Scott Huffman writes in a message to Joe Nicholson
 SH> I was referring to the fact that direct current gets attenuated 
 SH> at a much greater rate than alternating current. Every textbook 
 SH> since the 1900's mentions this. But not one of those books 
 SH> has ever explained.. WHY.. (according to the textbooks, if 
 SH> you take a DC signal, and an AC signal of the same power and 
 SH> send them along a line, tha AC signal goes much farther). 
I don't think you read that right.  (Remember that you can have the same 
power transfer with high voltage, low current and high current, low voltage 
situations.)  The main loss is resistive, which is the same for both AC and 
DC.  The convienience of being able to use transformers to convert AC power 
to high voltage/low current and back again is the controlling factor.
  
Theoretically, if AC and DC *voltage* were the same, DC power transmission 
should be less lossy, because you have no capacitive line-to-line loss with 
DC, as you do with AC.
  
However, it is very difficult to transform the DC to practical voltage levels 
for home and business use.
  
 SC> It's worse than "IR" - it's I SQUARED times R.  But you knew
 SC> this, right?
 SH>  
 SH>  For power loss, yes, but for the voltage drop across miles 
 SH> and  miles and miles of transmission line, it's simply IR. 
The controlling factor is the power loss, I squared R.  You don't give a 
rat's patootie about the voltage drop on the line - only the efficiency of 
*power* transfer from one point to another.  Trust me on this one.  Now go 
back and read my previous message again.
  
 SH>  It wouldn't be too advantageous to start out with 120 VAC 
 SH> at  the Hoover Dam and wind up with a trickle of 2 or 3 VAC 
 SH> in  Southern California after traveling across the desert. 
You still don't understand that the *current* on the line is the controlling 
factor.  Please do the math with the same resistance transmission line (try 1 
ohm for the transmission line and a load of .11 ohms for the 110 volt case 
and 110,000 ohms for the 110 KV case) at both 110 volts at 1,000 amperes and 
110 Kilovolts at 1 ampere.  You have 1/1000 the current in the line with the 
higher voltage and therefore 1/1,000,000 the power loss.  In both cases, you 
have the same power applied to the circuit.
  
In the 110 volt case, you couldn't even get the rated current into the .110 
ohm load because the 1 ohm in the transmission line would suck up almost 91% 
of the power!
  
Let me give you a hint, the 110 volt case would give you 99.1 amperes at the 
load or 1080 watts at the load.  the 110KV case would give you 0.999990909 
amperes at the load or 109,999 watts at the load, a loss of only 1 watt!  
These are extreme cases, but you can see the difference that the amount of 
*current* that a transmission line is asked to handle makes.
  
 SH>  Since power in equals power out, it's best to have a high 
 SH> voltage  and a low current, and reduce IR "drop" along the 
 SH> lines. 
  
As you can see from my math above, power in does **NOT** equal power out of a 
transmission line.  The voltage drop across the resistance of the line heats 
up the air around the line.  No other practical work is done.  In the 
hypothetical 110 volt case, you couldn't even force the rated power to the 
load unless you raised the input voltage to nearly 1100 volts - you would 
then get 110 volts across the load!
  
All of this holds true for transmission lines from antennas to our scanners 
too.  The higher the impedance of the scanner and the higher the impedance of 
the antenna and transmission line, the less signal is lost from the antenna 
to the scanner.  In other words, all else being equal, a 75 ohm 
antenna/transmission line/scanner would deliveer more signal power to the 
scanner than a 50 ohm setup.  50 ohms is used because that is a practical 
standard for practical antennas.  In other words, most resonant antennas 
present something around 50 ohms as a natural impedance to the transmission 
line without any kind of impedance transformer coils or baluns being 
necessary.  In fact some scanner hobbiests will use 75 ohm lines if their 
antenna to scanner run is very long.  They take a VSWR hit at both ends of 
the circuit, but even with that, the transmission is more efficient in the 
higher impedance line over the long length.  With a short line, it doesn't 
matter as much.
This is my last post on the matter, if you want more tutorial, send me 
netmail at 1:282/24.1.
 
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