Tip Of The Month Or SO...


    Although commercial coax cable was first introduced back in the 1930's, Mr. Quackenbush, an engineer with the Amphenol  Company, was responsible for the designing the first line of coax connectors, thus giving us the PL-259 and the SO-239 connectors for this new RF transmission line.
    Remember from our last Tech Tip, that the impedance of a coax cable is a ratio of the inside and outside diameters of the wires?  Well, if you do the calculations for pin and shell diameters of a PL-259 connector you get an impedance of 30 ohms.  Like radio waves, light waves do no like hitting a change of impedance.  When light goes through even the clearest glass at a slight angle, some of the light reflects.  When light hits water, some reflects.  Reflected waves are measured as an SWR. (Standing Wave Ratio)  By the time you get to about 200 MHZ, that 50 ohm-30 ohm-50 ohm change in a PL-259 connector causes some real SWR problems.
    In 1941 two RADAR engineers were independently working on this problem.
    Paul Neil tackled the problem by designing a coax connector that looked as much as possible like a piece of coax.  Mr. Neil's connector became known as the Type N connector.  Literally down the street, Mr. Carl Concelman tackled the problem a little differently.  Carl realized that where the center pins connected there would be some mismatch.  He also knew that a little capacitance would tune out this mismatch.  Using some carefully shaped Teflon, Carl was able to make a bayonet locking coax connector that worked well with the RADAR systems of the day.  This connector was known as the Type C connector.
    About a year later Paul and Carl were working for the same company.  They combined Paul's superior pin and grounding methods with Carl's Teflon tricks and locking pins.  Thus was born the Bayonet locking Neil Concelman connector.  Now you know where BNC came from.  Well, they quickly found that the BNC connectors kind of worked themselves loose rattling around in B-17's.  So they went back to Paul's original threaded ring, designing the connector that's popular on cellular telephones; the Threaded Neil Concelman TNC connector.
    Coax connectors also have some big mechanical challenges.  The cable TV companies use a solid shield aluminum coax.  All metals, especially aluminum expand when heated.  Baked under direct sun light, then cooled to pre-dawn sub-freezing temperatures, long runs of CATV coax can expand and contract as much as a foot.  Think about it as they're exposed day after day after day.
    The cable TV companies put big expansion loops in the coax and use connectors that strongly grip the coax, but loose connectors are a common problem with cable TV companies.
    For years all PL-259 connectors were designed to be soldered to the coax.  With mass production came the crimp-on connectors.  Crimp-on connectors do not work as well as the soldered type especially in the RF field.  The center lead may work itself loose or the crimped job may be sloppy leaving a gap in the center lead and the connector where weather can affect it.  Also if the crimped job is sloppy  there causes a impedance problem.
    When Mr. Quackenbush originally designed the PL-259 connector, he specified that bakelite be used as the insulator.  Later, Teflon was commonly used because of its superior heat resistance capabilities.
    Teflon insulated PL-259's are still the best PL-259's.
    Be very cautious of the non brand names or the inexpensive  connectors as the insulator seem to be made out of old melted plastic milk bottles or some similar  plastic material.
     Also make sure your PL-259 connectors have a ridge on either your Bakelite or Teflon insulator.  This ridge on the insulator prevents arching of high RF from center conductor to the shell of the PL-259.
      We here at R & R Communications, Inc. only sell the American made Amphenol Connectors.  Amphenol also has a line of PL-295's made overseas as well.  We do not carry these.
      When purchasing PL-259 connectors or any RF connector make sure you ask for American Made.