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.
1 Simple RF- choke
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.
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.