Wednesday, December 12, 2012

FEED LINE CHOKE FROM COAX


Over the years coiled coax has been used to choke off RF from the outside of coax feed lines.  Sometimes these are called 1:1 baluns, choke balun, but technically a balun is an impedance transformer while these coils are chokes.  (NOTE:  This discussion is limited to air core chokes only.)  The reason for a choke is to stop RF from coming down the coax, causing distortion of the antenna’s radiation pattern and RF problems in the shack. 

Coax line chokes work well if they are used correctly.  However, there are some things one needs to know about using this type of coax line choke.

1      1.       They are most effective on one band only below 40 meters.
2      2.       If carefully designed, they will perform on more than one band 40 through 10 meters (see chart below).
3      3.       Always “flat” wind these chokes.  Scramble winding adversely impacts their performance.

The most important performance limiting factor is inner turn capacitance, called parasitic capacitance. 
Remember that any time two conductors are next to each other, even if there is a DC electrical connection between them, there still is some capacitance between them at radio frequencies (RF).  

NOTE:  Do not confuse parasitic capacitance with the capacitance between the shield and the inner conductor of the coax.   The capacitance between the shield and the inner conductor does not contribute anything of significance to this discussion.

As parasitic capacitance gets larger the choke starts to look more and more like a capacitor to RF.  Remember that a capacitor's or an inductor's resistance to alternating current (AC) varies with frequency.  

Experience over the years has shown that 10 times the feed line resistance for a well match antenna is a good number to use for a coaxial line choke's reactance.  I used 500 Ohms for creating the chart below.  In the real world up to 25% either way is acceptable.  That is 375 to 625 Ohms.  A choke will remain effective so long as both its coil and parasitic capacitance remain in the acceptable range for the expected operating frequency.

The following chart gives a list of coaxial line chokes for various ham bands. 

      Turns         Diameter         Length     Coax type         Band Coverage

      13             8                   3.25           RG-174           160 meters only
                                                              RG-58
                                                              RG-8x
      9              9                    3                RG-174           80 meters only
                                                              RG-58
                                                              RG-8x
      8              6                    2                RG-174           40 meters only
                                                              RG-58
                                                              RG-8x
      9             4                    2.25            RG-174          40 – 30 meters
                                                              RG-58
                                                              RG-8x
      8            4                    2                  RG-174         30 – 20 meters
                                                              RG-58
                                                              RG-8x
      5            5                    2                  RG-174         20 - 15 meters
                                                              RG-58
                                                              RG-8x
     6             4                    2                  RG-174         17 - 10 meters
                                                              RG-58
                                                              RG-8x
     4             3                    1                  RG-174         6 meters only
                                                              RG-58
                                                              RG-8x


     Length and Diameter are in inches.

Be sure to match the length of the coil keeping the spacing even between turns.  This may require using a spacer between turns while winding the coil.  Once the coil is wound the spacer should be removed and the turns need to be glued in place.  Deviating very far from either length or diameter will chance the frequency at which the coil will work.

Here are three graphs that will help you see why a coaxial line choke is limited.  Note how rapidly the inductance and capacitance very with frequency.  Notice that on each graph inductive reactance and capacitance reactance cross on 500 Ohms.  Remember from above that this is the design criteria.









Notice the higher the band, larger the range of frequencies the choke covers.

Other coil sizes and coax types will work.  However, they are too numerous to post.  

73
Dave, K7PDW

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