Antenna Analyzer AIM4170 page 24
For example, if the loss at 10MHz is 1.5dB/100feet, enter 1.5/SquareRoot(10) = 0.47
dB/100 feet for the loss. This attenuation value is close to the value at 1MHz. You can
enter loss as either a positive or a negative number, it doesn’t matter.
When calculating loss, the physical length of the cable is used. Although the physical
length of the coax may not be known with great accuracy, the loss has a secondary effect
and it’s not as critical as the electrical length. Using 120 feet or 125 feet for the loss
calculation will not affect the final answer very much, but the ratio of physical length to
the velocity factor is important (as mentioned above).
The nominal impedance, Zo, of the coax is very important. This can be found from the
manufacturer’s data sheet. It does vary from one manufacturing run to another and it
varies somewhat along the roll of coax. This value may be fine tuned by using a known
terminating resistance of a few hundred ohms to experimentally optimize the transformed
value. After the cable is characterized by testing the actual cable or an equivalent piece of
cable (ideally from the same roll), the transformed impedance values will be accurate to
within a few percent.
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