[Elecraft] Accuracy of Wattmeter W2 under 2 Watt

[Ron D'Eau Claire]

The DL1 can be very accurate. As a QRP and QRPp enthusiast I have a homebrew
version that goes back decades before Elecraft, Hi! 

A simple RF voltmeter is a common way and inexpensive to measure very small
power levels. (For the more exotic measurements a bolometer or, optionally,
an accurate well calibrated oscilloscope is often used.)

The RF voltmeter system (which the DL1 uses) gets inaccurate as you get down
into the barrier voltage range of the diode. In that range the response of
the diode is no longer linear. My homebrew version uses a 1N34A Germanium
diode which is darn near "unobtainium" these days but a good PIN diode such
as Elecraft supplies in the DL1 is just as good. Both have a barrier voltage
of about 0.2V. 

But you can make very accurate measurements even at very low levels where
the barrier voltage makes the system non-linear with a little more effort.
Instead of applying RF to the device, connect an adjustable dc power source
of about a 1 watt. Use your accurate DMM to measure the voltage and current
applied to determine the output as you vary the d-c power applied.  The d-c
power reading is the same as the RF power reading from such a device. So, by
creating a calibration chart using accurate d-c power levels, you can now
measure similar RF power levels with excellent accuracy.

73, Ron AC7AC 

 

-----Original Message-----
From: Elecraft [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Rick
Robinson
Sent: Sunday, October 25, 2015 2:35 PM
To: Hajo Dezelski
Cc: elecraft at mailman.qth.net; Steih, Uwe
Subject: Re: [Elecraft] Accuracy of Wattmeter W2 under 2 Watt

I don't have the paperwork in front of me but the Elecraft DL 1 dummy load
has provisions to measure power up to 20 watts with a 10%  accuracy. Just
use a dvm. They also make a WM1 that handles more power with a .5 dbm
accuracy.

On Sunday, October 25, 2015, Hajo Dezelski <dl1sdz at gmail.com> wrote:

> Hello,
>
> I have been using a wattmeter from Fox for powers under 1 Watt, but 
> the display is too nervous and hard to read. So I stumbled over the 
> Elecraft W2 and read that with the 200 W sensor, you can read also 
> power levels under 2 Watt. This would be really helpful for I 
> experiment with power levels down to 0.5 mW.
>
> So before I invest, let me just ask some questions:
>
> 1. Has someone compared the power indicatores for the levels of 100 -
> 1000 mW with a precise wattmeter? As far as I understood each led 
> signifies 100 mW.
>
> 2. If I would use the computer readout: How precise is the output?
> Just 100 mW or ist it e.g. 147 mW?
>
> 3. I am working max. 10 Watt with the Kx3, but normally use only 
> between 100 mW and 5 W max. Could I adjust  with the calibration of 
> active sensor down to let's say 1 Watt instead of 2 Watt?
>
> 4. Is it possible to modify the transformer T1 and T2 of the sensor so 
> that the range would be (20W - 2 W and 200 mW. Or is there a sensor 
> like this available on request from Elecraft.
>
> I know a lot of questions, but as far as I can see this meter would be 
> a real champion for QRPP and could be used with a WSPR TRX.
>
> 73 de
> Hajo (DL1SDZ)
>
> ---
> Cela est bien dit, mais il faut cultiver notre jardin.
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--
Rick, W8ZT

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