T-MATCH   Construction

The T-Match with its balanced feed point is one method of feeding a Balanced Dipole.  A suitable 4:1 balun is used in conjunction with the T-Match to perform this function.  The T-Match, with a 4:1 balun and Balanced Dipole can be substituted for a Folded Dipole with its 4:1 coax balun. 

 

 SCHEMATIC:

Spacing between Arms & Dipole can be anything convenient for construction

Adjustment of the T-Match with a 4:1 balun is simple - just adjust the position of the shorts on the Arms until you get an acceptable SWR.  For the lowest SWR it may be necessary to change the length of the Driven Element. After changing the driven element readjust for the lowest SWR.


 

6m

 

                                                   ***** 6m Antenna T - MATCH *****

 

 

      FREQUENCY of the Antenna  (Mhz)                     50

      LAMBDA                                  5995.85  mm  or 236.057 ins

      DIPOLE                                  2836.04  mm  or 111.655 ins

      DIPOLE DIAMETER                           12.7   mm  or   0.5   ins

      ARM LENGTH                                   709  mm  or   27  ins

      ARM DIAMETER                               6.3   mm  or   0.25  ins

      ELEMENT SPACING: make it convenient for construction

 

 

                                                ***** 4:1 COAXIAL CABLE BALUN *****

              The frequency selected is:                50  Mhz

              The coax you selected is:                 RG-213/U

              The velocity factor of the coax selected is: .66

              The physical length of a half-wave coaxial (4:1)

              Balun is: 1978  mm  or   77  ins

 

2m

 

                                                           ***** T - MATCH *****

 

      FREQUENCY of the Antenna  (Mhz)                    144.300

      LAMBDA                                  2077.56  mm  or  81.794 ins

      DIPOLE                                   982.69  mm  or  38.688 ins

      DIPOLE DIAMETER                            4.765 mm  or   0.187 ins

      ARM LENGTH                                   245  mm  or   9  ins

      ARM DIAMETER                               4.765 mm  or   0.187 ins

      ELEMENT SPACING: make it convenient for construction

 

 

                                                ***** 4:1 COAXIAL CABLE BALUN *****

 

 

               The frequency selected is:                144.3  Mhz

               The coax you selected is:                 RG-213/U

               The velocity factor of the coax selected is: .66

               The physical length of a half-wave coaxial (4:1)

          Balun is: 685  mm  or   26  ins

 

 

  (NOTE: The PHYSICAL LENGTH is for the shield. Keep the center conductor as short as possible.  Keep the insulation   long enough for high power.  Example: for 432 Mhz use a 5 mm or .197 ins length of insulation.)

 

                                          ***** BALUN CHECKING PROCEDURE *****

 

     To make or check a 4:1 balun when the velocity may be unknown use a .7 factor and cut the coax longer than the formula indicates for the shield. Cut the leads as short as possible but to a useable length, short them together on one end.  On the other end connect the leads together to form a small loop.  Use a grid-dip oscillator (GDO) with minimum coupling to the loop and tune for a sharp dip.  Adjust the GDO while listening for the GDO on your receiver.  Reduce the length of the coax in small increments until the frequency on the receiver approaches the desired frequency.  With each adjustment keep the leads just long enough to make the connections. You may also use the above method to check any balun which was made using this type of BALUNs.  The velocity factor in some coaxial cable has been found to vary from manufacturers specifications so it is a good idea to check your balun to make sure it can do the job.

  KEEP THE LEADS SHORT AND USE MINIMUM COUPLING

 

70cm

 

                                                          ***** T - MATCH *****

 

      FREQUENCY of the Antenna  (Mhz)                   431

      LAMBDA                                   695.57  mm  or  27.385 ins

      DIPOLE                                   329.01  mm  or  12.953 ins

      DIPOLE DIAMETER                            4.765 mm  or   0.187 ins

      ARM LENGTH                                   82  mm  or   3  ins

      ARM DIAMETER                               3.175 mm  or   0.187 ins

      ELEMENT SPACING: make it convenient for construction

 

                                            ***** 4:1 COAXIAL CABLE BALUN *****

 

               The frequency selected is:                431  Mhz

               The coax you selected is:                 RG-213/U

               The velocity factor of the coax selected is: .66

               The physical length of a half-wave coaxial (4:1)

                    Balun is: 229  mm  or   9  ins