Cecil Moore wrote:
Richard Fry wrote:

There is a general belief that a balun should be used where coax
connects to the feedpoint of a 1/2-wave dipole (for example), to avoid
RF current on the outer surface of the coax outer conductor -- which
can affect antenna input match, radiation pattern etc.
But does even a perfect balun fully remove these effects?

No, it doesn't, for the reasons that Richard gave, and Cecil summarizes
below.

The reasons why these currents can appear is because the antenna, the
feedline layout and/or the environment is asymmetrical - and often it's
all three.

How much unwanted feedline current you have will depend on the physical
layout, how long the feedline is in wavelengths, and where and how it
may be grounded at the shack end. That's what makes it unpredictable, as
Cecil says below; so whether or not you need a balun will depend
entirely on your particular situation.


There are two sources for common-mode current. One is
conduction. The other is induction.

The 'conduction' component of the current is launched onto the feedline
where it connects to the antenna, and a good choke balun at the
feedpoint can effectively knock it out. The balun at the feedpoint
doesn't have to be perfect; only good enough to make sure that the
conduction current doesn't dominate any more.

That leaves the 'induction' component of the current, which is due to
asymmetrical electromagnetic coupling between the antenna and the
feedline. Unless the feedline is much closer to one side of the antenna
than the other, this induction component will generally be much smaller
than the conduction component was (before you mostly removed it with a
balun at the feedpoint).

If the remaining induction current is a problem, you may need to use
additional chokes at various places along the feedline. Depending on the
feedline length and the grounding arrangements, there will be current
maxima and minima at various places along the feedline. The most
effective place to put a choke is obviously at a maximum, where the
current is trying to be large; the least effective place is at a
minimum, where it's low anyhow.

The last traces of feedline current are very hard to kill completely.
It's rather like trying to squeeze the air out of a long balloon - choke
the current in one place, and a new maximum will try to pop up somewhere
else. Even so, a few chokes in the right places - and always put the
first one at the feedpoint - will almost definitely put you in control
of the situation.

But how do you know how much current you've got, and where the current
maxima are?

Since common-mode
currents are somewhat unpredictable, the best way to
ascertain if there is a problem is to measure the
current. A toroidal pickup slipped over the coax will
give one a relative reading and is super simple to
implement.

Here is a whole page about clip-on current meters, which have the
advantage that you don't have to disconnect the cable to feed it through
a toroid:
http://www.ifwtech.co.uk/g3sek/clip-on/clip-on.htm

The clip-on gadget is a bit more trouble to build, but you'll be very
glad you did! With one of these, you can actually *see* how much RF
current you've got, and where it is. Without it, you might as well be
investigating RFI problems with your eyes shut.



--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek