In general, I agree with Bob. For most situations, a 1:1 balun will
produce a good match - but it all depends.
The impedance you are matching is NOT the characteristic impedance of
the parallel feedline, but the impedance seen at the end of the parallel
feedline. That can vary from very low to very high depending on the
length of the radiator, the length of the feedline, and the frequency.
The feedline transforms the impedance of the radiator depending on the
feedline length and frequency. Study a bit of transmission line
principles to discover why that is true.
If you want to convince yourself, just connect an antenna analyzer to
the station end of the parallel feedline. If you find an extremely low
or extremely high impedance on any band you wish to operate, try adding
or subtracting 1/8 wavelength of feedline for that problem band and
measure again.
73,
Don W3FPR
On 5/24/2018 10:13 AM, Bob McGraw K4TAX wrote:
My findings and applications agree with Victor's statement that a 1:1
current balun, specifically a Guanella balun design with added common
mode choke is the preference for using a tuner to feed a balanced or
open wire line. For the life of me, I have never been able to
understand the general belief that a 4:1 balun should be used with open
wire feed line. Maybe it is the "400 ohms" or "450 ohms" that is
attracted to the number "4" in the balun ratio that is the attraction. I
view it the case of the "tuner lemmings" where one started it
{incorrectly as it may be} and the rest followed.
By now most of us are well aware of the need for a 4:1 Guanella balun to
be wound onto two separate cores, not just a single core. Unfortunately,
most balun manufacturers are clueless and continue to promote these
single core baluns for OCFD antennas. A few (e.g., Balun Designs and
Palomar Engineers) have either dual-core’s or even hybrid baluns.
The secondary reason that the single core 4:1 Guanella is not
appropriate. If you wrap both winding's onto a single core, each
winding has half as many turns as when you wrap them onto separate
cores. Since inductance (and CMI) increases with the square of the
number of turns, it is obvious that a single core balun could never work
anywhere nearly as well as a dual-core balun.
But the primary reason is, in HF antenna applications, a 4:1 Guanella
Balun wound onto a single core has no CMI at all; ZERO. In fact it can
even generate CMC. (Source: G3TXQ). Therefore, by definition, it is
not even a balun at all. – Yes I have read Sevick too and he shows a
single core 4:1 Guanella balun as well as a dual core, but he fails to
explain the difference. The single core will only work in applications
that are 100% ground independent, or “floating.” No HF antenna
located here on mother earth is ground independent; thus the single-core
4:1 Guanella is not suited for HF antenna applications.
As to tuner ranges, add a length of coax or insert a piece of equipment
in the path and the Z +j is now different. Hence the appearance match
range will be different.
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