Reisert Choking balun for HF and upper MF bands. (1.5MHz - 30MHz). FT140-43 Ferrite Toroid Core

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.

The antenna is a 40m band half sloper that is it is a quarter wavelength on the primary band of 7 MHz and with a third harmonic band near 21MHz. This antenna can also be quite useful on other bands apart from 40m and 15m.

The antenna is an antenna of convenience as it is made by cutting two insulators into a tower guy wire and despite this simple approach the antenna has some good performance characteristics that have been confirmed by measurement with WSPR. 

The antenna is simply fed with coax cable to the apex of the antenna and while not a good design it is not an uncommon amateur radio antenna.

The problem with the antenna is due to the direct coax feed connection the antenna, the antenna exhibits some annoying multi-mode RF currents on the coax that manifest themselves as RF feedback, such as crackling in unconnected speakers, that is speakers not connected to the radio in question.


  • Prevents unwanted RFI by eliminating feed-line 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 1.5 ~ 30MHz
Choking Impedance 3k Ohms (-30dB) min to >10k Ohms (-40dB) Ref: Figure 3
Core Used FT140-43 Ferrite Toroid Core
Number of turns 19 (9.5 + 9.5). Ref: Figure 2
SWR 1:1 Ref: Figure 5




Figure 2 Winding details of the 1:1 choking balun  




The construction was simply to wind 9.5 turns of RG316 coax with a cross over and another 9.5 turns onto a FT140-43 Ferrite Toroid Core. The result of 19 turns achieved an average of 40uH lumped value inductive reactance to common mode RF currents from approximately 1.0MHz to 30MHz.

Parts list.

  • FT140-43 Ferrite Toroid Core
  • About 1.0mtr of RG316 coax.


Photo 1 Choking balun assembled



Photo 2 Choking balun assembled






The mini-VNA antenna analyser data points recorded and graphed in a spreadsheet in Figure 3 show the attenuation as measured in the coax shield presents a minimum of -28dB at 1.0MHz to -33dB at 30MHz and much greater attenuation of better than -40dB from 4MHz to 18MHz.


Figure 3 The evaluation of the choking attenuation 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






Page last revised 09 August 2020  


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