Common mode chokes are added as series elements to a transmission line to kill
common mode current. The line may be a short one carrying audio or control signals between a computer and a radio, video between a computer and a monitor, noisy power wiring, or feedlines for antennas. This application note focuses on the use of chokes on the feedlines of high power transmitting antennas to suppress received noise, to minimize RF in the shack (and a neighbor’s living room and television sets) and to minimize crosstalk between stations in multi-transmitter environments.
I am located nearby 400.000 Volts power line and the noise on 80m without the CMC is about S9 (-69dBm) and the addition of the CMC with a proper grounding system is decreasing the noise level significantly. Nowadays with the CMC my noise level on 80m band is below S6 (-90dBm))
Enfed or EFHW antenna are usually generated by design some Common Mode Currents on your feeder and using CMC-Common Mode Choke/Line isolator will assist you to reduce and even supress spurious and EMI. It will also help you receive by filtering induced noise from various sources.
The most common uses are to block/eliminate common mode current (CMC) from traveling along the shield of your coax back into the shack. While they don’t “solve” an SWR problem, you will see reduced reflected power in the shack caused by current on the shield of the coax. A choke has also been known to help reduce noise on the band by absorbing static buildup in the antenna system.
Since the coaxial shield is acting also as a counterpoise CMC should only be placed on the feed line if needed, & if you are using a EFHW antenna it needs to placed 0.05 of a wave length of the lowest band. If you are using a random wire antenne , the CMC should be installed at about 6 to 9 meters from the transformer (Mine is installed at 6.5m from the transformer).
Consider a simple dipole fed with coax. In the common mode circuit, the coax shield becomes part of the antenna, acting as a single wire connected between one side of the center of the antenna and ground. As a common mode circuit element, its VF is near 0.98 (depending on the diameter of the shield and the dielectric property of the outer jacket). In the common mode circuit, this wire (the coax) has some impedance, (RS +
jXS), by virtue of its electrical length, which is different at every frequency. At some frequencies, XS will be positive (inductive), at others it will be negative (capacitive).
Without a choke at the feedpoint, the feedline becomes part of the antenna; if the antenna system, including the feedline, is unbalanced, this causes the feedline to radiate part of transmitted power; when receiving, signal and noise picked up by the feedline is
coupled to the antenna. This is most easily understood with coax, where skin effect and proximity effect combine to cause common mode current to flow on the outside of the shield and differential mode current to flow on the outside of the center conductor and return on the inside of the shield.
In oder to achieve a CMC , few solution are commoly used and the one i am using is using Toroids or Fair-Rite type : 43, 31 , 52 and 61.
According the table below , it is noted the toroid you should use in order to achieve an efficient CMC device for your EFHW or Enfed Antennae.
According this table , using only one choke will not cover the entire HF and a combination of Toroids should be used.
Common mode current is carried on the outside of the coax shield, or as the difference of unequal currents on the two conductors of 2-wire line. A line carrying common mode current acts as antenna for both transmit and receive. Common mode current that couples to the antenna changes the directional pattern of an antenna by filling in the nulls of it’s directional pattern. In simple terms, the common mode circuit becomes part of the antenna – the part of it that is close to noise sources picks up that noise; and when transmitting, radiates RF that poorly designed equipment will hear as RF interference.
Because a Fair-Rite choke can be inductive or capacitive, and because the common mode circuit will be inductive at some frequencies and capacitive at others, the
choke can cancel part or all of the of the common mode circuit
For an antenna from 160m up to 10m bands , the CMC shall be located about 4 meters from the 1:49 or 1:64 transformer.
DESIGN
In general, any combination of chokes can be used in series to provide the
desired choking impedance over the desired bandwidth. Their combined choking impedance, RS, will be the sum of their RS values on each band.
According the table underneath , the desired and highlighted in red will be a good choice in order to achieve a great CMC for your EFHW antenna.
It is highly recommended to use RG400 for the choke winding , therefore according some experimentation , RG58C is not a bad choice since RG58 has less losses on the HF band. If you are using RG58 , do not tight it too much on the Toroid because the of radius bends limitation for RG58 and to prevent any short circuit with the center conductor and the shield.
According the Table i am using : 17T on FT240-43, 10T on FT240-31 and 11T on 2xFT240-52 with a PVC pipe.
This Combination is providing me a RS total average value of 6,000 Ω from 160m up to 10m which is excellent.
This design is able to handle up to 1kw with ease and the separation between the coils is not really critical.
The Choke shall be located 4m from the feeding of the transformer. Why 4 meter , simply because the 4 meters braid (shield) is acting as a counterpoise according 1/20 wavelength which is the minimum lenght regarding the lowest frequency band (80 x 0.05 = 4 meters).
Alternative Combination
Another relevant combination : Cores are (3) 31 mix with 15 turns on FT230-31 , 12 turns on FT230-31, another 12 turns on FT230-31 and (1) 43 mix with 5 turns on FT230-43 for 20m.
CMC Choke Summary
I do have any RF feedback issue even using QRO amplifier , thererfore since EFHW or ENFED antennae are subject to provide interference , it is mostly a safeguard system . Even when using my QRO HF Amplifier.
This CMC did not provide significant losses and the SWR remains very good.
The CMC is providing a high impedance circuit for the coaxial shield and will prevent spurious and RFI to come back inside your receiver and acting as a trap. The coaxial center conductor will not be affected.
F5NPV - V85NPV 