Do you really need one? We don’ t think so!

Numerous grounding standards require a copper down conductor run along the tower length to a ground rod or to the grounding system. Perhaps the writers of such standards thought that copper was a better conductor than the tower, or since the tower was made of steel (ferrous material), the added inductance would impede the lightning strike energy from flowing to ground. The joint between the tower sections could develop resistance?

Regardless of the reason why, such standards still exist. These standards address both types of installations: one with the top lightning rod grounded to the tower and one with the lightning rod insulated from the tower. With the latter, it is impossible to prevent the insulated rod/down conductor from arcing back to the tower. The instantaneous voltage drop due to the inductance of the wire can be hundreds of kilovolts. Much more current will flow down the tower than the isolated conductor/inductor. 

To prove this premise, we placed a 10-foot section of Rohn 25 tower over PolyPhaser’ s "Big Bertha" lightning simulator. We bonded a 10-foot section of insulated #2/0 cable to the surge input side of the tower. The other ends were monitored for the amount of surge current for each path.

The setup was ‘ shot’ with 5000 volts. The results show the tower section had four times the #2/0 cable current! Only 19.7% went down the #2/0 cable and 80.3% traveled on the tower

If the copper wire is bare, naturally occurring rain, which is slightly acidic (pH 5.5-6.0), will remove some copper ions on contact. When these ions drip onto the tower, the galvanized coating (zinc) will wash away, resulting in rust and decreasing the life of the tower.

In another test, we took a 10-foot section of Times Microwave LMR-1200 Coaxial cable and bonded it to the same tower section over Big Bertha. This coax has a larger diameter than the #2/0 cable but we wanted to show the advantage of using a coax cable rather than a grounding conductor. The coax conducted 30.9% of the current while the tower conducted 69.1%. Using a good coax cable can do double duty - as a useful transmission line and a conductor for lightning current. The next step is to properly protect the equipment using PolyPhaser products and proper grounding techniques.

After a lightning discharge has jumped 2 miles to hit your tower, do you really believe a 3 inch standoff insulator can direct it to a different ground connection? 

There could be other reasons for a separate downconductor such as:

• Making a low noise HF receiving antenna from an insulated tower in a high rf environment.

• An impedance matching (Gamma Match) connection to a grounded tower used as an antenna.