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


Technical Tips from Mark W. Persons
AM RF Junction Box

Wednesday, August 6, 2014:  Here is a good way to join a 1/2", 7/8", or larger coaxial cable to an N connector.  Normally you would use a connector, but in this case, the 7/8" cable has a "spiral wound" outer conductor.  Connectors for that style of cable are no longer available.  The answer was to run the cable into an 8" x 8" x 4" electrical junction box. 

The cable was brought through a standard electrical strain relief at the left side and then a ground strap was tightly wrapped around its outer conductor.  That strap runs to a 1/4 inch bolt on the right side, which in turn connects to the station ground outside the box. 

The cable center conductor is run to an N connector on the right side via a copper strap.  Yes, N connectors are OK for power levels of 1000 watts and below.  Many 1 KW transmitters use N connectors as their RF output fittings nowadays.  One added benefit of using this electrical box is that there is room for an arc gap.  You can never have too many arc gaps in a system!  They are easy to custom build.  Adjust the gap until there is an arc-over normal power and full modulation of the transmitter.  Then widen the gap to twice the distance.

This type of construction applies to AM in the 530 to 1700 KHz range.  It DOES NOT apply to FM in the 88 to 108 MHz range!  Lead lengths of this size have a lot of inductance in the FM broadcast band. 

Put a label on the box that says "Remove power before opening cover."    

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