starman wrote:
John Doty wrote:

Consider a lamp dimmer that generates 10 mW of RFI, which rides out in
common mode on the mains, finds its way to the power cord of your
transceiver, rides out on the feedline to the antenna, and then couples
back through differential mode to your receiver input. That's not a very
efficient coupling path, so suppose it has a loss of 60 dB. You'll still
get 10 nW to the receiver. This is a lot: even if it's spread over 30
MHz, it's still 10 uV in a 6 kHz channel. That's S6 on my Drake R-8, a
very serious quantity of noise.


What is the actual coupling process between common and differential mode
at the far (antenna) end of the coax?

Consider first an open circuited coax cable hanging in mid air. In coax,
the common mode is carried only on the shield, while the differential
mode is carried by opposing currents on the shield and center conductor.
Since the shield is open circuited, the sum of the common mode and
differential mode currents on the shield must be zero at the end. But
the differential mode current must also be zero, since the center
conductor is open circuited. We therefore conclude that the common mode
current must also be zero at this point: the shield current cannot be
balanced by driving a differential mode current.

Now attach a wire to the center conductor. Now a current can flow out
from the center conductor to the wire, so the common mode shield current
can be balanced by a differential mode shield current: the common mode
energy thus drives a differential mode current.

The misnamed "magnetic longwire balun" doesn't help here, since it can't
suppress this coupling without also suppressing the coupling of the
antenna to the differential mode: it has no way of distinguishing the
current from energy coming down the wire from the current due to energy
coming up the coax.

If you connect the shield to an infinite, perfectly conducting ground
plane, all of the common mode current flows that way. This is why a
ground stake at the feedpoint helps (although in real life it's not
perfect).

With a balanced antenna things are generally better, but more
complicated: without a balun, the common mode on the coax will excite a
combination of common and differential modes on the antenna. A balun can
help, but practical baluns are not ideal devices.

Furthermore, the common mode on the coax can couple electrostatically or
magnetically to nearby conductors like the antenna. Careful orientation
of the line with respect to a balanced antenna can minimize this, but
it's difficult to avoid some coupling in practice.

Given that the methods for decoupling the common mode from the
differential mode at the antenna are imperfect, it's often a good idea
to try to keep the common mode energy away from the antenna in the first
place.

-jpd