On (01 Apr 97) Charles Moody wrote to Ray Wade...
 CM> RW>Do you know the formula for calculating forward and reflected
 CM> power? It RW>has little to do with ohms law. And SWR is the ratio
 CM> between forward RW>power and reflected power.
 CM> ......................................................................
 CM> ..... I stand corrected, and thanks for being patient as I learn.
 CM> What has thrown me is the fact that many, and I do mean many SWR
 CM> meters and the like seem to need certain amounts reflected power to
 CM> move their needles.
As I have already said, many meters are innacurate. The biggest problem
with most is that they tend to *add* any reflected power to forward
power (and vice versa) thus showning incorrect power levels in both
directions (forward and reflected).
A Bird watt meter, because of its unique element design, *mostly*, but
not completely, overcomes this problem. That is why a Bird is considered
to be the best and is the instrument found in most repair shops.
 CM> .05 watts to 5 watts shows a flat match, but at 50 watts you get 1.2:1
 CM> On a MFJ antenna analyzer it showed my antenna at 1.2 across the whole
 CM> band. But using both a Micronta, and a Para Dynamics SWR meters it
 CM> took 45w to get enough refected power to move the needles.
Here the problem is with meter *range*. If your meter can measure 100
watts of forward power (full scale of the meter), then, for example, if
your true reflected power is 1/10 of the true forward power
(approximately a 2:1 SWR) and you have a forward power of 50 watts, your
true reflected power will be only be 5 watts or + or - 5% of the entire
meter range. Most *ammeters* (which is what the meter is in *any*
meter) have an *accuracy* of no more than 5% of the *full* range (in
this case 5 watts of the 100 watt range) . Thus you may not see any
movement in the reflected power position at all (error of - 5%)! Or, you
may see 10 watts reflected (the + 5% meter error)!
Bird watt meters utilize different elements to measure different
ranges. These elements can measure *full scale* meter movement from 5
watts to 5000 watts with *standard* elements. So, for example, if a
transmitter forward power is measured at 5000 watts (using a 5000 watt
element) and the reflected power is only 50 watts (1.07:1 SWR), you
couldn't *see* the 50 watt reflected power using the 5000 watt element.
But if you keep the transmitter power at the same level (5000 watts
forward) and put a 100 watt element in the Bird to measure the reflected
power, the 50 watts reflected would move the meter to 1/2 of the total
scale and show the true 50 watt reflected power.
Bird also makes *non-standard* elements that can measure even larger and
smaller power ranges.
 CM> HmmMmmm?  Big confused look on my face....
Again I repeat. If your SWR is less than 1.5 to 1, QUIT SCREWING with
the antenna! It is a complete waste of time!
... Confucius say: Man who run behind car get exhausted.
--- PPoint 2.00
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