TCS External Loading Coil Information


      The familiar L701 external loading coil. Most TCS equipment owners have one sitting on the shelf but have never attempted to use the coil.

                                    Click to enlarge photos.


                                                    A very robust and well built unit.

                                                A strange dual wiper switch selects the taps.

          There are 7 switch positions. max inductance is CCW on Zero (0), minimum inductance is switch position 6. Inductance values of each position are:
                       (measurement taken are + - 5 uH)
                0- 100 uH
                1-  76
                2-  58
                3- 41
                4- 25
                5- 12
                6- > 1uH


               Switch position  "0" is maximum inductance          

                                         Click to enlarge

                The external loading coil was intended to be use to aid matching the TCS transmitter to a 20 foot whip on the lower frequency ranges.



                                           Click to enlarge

                      Note on the 20 feet whip chart that the coil is only used on the lower frequency ranges. Switch Position 2 is approximately 58 uH.   Position 6 is used for 2.25 Mcs and above and when the switch is position to 6 all coil windings are bypassed and in this case unless the operation needs to rapidly QSY the a frequency below 2.25 Mcs then the coil could be removed.






      Note the winding method, a winding is rewound over its self to conserve space and allow more turns. A typical coil wound on this size of coil form (2 1/2 inch diameter) would have an inductance of approximate 30 uH but the TCS L701 coil has a total inductance of 100 uH.



      As the heavy duty switch is turned to the right (clock wise) a sliding contact shorts out turns and the inductance is decreased. Position Zero (0) is max inductance not minimum. Note that the switch has dual contacts that select the taps. See next photo.


               Why dual contacts? Another TCS mystery.



                   The rear of the switch also has multiple contacts (4) which slide on a single ring which is connected to zero(0), Two (2) contacts are shown above and the other two(2) are hidden from view.


                                      The four rear switch contacts are all in parallel.

                The coil presents a very high Q, approximate 170 at 2 Mcs

                   Additional odd ball or in between inductance values can be obtained by shorting out tap positions.


                       Example: Position the alligator clip jumper on coil tap five (5) and the end of the coil results in a value of 71 uH at switch position "0". Switch position "4" results in a low value of 6 uH. Etc.


                                 Add a "clip on" variable cap at either side or in series and you can match a
wide variety of loads. An extra L701 TCS coil would be handy in your "Matching Strange Loads" tool box


                                        Return to TCS Index
                                            Return to K4CHE index