Originally Posted by SailorKeddy
So I think I have RG8 cable going from my VHF to antenna ( based on 0.4" OD) and I need to connect two ends together before it goes in mast. There is an old screw type connector on one side that I snipped off (see pic)...what type of connector is this? How else could I connect some RG8 coax cable?...
The connector you show is some variant of the PL-259 connector, part of the UHF-Series. I believe the nomenclature PL-259 was the USA Army part number, probably a holdover from wartime production in the 1940's. The series designator UHF is a bit misleading, as today no one would use a PL-259 connector at UHF frequencies. The PL-259 is a PLug
and it makes with the SO-239, a SOcket.
To connect a PL-259 to another PL-259 requires an ADAPTOR, not a connector. The nomenclature used is sometimes 83-1J, the part number of a major manufacturer (Amphenol) for an in-series plug-to-plug PL-259 adaptor. The Army part number is PL-258, but this is not in common use in my experience (which goes back to the 1950's).
You mention two generic types of coaxial transmission
line, RG-8 and RG-8X. Both these designators are now generally meaningless as there are no military specifications for them. You can buy coaxial cable marked RG-8 or RG-8x and get a wide variety of products of various quality.
In general, the RG-8/U Mil-C-17 specified cable was a single-shielded 0.5-inch OD 50-Ohm impedance cable with a contaminating black vinyl outer jacket. This has now long been superseded by RG-213 cable, a similar cable except the outer jacket is a non-contaminating black vinyl, often called a Type-II. In coaxial cable to be used outdoors in weather
and sunlight, it is much preferred to use cables
with non-contaminating jackets. There are also many very similar cable to RG-213 made by many cable manufacturer with their own designations. Cables
with contaminating outer jackets tend to see increases in loss as the outer jacket vinyl contaminates the copper shield conductor.
The RG-8x cable was something of a hybrid cable in which the general dimensions of RG-59/U, a 75-Ohm cable were used, but the dielectric was changed to a foam instead of polyethylene and the center conductor diameter increased to get the cable impedance back to 50-Ohms. There has never been, to my knowledge, any Military Specification for an RG-8x cable. This cable is often seen in marine applications and comes is a very wide variety of quality. In my experience RG-8x is not a good cable to use--unless you buy it from a major cable manufacturer--as it has a non-contaminating jacket and the foam dielectric must not be subjected to any sharp bends. Migration of the center conductor in the foam is a problem with this cable, and particularly in hot climates. No professional antenna installer would ever use this cable.
In general for installing a VHF Marine Band antenna atop a tall mast, the transmission
line loss should be carefully considered. A typical goal in any transmitter installation
is to limit transmission line loss to less than 1-deciBel.
Here are some typical characteristic transmission line losses at 150-MHz for 100-feet of line for several grades of cable:
RG-58/U = 6.2 dB
RG-8x = 4.7 dB
RG-213 = 2.8 dB
LMR400 = 1.5 dB
With this data we find the maximum length at 150-MHz for a loss of 1-dB to be:
RG-58/U = 16-feet
RG-8X = 21-feet
RG-213 = 35-feet
LMR400 = 67-feet
If you want an optimum installation
, choose the transmission line accordingly.
For connectors, the transmission line at the antenna must be fitted with the appropriate mating connector for attaching to the antenna. It sounds like a PL-259-type connector must be used in the situation under discussion.
There are many choice for a PL-259-type connector, depending on what transmission line you choose. Get a quality connector that is specifically designed for the transmission line. I am old-school and prefer solder connectors for the center conductor. Center contacts should also be captivated so they do not retract in cold weather
and break contact.
If you buy transmission line from a vendor who offers a service
to install connectors, I recommend taking that option. Most boaters do not have specialized tooling for installing modern connectors using crimps, and solder connectors can also be a problem, as they require some skill and possession of proper soldering irons.
If there is any weak point in a boat radio
system, it seems like the transmission line connector is the number one suspect. In the case of a connector installed at the top of a tall mast, make every effort to use a proper connector, to remove strain from it, and to protect it from ingress of water