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
On 11/17/2013 2:25 PM, Don Wilhelm wrote:
In addition, many wattmeters are woefully inaccurate when reading SWR.
Most All will be quite accurate for an SWR of 1.0, but at anything other
than that, you will find a variation in the readings on amateur grade
wattmeters.
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