Hi Mike.
Mike
Gary mentions that his resistance is directly connected to the meter and is
non-reactive .
There wouldn't be any reflected power in this situation and the load and tuner
output would be at 1000 ohms.
I don't know if you have been following last two weeks discussion on the Bird
43(tm ) mechanics but it has been interesting.
---
Ron
Mike wrote:
> Gary wrote:
>
> > By adjusting the tuner for zero reflected power on the first watt meter
> that
> > leaves the drake watt meter and a 1000 ohm resistive load that should have
> > no reflected power.
>
> Huh, 1000 ohms is a bad mismatch against a 50 ohm characteristic
> impedance, whether its purely reactive, purely resistive, or some
> combination of the two. I would expect the second meter to indicate a
> large amount of reflected power under this condition. In fact a 1000
> ohm load produces a VSWR of 20:1 in a 50 ohm system. This
> corresponds to a voltage reflection coefficient of around 0.9. This
> corresponds to a power reflection coefficient of 0.9^2 = 0.81,
> which is consistent with your measurement of 100 watts forward
> and 80 watts reflected. Remember the 80 watts isn't real after the
> first couple of bounces of the RF signal. The reflection from the
> 1000 ohm load combines with the incident wave at the tuner output
> such that the net steady state impedance seen by the tuner is 1000
> ohms (I am assuming very short cable lengths). The tuner then
> transforms the 1000 ohms into 50 ohms so that the first meter
> indicates a 1:1 VSWR.
>
> >
> > With 20 watts indicated on the first watt meter the drake watt meter
> showed
> > about 100 watts forward and about 80 watts reflected. The difference in
> > forward and reflected readings is the power delivered to the load. But
> there
> > should be no reactive component in the load in this setup. There should
> not
> > be any reflected power from the load. There should be no "apparent power".
> >
> > The reading of high forward and high reflected power on the meter has to
> be
> > meter error when not operating at the design impedance.
> >
> > This brings us back to the question of how do we determine how much
> > reflected power we have, if any, when looking at a watt meter?
> >
>
> When it comes to amateur systems where the rise time of the RF envelope
> is long compared to the settling time of the transmission line's transient
> response
> due to reflection/re-reflection of the RF signal, reflected power is a red
> herring. It only has useful meaning with respect to the transient response
> of the RF waveform. As it turns out, there is a direct mathematical
> relationship
> between this reflected power, VSWR, and load mismatch. Wattmeters really
> indicate power delivered to the load and VSWR/reflection coefficient as
> expressed in terms of forward and reflected power. The concept of reflected
> power flowing back into the source is misleading in this context and should
> be avoided.
>
> > In this case with the 1000 ohm resistor there should be no reflected power
> > in the circuit. This indicates a gross error in the meter reading.
> >
>
> The meter readings you observed are consistent with the amount of power
> that would be initially reflected if a 100 watt signal traveling along a
> long 50
> ohm transmission line encountered a 1000 ohm mismatch. These readings
> are also consistent with the 20:1 VSWR that is created when a 50 ohm
> line is terminated with a 1000 ohm load.
>
> 73 de Mike, W4EF.............
>
> > Gary K4FMX
>
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