On Nov 8, 11:58*pm, ashwanthh
wrote:
Hi all,

For my RF project, I want to measure vswr readings, in order to
that I was planning to construct a VSWR meter. The concept of VSWR is to
calculate the ratio of Voltage transmitted to the Voltage reflected. So
my basic question is,

1. What sensor/instrument is used to measure the voltages from the coax
cable(in my case)?

Please some one help me with some ideas

--
ashwanthh

What is your "RF project"? What frequency or range of frequencies?
What power level?

Pretty much all SWR meters don't actually measure the forward and
reverse voltages directly, but deduce them from measurement of the
voltage and the current on a transmission line at a point along the
line. Then, knowing that the forward voltage divided by the forward
current (including phase) equals the line impedance, and also equals
the reverse voltage divided by the reverse current, you can deduce the
forward and reverse voltages (and forward and reverse currents). You
use the line impedance in the calculation, and if you've assumed a
line impedance different from the impedance of the line you're
actually measuring, you'll get an answer that's not correct for your
actual line. The "calculation" is commonly done with analog
electrical parts in typical ham SWR meters, and there is generally a
way to adjust the parts so that the meter reads correctly for the line
impedance you want to use--but you must actually do the check/
calibration or you won't know. (The calibration may involve just
adjusting a variable part, or it may involve replacing a fixed-value
part with one of a different value...not nearly so easy to get it
right.) Also, the accuracy of many ham-type SWR meters suffers if you
don't operate them at the right power level, because of non-linearity
in the detector diodes; there are ways around that (e.g. using non-
diode power detectors or operating the detector at a fixed full scale
range).

It's also possible to measure just the voltage at a few appropriately-
spaced points along a line and deduce the SWR more directly from those
readings, but this is very seldom done in practice. See "slotted line
measurements" for more details.

Cheers,
Tom