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Originally Posted by andrew_wallasey
Depends on what size of coax is used.
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Spot on, also there are 2 seperate issues here. The coax between node and DA/DP and the coax from DA/DP to the customer.
Most of us are now aware that distance from the home to the cabinet and level available at the tap port affect what coax is used for the drop cable to the customer. Also remember that attenuation at higher frequencies will also affect the choice of cable, to put it into simple terms a customer with x amount of level with a cabinet outside his front door compared to a customer with x amount of level and a far away cabinet would need different cable. The customer with the cabinet directly outside would not be affected as much with slope attenuation as frequency increases.
(The type of cable used has very little affect on attenuation at the return path/US frequencies because the frequencies are low)
The choice of cable between cabinets is a similar situation, the original ntl areas can generally use around 2 or 3 different sizes, and the choice is more to do with distance between cabinets than levels. The Distribution amplifier not only has to amplify the signal but also has to compensate for the slope by amplifying more at higher frequencies. The amplifiers have a device known as an "cable equaliser" plugged in to compensate for the cable loss. The frequency response of the equaliser would be exactly the opposite of the cable it is compensating for. Another point to consider is how many times they are splitting the signal from the Node/DA cabinet to feed other cabinets, every time you split in the real world you will lose aalmost 4dB. Sometimes the split is not equal as one cabinet may be much closer than the other to be fed, in this case they could use a directional coupler. If it was say an 8dB directional coupler there would be about 1.5dB loss in the through leg and 8dB on the tap leg. All of this would depend on the planning and topology of the area, but generally cabinets further away would have larger hardline coax (750 or 860 series) to keep the slope attenuation within a manageable figure, and cabinets closer would use smaller hardline (500 series) for cost efficiency.
The HFC network is far more complex to understand than first appears, there are lots of other consideratiuons for derating amplifiers in cascade, cascade noise figures, and thats without even thinking about all the return path headaches.
