At my age, I've not kept up with the theory and forgotten much of what I
used to teach from lack of use. I hope I phrase this correctly.
I think your tutorial would make good reading for those entering into
this discussion and label which type of balun is being described.
The way I interpret what he wrote (with my basic understanding of the
subject), is similar to what you have said about SWR. The balun does
not change the characteristics of the antenna, but rather what the feed
line and rig see. His remarks do not seem to apply to a choke which is
all many antennas now use, such as (many, most, or all) of the Force 12
family, but rather the voltage baluns which are usually transformers,
either auto, or isolation. The preferred baluns shown are the so called
bead baluns (or open chokes as in your tutorial) which do not act as
resonant circuits over their operating range, but rather as a fixed
resistor, or inductor for common voltage, but do not affect differential
voltage (the signal). Ideally the maximum and desired isolation can be
around 5000 ohms for common mode voltages / signals at the desired
operating frequency. Useful isolation is available over an octave,
depending on what is needed. There, because of the generic use of the
term, "balun", I think it becomes confusing and only applicable to a
subset of baluns?
The choke appears to be unbalanced to unbalanced, but it can offer
enough common mode rejection to appear to work well as a true balanced
(the antenna) to unbalanced (the feed line) balun.
I disagree with his # of turns analysis as with the chokes more turns
and increased inter turn capacity lowers the frequency of maximum
isolation at the expense of higher frequency isolation, but that
isolation still covers a wide range,. I believe the curves in your
tutorial demonstrate this with no resonances involved within the
operating range of the choke. The isolation just drops off as I
understand it.
What he says might possibly be attributed to transformer / voltage
baluns which could / might have have high frequency resonances. With
out attributing characteristics to a particular type of balun, it does
become confusing. When a specific term like "balun" becomes generic the
use and meaning can often become muddied.
A choke is a choke, is a choke. It is not normally resonant as we use
them. It has a wide frequency range of relatively high isolation that
tapers off in both directions outside of its designed range as I
understand it.
73
Roger (K8RI)
On 11/13/2015 3:38 AM, Jim Brown wrote:
On Thu,11/12/2015 10:07 AM, Earl Morse wrote:
The first problem is that the word "balun" is used to describe at
least a dozen very different things. Thus, one or more of the answers
below sort of make sense for one of those things but not for the other
things.
That said, most of what is written below makes no sense to me for any
of those "things."
73, Jim K9YC
The antenna impedance won't change, it will still look like a typical
dipole with or without the balun.
The balun will act like a transformer over a fixed set of frequencies
depending on core material, # of turns, and inter-winding
capacitance. The high frequency limit would be determined by core
material and inter-winding capacitance and the low frequency by the
core material and # of turns.
# of turns and inter-winding capactance are mutually exclusive
meaning that for the improvement you get at lower frequencies by
adding more turns you will lose at the higher frequencies due to the
increase in inter-winding capacitance.
A typical balun when terminated in its design load impedance would
show less than 50 ohms at the low end of its frequency range,
maintain 50 ohms through its usable frequency range, then go high
when it hits its self resonant frequency at the high end of its
usefulness.
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73
Roger (K8RI)
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