Bravo and kudos to Mr. Robbins for his comments below.

To elaborate, the equation to calculate the loss in a specific length of
coax when its loss per 100' is known for a given frequency is:

Loss in decibels = (published loss per 100 feet) (decimal number of
hundreds of feet in the system)

When the total loss of the coax in decibels is known...

Coax system efficiency = 1/10^(loss, total/10)

These equations are based on 1:1 SWR within the coax, and its terminations.
While this is not likely in the real world, nevertheless it is the basis
upon which the FCC sets and licenses the operating powers of AM, FM & TV
broadcast stations in the US.

RF

Visit http://rfry.org for FM broadcast RF system papers.

____________________________

"David Robbins" wrote in message
...

"Michael Melland" wrote in message
...
If a cable, any cable ..... for argument sake... say Andrew LDF2-50...
has
a
specified loss at.... say 1000 MHz.... of 3.52 dB in 100 ft... is there
a
way to calculate what the loss would be in different lengths ? Say for
50
feet ?

The cable above is an example.... but is there a rule for calculating
loss
for cables of any kind under 100 feet ? Would loss at 50 feet be
exactly
1/2 of that at 1000 feet ?


for a 1:1 swr the loss is a linear function of length. so the loss at 50'
would be 1/2 that at 100' (not 1000' as you typed above!). if the swr is
greater than 1:1 its not so straight forward.

oh, just fair warning... questions like this often attract nit-picky
trolls
who will turn this thread into a never ending discussion of why swr isn't,
and how lossy cables change the world... ignore them for your own sanity.