Last Modified: Sat, July 23, 2011
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. In fact, you can use band noise as a signal source for comparing antennas, but you obviously need a QRM free band!
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 element does help some. Since it is a precursor to rain and possibly lightening, it is time to QRT and pay attention to your driving.
It should be mentioned that dust, and some airborne pollutants can also cause static build up.
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 highpass 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 previous HF antenna was over 16 feet. If I were still living in Kansas City, where I grew up, I couldn't extend to half this height due 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 is not something to be complacent about.
There is another related issue which needs to be mentioned at this point, and that is lightning safety. Contrary to popular belief, inductive shunt matching, does not increase the likelihood of lightning damage, should a strike occur. If anything, it reduces the possibility. Yet, in my 39+ 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 the whip. This despite having the antenna well DC grounded. I shudder to think what could have happened had it not been grounded to the body.
There are other static problems mobile operators should be aware. Dust and other atmospheric pollutants can become charged when they rub against one another. They collect on 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.
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.
It is interesting to note, that most newer vehicles are being factory-equipped with ceramic brake pads. They last a lot longer, provide better stopping power especially when hot and/or wet, and do not contain any asbestos (a good thing). However, ceramic pads have metallic particles embedded into them to aid in reducing static build up. As brake pads wear (metallic or ceramic), minute amounts of metal are scrubbed off both the pads, and the drums (disks) in the process. These particles are part of the brown-cloud smog all too many cities have to deal with. Since they are magnetic, they cling to antenna mag mounts like glue. It is these trapped particles which cause mooning (scratch swirls) in the painted surface under the mounts.
Atmospheric static (band background noise), can be caused by a myriad of sources, most of which we can't control over. For example, it is not uncommon for thunder storms several hundreds of miles away to wreak havoc with low band communications. There is no practical way to deal with it, save turning off the radio or switching to localized FM operation.
The best way to deal with rolling, and dust static is to bleed it off as best we can. Corona balls, and static drains work well in reducing the perceived level. However, no amount of prevention will eliminate all of it.
Corona balls are the little round things on the tips of most mobile antennas. Remember, the tip of the antenna is its highest voltage point, so we don't want the end of it being sharply pointed. We want to dissipate the corona, not invite it to 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 often have sharp points like protruding set screws which negate their purpose 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 right 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 a one inch ball.
If you use a properly-mounted cap hat, there is no reason to use a corona ball. Properly mounted means, at the very tip of the whip. This can be difficult to do, but isn't impossible. Further, any whip above the cap hat, will have very little effect on the resonant point. If it does, it means the cap hat is improperly mounted!
If you're still bothered with rolling static after installing a decent-sized corona ball (and DC grounding the antenna element), I suggest a static drain. The one shown at right 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.
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.