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Technical Tips from Mark W. Persons
AM Tower Welding

Whenever you have an AM radio station, it is extremely important that the tower or towers have their sections welded to each other.  This makes a tower a "continuous electrical conductor" from the top to the bottom.  Towers are typically built using 20-foot sections.  Remember, the tower IS the AM radiator, it is not a support for an AM antenna. 

You would think that tons of downward pressure on each section joint would result in a good electrical connection.  Well, corrosion increases the electrical resistance of the connection.  With bad connections, the antenna impedance can change, resulting in antenna current readings that are not consistent with FCC licensed values.  Corrosion can also cause RF harmonics of the operating frequency to be higher than the FCC allowed limit.  Any harmonics IE: 2, 3, and 4 times etc. the operating frequency, must be attenuated by more than 73 dB when the station is running 1000 Watts of power or 80 dB when running 5000 Watts or more of power.

This AM tower is nicely built as it has 18-inch fiberglass insulators at the top of each guy line.  If the insulators had been porcelain, then a shorted insulator could cause problems with the tower impedance to.  A short happens when an insulator cracks and fills with debris making a path for an arc.        

The first photo shows a tower worker arc-welding sections together.  After the work, the welds were covered with cold galvanizing as seen in the second photo.  If the tower is over 200 feet with required marking, the galvanizing is painted over with orange or white to match the rest of the tower.

The stories go on and on.  Stop in again sometime.  I'll leave the soldering iron on for you. 
Mark W. Persons   ham W0MH      

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