DIRECTIVE SYSTEMS www.directivesystems.com
An examination of water entry problems with VHF and UHF arrays.
There are as many different strategies recommended by as many people as you can ask, about weatherproofing antennas. It seems that everyone has their own best method of keeping water out of your prized antenna system. Lately there have been some rather ambitious commercial methods employed in the cellular and two way communications industries. These methods employ narrow and wide strips of black vinyl tape combined with a layer of butyl rubber. These choices of weatherproofing materials are very effective in keeping water out of coaxial connections that are exposed to the elements.
Certain areas of the country are more prone to precipitation than others, and what may work in New Mexico may not be adequate in Seattle, WA for example. Your best bet is to ask others in your area what they use, and how successful they are, but we have seen some interesting phenomena here in wet and wild Maine, that should be of interest to all.
All of our VHF yagis employ N connectors on T match feeds. The N connector is supposed to be weatherproof, and , for the most part, they are. I would not, however, bet my life that they will stay dry without a lot of help. THINGS CAN GO WRONG THAT MAY NOT EVEN BE VISIBLE TO YOUR EYE. A few comments about these situations may help you achieve more success in keeping your antenna system at top efficiency. The first thing to realize is that there can be drastic differences in air pressure over a short space of time. This can cause water problems with antenna systems. Air temperatures can vary dramatically as well, with the same effect.. What do these situations have to do with water entry? So how does this affect your cable connections? Lets take varying air pressure first.
Anytime the air pressure outside changes during periods of precipitation, a gradient will develop across any barrier. For example, The connector that you sealed up on your 432 yagi has an air space inside the connector. As the outdoor air pressure starts to rise, there will be the tendency for air to leak into the connector by any means available. (The inside of the connector is at a lower air pressure at that time.) Even though the amount of air inside is small, it is still possible for a change in pressure to allow water vapor to be sucked in through the back of the UG-58/U N panel fitting. They are sealed, but are by no means a hermetic seal and it has been known for water to enter by that route especially if the plug portion of the connector has been vapor wrapped with a good layer of vinyl tape and a layer of butyl rubber. A tightly sealed connector may make the problem appear worse in this case by allowing a higher pressure gradient. The illustration below illustrates this effect. We have colored the air chamber blue, while the teflon insulation is a light gray color. The gasket inside the UG-21 type plug is shown in red. This prevents water from entering the connector via the threaded portions of both connectors. The only other entry point is the teflon to center pin contact area. This is usually a press fit and may not be vapor proof!
Temperature changes also cause trouble. The effect can be more severe than with a simple air pressure change, since temperature swings can cause larger changes in pressure as any air is heated or cooled. With rain showers, drastic cooling of metal connections is commonplace. With the cooling comes a contraction of any air mass that is trapped inside the connector. All of a sudden we have another pressure gradient! If it is raining, or if things are just wet, it is possible for water vapor to be sucked inside. This is definitely not good. One way to prevent this from occurring at the antenna feed connector, is to spray the back of the connector with several light coats of a good clear sealant such as Rust O Leum Crystal Clear # 7701. This spray will work its way into any tight spaces and seal them rather effectively and prevent water migration in through the back of the connector. The coating will eventually break down in direct sunlight, but the sealed cracks will stay sealed for many years with one application. Other materials may also work. A thin coating of RTV may be applied, but you should remember that RTV is not a great dielectric, so use it very sparingly, just to seal any potential leaks.
