On 9/9/2010 6:50 PM, lu6etj wrote:
On 9 sep, 15:48, Roy wrote:
Before beginning a discussion about what constitutes a balanced load and
what doesn't, how about answering these questions?
1. What is "balance"?
2. What defines a "balanced" feedline?
3. What are the properties of a "balanced" load?
4. How can you tell when a line, load, or transmitter is "balanced"?
5. What's the big deal about being "balanced", anyway?
And finally,
How does a balun achieve "balance"?
Roy Lewallen, W7EL
Hello Roy
Why? that way we can end up having to define all words we use :). I
learnt a two terminal balanced circuit basically have the same
impedance respect to ground in its terminals, if Windom Carolina not
have it, well... we have missed the "bal" part of the equation :)
I recognize the other items are interesting to analize too but in
diferent sense that the proper use of the term "balun" towards I
pointed my little observation.
Years ago I have read your very good article "Baluns: What They Do
And How They Do lt" (until today I keep it safe in my computer for
reference) and I believe I understand where you point to with the
other good questions.
For John: Dear friend I could not translate well your post, may you
repeat in a little more Tarzan english for me?
Miguel
Suppose you connect a transmission line to a perfectly symmetrical,
horizontal antenna. The antenna and feedline would be a "balanced
circuit" by your definition, since the two conductors of the
transmission line have equal impedances to ground. But the transmission
line will radiate.
Now connect one conductor of the transmission line to the center of your
rig's coaxial connector, and the other conductor to the rig's chassis.
(This is Fig. 2 of the article you mention, which by the way is
available at
http://eznec.com/Amateur/Articles/Baluns.pdf.) Is it still
a "balanced circuit"? Why or why not?
Or suppose you take two signal generators which are perfectly coherent
(i.e., phase locked to run at exactly the same frequency) and exactly in
phase with each other. Each has a 50 ohm output impedance and each
produces exactly 1 volt RMS of RF when open circuited. Connect one of
these to each of the terminals of the feedline instead of connecting the
feedline to your transmitter.
Now,
-- The impedances to ground looking toward the antenna from the feedline
are the same for the two feedline terminals.
-- The impedances looking back toward the generators from the feedline
are the the same for the two feedline terminals.
-- The two feedline conductors have equal voltages and currents.
-- The circuit is surely balanced by your definition.
Yet the feedline will radiate. Change the generator phasing any other
angle except 180 degrees, and the feedline will radiate. Only when the
two generators are exactly out of phase will the line cease radiating. I
call that condition "balance" for the reasons explained in the article,
but it's quite different from your definition.
If we're to use your definition of "balance", we have to conclude that
balanced transmission lines radiate some times and some times they
don't. If the system is already "balanced" when the generators are zero
or, say, 90 degrees out of phase, would a balun do anything if connected
between the generators and transmission line?
Roy Lewallen, W7EL