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W2IK's
"OVERLOOKED ANTENNA
CONSTRUCTION TIPS"
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By Bob Hejl - W2IK c2004
Many ham radio operators enjoy the triumph of constructing or repairing their own antennas. As guidelines, they often use one or more of the many antenna designs which are available in both books and on the Internet. Sometimes they might even take a basic design and adapt it to increase it's efficiency or to make the antenna fit within the physical limitations of their property. It is unfortunate that in these designs little thought is ever given regarding material selection beyond it's cost, weight and strength. There is another important detail that must be taken into consideration during selection and before construction is to begin. The best antenna design using the strongest materials available may be a fruitless effort if you expect your antenna to perform for an extended period of time. One of the most overlooked antenna construction considerations involves the action that usually occurs when two dissimilar metals are brought into physical contact (mated) during the assembly process and exposed to the weather. There can be a serious onset of a type of corrosion at their respective contact points.
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This corrosion, also called "galvanic corrosion", can create havoc in as little as three months. Galvanic corrosion is a process that occurs when dissimilar metals are connected to each other and moisture, such as rain, condensation or even high humidity, is introduced. When all of this is present, electrons flow between the two metals (also called "ionic flow"), which causes the most chemically active metal to change. When galvanic corrosion occurs, it can usually be identified by characteristic surface blistering or white powder corrosion around dissimilar metal points. (Even the national organization that supposedly represents tons of ham radio folk fails to discuss this concern in their "Really Big Book On Antennas". The closest they come to talking about it is just a few lines about rust! Which is NOT the same thing..)
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If left on its own, the corrosion process will continue, eventually pitting of the metal will occur and electrical or RF conductivity will fail. It's problems can be magnified when an antenna is used in humid, windy, intense heat or cold (thermo-galvanic corrosion) or other harsh conditions. Even an antenna encased in a fiberglass shell, such as a vhf or uhf repeater antenna, can suffer from the effects of this corrosion.
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The corrosion pictured on the left is very visable. The corrosion pictured on the right might be unseen if covered by a washer and screw.
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There is a direct relationship between various types of dissimilar metals when they are mated. Some dissimliar metals, such as copper and brass, when mated cause very little corrosion. There are other metals, however, that react most harshly when matched. Zinc and brass, for example, will cause corrosion with the zinc metal quickly breaking down. If you wish to prevent galvanic corrosion the best means, of course, is to use the same metal throughout your construction. If this is not possible your next course of action would be to assemble materials that have a close relationship on a galvanic metals table. Here is a descending list of metals and their associated relationships in order of the most "noble" or least active.
least active gold
silver
silver solder
bronze
copper
brass
nickel (plating)
tin
lead
lead-tin solder
stainless steel
iron/steel
aluminum alloys
aluminum
zinc-galvanized steel
most active zinc
When choosing materials you should try to pick a metal part made from the type closest to the other metal parts you will be using as charted on the above list. It is very important to consider EVERY part of your antenna including clamps and washers. For example, if brass screws are used to hold aluminum tubing in place they will cause a headache when corrosion takes hold. The better choices would be either stainless-steel screws to secure aluminum tubing or brass screws to hold copper tubing.
Remember that if you are refurbishing an antenna system you might be locked into using a hard to find piece of hardware based upon the original construction materials. Some antenna companies skimp by using the cheapest, but improper mating metals. For example, if you are servicing a tall two-meter base or repeater antenna and you notice that the antenna company used stainless steel couplers and set screws to link two sections of brass or copper rods it would be wise to replace the couplers and the set screws with brass components. If you don't, you might find that the antenna will exhibit intermittent high swr when unseen corrosion takes place between the set screws and the rods. If any wind flexes the antenna the flexing will cause the corrosive joint to change the swr and it will not only effect your signal but will also stress your transmitter as the load it sees will constantly change with the wind. You don't want to "lose" a set of repeater output transistors because of antenna joint corrosion that could have been prevented. It's worth the extra time to locate hardware made of either the same metal or one closest to the antenna metal as outlined on my chart. In doing so you will avoid having to re-fix the problems caused by improper repair.
