Paul Christensen wrote:
> > If all the reflected power was absorbed by the transmitter PA then there
> should
> > be no increased forward power reading on the bird meter whether there is
> > reflected power or not?
>
> I ran some tests this morning using my Ten-Tec Omni Six and Kenwood
> TS-850/SAT with auto-tuner. I used my Bird 43 wattmeter as the measuring
> device at the source end into 100-feet of LMR-400 and terminated with a
> 20-meter half-wave diplole which was removed to create an open termination
> during testing.
>
> First test:
>
> I used my Omni Six to establish that the antenna and line were working
> properly. I removed the antenna from the line and replaced it with a
> combination 50-ohm Bird Termaline load and wattmeter (Model 6154).
> Transmitting with the Omni Six at 100-watts as read on the Bird 43, the
> measured power at the line end read just under 100-watts. Reflected power
> on the Bird 43 measured zero watts. The lower reading is due to a
> combination of slight loss in the LMR line and the fact that the Bird 6154
> is only accurate to within +/- 10-percent or so.
>
> Next, I removed the load and transmitted into an open termination. The Bird
> 43 measured 100-watts forward power and just under 100-watts reflected (the
> difference being attributed to power absorbed in the line due to line loss).
> I am assuming that since the Omni Six, having a reasonably close 50-ohm
> resistive source, and feeding a 50-ohm characteristic impedance line, that
> all power is being absorbed by the Omni's PA section, less the line loss
> power absorption. Where else is the power going? As the forward wave
> reaches the unterminated end of the line, a reflected wave is produced, but
> exactly 180-degrees out of phase with the forward wave. The phase reversal
> is always 180-degrees regardless of the degree of mismatch. The amplitude
> of the reflected wave IS a function of the degree of mismatch.
>
> As the reflected wave travels back to the Omni Six, it too is travelling on
> a 50-ohm characteristic-impedance line and is terminated into the Omni's
> 50-ohm source. The reflected wave sees no mismatch and is completely
> absorbed by the Omni's PA.
>
> Second Test:
>
> I replaced the Omni with my Kenwood TS-850/SAT. I disengaged the the
> auto-tuner. Using the same procedure as with the Omni Six, my Bird 43
> measured 25-watts forward power and just under 25-watts reflected power
> (Again, the difference being attributed to line losses). Unlike the Omni,
> the TS-850 incorporates a SWR foldback/protection circuit to deliberately
> limit the output to a fraction of 100-watts. The degree of power foldback
> is a function of the the mismatch as seen by the transmitter's SWR circuit.
>
> Third Test:
>
> I engaged the auto-tuner on the TS-850 which was pre-adjusted for
> 20-meters. Since, the tuner will not tune into an open termination, I used
> the saved tuner value. I have no idea what complex network setting the
> tuner was saved as. The Bird 43 measured 10-watts forward and just under
> ten watts reflected. Again, without a load at the terminating end, the
> forward and reflected reading measure approximately the same. I tried this
> on several bands and the results are the same.
>
> Fourth Test:
>
> I re-connected the 20-meter dipole and tuned the TS-850 off the dipole's
> resonance frequency to produce a 3:1 SWR reading. With the auto-tuner
> disengaged, I read 35-watts forward power and approximately 10-watts
> reflected power.
>
> Next, I engaged the TS-850's auto-tuner and did not touch the power control.
> Now, the Bird 43 reads just under 45-watts of forward power and reflected
> power satyed the same at 10-watts. However, the output power on the TS-850
> still reads 35-watts. I suspect this is because the TS-850's SWR circuit is
> not a true directional coupler.
>
> My conclusions:
>
> 1) As expected, the Bird's directional coupler adds the amount of reflected
> power to the forward power reading. Caveat: the addition is only true up to
> some limit of mismatch. Into an open termination, no power is being
> absorbed by a load. At this extreme, reflected power does not add to the
> forward power reading.
>
> 2) The TS-850's power indicator is not based on a true directional coupler
> and hence the forward power indicator never adds the amount of refelcted
> power to its readings.
>
> 3) A 50-ohm resistive source, fed into a 50-ohm characteristic impedance
> line with an open termination, yields total absorption of the reflected wave
> into the source and hence all reflected power, less power absorbed in the
> line due to line loss. The Bird reads 100-watts forward power and close to
> 100-watts reflected power. Again. the difference being power lost in the
> line due to line loss.
>
> 4) Here's where things get complicated: A mismatched impedance from the
> source to the line (e.g., auto-tuner, ATU, pi-network), and transmitting
> into an open termination creates some intersting results. Power cannot be
> absorbed by an open load; there is none. Power can be absorbed by the line
> to the degree that the line has loss. If the reflected wave sees a complex
> impedance back at the source, the wave sees this as another mismatch,
> reverses itself precisely 180-degrees and travels back to the open
> termination. The amplitude of this re-reflection is also governed by the
> degree of mismatch at the source. Again, in this example, I have to
> conclude that 100 percent of the reflected power is absorbed at the
> transmitter (less line loss), despite the attempt by the transmitter's
> complex network (again, the Pi, ATU, or auto-tuner) to redirect the wave
> back to the unterminated load.
>
> -Paul, W9AC
If all the power was reflected back and absorbed by the ten-tec finals with an
open termination on the line then it would be dissipating the full 100 watts in
the final?
How about when there is no line at all connected? What about if the line was a
quarter wave length and open at the far end. The radio would see a dead short.
If the reflected wave was always 180 degrees out of phase with the forward wave
when the load is other than an open or short then there would not be any peaks
and nulls along the transmission line that we see.
With an open termination at the end of a line the voltage and current are 180
degrees out of phase with each other so there is zero power. If there is zero
power at the termination then there is no power to be reflected.
Adding the tuner in line between the watt meter and the transmitter changes the
impedance that the watt meter sees and therefore gives a different reading.
Could we conclude that the bird meter is not a true power measuring device but
dependent on matched impedance's? So really the only time that it will read true
power is when the impedance presented it is 50 ohms resistive.
73
Gary K4FMX
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