Marine VHF antennas are sometimes mounted near a radar dome: an open-ended antenna would pick up the high power radar transmitter energy and blow up the VHF set instantly.