Coaxial Transmission Line Outer Insulating Jacket Damage

VHF Marine Band radios, protocol, radio communication theory, practical advice; AIS; DSC; MMSI; EPIRB.
moabelite
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Coaxial Transmission Line Outer Insulating Jacket Damage

Postby moabelite » Mon Dec 28, 2020 5:58 pm

Our recently acquired 1996 Boston Whaler Outrage 24 has a cracking [of the outer jacket insulation on the coaxial transmission line cable] between the marine radio antenna and the connection into the gunwale.

tranmissionLine.jpg
Fig. 1. Transmission line outer jacket cracking.
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ratchetMount.jpg
Fig. 2. Ratchet Mount and gunwale fitting.
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After looking here on ContinuousWave I don't see any experiences with wire wrap that might help to protect and preserve the wire. It is currently operational and my hope is that some attention in the near term can avoid or at least delay a complete replacement of the run back to the center console.

[Implied question:] What is an effect product for repair of cable with cracked outer jacket insulation?]

The boat now lives indoors year round (except when on the water) so future UV damage should be sharply curtailed. See attached photos for the current condition.

{Moderator's note: the coaxial transmission line cable shown in Figures 1 and 2 was initially referred to as "wire." The coaxial cable transmission line is not a wire.]

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Phil T
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Re: Coaxial Transmission Line Outer Insulating Jacket Damage

Postby Phil T » Mon Dec 28, 2020 6:31 pm

Consider liquid electrical tape as a short term fix.

It really needs to be replaced.

You don't need to replace the entire run. Only the ~1'ft that is exposed.
1992 Outrage 17
2019 E-TEC 90
2018 LoadRite 18280096VT
Member since 2003

moabelite
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Re: Coaxial Transmission Line Outer Insulating Jacket Damage

Postby moabelite » Mon Dec 28, 2020 7:57 pm

Thanks Phil - The alternative to liquid tape is to splice in a new section for this +/- 1.5 feet? I assumed (likely incorrectly) that splicing would be difficult on the antenna end and could [affect] performance.

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Phil T
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Re: Transmission Line Splice Connector

Postby Phil T » Mon Dec 28, 2020 9:13 pm

Use "Liquid Electrical Tape" (a product name) to get another few months of service until you can round up the appropriate connectors to cut away the degraded cable and install a new short section. This assumes there is no existing slack that can be found to cut out the degraded section and reconnect to the antenna base. There are several how-to videos on Youtube.

connectors.jpg
Fig. 3. Diagram for use of splice or barrel connectors
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Here are instructions on using the Shakespeare PL-258 connectors
1992 Outrage 17
2019 E-TEC 90
2018 LoadRite 18280096VT
Member since 2003

jimh
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Re: Coaxial Transmission Line Outer Insulating Jacket Damage

Postby jimh » Tue Dec 29, 2020 10:15 am

The antenna transmission line shown in Figures 1 and 2 from a 1996 boat is probably from that same epoch, and is thus approaching 25-years or service.

Because the portion of the transmission line from the base of the antenna to the the pass-through fitting on the gunwale is subject to flexing when the antenna ratchet base mount is moved to lower the antenna, the motion has caused cracking of the insulation when the insulation lost its original flexibility. Also, there was probably insufficient slack in the cable so the cable was flexed too much when the antenna was lowered.

TEMPORARY REPAIR
To make a very temporary repair, I suggest using 3M SCOTCH 35 Vinyl Electrical tape in the white color. This tape is sold in a width of 3/4-inch. To create a flexible repair, unroll a suitable length of the tape. Using a sharp knife (an Exacto blade is highly recommended) cut the tape width down to about 1/4-inch. Apply the tape in 1/4-inch-wide segments, not in a spiral winding, with a thickness of not more than two raps. Use many short individual windings. This will create some protection for the transmission line but allow it to remain flexible. If there are areas where all the original insulation is removed, apply tape to those areas first, then re-cover with a second layer when repairing the whole length.

I do NOT recommend trying to make a repair of the cable as described and illustrated above with various splice or barrel connectors. The deterioration of the original transmission line is too great to permit such devices to be used effectively. And adding unnecessary connectors to the transmission only increases chances for failures and problems.

