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Radio Broadcast Technical Consulting and Sales
 
10032 Island Drive, Brainerd, MN  56401

 


Technical Tips from Mark W. Persons
Gas Tube and Air Gap Arc-Gaps for an AM Station


For AM stations with solid-state transmitters, I have been experimenting with gas-tube arc-gaps to protect the transmitter from excessive voltage spikes coming in on the transmission line.  The goal is better lightning protection. 

Shown here are two sets of 1000 volt arc-gaps sandwiched between and soldered to pieces of copper strap.  There are 20 of the A71-H10X arc-gaps made by Epcos, Inc., available through Digi-Key as part number 495-1469-ND.  It took a 100-watt American Beauty soldering iron to do the soldering on these.

The copper pieces are 1 inch by 2.5 inches with brass 6-32 bolts and nuts at each end.  I use brass hardware in RF situations.  One end should be bolted to ground and the other should be connected to the center conductor of the 50-ohm transmission line leading to the transmitter.  For safety reasons, this must be done inside an enclosure, such as where a main/auxiliary transmitter relay is located. 

These gas-tube arc-gap assemblies are designed for 1000 watt transmitters and have only 10pf of capacitance to ground so upsetting the impedance is not an issue.  For a 5000 watt transmitter, 2500 volt arc-gaps are probably the best choice.  A standard air-gap lightning-gap should also be installed in the system, closer to the antenna, to take the brunt of a high voltage surge.  You can never have too much lightning protection.          

Here is an air arc-gap as seen on the inside of a Nautel J-1000 AM transmitter.  It consists of two acorn nuts that are rigidly held in place.  One is on chassis ground and the other is connected to the 50 Ohm antenna point.  The arc is being checked by a wire gage tool that was originally intended for setting gaps on gasoline engine spark plugs.  For 1000 Watts, the gap is .030 inches.  

For additional arc-gap settings, see Standards.            

Here is a custom-built arc-gap that is very inexpensive.  Constructed from standard angle-iron hardware, a porcelain insulator, and bolts with two acorn nuts, it is sturdy.  The important part is that it is rigid so it holds the gap spacing.               

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

Questions?  Email Mark Persons:  teki@mwpersons.com         

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page last edited 02/22/2016