[Tech-Assist] Marti STL funny readingsHere's a head scratcher.

[Cowboy]

On Tuesday 26 July 2011 01:09:14 am Ronald J. Dot'o Sr. wrote:

> Both thrulines show 24 W into a 50 ohm dummy load and almost double that 
> into the antenna.  I'm sure that the STL-20 isn't magically almost doubling 
> the TPO into the antenna.  Both thrulines show about 1 to 1.5 W reflected.

 Alan gave you the short version.
 Here's a little longer version.
 ( ignoring frequency, which compounds the indication )

 The ONLY meter that measures power is a calorimeter.
 That's because energy isn't power until it's dissipated, and a calorimeter
 measures power by dissipating energy.

 Some meters, such as an iron vane, measure voltage, current, and angle,
 so can indicate the energy in the line that will dissipate, and are calibrated
 to read the power that would be dissipated from that much energy.
 Assuming zero loss between the meter and load, they are accurate.

 Got it so far ? Good.

 Most "engineers" assume a line carries power, and that "power" is
 measured directly. The rest of us, being the small minority, acquiesce
 to this terminology, which does nothing to clear the confusion, but
 does comply with conventional wisdom.
 OK, even if this were true, we still need at least *three* parameters to
 measure "power" if we're using anything other than a calorimeter.
 We need three of voltage, current, phase angle, resistance.
 
 The thruline type of meter inductively samples current only.
 Using one assumes a true 50 ohms real resistance, and a zero
 phase angle, such as produced by a 50 ohm dummy load.
 These types of meters are incapable of sampling voltage, so
 voltage is ignored.
 If the resistance is not 50 ohms, and/or the phase angle is not zero,
 simple ohms law will tell you that the current ( and voltage ) is something
 other than what it would be if the load were a true 50+j0.

 All of this assumes that the transmitter is a constant "power" source,
 which of course, it is not. the transmitter will only make that power
 into a very limited range of load conditions.
 Due to reflected energy, the load seen by the transmitter is not
 the load it's designed to work into, so the transmitter is, in fact, 
 delivering more or less energy into the line.
 Additionally, reflected energy is re-reflected at the transmitter, and
 either adds to, or subtracts from, the incident transmitter power
 depending on the angle, or is partially reflected, and partially dissipated
 as additional heat in the final amplifier, or matching circuitry,or line.

 All of this taken together, tells us that power meters, the Thruline
 inductive type, or the voltage sample type built into some transmitters,
 are *only* accurate when the transmitter sees its designed load Z, and
 at no other time, under no other conditions.

 If the Thruline were calibrated in amps, it would be easily possible to render
 accurate power measurements into any known load, just as we do in AM radio.
 It's still possible, but the reverse math gets confusing quickly.

 As such, you have a few knowns. You know the approximate power output
 of the transmitter. You know the calibrated conditions of the meter, and
 you know the meter reading.
 Taken together, you know that the line is not matched at the point
 it is being metered, and so is not terminated in its characteristic Z.
 You do not know the antenna Z, unless you know the exact electrical
 length of the line between the meter and the antenna.
 It may be higher, or lower than 50, depending on what it's being
 transformed into by the line at the metering point.

 The directional nature of the Thruline does give an accurate relative
 indication. You've measured the line loss, so it is possible to use the
 forward and reflected readings, factor in the loss in both directions,
 and derive the true SWR mismatch at the antenna.
 Once you know that, you can then calculate the four possible limits
 of the mismatch.
 Obviously, it's worse than simply reading the meter would suggest.

 Additionally, this is all academic. You know the antenna is not matched
 to the line for whatever reason. Could be a bad antenna, could be a bad
 connector. That's all you really *need* to know to address the real problem.

 Hope this helps.....


-- 
Cowboy