[Elecraft] Internal K3 SWR vs Extermal SWR Meter

[ab2tc]

Hi,

I am not aware of any "Wattmeters/SWR Bridges" that work by reading E and
estimating power as (E^2)/50 ohm. If it did, it would not be able to
determine any kind of estimate of SWR or reflected power. All the
"Wattmeters/SWR Bridges" I am aware of are constructed with a bridge circuit
that is in principle capable of correctly displaying forward and reflected
power even with a complex load impedance. But they do rely on the
performance of at least one transformer, often a capacitive divider for
sensing voltage and diode rectifiers with their inherent non-linearities,
especially at the low power levels preferred for tune-ups. These components
are often chosen for their lowest cost rather than performance and
calibration may be sloppy. Even with quality RF components, unless the
device has a microcontroller that can store and apply corrections for
frequency, power level and temperature, accuracy is generally poor, easily
exceeding 5-10% of full scale. High end devices that do much better than
that are available but they come at a cost. Nothing in the instrument can
compensate for the errors caused by common mode currents in the coax though
so it's always important to keep the RF out of the shack. Three instruments
in a line next to each other *should* read the same forward, reflected power
and SWR under ideal conditions.

AB2TC - Knut


k6dgw wrote
> Most ham-grade "Wattmeters/SWR Bridges" that I've encountered read E, 
> assume R, and "calculate" P in an analog fashion.  When SWR is not 1:1, 
> Z is not equal to 50 ohms, and in most cases, the higher the SWR, the 
> higher the power indicated on the meter.  For these instruments, nothing 
> you see on the indicator is true unless the SWR is 1:1 [i.e. the 
> impedance is 50 ohms].  Even then, many are sensitive to common-mode 
> current on the outside of the coax shield, and most of us have *some* 
> common-mode conduction on our feedlines.
> 
> I once put three ham-grade "wattmeters" in series right next to each 
> other on 20m, and they all read differently.  When I changed their order 
> in the feedline, they all read differently ... and different from what 
> they had before.  I concluded that, unless I was willing to spend the 
> money to acquire test equipment that actually sensed and displayed watts 
> and not E^2/R, I'd have to be satisfied with approximations.
> 
> 73,
> 
> Fred K6DGW
> - Northern California Contest Club
> - CU in the 2014 Cal QSO Party 4-5 Oct 2014
> - www.cqp.org
> <snip>





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