ANTENTOP- 01- 2020 # 024

Ferrite Clips and RF Current Sniffer

Ferrite Clips and RF Current Sniffer

By: Igor Grigorov, VA3ZNW

Practice shows that even in case of a well matched with coaxial cable antenna there is some RF current flowing on the outer braid of the coaxial cable. It is so called common mode RF current. Causes of the current are very different. It may be induced current from radiated antenna (in this case balun at the antenna cannot save the situation), it may be not correct placement symmetrical antenna in the outer space, it may be reason that the coaxial cable braid believe that it is a part of the antenna.. lots of the very different reasons.

Anyway, the RF current on outer braid of the coaxial cable, at work on transmitting, going in to the shack and may cause lots of harm inside. It is RFI that may hinder any electronic equipment, transceiver may begin to bite... Transmitting signal may be go to worst at every pressing of the key. So in this case before a ham there is a task to eliminate the parasitic current as much as possible.

Figure 1 Simple RF- choke

Figure 2 Ferrite Clips on the Coaxial Cable

To clarify this method I made two RF Current Sniffers. First has LED for indication of the RF current. The second one has connection to internal meter to shows the RF current. Figure 3 shows schematic of the RF Current Sniffer with LED indicator. Figure 4 shows schematic of the RF Current Sniffer with meter indicator. Sniffer with meter indicator is more sensitive compare to sniffer with LED indicator.

Radical preventing measure to combat common mode current that is installing the coaxial cable underground in metal pipes, using symmetrical antennas and antennas that do not recognize the outer braid of the coaxial cable as part of the antenna, often is not available to the radio amateur. However there is a simple method that may work to prevent flowing of the RF current on the outer braid of the coaxial cable. It is using an RF- choke that installed on the coaxial cable before this one coming to the shack. Figure 1 shows the simple RF choke. It is a coil wound by the coaxial cable. The coil may contain 5- 15 turns of the coaxial cable, diameter of the coil can be 10..15- cm.

However this RF- choke is not enough to eliminate the RF current on the outer braid. Inside of the shack it should be installed several rows of ferrite clips on the coaxial cable. The first row is a long one- it may contain 10 clips. Figure 2 shows ferrite clips on the coaxial cable. Through one meter it is possible install a second row in five clips, then through one meter one more row in three clips and then one- two clips on distance 50- cm along the rest of the coaxial cable length. The ferrite clips works good enough to kill RF current on the outer braid of the coaxial cable that removes RFI to other electronic equipment and eliminates the biting of the transceiver.

However, before putting on the coaxial cable the ferrite clips I still have a question- how efficiency the clips are.

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