This letter is in response to the second sentence in the second paragraph of the October 2001 Product Update (see "Coaxial cable: behind the video," page 58). I thought it was a great article, but I found this one theoretical "boo-boo." It should read that the load impedance equals the source impedance for maximum power transfer. If you can vary the source, then it equals zero for maximum power transfer.
However, the problem with transmission lines is the reflection off an unmatched line, either the source or the load direction. In any unmatched transmission line, the video signals are reflected back-and-forth and can result in false triggering of digital circuits. To a lesser degree, an increase in voltage standing wave radio (VSWR) causes a slight increase in the coaxial cable attenuation.
With both source and load matched, there are no reflected waves in either direction; and therefore, no chance of false triggering. So, maximum power transfer, although important, must sometimes bend to other system requirements.
Raymond P. Meixner
RF/Microwave consultant
[email protected]
A source for the article replies:
The only way I would modify the sentence in question would be to address signal reflection separately. The whole point of optimizing a transmission line is to maximize the signal at its destination. Matching the source impedance with the load impedance is how to accomplish that. In the case of a transmission line, the cable is the load. Reflections are created at points of impedance mismatches. Some portion of the signal is reflected at mismatch points, thus reducing the signal at the destination.
Cablofil a cable-tray player
The article "Cable trays bend for on-the-job retrofits," (October 2001, page 33) did not reference Cablofil. Cablofil is an active player in the industry; we recently partnered with ETL Semko to measure the impact of cable tray on telecommunications cables.
The article talked about the "European flavor" of cable tray, but did not mention that Cablofil, Metal Deploye developed wire-mesh cable tray in Europe. In fact, Cablofil developed wire-mesh cable tray in 1971, debuted the technology in the United States in 1995, and holds the original patents for wire-mesh cable tray and accessories.
Dawn Campbell
Marketing communications manager
Cablofil Inc.
We appreciate the opportunity to publish this information on Cablofil's history and current endeavors in the cable-tray industry, and this information was not intentionally omitted from October's Technology feature.
The reason that Cablofil's information was not included is that the Technology feature, unlike our monthly Product Update article, is not meant to be a comprehensive round-up of all the products in a given class. Rather, the Technology article looks at a representative sampling of vendors, and focuses on the technological innovations and product improvements they have brought to a class of products.
Our Product Update is meant to cover all products of a particular type. This is why we include a product table with the Product Update, and not with our Technology features. -Ed.
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