Common mode balun for HF antennas
It has many other names: Common mode choke, current balun, line isolator etc.

Every antenna needs a common mode choke in the feeding point! This applies to all antennas like dipoles, OCF dipoles, ground planes, slopers etc. Without a common mode balun there is always some common mode RF current in the feeding cable which will disturb the antenna radiation pattern and the very important thing is that the feeding cable picks up local man made noise during reception. Additionally a common mode balun is reducing the RF field in ham shack and helps rigs to work properly.


You just need a ferrite toroid and wind the feeding coaxial cable thru it. The purpose is to achieve enough common mode impedance to attenuate common mode RF current in the feeding cable. 1000 ohms or more is excellent and 500 ohms is much much better than no balun.


The basic construction principle of a 1:1 common mode balun. Note the two different winding styles.

RG-58 feeding cable can be wound on a ferrite toroid. A healthy RG-58 can stand 1kW on 160m and 80m. With higher QRO power and on higher bands it is recommended to use a Teflon insulated coaxial cable or wind a thicker coaxial cable in a wider style thru a ferrite toroid. If you use a continuous feeding cable for a balun there is no need for an enclosure. You can also consider to stack two toroids to get more margin for extreme QRO power. Two stacked toroids provide double the listed impedance. Here are few examples of ferrite toroids, number of turns and measured common mode impedance.

Amidon FT-240-43 is made by Fair-Rite as a part number of 5943003801
Amidon FT-240-31 is made by Fair-Rite as a part number of 2631803802
Amidon cores can be ordered eg from here www.rf-microwave.com or here www.amidoncorp.com
Fair-Rite and Wurth cores can be ordered eg from here www.mouser.com or here www.digikey.com
Ferrite Toroid
Turns
Ohms at 1.8 MHz
Ohms at 3.5 MHz
Ohms at 7 MHz
Ohms at 14 MHz
Notes
Amidon FT-240-43
6
500
700
1000
1300
Same as Fair-Rite 5943003801
Amidon FT-240-43
8
800
1200
1600
1400
OD=61mm, ID=35.5mm, H=12.7mm
Amidon FT-240-43
10
1400
2100
3300
1600

Wurth 74270191
6
500
1000
1700
1500
OD=61mm, ID=35.5mm, H=20 mm
Wurth 74270191
8
800
1800
2400
1500

Wurth 74270191
10
1300
2600
2500
1400

Amidon FT-240-31
6
700
1100
1300
1700 Same as Fair-Rite 2631803802
Amidon FT-240-31
8
1400
1900
2400
2900
OD=61mm, ID=35.5mm, H=12.7mm
Amidon FT-240-31
10
2200
3200
3900
4000


Here is a more compact construction which is using a ferrite tube (Axial Ferrite Bead) instead of a toroidal core. See an 3.5MHz application here.
 
7 turns of RG-316 Teflon coaxial cable wound on Wurth 742700790 Axial Ferrite Bead               5 turns of RG-58 wound on a Wurth 74270096
Ferrite Tube
Turns
Ohms at 1.8 MHz
Ohms at 3.5 MHz
Ohms at 7 MHz
Ohms at 14 MHz
Notes
Wurth 742700790
5
500
1000
1500
1700
OD=26mm, ID=13mm, H=28.5mm
Wurth 742700790
6
700
1400
1900
1600

Wurth 742700790
7
900
1900
2200
1500

Wurth 742700790
8
1250
2500
2700
1500


Update 2020-06-28

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