If you need to clean off a metal surface it is important to use a "like metal" brush to do so. Using steel wool to clean a brass rod that might be part of a vhf/uhf repeater antenna will embed small particles of steel into the rod and create a corrosive atmosphere. Use a small brass brush instead. Be aware that even when you solder, the type of solder used (silver solder or tin-lead solder) needs to be considered based upon what you are bonding. When you construct an antenna also be aware that the surface area of each contact point is very important. Making one small point of contact, using one set-screw for example, will increase the possibility of galvanic corrosion creating problems. This is compounded by the fact that any RF energy will have to pass through a smaller contact point causing additional resistance. If you must assemble two pieces of your antenna either design a larger overlap or use two or MORE flat tipped set-screws to hold the joint. Do not use plated set-screws because as soon as they are tightened they will lose some of their plating at the contact point possibly exposing the metal below the plating. This may cause a corrosion problem unseen to the naked eye. Multiple set-screws within the same collar are a must. This not only reduces the problem but also makes a better physical connection that won't be as easily loosened due to winds or vibrations.
Whenever you are inspecting an antenna for refurbishing, the tiny corrosion on the contact point of a set-screw may go un-noticed. Many problems can arise due to galvanic corrosion including high swr, lowered output power, reduced receive capability, introduction of intermod in vhf and uhf operations, induced static in receive, an increase in "stray current corrosion", "telluric effect corrosion" and harmonic interference being generated. If you find it necessary to combine metals not closely linked on the above list, use an intermediate metal, such as if you must attach a copper wire to aluminum tubing, use stainless steel or nickel washers and a stainless steel screw or a tinned solder lug with a stainless steel screw. This isn't the best solution, but it will reduce the chance of corrosion. The best answer is to always use "like metals" where possible.
In every case of bonding or connection, in order to reduce the galvanic corrosion, coat all of your antenna's joints and connections. This can be done with a joint sealing compound, but it is important that the compound is non-hygroscopic (not waterbased) and does not contain any aggressive ions that can be leached during service. Spray or paint on coatings that contain zinc or aluminum powder should be avoided. Epoxy-based coatings are much better. If possible, use a clear type as this will aid in joint inspections. If, during any inspection, you see a peeling of the sealer, remove it completely and re-apply. These epoxy coatings can be obtained at most marine supply companies as they are made for both marine and atmospheric applications. Make sure that the surface you wish to protect (the joint) is clean and dry. The spray type allows you a quick seal coating "in the field". In humid conditions, use a hair dryer to remove any mositure. You do NOT want any moisture trapped under the coating. If you are working with tubing, remember that moisture can be introduced through the ends of the tubing into the interior of the joint so make sure the tubing ends are also sealed. The less moisture the better the joint will be protected. BEFORE YOU COAT: Make sure that no corrosion has taken place at the joint. If it has, the corrosion that has started will continue. Take the time... inspect, clean and re-assemble any questionable joint. Also be aware that even when bolting your antenna to a tower you need to use the right hardware. Refrain from using stainless steel "U" bolts to connect an antenna to an aluminum tower even though the two metals are close on the list. Use aluminum hardware or galvanized "U" bolts. So the next time you plan on building or repairing that antenna, consider your material selections accordingly.
BEFORE YOU LEAVE:
While I have your ear there is something else you should also know. This same principle also should be applied when repairing or installing a grounding system for your shack. Don't use copper grounding braid and attach it to steel (or even copper-clad steel) ground rods!! This creates a wonderful corrosion point that not only affects your system from a safety standpoint, but also RF may be reintroduced back into your shack and give you a nasty RF burn. Ouch ! Your best bet: Use several 1/2 inch copper tubing sections to act as ground rods, then attach your copper ground wire or braid using copper or brass clamps or wrap it around the tubing and solder it using silver solder. Another reason for doing this is that the copper tubing has more surface area exposure to the ground and is, of course, a better conductor. Use more than one and stagger them so they are not in a straight line and space them differently and not a fractional wavelength such as 1/4 wavelength from each other.
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