"Carl / KG6YKL" wrote in message
news:qtY8e.56016$lz2.20887@fed1read07...

I don't understand the question. Antenna VSWR doesn't change from
microwatts to megawatts (with the possible second order effect of the
megawatts heating the elements to the point that they expand in diameter
and length).

Well, if you don't understand the question it is only because I don't
understand enough to ask an intelligent question. When I look in the HRO
catalog may of the SWR meter ads have multiple power settings
(15/150/1.5KW). I assume that you simply select the appriate range that
is just greater than your rig. I was just wondering why the range
mattered. Why not just use the highest power range if the sensitvity
doesn't vary significantly with power.

Even if they cost the same, I'd rather build a nice kit than buy one. I'd
like to understand more about how they work and there is nothing like
getting your hands dirty to understand something.

Carl

VSWR is a ratio. It's the ratio between the power being sent to the antenna,
and the power being reflected back from the antenna to the transmitter due
to mis-match. Measuring VSWR is usually a two-step process. You first set
the instrument to measure transmitted power by adjusting a potmeter so that
the needle reads full scale. You then flip a switch to measure the relative
amount of power coming back. There's different versions that use two needles
or two instruments at the same time, but the principle remains the same.

The reason for having different power ranges is that first step of setting
the instrument to full scale. It would be near impossible to adjust with
1.5KW input if the instument was sensitive enough to also reach full scale
with just 5W. At least not with the parts used in a $20 Radio Shack model
;-) A slight error would smoke the instrument.. On the other hand, if the
instrument would be fixed to 1.5KW, the 5W handy would never get the
instrument to read full scale. So you have to manually pre-select the proper
range.

There's electronic versions out there that can auto-range but they are
usually not available to mere mortals.

If you want to build one, get the ARRL Ham Radio handbook, they usually have
one or two designs in it.

Rob