Clarification
Aug 1, 1999 12:00 PM, Alan Kruglak
A number of questions have arisen regarding my feature, "A Question of Balance," published in the May 1999 issue of S&VC. Generic video baluns do not offer common-mode isolation, a point that I did not clarify sufficiently. The illustrations on page 34 are used to show the basic concept of balanced/unbalanced signal transformation, but neither of the baluns depicted will isolate common-mode current problems. Only a solid-core or air-core transformer with two separate windings will suffice.
With proper design and selection of low-, high- and bandpass/band-reject filter elements, one can mix several AC (audio through RF) signals on the same transmission line as long as frequencies do not overlap. Because baseband video contains modulation information that overlaps baseband audio frequencies, mixing them is impractical.
We can, however, multiplex audio and RF-modulated video (not baseband, as was stated) on the same pair of wires by using low-pass and high-pass filters. The RF-modulated signal would use a different pair of windings on the balun from the audio signals to couple into the CAT 5 wiring system to achieve the proper transformation ratio.
ALthough line-level audio can be fed directly to CAT 5 wire, I would use at least 1:1 ratio transformers at the source and load to minimize such unwanted common-mode voltages, as those inductively coupled from power lines, motors or other adjacent 60 Hz/120 Hz electromagnetic fields. Interference from these sources would not be heard on a communications system with a narrow audio bandwidth, but it would come through on a full-bandwidth audio reinforcement system.
The discussion of signal loss in CAT 5 wire is intended to point out that CAT 5 is not a practical alternative to coaxial cable. Using the combination of a balun and CAT 5 wire results in greater signal attenuation than conventional coax or even open wire. Not that the balun in particular is the culprit here, but because one must use a balun to connect coaxial cable to a twisted pair in any case, the combination makes for a lossy solution.
It appears that there is a minor disagreement on semantics. Having been an amateur radio operator for nearly 30 years and a builder of antennas for HF, VHF and UHF operation, I interpret a balun's ratio to describe the impedance transformation. When we needed to match a 200 V dipole driven element on a yagi to a 50 V transmitter, we called for a 4:1 balun, not 2:1. I defer to numerous editions of The Radio Amateur's Handbook, RSGB VHF/UHF Manual and The ARRL Antenna Handbook for this common usage.
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