For decades, people have been slipping tennis balls over their antennas as a means of preventing the antenna from slapping into their vehicle. It is a quick and inexpensive method ... just be aware of the effects it will have on the antennas characteristics.
Antennas that are connected to equipment that transmit audio, video or digital data accomplish that feat by radiating an electromagnetic field. Everything and anything between the transmitting antenna and the receiving antenna or device will have some effect on the radiated electromagnetic field. It could deflect it, reflect it, distort it and/or absorb it and the closer to the radiating source the greater the effect will typically be. Putting a tennis ball over the radiating device is as close as you can get. And, where along the length of the antenna that you place the tennis ball will have a varying effect on the antennas characteristic and functionality.
A tennis ball added to a transmitting antenna will add capacitance, which, on a previously tuned antenna, will cause a resonant frequency shift and a change in the standing wave ratio (SWR) at that frequency. And, as the tennis ball is moved up or down the antenna the effect will change the SWR. On a top loaded antenna, such as the Firestik, the more towards the top of the antenna that the tennis ball is positioned, the greater the effect it will have on the antennas characteristics. The reason being, as the current becomes restricted due to the increasing electromagnetic flux in the close wound area of the antenna, the voltage increases and with that the environment becomes more reactive to anything in what is known as the near field of radiation. But ... who really cares about all of that? Really! Here comes the part that you must know.
You can use a tennis ball if you want but you must tune the antenna with the tennis ball in its desired resting place. If you add it after you've tuned the antenna ... you get to tune it again. Also, you should put a mark of some sort on the antenna so you know exactly where the bottom or top of the tennis ball was when you tuned it because inadvertently, some knothead will start fussing with the tennis ball and by doing so will be changing the SWR. And even if you avoid knotheads, just bouncing down the road or trail can cause the tennis ball to change its position and the SWR thereof. Knowing where its home is (the mark) will allow you to leave your SWR meter on the shelf.
And while we're at it ... keep in mind that the antenna tip will also change the resonant frequency of the antenna. If you tune the antenna with the tip off then put it on afterwards, it will not be the same. The tip isn't necessary but if you are going to use the tip ... all SWR measurements must be taken with the tip on. You take the measurement, remove the tip and make the adjustment then replace the tip and recheck. Also, the shorter the antenna, the greater the tip changes the resonant frequency. If you lose the tip it would be a good idea to retune the antenna until you get a replacement.
Updated April 16, 2014 - This article was written and posted at the requested of "Fifty Caliber"