Proper installation techniques are essential for reliable equipment operation. This is particularly true when the electronics is exposed to the elements. In this engineering brief we will explore a simple method to protect outdoor cables. The general principle may be applied to any exterior connection including cable splices or as extra protection for the wire to junction box pass through. To demonstrate we will use a short piece of LMR-400 coax cable and the supplies as shown in Figure 1.
On a personal note, I have used this technique on land and sea with radio frequency, video camera, and multiwire control cables. The technique is simple and provides excellent protection against the elements including salt spray and the occasional green water wave sweeping over the deck.
Figure 1: Picture of the LMR-400 cable with TNC connectors along with supplies used in the article.
Tech Tip: A variety of coax cable is available for radio frequency work. Some common examples include RG-58, RG-8, and LMR-400. Each cable has a given power handling capacity and attenuation. LMR-400 provides good performance for extended wire runs.
Parts used in this demonstration
The following components were used in this demonstration:
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3.3 ft LMR-400 extension cable with M and F TNC connectors. The LMR-400 cable provides a convenient right-sized demo.
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Silicone self-fusing tape.This self-fusing tape provides an excellent shield against the elements. However, it is relatively delicate. As will be shown, it is easily cut, and certain types may be sensitive to ultraviolet radiation. Note that this tape was chosen for its desirable physical properties and visual impact with the easy to see blue stripe. Lower cost alternatives may be found here.
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Black electrical tape provides a shield for the delicate self-fusing tape. It protects against abrasion, puncture, and the sun’s ultraviolet rays.
Steps to weatherproof the cable
There are several simple steps to weatherproof your cable.
Step 0: Inspection
Begin with a clean and dry cable. Be sure to inspect the cable especially if it has been exposed to the elements.
Tech Tip It’s been my experience that corrosion works its way into a cable from the open ends. Green copper is the first giveaway that something is wrong especially in the braid of coax cable. If you find corrosion, consider cutting a foot from the cable end and reterminating the ends. It’s not a pleasant operation. However, you are better off doing it on your time than when the system is down on a cold rainy night.
Step 1: Self-fusing tape
Wrap the connector using the self-fusing tape as shown in Figure 2 and 3. It’s important to provide sufficient overlap for the self-fusing tape to function properly. In Figure 2 we see that the tape itself has a blue guide in the middle of the tape. The installation is successful when each consecutive wrap covers the blue line.
In Figure 3 we see that the self-fusing tape extends beyond the connector. In this example, we extend an inch beyond the heat shrink for the TNC connector.
Figure 2: Picture of the partially wrapped TNC connection. The proper overlap is ensured when each consecutive wrap covers the blue line. The clear plastic covering prevents the tape from self-bonding while in storage.
Tech Tip: The self-fusing tape must be installed under slight tension. While the tension has been removed from Figure 2, we can see the narrowing distortion as the completed wrap is thinner than the original tap width.
Figure 3: Image of the completed wrap. Note that the self-fusing tape extends beyond the TNC connectors by about an inch.
Step 2: Protective overwrap
The self-fusing tape provides excellent weatherproofing. However, it is easily cut, abraded, and some tapes will break down under UV light. A simple solution is to add a protective layer of black electrical tape as shown in Figure 4. The chosen 3M Super 33+ tape is rated for extreme temperatures.
Be sure to extend the electrical tape about an inch past the self-fusing tape as shown in Figure 5.
Figure 4: The self-fusing tape is protected by a coving of black electrical tape.
Tech Tip: Slight tension improves the integrity of the wrap. However, as the task is completed, be sure to add an additional wrap with zero tension. This will prevent the end from lifting. This is especially true for electrical tape, as we do not want the end to be whipping in the wind.
Figure 5: The black electrical tape extends about an inch beyond the self-fusing tape.
Tech Tip: The technique may be used for cable penetrations such as the Sparkfun TOP106 antenna as shown in Figure 6. This antenna is included as part of the RTK Express GNSS Receiver kit.
Figure 6: The self-fusing tape may be used to weatherproof cable penetrations such as this Sparkfun “UFO” antenna. The black electrical tape should be added to protect the self-fusing tape.
