Hello Richard,

Please find comment under your text.

On 7 sep, 18:29, Richard Clark wrote:
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.

Correct.

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.

For cable with solid screen, Iscreen = I_inner + I_outer.
To go in more detail (I thought) was not relevant for the original
question.
For radiation, and distance between currents WL, only common mode
current counts (and you can measure that with a current probe).


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.

A fully balanced load can be connected to an unbalanced feed line with
a voltage balun (for example center tapped transformer), a choking
type balun is not necessary for this. So I do not agree on your
point.


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.

Did I mention frequency under point 1? I ment to say that requirements
for the balun (how good it must be) depends on your application. For a
field day you may ignore the balun function at all (you may burn your
fingers), but for an Antenna Range, you need a good one with high
common mode attenuation (when you use a balanced reference antenna).


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.

Impedance transformation is a "byproduct", main function is to couple
balanced circuit to unbalanced circuit without feed line radiation.
The stress applied to the balun depends on the impedance level. I
mentioned this to show that even with dipoles (and same power) you can
have different designs (or size). Yes, a choke type balun that can
handle max. 1kW into a HW dipole, will probably run hot (or flashes
over) when used on a thin FW dipole with same input power.


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.

I designed some PCB baluns for feeding PCB antennas and wire antennas
connected to the (tiny) PCB, because of ease of production (no other
handwork required). They perform not as good as a coaxial type, but
good enough for the application.


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.

This depends on the type of ferrite, frequency and final geometry of
the balun. Although not preferred (possible change on parasitic
resonance with capacitive common mode impedance), an inductive common
mode choking action can provide good isolation (as long as reactance
is high enough with respect to the impedance at the balanced side).
For JOTA I use a choke type balun that is inductive (with low Q
factor) for 80m, while it is almost resistive for 40m.

73's
Richard Clark, KB7QHC

Richard, I only wanted to make clear that there are many solutions for
the same problem and that one solution cannot fit all problems. It was
not my purpose to be complete.

Best regards,

Wim
PA3DJS
www.tetech.nl
without abc, the address is correct.