PERMANENT REPLACEMENT
Because I consider having a 25-watt fixed-mount VHF Marine Band radio with a proper fixed antenna as essential safety gear, I recommend for the long term solution to this problem the entire transmission line length should be replaced. However, since for many VHF Marine Band antennas the transmission line is integral with the antenna, this may mean you will be replacing the antenna.

A service life of 25-years for a VHF Marine Band fiberglass antenna is outstanding, so I would not fret about replacement of it. The cause of the problem is likely due to insufficient slack in the transmission line between the antenna base and the gunwale fitting. When installing a new antenna, leave more slack at this point.

COAXIAL TRANSMISSION LINE OUTER JACKET
The outer vinyl insulation jacket on coaxial transmission lines that will be used outdoors MUST be made from a compound generally described in radio transmission line literature as "Type-II Non-contaminating vinyl jacket." This type outer jacket for coaxial transmission lines is generally also resistant to decay from UV.

In the marine electronics business there are many coaxial transmission line cables sold under the description of "marine grade", but there really is not a particularly recognized standard for a "marine grade" cable, and this term is often used just to indicate a cable with a white vinyl jacket.

When choosing a new antenna, look for a manufacturer that specifies the transmission line provided will be use a Type-II Non-contaminating vinyl outer jacket.

CAPTIVATING TRANSMISSION LINE
When an antenna transmission line is passed through a small gland or fitting in a bulkhead, the opening is generally too small to allow a connector to be passed through. (In electronic installation, we say the cable with connectors has been captivated by the fitting.) This means that the connector for the transmission line at the radio must be installed after the cable has passed through the fitting. This means installation must be done on the boat. Or, for removal of the cable, the connector must be cut off. For some advice on how to avoid this problem, see my article on this topic at

A Captivating Saga of VHF Antennas
viewtopic.php?f=13&t=5734

The above article suggest use of a particular bulkhead fitting or gland (Blue Sea Systems) and a particular type of very small diameter connector (the FME female).

I also note that in the installation shown in Figure 2 above, the antenna cannot be unthreaded from the mount until the transmission line is snaked back from the radio, and its connector cut off. This makes servicing the antenna quite difficult.

jimh
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Re: Coaxial Transmission Line Outer Insulating Jacket Damage

Postby jimh » Tue Dec 29, 2020 10:32 am

I note in Figures 1 and 2 that the ratchet mount provides a hole for the transmission line to pass through. You can consider routing the transmission line through the hole to eliminate the short-radius 90-degree bend of the transmission line at the base of the antenna.

Depending on the diameter of the hole in the ratchet mount and its orientation, a small diameter connector like an FME cable-end female may be able to pass through the hole. Generally an FME cable-end female connector can pass through a hole of 3/8-inch diameter. If the hole is drilled at a right angle to the ratchet base fitting, the FME connector may not be able to fit through due to lack of room to bend inside the mount.

moabelite
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Re: Coaxial Transmission Line Outer Insulating Jacket Damage

Postby moabelite » Thu Dec 31, 2020 7:24 am

Phil and Jim - Thanks for the information and link - I'm going to plan on making the temporary repair using the liquid tape as a temporary measure while planning for the full replacement. I will report back with updates as progress is completed. Thanks again!

jimh
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Re: Coaxial Transmission Line Outer Insulating Jacket Damage

Postby jimh » Thu Dec 31, 2020 1:30 pm

I doubt the liquid tape will be sufficiently flexible. If the transmission flexibility is decreased, it will just break at a new spot. The stiffer the transmission line becomes, the more leverage the stiff part will exert on the still-flexible part.

The weak point in the transmission line as seen in Figure 1 is at the sharp 90-degree bend the transmission line makes where it enters the metal ferule at the base of the antenna. If the other part of the cable is made stiffer, it will just create greater leverage of the cable at the bend point, and will tend to work harder to crack or damage the cable at the bend.

Also, with the arrangement shown in Figure 2, removal of the antenna from the threaded mount will be impossible unless the transmission line is fished back from the pass-through at the gunwale and from the passageway to the radio (presumed to be at the helm console) and will likely need to have the connector removed. These are big problems for servicing the radio antenna. See my article on transmission line captivating (linked in an earlier reply).