Contents: Basics; Other Static Problems; Some Things That Help; Conclusion;

Every single mobile operator is plagued with static. There are several kinds of static, some of which we can control, and some we can't. Knowing the difference is 90% of the cure.

Atmospheric static is the background noise we hear when we're listening to a clear frequency. It is caused by all manner of natural and man-made electrical discharges. Even the stream of electrons from the sun, and solar system add their toll. It can be soft, or so loud as to block all but the strongest signals. Although the strength (loudness) varies over a wide range, short-term changes aren't evident.

We hear static because electrical discharges by their nature have very fast rise times. In basic terms, it is nature's version of high frequency interference. Further, their frequency bandwidth can extend well into the UHF region. Noise blankers are nearly worthless in curbing atmospheric static. DSP (Digital Signal Processing) and audio filters can help reduce the perceived level, but little else can be done.

Nature also generates so-called rain static. It doesn't have to be actually raining to have it occur. It is caused by a difference in potential between the earth and moisture-bearing clouds, and individual particles (and/or molecules) of moisture, dust, and even pollutants. It usually starts as a small hiss building in crescendo until a discharge occurs. The discharge usually is a lightening strike, although we might not actually see or hear the results of one.

Some people believe it is caused by moisture molecules physically hitting antenna elements. This is a false assumption. It is, however, more prevalent when the humidity is high and/or when there is a near by rain or snow storm. It is especially bothersome if there is virga present. Virga is falling moisture (snow, rain, sleet, etc.) that evaporates before hitting the ground. There isn't much one can do to control rain static, although DC grounding the antenna does help some. Since it is a precursor to rain and possibly lightening, it's time to QRT and pay attention to your driving.

Tech Talk: Any decent mobile antenna will require some form of impedance matching. The best way to accomplish the task is with a shunt coil as shown at right. The coil, along with a little capacitance borrowed from the antenna (tuned slightly above operating frequency), form a lowpass LC network. This not only transforms the low input impedance (≈25 ohms) to that of the feed line (50 ohms), it provides a DC path to ground.

While DC grounding the antenna does help control static, the real reason to DC ground an antenna has more to do with the following two possibilities. First, if the antenna were to come in contact with a high voltage overhead line, proper DC grounding will shunt the energy to the body of the vehicle, and hopefully not through your favorite radio. While rare, this sort of thing does happen. The tip of my HF antenna is just over 16 feet, but if I were living back in Kansas City, where I grew up, I couldn't extend to this height because of the low wires and tree limbs. In some areas of the country, I'd be hitting the overhead feed lines for light rail. Believe me, it's not something to be complacent about.

In my 37+ years as a mobile operator, I've had my HF mobile antenna struck by lightning three times. One of these destroyed the radio, and burned off two feet of whip. This despite having the antenna well DC grounded. I shudder to think what would have happened had it not been grounded.

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There are two other static problems mobile operators should be aware of as well; rolling static, and dust static.

A vehicle in motion picks up electrons from the atmosphere which build up on the surface of the vehicle and the antenna. It sounds like rain static (a loud hiss), but it is steady in tone and volume with an occasional discharge pop that negates the noise. It soon returns, and sometimes is so pervasive that communications are impossible. It is more evident on the higher HF bands, and it is more prevalent in dryer climates like the desert southwest.

When vehicle AM radios first came into vogue, manufacturers were hard pressed to solve the problem. As a solution, they installed special springs inside the front axle grease cups to electrically connect the axles, and the brake drums and tire rims. They also placed graphite inside the tires. The thought was to shunt the static to ground through the various pieces mentioned. It worked, to a point. Nowadays, tires contain conductive rubber compounds and coatings, and metallic disc brakes provide a better solution. Nonetheless, you can still suffer the malady.

Dust and other atmospheric pollutants can become charged when they rub against one another, and/or the surface of our vehicle and our antennas. Driving along dusty roads can cause dust static, especially when the humidity is low. Dust static is just a steady buzz that never goes away until you stop wherein there is usually a pop, and the static stops. Once you're underway again, it starts back up. The perceived loudness usually increases with speed.

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The best way to deal with rolling and dust static is to bleed it off as much of it to the atmosphere as we can. Corona balls and static drains work well in reducing and/or eliminating the perceived level.

Corona balls are the little round things on the tips of most mobile antennas. Remember, the tip of the antenna is it's highest voltage point, so we don't want the end of it being sharply pointed. We want to dissipate the corona, not invite it ride along.

The problem is, most commercial corona balls (typically 1/2 inch in diameter) are too small to be effective, especially if you run high power. What's more, they have sharp points like protruding set screws which negate their premise in the first place.

Nowadays,I use a 1" aluminum ball made by Naugatuck Manufacturing. They cost about $3 each, but be advised they have a $25 minimum order. These units come predrilled and tapped for a 10x32 set screw. I use a homebrew wooden jig to drill a 1/8" hole about 15° from the predrilled hole. This allows the set screw to wedge very tightly against the stainless steel whip. The left photo is worth the proverbial thousand words. Note the dimple in the flat to center the drill. By the way, a number 7 drill bit (.201"), or a number 6 (.204") are about right for the end of an 8 foot CB whip (nominal .200"). If you cut the whip to 6 feet, you'll need a size 3 (.213").

If you're not into making your own, or want a shiny one, Ed Helms, N4ZVN, makes polished aluminum corona balls (as shown at left), in both 3/4 and 1 inch. They're held on by a set screw.

It should be noted that a 1 inch aluminum ball is a good compromise between weight and size, but only if you use a standard (or shortened) 102 inch whip. Whips like those supplied with some commercial VHF antennas are not strong enough to properly support the ball.

If you're still bothered with rolling static after installing a decent-sized corona ball, I suggest a static drain. The one shown is made from an 1/8" diameter piece of vinyl covered stainless steel aircraft cable purchased from my local Ace hardware. It is attached to the transport tie down using a heavy-duty lug which is crimped and soldered. About 1" of the cover is removed from the outer end and the individual wires are splayed out. A large, flat ground braid will work well too.

The point is to have the drain the last thing in the slip stream. By necessity, the ends of the wires must be sharp which aides the flow of electrons to be dissipated into the slip stream. It works just like static drains found on every airplane flying. The result is a series of pops and snaps that are easily handled a noise blanker, rather than a steady roar that blocks out all signals.

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I like to think that controlling static is a trick of the trade. I say this, as I see very few amateurs using large corona balls and/or static drains. And if my e-mail in box is an indication, it isn't a very well known trick either. Hopefully this little treatise will help.

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