Marc-Oliver Hentschel, market segment manager, Draka Comteq analyses the debate over which right
transmission length is best for broadcast situations.

There has been much recent industry debate around the issue of 'ideal' transmission lengths of coaxial HD cables. In our opinion the right transmission length is the one that's best for practical use in real world broadcast situations, and not specifically under 'ideal'conditions. Draka Comteq's track record has always produced future-proof cable solutions
Our pragmatic approach was validated from our user experiences during last year's HD broadcasts of the World Cup Football competition. And in many other real-life situations with users. But let's look at a few of the practical considerations that form our belief that the truth about transmission lengths must be based upon facts and not on marketing hype or theoretical perfect conditions of use.
The maximum transmission length of a cable essentially depends solely on the attenuation value at the frequency to be considered. The attenuation values of coaxial cables are determined by:
• The diameter of the inner conductor and the way it is constructed
• Braid (braid angle and diameter in proportion to the diameter of the dielectric insulation)
• The construction and thickness of the foil
• Dielectric losses at high frequencies (loss factor tan _).
If the physical conditions are identical with different manufacturers, it will be the same for the electrical properties. Very often, thin inner conductors are compared with thick ones. It is important that when cables are compared, the same inner conductor size should be used.
During the 2006 Soccer World Cup and the requirement for 1080i HDTV signals, a number of tests have been conducted from reputable independent institutes, appliance and broadcast van manufacturers concerning the issue of transmission length.
It has been shown that maximum cable length measurement (x1) is subject to certain conditions:
• Laboratory conditions (constantly low humidity, constant room temperature, etc.)
• New, optimum condition of the assembled cable
• Both cable and connector have been tuned
A further HD test showed that a vtr with several outputs had different transmission lengths at the individual outputs. At one output, the 0.6/2.8AF cable achieved 70m whereas at another output 90m could be reached. Even an exchange of cables did not lead to different results. Here the different hardware at the output becomes apparent.
The transmission length has additionally been tested by a renowned institute which estimated the maximum length of Draka's 0.6/2.8AF cable at 90 m.
But which value can a user depend upon for the calculation of transmission length? To be definitely on the safe side our advice would be to reduce the calculated transmission length by 10%.

A question of definition?
We are in a precision business so to be realistic about this issue of 'longer' transmission lengths, the real quality of a cable can only be accurately represented by the following conditions:
• The homogeneity of the inner conductor, dielectric and outer conductor
• The impedance with the lowest tolerance (typically 75ohm ±1%)
• (Lowest tolerance means optimum adjustment, i.e. lowest transmission loss)
• A high screening factor (typically > 100dB)
• (High screening factor means protection of the signal against noise from the outside. This is reached by a double-laminated aluminium foil plus an electrically optimised braid relating to the transfer impedance).
So our advice to customers is not to be sidelined by the issue of longer transmission lengths under 'ideal' conditions but to consider the other parameters which together make up a quality solution for the broadcast industry as we move into an HD world.
Draka Comteq's 0.6/2.8AF cable has proven itself in the field over the last 25 years and has been a dependable performer for four generations of broadcast evolution. More than a question of definition, it's also a question of the industry acceptance of a future-proof, reliable component in the broadcast chain which continues to get the job done in less than ideal conditions to the highest possible standards.

Your Connectivity Team