Jim,
What "bugs" me most about this issue is that it isn't Rocket Science -
simple application of Ohm's Law will give you the answers.
Take the figures for Wireman 553 ladderline which has #18 conductors:
100ft of #18 copper wire has an RF resistance of 8.59 Ohms at 10MHz
If a current of 1A flows into 100ft of that ladderline, the power loss
will be 8.59W in each wire - a total of 17.18W
Because the surge impedance of 553 ladderline is about 400 Ohms, the
applied power must be 400W to drive the 1A current
So, 400W applied and 17.18W copper losses gives us a matched loss of at
least 0.19dB; that's about 2.4 times the Antenna Book figure of 0.08dB.
It's a much bigger error than can be explained by any "rounding".
Some may think that's an insignificant difference and that we are
"nit-picking"; but now imagine you are running a long length of that
ladderline at high VSWR based on an Antenna Book loss prediction of 2dB
- would you be happy if you discovered that the real loss was at least
4.8dB? Or that what you thought was a 4dB loss was actually nearer 10dB?
73,
Steve G3TXQ
On 27/01/2012 17:49, Jim Brown wrote:
On 1/27/2012 5:10 AM, Steve Hunt wrote:
The ladderline loss figures quoted in the Antenna Book - which I believe
are also repeated in TLW - are out by a factor 2. It's very easy to
demonstrate that with a simple I^2R loss calculation. I'm happy to show
the sums if you need convincing.
One of the major deficiencies in published data for transmission lines
is good numbers for loss and shielding effectiveness in the MF and HF
spectrum. Loss is most often quoted in dB/100 ft and rounded to one
decimal place. This yields values of 0.1 dB/100 ft and 0.2 dB/100 ft for
most coax at 1 or 2 MHz, which is lousy resolution (0.1 dB is the result
of rounding between 0.05 and 0.149; 0.2 the result of rounding from 0.15
to 0.249). Now, loss at 1 or 2 MHz is quite difficult to measure, first
because it is small, but also due to termination effects (that is, Zo is
not a constant 50 ohms, but rather varies with frequency and is
increasingly reactive below HF, but generators and loads are a pure 50
ohms resistive). I've attempted it, and you've got to measure at least
500-1,000 ft to get reasonable precision. Why does this matter? Several
reasons, Some hams need to run 500-1,000 ft to their antennas, so the
difference can matter. Second, many hams are spending big bucks for
premium coax when they may not need to.
|