On Sun, 7 Sep 2008 06:54:49 -0700 (PDT), Wimpie
wrote:

Most baluns (in antenna systems) are used to make sure that the
current in the antenna structure is far higher then the common mode
current in the feedline.
Common mode current = Icenter + Iscreen.

Hi Wim,

I presume "screen" is the coax shield. If there is any Common Mode
current, it is going to reside on the outside of the shield in a
different degree than inside the shield where it will be combined with
Differential Mode current.

In fact it would be simpler to measure it there too.

To say in other words: function of balun is to make sure that the feed
line does not take part in the radiation process.

Only if the BalUn is designed with choking properties, otherwise it
will make no difference at all in feed line radiation. To quickly
summarize from popular labels, you have Current BalUns and Voltage
Baluns. Such labels differentiate those that choke and those that do
not, but they are often misapplied and represent no real guarantee.
The informed buyer should always examine the construction details.

Some issues that may affect the type and actual design of the balun:

1. "far higher" (how good it mist be) is not a hard figure and depends
on the application. Requirements for a field day will be different
then for an EMC measuring antenna in an anechoic chamber.

This is an unusual issue. There is no correlation to need and
frequency. There is no correlation to application and frequency.
Motivation may even dictate you ignore chokes and BalUns altogether
during field day for simplicity's sake. If you demand performance,
then all the rules of choking and BalUn application apply at all
frequencies.

2. The behavior of the balun depends on the impedance levels (both
common mode and differential mode) on both balanced and unbalanced
side. A balun for a full wave dipole "receives" more voltage stress
than a dipole for a HW dipole (at same power level).

The point of a BalUn is to transform from the source Z to the load Z.
If the load Z happens to be high, you select a BalUn with its ratio
designed appropriately. Hence the variety of ratios as no single
design is appropriate at all frequencies for a single antenna. Voltage
(or current) stress is part of the design criteria, not a limitation
to its application.

5. Financial issues may play a role (especially in mass volume
products). A balun on PCB is far cheaper than a coaxial one with
ferrite cores.

This point, and two before it (not quoted here) seem to speak to small
signal, board level applications. They have no application with
antennas. For instance, no one is going to find any application for a
BalUn on a PCB (for which I see no distinction) unless the antenna is
also on the PCB.

6. How much insertion / mismatch loss is allowed, what about power
handling?

BalUns that are appropriately selected to the load have a far greater
chance of performing without issue than many other matching solutions.
Consider the insertion loss of a typical, external tuner compared to
that of a BalUn that satisfies the same mismatch. I suspect you will
probably lose far more power in the tuner, or at best achieve parity
with the BalUn. This is not to say that a BalUn solves all problems
and is the universal solution, however.

I prefer the current
choke balun (the one with the ferrite cores). If possible with low Q
factor for the common mode inductance.

In fact, the 1:1 Choke BalUn has very little inductance to offer, and
some formulations of ferrite may even present a minor capacitive
reactance. The major characteristic of ferrite is Resistance, and it
is the resistance that offers the isolation (choking) from input to
output.

73's
Richard Clark, KB7QHC