From: Larry.Mc.Donald@ofa123.fidonet.org
Newsgroups: rec.radio.amateur.misc
Subject: 6m Duplexer, 7-bit ascii
X-Sender: newtout 0.05 Dec 30 1992
Date: 31 Dec 92  10:25:08

                   SUCK OUT CAVITY DESIGNED TO BE A DUPLEXER 
                            FOR SIX METER REPEATERS
                            By Will Anderson, AA6DD 
                (From the Southern California Six Meter Club.)
    [This  post uses the 7-bit ascii characters for the graphic renditions 
    of  the figures in the original article.  If your system is capable of 
    the IBM PC extended character set, please see the article posted under 
    the subject "6m duplexer, IBM chars".] 

       Here's  a  solution  for a 500KHz repeater  duplexer.    Commercial 
    cavities for six meters are usually the size of water heaters and  you 
    need four to five of them. 
       When we implemented the 500KHz band plan,  Dan N6BKL,  Woody WB6ZCO 
    and  I  started researching and building prototype suck out  cavities.  
    Woody was successful in utilizing coffee cans soldered together.   Dan 
    followed  this up with the #10  cans and was able to build  successful 
    cavities. 
       I  first started out with a band pass filter described in the  ARRL 
    VHF Handbook.  This was a band pass filter.  I then asked Woody how to 
    build  it into a notch filter.   He described for me how their circuit 
    in  the  coffee  can was.   It consisted of  a  single  link,   series 
    capacitive tuned.   We removed one link and connected the input to the 
    output.  I achieved a 12 db notch with about 3 db of loss. 
       I  soon  discovered  from the RSGB Handbook that maximum Q  in  the 
    cavity is when it's impedance is 77 ohms.   For a circular cavity with 
    a  circular tuned resonator the ratio of the diameter of the resonator 
    (d)   to  the  internal diameter of the cavity (D)   must  be  3.66/1, 
    according to the following formula: 

                             Z  = 138 * LOG  (D/d) 
                              0            10 
       (Cross section is two concentric circles,  D  =  inside diameter of 
    outer circle, d = outside diameter of inner circle.) 

       In  order to make my LMB box "band pass filter"  from the  Handbook 
    optimum Q, I would have to put a one inch (1")  outside diameter (3/4" 
    nominal)   pipe down the center of the segmented box.  (See figure A).  
    Here  is the formula for calculating the impedance of the  three  inch 
    (3") square segments of the box (D): 

                          Z  = 138 * LOG  (1.178*D/d)
                           0            10
       (Cross  section is a circle contained within a square,  D   =   the 
    dimension  of the square's side,  d  =  outside diameter of the  inner 
    circle.) 

       The total length of the 3/4"  diameter pipe is 57" long.  A  copper 
    coupler  can be used to slide back and forth to tune 1/4   wave length 
    line.    One inch (1")  ceramic standoffs are used to support the pipe 
    the 3" box.  The most critical part of the tune up is the distance the 
    link coupling is from the pipe.   It must empirically measure the loss 
    and  the  depth of the notch to determine positioning.    Because  the 
    length has such large currents running in it,  it must be at least #12 
    TW wire.  A brass or copper strap would work even better.  I  was able 
    to achieve over 25 db of notch with .7 db of loss. 
       Two  cavities  and  two  antennas can be used to  create  a  usable 
    repeater   system.    Adjust  the  two  cavities  to  suck   out   the 
    transmitter's frequency,  couple the two cavities with a 37" length of 
    RG213   or  RG9  coax.   This is an electrical 1/4  wavelength.    The 
    cavities  are placed between the receiver and it's antenna.   The  two 
    antennas must be vertically spaced at least 40 feet apart. 
       The  complete duplexer can be built by using three notches tuned to 
    the  transmitter frequency on the receiver line and two notches  tuned 
    to the receiver frequency on the transmitter line.   This will achieve 
    about 80  db of isolation and can be fed with one antenna as in figure 
    B. 
       Box specification:  LMB aluminum chassis,  12"W x 17"D x 3"H.   The 
    lid (cover)  is 12" x 17".  The segment dividers are made from a 15" x 
    14" cover material. 


                                    FIGURE A:
   ---- *****************************************               *************
   |    *                   *                   *               *           *
   |    *    ###########    *    ###########    *               *           *
   |    *    #         #    *    #         #    *               *           *
   |    *    #    *    #    *    #    *    #<-- * -- 3/4" pipe  *           *
   |    *    #    *    #    *    #    *    #    *     57" long. *           *
   |    *    #    *    #    *    #    *    #    *               *           *
   |    *         *    #    *    #    *    #    *               *           *
   |    *         *    #    *    #    *    #    *               *           *
   |    *         *    #    *    #    *    #    * RF            *           *
  17"   *         *    #    *    #    *    #    * Conn.         *           *
   |    *         *    #    *    #    *    #    ***             *           *
   |    *         *    #    *    #    *    #  ----* ---         *  O --- O  *
   |    *         *    #    *    #    *    #  | ***   |         *     |     *
   |    *         *    #    *    #    *    #  | *     |         *     |     *
   |    *         *    #    *    #    *    #  | *     |         *     |     *
   |    *         *    #    *    #    *    #  | *     |  #12 -- * --->|     *
   |    *         *    #    *    #    *    #  | *     7"  wire. *     |     *
   |    *         *    #         #    *    #  | *     |         *     |     *
   |    *         *    ###########    *    #  | *     |         *     |     *
   |    *         *                   *    #  | *     |         *     |     *
   ---- ***********************************#**|** -----         *************
        |         |         |         |         |               |           |
        |--- 3" --|--- 3" --|--- 3" --|--- 3" --|               |           |
        |                                       |               |           |
        |----------------- 12" -----------------|               |---- 3" ---|

              Adjust total length for resonance notch 29 - 30 db.
              Adjust link coupling for no more than 1 db of loss.


                                  Figure B

                                   Antenna
                                     /^\
                                      |
                                      |
          *************************** | ****************** -----------
          *       **       **       * | *       **       *           |
          *       **       **       * | *       **       *           |
          *       **       **       * | *       **       *           |
          *       **       **       * | *       **       *           |
          *       **       **       * | *       **       *           |
          *       **       **       * | *       **       *           |
          *       **       **       * | *       **       *           |
          *       **       **       * | *       **       *          17"
          *       **       **       * | *       **       *           |
          *       **       **       * | *       **       *           |
    RX ---*-O   O ** O   O ** O   O-*-^-*-O   O ** O   O-*---- TX    |
          *     | ** |   | ** |     *   *     | ** |     *           |
          *     | ** |   | ** |     *   *     | ** |     *           |
          *     | ** |   | ** |     *   *     | ** |     *           |
          *     | ** |   | ** |     *   *     | ** |     *           |
          ***************************   ****************** -----------
          |     |    |   |    |               |    |     |
          |     |____|   |____|               |____|     |
          |       37"      37"                  37"      |
          |                                              |
          |                                              |
          |-------------------- 15" ---------------------|




... Larry.Mc.Donald, N6ZMB, Fullerton, California!  <cough, cough>
___ Blue Wave/QWK v2.10

--- Maximus 2.01wb
Newsgroups: rec.radio.amateur.misc