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CHOKING BALUN FOR HF BANDS

Choking balun for HF and upper MF bands. (1.5MHz - 30MHz).


Requiring a choking balun to isolate the potential feed-line common mode RF on the coax cable for HF/MF bands 1.5MHz - 30MHz. A simple coax cable wound on to a ferrite toroidal core was chosen.

  • Prevents unwanted RFI by eliminating feedline common mode currents and radiation
  • All power goes to the antenna, improving efficiency
  • Reduces noise or EMI picked-up by your coax shield
  • Power is balanced between driven elements of antenna

 

 

 

Figure 1 Schematic of the 1:1 choking balun  

 

Type  Choking Balun
Ratio 1:1
Frequency Range 1.0 ~ 30MHz
Choking Impedance 1k Ohms (-21dB) minimum. Ref: Figure 3
Core Used FT240-43 Ferrite Toroid Core
Number of turns 14 (7 + 7). Ref: Figure 2
SWR 1:1 Ref: Figure 5

 

 

 

Figure 2 Winding details of the 1:1 choking balun  

 

Construction

 

The construction was simply to wind 14 turns of RG58 coax onto a FT240-43 Ferrite Toroid Core. The result of 14 turns achieved an average of 40uH lumped value inductive reactance to common mode RF currents from approximately 1.0MHz to 30MHz.

Parts list.

  • FT240-43 Ferrite Toroid Core
  • About 1.0mtr of RG58 coax.

 

Photo 1 Choking balun assembled

 

 

 

 

Testing

 

The AIM 4170C antenna analyser recorded in Figure 3 show the Impedance as measured in the coax shield presents a minimum of 1000 Ohms from 1.0MHz to about 30MHz and much greater for the 1.8MHz to 14MHz amateur radio bands.

 

Figure 3 The evaluation of the choking impedance of the balun over a bandwidth from 1.0MHz- 30MHz.

 

 

Figure 4 Choking impedance to dB of choking. 20dB attenuation should be considered the minimum.

 

 

Figure 5 SWR.from 0.1MHz- 30MHz

 

 

Also see other baluns and ununs:

1:1 Choking balun Choking balun for lower HF and MF bands. (1.8MHz - 10MHz) T250-26 Powdered Iron Toroid Core.

1:1 Choking balun low band VHF Choking balun for lower band VHF. (14 ~ 54MHz) FT140-43 Ferrite Toroid Core.

1:1 Guanella current balun 1:1 Guanella current balun (1.8 - 30MHz) L15 ferrite toroid core.

1:4 Guanella current balun 1:4 Guanella current balun (1.8 - 30MHz) L15 ferrite toroid core.

1:1 Ruthroff voltage balun, 1:1 Ruthroff voltage balun (1.8 - 30MHz) T-200-2 powdered iron toroid core.

4:1 Ruthroff voltage balun  4:1 Ruthroff voltage balun (1.8 - 30MHz) T-200-2 powdered iron toroid core.

6:1 Ruthroff voltage balun  6:1 Ruthroff voltage balun (1.8 - 30MHz) L15 ferrite toroid core.

1:9 voltage unun_v1  9:1 voltage unun (1.8 - 30MHz) T-200-2 powdered iron toroid core. Version 1 

1:9 voltage unun_v2  9:1 voltage unun (1.8 - 30MHz) L15 ferrite toroid core. Version 2

 

 

 

 

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Page last revised 20 November 2019  
 

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