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hardline



>What i don't understand is why people are designing antennas and perhaps
>also PA's, to be 75 ohms?  At least then, the mismatch isn't on both ends
>of the feedline.  Maybe the engineers around here can explain that one to
>me??

It turns out that all coaxial cable has minimum loss if designed for a 70
ohm impedance and maximum power-handling capability if designed for a 35 ohm
impedance. Traditionally, equipment that transmits significant power, such
as transceivers and their antennas, is designed for a compromise impedance
of 50 ohms so that the coax can be smaller and cheaper for the power
handled.

CATV systems were designed for 75 ohms to minimize loss in the coax as the
amount of power sent down the cable is minimal. This was a compromise to
allow use of a simple 1:4 balun to convert 75 ohms to the 300 ohm input on
most TV sets in the 1960's. Consequently there are 2 types of coax available
and if you use one on the other type of system you will have matching
problems.

75 ohm coax may be easily matched to a 50-ohm system at each end with a
low-loss L-network, but it only works over a narrow band of frequencies.
Transformers will work over a wider frequency range but have a higher loss
at VHF and UHF. Unless you are reducing cable losses by 2 dB or more when
converting to hardline the loss in the transformers isn't worth it.




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