Thanks Don! I aggree with you. I used to calibrate RF-electro-surgery devices in a former life :) Thanks for the tip on the non-reactive power resistor. I think I will look to get one.
Good stuff!. Duane On Mon, 20 Jul 2009 21:07 -0400, "Don Wilhelm" <[email protected]> wrote: > Duane, > > While you are correct on the meter movement specs, take a hard look at > the wattmeters available to hams. > > The LP-100 wattmeter does almost what you are referring to. It is quite > accurate when calibrated. > > I have a general mistrust on analog reading wattmeters. Many wattmeters > on the ham market are speced for 20% of the full scale reading - and > that means an error of up to 20 watts on a 100 watt scale (often the > lowest scale). > Even the revered Bird wattmeter is speced for 5% of the full scale > reading right after calibration, and that is up to a 5 watt error with a > 100 watt slug - and how many Bird wattmeters in the hands of hams have a > current calibration sticker? > > The Elecraft W2 wattmeter promises to make the power measurement > capability better, but it is not yet available (ship in August was the > target). > > You can certainly do better than that with an oscilloscope to read the > RF voltage across a precision 50 ohm non-inductive resistor. Caddock > makes 50 ohm Thick Film Power resistors that are good to 100 watts with > a heat sink - attach one to a heat sink salvaged from a defunct computer > CPU cooler and attach a BNC connector with zero length leads and you > will have an inexpensive precision dummy load that is flat to at least > 200 MHz. Ridge Equipment (use Google) often has dummy loads at a very > inexpensive price that are good to at least 200 MHz as well. So for > under $20, you can have a piece of precision equipment that is the basis > for power measurement (just measure the RF Voltage across the precision > dummy load and compute the power). That device can be used for > calibration of wattmeters, and the result should be within 5% even > allowing for errors - and the results have nothing to do with the "full > scale". > > 73, > Don W3FPR > > dw wrote: > > If I am correct, most manufactures claim a 10% tolerance on most > > d'arsonval type watt meters. > > I believe there are also some caveats. > > That 10% is at a specified level of applied wattage, and of course > > applied into a 50 ohm load. > > This means that at 100 watts applied power, the meter can read +/- 10 > > watts. > > This however, does not take into consideration the non-linearity > > characteristics of a d'arsonval meter. > > And so the manufacture will take a specific meter as his prototype and > > mark various wattage readings up through its range. > > And those physical points will become the template for the numbers > > displayed behind the meter when it goes into mass production. > > If he is really fussy, he will take 10 of those meters on the prototype > > bench, and record their physical readings and then make his template > > based on the average of those ten prototype meters. > > > > We then have to factor in the variance of tolerance for every d'arsonval > > meter assembled into future boxes on the assembly line. > > What we might end up with then, in the real world, is a meter, when > > activated by the user, points to various numbers on its display with an > > inaccuracy that may be 20% or more, based on its individual mechanical > > response to various wattages up its range. > > > > The d'arsonval meter is driven with a series resistor (usually a pot) > > from the rectified power source. > > A manufacture may be so inclined to visibly fix calibration pots with > > the eye during the assembly process, and then spot check the meter with > > 100 watts applied. If it falls within +/- 10 watts during the > > spot-check.....ship it......it meets its advertised accuracy. > > > > > > This is where the micro-chip can give you an edge up. > > The micro-chip can hold within its memory a table of values programed by > > the user. > > Remove the d'arsonval meter and its series-R and connect the > > micro-chip's analog input instead. > > As a calibrated applied wattage source is used, "teach" the micro-chip > > how to interpret the voltages it sees as the wattage is increased up the > > scale. > > Now you have a watt meter that is calibrated at multiple points up its > > scale instead of at a single spot-check point. > > A microchip watt meter can then approximate the accuracy of the bird > > watt meter. > > Since ( I think this is true ) the bird meter is also rated at a certain > > wattage point and that accuracy is (due to the use of a d'arsonval > > meter) diminished below that wattage point. > > > > Some day a smart ham is going to manufacture and sell a mico-chip based > > watt-meter which the user can calibrate himself using a bird meter, or > > better yet...an o-scope. > > Of course that is... if there is a marketable demand for higher accuracy > > than currently exists. :) > > > > -- [email protected] ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