LOOP YAGI FEEDPOINTS
Loop Yagis up until February of 2007, have all utilized a UG-58/U female N type connector and an aluminum bracket, coupled with a metallic "hood" to seal the back end of the connector. This effectively keeps water and weather away from the center conductor and semi rigid coax that leads up to the driven element. The hoods employed, (UG-107/U) are designed to seal UT-141 coax, and, when soldered, present a hermetic seal. The connectors and hoods are further sealed with an RTV gasket that defies water entry. The same problem can develop here, however, that has vexed connectors on all yagi antennas. Water may still enter through the teflon metal interface of the mating male N plug around the center pin. If you do not effectively seal the male connector that attaches to the loop yagi. Any water entry there, may "wick" into the back of the UG-58/U female connector as the air pocket inside the hood contracts from pressure or temperature change. Another ingress point may be a cut or knick in the attached cable that allows water vapor to enter under the plastic sheath and migrate into the connector, with disastrous results. Water entry here is not a very common occurrence, but it has happened. When it does, the antenna becomes impossible to tune and has a terrible VSWR all of the time. The only simple solution in the past has been to discard the old connector assembly and substitute a new one. While it is possible to repair old ones, the trouble is not usually worth the effort.
Looking at the diagram above, with that air pocket inside the connector assemby, any cooling of the air will cause it to contract and develop a negative pressure inside. this can pull water in from the inside of your type N connector that you forgot to seal properly. As we commented earlier, this is a rare occurrence, but it has happened on a few occasions
In February, 2007, Directive Systems switched over from using the Amphenol UG-107/U hoods on loop yagis. The reasons were twofold. The first was that the price of UG-107's went into the stratosphere, as Amphenol raised their price by about 257%. The second reason involved the electrical improvements gained from dumping the hood and going with a system that did not have any free air that could expand or contract, thus preventing water vapor from moving at all. The end result should be less problems with water vapor in the future. For more information on the improvements check out our latest APP Note!
Power dividers make for a potentially bad experience as well. There is a large air mass trapped inside the power divider. An air dielectric makes the power divider have very low loss, wide bandwidth, and good power handling, but it also raises the problem of varying air pressure within the PD body. Anytime the air contracts within the power divider, it can perform in the same manner, and pull any water that may have leaked in to your poorly sealed connector directly into the inner body of the power divider. Over a period of time, the power divider can start to fill with water! Amazingly, the connectors attached to it may appear to be bone dry and not indicate that there is any problem. Many people have found that drilling a very small hole on the bottom of the power divider will always prevent any build up or reduction of air pressure within the divider and eliminate the water entry problems such pressures can cause. The trick is to make a tiny hole so that insects cannot enter and build a nice split level home for themselves inside. Few insect homes maintain a 50 ohm impedance!! Another good idea is to make the tiny weep hole on the underside of the divider! We don't want to funnel water into our system! (D'oh!)
Summarizing the above llustration, the Type N panel connectors used in all power dividers are some relative of the UG-58/U connector. These connectors consist of a silver or gold plated center pin that is press fit into a teflon dielectric that positions it concentrically inside the metal connector. The center pin body is lightly knurled usually, to allow it to more easily grab the teflon and hold in place. Water will not flow through this teflon "dam" easily, but if water vapor is encouraged by some pressure or suction action, it can penetrate the teflon/metal interface quite readily! Any water that leaks into your Type N plug can be sucked into the power divider when these conditions are present. The trick is to make sure that no water can enter the cable by any means. The rubber gasket inside a typical Type N plug, such as a UG21/U or UG1185/U can keep water out, but they are not anywhere near 100% effective. The only remedy is to apply a vapor barrier to the outside of the connector, and pray that the water will stay out where it belongs. If the cable connector stays dry, the power divider will remain dry. A small weep hole in the bottom of the divider will lessen the suction effect and reduce the chance of water entry. A 1/16" (1.6 mm) or less hole diameter is sufficient, and will not allow insects to enter. Air dielectric power dividers are extremely low loss. They do require some thought in their implementation. The weep hole idea is a good one. Proper sealing is mandatory in wet climates.
