[Elecraft] High Accuracy Wattmeter

Mon Feb 11 19:56:52 2002

I would suspect your problem is a shielding problem.  If your reading is
jumping all over the place then it's not well shielded.  Adding more
shielding should tame it as well as using temperature compensated components
such as NPO capacitors.
You can also use a small toroid with 5-10 tuns on it and have the center
lead of coax run through it.  This will act as an inductive pick-up.  Then
use a capacitor diode combination to turn the raw RF that is picked up into
dc which can then be read from a meter.  Calibrating this would be a matter
of using a small opamp to amplify the DC and vary the gain of the opamp so
your reading a true representation of power.  The problem arises as to what
will you use a reference to ensure your readings are accurate ie calibrated
wattmeter to ensure correct readings?
If you wish to discuss this further contact me offline.

I hope this helps,
Robert
VE3RPF

----- Original Message -----
From: "Charles Greene" <[email protected]>
To: <[email protected]>
Sent: Monday, February 11, 2002 7:29 PM
Subject: [Elecraft] High Accuracy Wattmeter

> Hi All,
>
> I need a high accuracy wattmeter for reading power from 1 to 10 watts into
> a 50 ohm load with a precision of 0.05 watts or less over the entire
> scale.  I tried a RF probe and a DVM, but the voltage is all over the
> place.  I have a scope, but reading that is like reading a meter.  If I
set
> the K2 control, it drifts up pretty much.  Anyone know of any circuits off
> hand that will do this?  How about a DVM connected to my OHR WM1?  I can
> accurately calibrate the voltage reading using a service instrument, but I
> can't use that for the power reading unless the power is going to the
> instrument which it isn't.
>
> TNX for your thoughts.
> 73, Chas, W1CG
>
>
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