Tower Down Conductors Prove Useless
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. 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 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. The former will not have much going down the wire conductor compared to the tower.
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 insulate #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 4 times the #2/0 cable current! Only 19.7% went down the #2/0 cable and 80.3% travelled 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 galvanised coating (zinc) will wash away, resulting in rust and decreasing the life of the tower.
In a subsequent test, we took a 10-foot section of Times Microwave LMR®-1200 coax 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 had 30.9% while the tower had 69.1%. This shows how using a good coax cable can do double duty - as a useful transmission line and a conductor for lightning. The next step is to properly protect the equipment using PolyPhaser products and proper grounding techniques.
Silver oxide is the only oxide (that we know of ) that is conductive. This is one reason why PolyPhaser’s N-type coax connectors are all silver with gold centre pins. Copper oxide is not conductive and the proper application of joint compound will prevent oxidation.
Knowledge of corrosion can make the difference between a good site that stays on the air and one which needs a lot of maintenance after a short period of time.
…This tech tip courtesy of PolyPhaser Corporation newsletter "Striking News"
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