If you seal all connectors attached to the power divider, you would think that water would have no avenue to enter the power divider, but such may not be the case. Water can enter a cable through a crack or slight cut in the outer jacket. The water can migrate down the cable under the outer plastic sheath, and fill up a connector no matter how well you have wrapped the connector at that point. It is always a good idea to make sure all of your connecting cables are in good mechanical condition. There should be no scuff marks or scratches on the outer protective plastic jacket. Any gaps in the outer jacket can allow water to enter. If the cable is long and aligned vertically, the water may be able to build up a significant hydrostatic head. This causes water pressure to increase at the bottom of the cable run. It is possible for water vapor to be pushed through rubber seals inside the connector and flood the connector itself. Once the water is in the business end of the connector, it can be drawn into the power divider with any change in air pressure. If water gets into the outer sheath of braided coax, there is not much that can be done. Usually the cable is ruined. It is almost impossible to dry it out. Solid copper Heliax type cables can suffer from the same effects. Water gets inside the outer plastic sheath and migrates down inside the plastic, but on the outside of the outer coaxial conductor. The water can still build up a hydrostatic head when it reaches a connector and flooding may result. One solution is to remove an inch or two of outer jacket near the connector, to expose the copper shield. Apply a water barrier directly on the copper shield by using butyl rubber and then electrical tape, to prevent any wicking of water into the connector. The sticky rubber compound will seal the path from water. Water that easily flowed along the outer jacket and under the plastic sheath will be stopped in it's tracks! This trick has proven effective in cases where water was entering connectors that seemed to be correctly sealed but kept filling up with water. Of course, if the outer copper jacket is breached as well, at some point above the connector, water can enter the dielectric, and flow down along the inside of the outer copper sheath. This will ruin the cable as well as fill your connector with water. Soggy cable will exhibit poor return loss and elevated attenuation amounts. When that happens, the cable must be replaced.
Always be sure to introduce a strain relief on any cables entering a power divider. This is especially important when large semi rigid cables are employed. An errant foot may kick the cable and subject the brass N connector on the power divider to extreme forces. A bent or broken flange may result, and a leak is sure to develop. The strain relief will absorb any forces and prevent damage to the power divider. The strain relief may be a heavy wrap of tape to anchor the cable, or a metal support bracket if a suitable surface is not available for taping. The strain relief will also help in reducing vibration of your cable and lessen the chance of your connector from working loose.
CABLE SEALING METHODS
These techniques are becoming standard procedure in the cellular industry
Sealing materials are applied to outdoor RF connectors to accomplish several goals. First and foremost is to prevent water from entering the rf connector. The second reason would be to prevent UV radiation from degrading the connector over a period of time. Sunlight and weather will oxidize the metal surfaces and cause DC resistance to increase on exposed mating surfaces. It is true that Type N connectors are supposed to be weatherproof, but if left exposed to the elements, the connector will tarnish and start looking rather ugly in a short time. Any good taping job will also prevent the connectors from loosening up in the wind. This is a common occurrence for cables exposed to high winds and vibration.
A few general rules about weatherproofing. Always apply tape above 32 degrees F. Even then, always protect your tape rolls by storing it under your coat and next to your body to keep tape flexible in cold weather. If the tape cannot stretch elastically, it will not seal properly. By stretching elastically, we mean that the tape stretches slightly below it's elastic limit. You do not want to stretch the tape to the point where it distorts. Only apply enough pressure to get a smooth wrap on an antenna or cable splice point. Smooth each wrap of tape with your hands to assure proper adhesion. Always cut the tape with a sharp knife and never stretch it to break it. A stretched tape will most likely unravel.
For vertical runs of cable, always run the final wrap from the bottom to the top, and overlap about 50% of the width of the tape. This will provide the same effect as shingles on a house. The water will run down across the joints without trying to go into the joints. In warm climates with much sunlight, it is a good idea to wrap an extra layer or two of vinyl tape over the completed splice to enhance UV protection. Your splice will last longer as a result!
The first step in weatherproofing cables, such as the flexible jumpers around rotators, is to make sure the cable is absolutely dry and the connectors are torqued to their proper limits. For N connectors this is about 15-25 in. lbs. Hand tightening is usually not good enough. A pair of pliers will get you to the proper torque amounts. Step 2 involves wrapping 1/2" or 3/4" wide wraps of vinyl tape, starting 2" below the main feedline, and extending the wrap to 2" above the flex jumper cable connector or strain relief.
