As mentioned earlier in the series, the use of ground rods and their length is related to the sphere of influence. I can't over emphasize enough the necessity of a good ground rod system. This is the connection portal to earth. Your grounding system is only as good as the ground rod. All your bonding, surge protectors, ground bars in the station will be useless if it has a poor connection to earth.

I was asked by a new tower owner which ground rod to buy. The eight foot or the ten foot rod. Let's look at the mathematics.

The volume of the sphere of influence is calculated as the length of the rod squared times Pi, times twice the depth of the rod, i.e. (length)²*π*2(length).

For an eight foot rod it would be 8² x π x 16 = 3215 cu. ft.

For a ten foot rod it would be 10²  x π x 20 = 6280 cu. ft.

As you can see above, a 20% increase in the rod length will give you almost double the sphere of influence volume for ground conductivity.

Installing a deep ground rod of 20 feet will increase the sphere of influence to a whopping 52,240 cu ft. volume. This is over 16 times the coverage of a single eight foot ground rod.

Bottom line is that the lower the ground impedance the lower the lightning voltage will be at the base of the tower. Think of the tower and ground relationship as a resistive voltage divider. The lower the ground resistance the lower the lightning surge voltage at the tower base.

Many times a ham will install a ground rod at the tower base and because the ground resistance is still very high, due to soil conditions, the current grounding will not sufficiently mitigate a lightning surge. Then the ham will post on the WEB that surge grounding is worthless.  In this case the problem was in the implementation - for example, using a parachute that is too small is also useless but the problem is not the parachute.

Tower Top Grounding

Many hams will install their rotor with a thrust bearing at the tower top and forget the bonding line from the rotor mast to the tower structure. Should your tower take a strike the surge current will be looking for a path to ground. This will take the form of traveling via the antenna connections, rotor wiring or traveling down the tower structure itself to the base of the tower. With the exception of crank up towers, the tower structure will provide the lowest impedance connection to the tower base.

Frequently, the tower rotor and the thrust bearing are forgotten. Without a high current bypass cable, the surge will travel through the bearing surfaces and weld the rotor and thrust bearing together during the strike. To prevent this you must install a bypass cable from the mast to the tower structure. This protects the bearing surfaces. Use at least a flexible number 2 copper multi strand cable for this application and let it be part of the rotor loop. 

For crank up tower a number 6 gage messenger grounding cable must be run from the top of the tower to the tower base.

Station Accessories

Lightning surge protectors come in many types. Most coax surge protectors are mainly a flash over tube with the ionizing voltage set to be above the operating wattage voltage for the operational impedance of the transmission line. Should a direct strike or conductive near field strike produce a magnetized field into the center conductor, the surge protector will ionize at a set voltage and conduct the surge to ground.

Several factors will alter the performance of a flash over tube surge protector. Line VSWR can increase the working voltage and have the surge protector flashing over at lower wattage ratings that thought possible. Frequency of operation can have a flash over surge protector operate at lower wattages than thought possible. Frequency is also a factor, for example a flash over tube surge protector that would flash over at 2000 watts at 30 MHz will flash over at 1500 watts at 50 MHz. This is due to the tube capacitance becoming more reactive as frequency is increased.

It must be noted that a flash over tube is not instantaneous and requires many microseconds before is ionizes. As a result some surge voltage will be present until the tube conducts. The more expensive surge protectors will also have a ground shunt inductance to remove the lower frequency out of band surge voltage. This is why many surge protectors are frequency banded. Also surge protectors may have a series capacitance to remove the DC component from the surge. Depending on the design they may read as a short or as an open under normal conditions.

Make note that a lightning surge has a very high RF component to the power bandwidth and as a result the surge must be treated as very high power RF. This is why the ground connection must be kept short. Components and design must not only be looked at from a voltage perspective but as a conducted RF signal going to ground. The RF component can be as high as 500 MHz.

Many coax switches have a surge protector pill installed into the switch. During a surge event you may think your switch has been shorted and it may only be the flash over pill is shorted. These pills are located under a large flat screw above the selector knob as in the Alpha Delta switches.

Ground Conductive Testing

Over time your ground rods and system bonding may become ineffective due to natural corrosion and poor bonding connections. The only way to ensure proper grounding is to check your ground system with a Meggar ground conduction meter. Depending on soil and weather conditions this should be done at a minimum of once a year.

Fluke, AMEC, Biddle, Meghom, DLRO and other grounding impedance meters can be purchased for this application. They have multiple leads and attach to your ground and to soil probes set at specific distances from the ground system to test soil conductivity.  These meters are not a DVM - they operate by producing a high frequency low voltage AC signal to do the testing. They also come in a clamp on variety but those are more expensive.

This completes the three part series on grounding. Visit the PolyPhaser web site and read all the tutorials on grounding to fully understand the complexity of proper station grounding. As mentioned earlier in the article for grounding to be effective a whole system approach must be used to protect your station from lightning and power line surges.

73, Mike - K6AER