RadioBanter - View Single Post

September 16th 04, 05:14 AM

In article ,
says...

Hi,
I am contemplating a pep reading wattmeter so I can check the output of my
HB
amplifier. Consequently, I am putting it right on the output of the amp. The
output of the amp should always see 50 ohms because it will be feeding either
a
50 ohm dummy load, or an ATU tuned to 50 ohms. With that in mind, I am simply
using a resistive voltage divider, to get a voltage sample, and squaring it
with an AD633 multiplier. This seems like a simple, cheap way to get watt
info. assuming you will always be working into a near 50 ohm resistive load.
Anyone see any reason why this will not give you a pretty good indication of
your power output? I realize that the load may not always be exactly 50 ohms,
and that there are losses in the ATU.
The reason I have not tried getting a current sample and using the
conventional VI COS Theta with the multiplier is due to the additional
complexity of circuitry. It is also difficult to get accurate current samples
over a wide frequency range. By making everything resistive it somewhat takes
the frequency dependency out of the problem. Thanks.
73 Gary N4AST

Resistive dividers don't work worth a crap at rf, unless you use
non-inductive resistors, and then you must consider that your input
to the squaring circuit will have some capacitance to ground, who's
ability to screw up your higher frequency readings will be astronomical.
Why the squaring circuit?

Bottom line is this, all the resistive ladders I have played with
were worthless unless they contained capacitive compensation, which
can be found only by trial and error. I prefer a 'T' connection with
a good quality 'coaxial pad' of 10 db, then I feed it to my Tek 2215A
scope. It's deadly accurate from D.C. to at least 30mhz. More accurate
than any of my inline wattmeters, with the exception of the 'BIRD', and
that is only good across the spectrum of the slug I use.