The next step requires the use of butyl rubber tape. This is usually available in widths of about 3" . Cut a length of rubber tape to approximately the length of your taped splice, and lay it against the connection along the long axis of the cable. Wrap the rubber tape around the cable connector joint, stretching it as you go, and overlay several lengths, if needed to accommodate a complete wrap around the splice. Small connectors up to 1/2" coax sizes will need only one piece.Form and knead the rubber along the overlap, so that the individual pieces adhere to adjoining pieces securely for a watertight seal. If you are sealing big connectors and large coaxial cable, the back end of the connector presents a large change in wrap diameter that is hard to cover effectively with vinyl tape. For those situations, one solution is to cut a short piece of butyl rubber, fold it in half, and wrap it around in this area to fill the area in and make a gradual transition between the two sizes. It makes for simpler tape wrapping on subsequent tape wraps!
Wrap this butyl rubber layer now with wide 2" vinyl tape along the entire length, being careful to not stretch the tape beyond it's elastic limit, and maintain a 50% overlap as you wind. Extend the 2" tape about 2" beyond the previous wrap. Finish the splice with three wraps of 1/2" or 3/4" vinyl tape, following the same procedure. For vertical runs, start the first wrap from the bottom about 2" beyond the last wrap of 2" tape. Finish with wrap #3 proceeding "uphill" from bottom to top to make the roof shingle effect. It will shed water more effectively.
For antenna connections, the same techniques may be applied. In many cases there is restricted access around the antenna connector. This is true with our yagis and loop yagis. It is very difficult to manipulate a vinyl tape roll in such areas. A different strategy is required in such situations. Most of the weatherproofing of connecting cables must be accomplished before you attach the cable to the antenna. For these situations, start with a single wrap of 3/4" vinyl tape about 2" below the coaxial cable connector or any strain relief, and proceed with the wrap up to the clamping nut. Do not cover the clamping nut. Leave it free to turn. Then apply a wrap of butyl rubber as shown above, up to the clamping nut as well, taking care to seal all points on the rubber wrap. Then apply 2" wide tape to cover the butyl rubber, extending an inch or more past the butyl rubber layer, followed by a wrap of 1/2 or 3/4" vinyl tape that extends 2" past the previous wrap on the cable side, but again, do not cover the clamping nut. End the final wrap at the clamping nut. Now install the connector on the antenna, using the correct tightening guidelines to get proper sealing of the connector. It must be tight!
Applying tape around the connector clamping portion of the connector may be accomplished in three ways. First, you can hold the tape roll next to the connector and push the tape through the restricted area. If you unroll enough slack it is oftentimes possible to relocate the roll to make a continuous wrap using this method. The second method is to cut strips and apply them in overlapping sections as you go around the connector. The third method uses a special tool, such as a pencil, or any small metal rod, to accomplish the wrap. Unroll several inches of tape from the tape roll onto the special tool in a uniform wrap, then unwrap it around the connector. The pencil or metal rod can easily fit around the connector in tight areas. Wrap three layers of 3/4" tape around the clampuing area and extend it to 1" beyond the cable clamping nut onto the previously sealed cable. Now cut a 2" long length of butyl rubber and wrap it around the clamping nut area, taking care to seal it properly. Finish with 2 layers of 2" vinyl tape, followed by three layers of 3/4" tape, always starting each wrap 1" beyond the previous wrap. If you are in an area of intense UV, such as Florida, more wraps of 3/4" tape will not hurt and will provide additional UV protection to the joint. In some cases, however, you may run out of room! The secret is to use just enough butyl rubber to make a contiguous seal, and no more. care at this step will assure success later on.
Hopefully we can all gain reliability with our antenna connections, and as a result, your antenna will not fill up with water during a bad storm, the Friday night before the January VHF Contest! (We can always hope!)