Step 3: Periodic inspection and removal
The cable connections should be inspected periodically. The timing depends on the importance of the system and the local weather. Some inspections should be conducted yearly, while others can wait a few years. As suggested in the Tech Tip, it’s best to conduct the inspection on a nice day of your own choosing, as opposed to fighting corrosion on a cold and rainy day.
The cable wraps are easily removed by a sharp knife as shown in Figure 7. Observe that the self-fusing tape is under a certain amount of internal tension. It tends to split and pull back thereby exposing the underlying cable. Protection against this property is one of the reasons we add the electrical tape.
Observe that the self-fusing tape is not sticky. It does not leave a residue like the black electrical tape. This is highly desirable as the metal and underlying cable are clean. The only stickiness is several inches away from the connection where the black electrical tape extended beyond the self-fusing tape.
Figure 7: The self-fusing tape is easily removed with a sharp knife. Observe that the connection is clean as the self-fusing tape does not stick to the cable or metal components.
Conclusion
Over the years, I’ve encountered a few poor exterior cable installations. It’s no fun, especially if the corrosion has worked its way deep into the cable. Sometimes there is slack, and the problem may be repaired by shortening the cable. Sometimes it takes the entire day with all the added time and expense of pulling to pull a new cable.
The techniques shown in this article can save considerable time and effort. Yes, the material does cost money, and yes, it does take time to inspect the cable and properly weatherproof the connection. However, that’s nothing compared to the alternative.
Please share your thoughts about this topic in the space below especially if you have good corrosion stories. Also, be sure to test your knowledge by answering the questions and critical thinking questions at the end of this note.
Best Wishes,
APDahlen
About this author
Aaron Dahlen, LCDR USCG (Ret.), serves as an application engineer at DigiKey. He has a unique electronics and automation foundation built over a 27-year military career as a technician and engineer which was further enhanced by 12 years of teaching (partially interwoven with military experience). With an MSEE degree from Minnesota State University, Mankato, Dahlen has taught in an ABET-accredited EE program, served as the program coordinator for an EET program, and taught component-level repair to military electronics technicians. Dahlen has returned to his Northern Minnesota home and thoroughly enjoys researching and writing educational articles about electronics and automation.
Highlighted Experience
Leveraging his military engineering experience, Dahlen provides unique insights into rugged and reliable electronics solutions suited for extreme environments. His articles often reflect the practical, hands-on knowledge gained from his time in the U.S. Coast Guard. At the time of this writing, he has created over 148 unique posts and provided an additional 490 forum posts.
Connect with Aaron Dahlen on LinkedIn.
Questions
The following questions will help reinforce the content of the article.
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What is meant by the term self-fusing?
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Suppose we found a UV resistant self-fusing tape; would it still be necessary to add an exterior wrap of electrical tape?
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Upon inspecting you notice a green liquid in the cable connector. Describe the appropriate action.
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How often should your system be inspected. For full credit, be specific to your local weather conditions.
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Why do we apply the self-fusing tape before the electrical tape?
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Select an appropriate tape to provide improved abrasion properties when compared to the Super 33+ electrical tape. Hint: Review this 3M Tapes to complete your toolkit document.
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This article features a joint using TNC connectors. Why are all such mid-cable joints undesirable? Hint: What can go wrong with the transmitter? Also, how will a technician spend their time over the lifetime of the installation?
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Research and then explain how mastic is applicable to weatherproofing cables.
Critical thinking questions
These critical thinking question expand the article’s content to develop a big picture understanding the material and its relationship to adjacent topics. They are often open ended, require research, and are best answered in essay form.
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Construct a table showing the properties of RG-58, RG-59, RG-6, RG-8, and LMR-400. Include aspects such as impedance, power handling, type of center conductor, attenuation per 100 ft, and max frequency.
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This engineering brief assumes the cable joint is completed in the open. Research and then select suitable products for an underground joint. For full credit, start by explaining if LMR-400 is appropriate for underground installations.
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With respect to a coax cable, what is a TDR? How is the device used for maintenance?
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Occasionally we encounter installation where coax cable is installed in a PVC pipe. This allows the cable to be easily removed as opposed to trenching. Identify the gremlins that will act on a cable installed in a 2-inch diameter pipe.