The setting of the spacing of the AM tower spark gap (arrester balls) is very critical for a solid state transmitter. The tower spark gap is the most critical defense against voltage transient damage to the transmitter via the antenna. The spark gap needs a low resistance path to ground, which is why a good ground system at the transmitter site is so important.

The spark gap needs to be adjusted to handle the voltage peaks of the carrier with full positive modulation which can be as high as 125%. This is calculated by taking the RF carrier voltage with no modulation and multiplying by 2.25. For example, a 1000 watt carrier has 224 volts rms and 317 volts peak output into a 50 ohm load. At 100% modulation this is 634 volts peak, and at 125% positive peak modulation this is 712 volts peak. To determine the voltage at the tower spark gap the calculation needs to be done for the tower impedance. Impedance can be calculated by taking the square root of the sum of the square of the resistance plus the square of the reactance.

The peak voltage at the tower can be figured in several ways. It can be calculated by the tower impedance in ohms, multiplied by the tower base current in rms, multiplied by the square root of 2 (to convert rms to peak), multiplied by 2.25 (to convert CW to 125% modulation peak).

A rule-of-thumb is to use 25 kilovolt per inch for spacing the gap at sea level. Therefore, if the tower is at 40 ohms impedance (not resistance) and your base current is 5 amps rms, then, the tower base voltage is: 40 * 5 * 1.414 * 2.25 = 636 volts and the gap is 636/25,000 = 0.025 inches

An alternative way to set the gap is to bring the arrester balls (spark gap) on the tower together with the transmitter at full power and full modulation until the VSWR trip occurs, and then open the gap up by an additional 0.01 inch. My friend, Buc Fitch=s final test for checking the ball gap is to send a fine spray (a mist of super saturated air) of distilled water into the gap. If the VSWR protection trips with normal programming modulation, the gap is perfect.

To minimize the effect of precipitation on the spark gap, the arrester balls should not be set vertically on top of each other but horizontally next to each other so that the gap cannot be filled with water drops. The arrester balls need to be welded to the tower and its base. A mechanical connection using nuts and bolts to support the arrester balls is not acceptable, they need to be welded to the tower because mechanical connections deteriorate with time. It is important to keep the gap clear and clean. Also, the adjacent surfaces should have no rust.