Friday 7 August 2015

Sandford Wattmeter

Just made up the Sandford Wattmeter kit from Kanga UK, which has been sitting in my "in tray" since last year...

This meter uses some 50 Ohm thick-film resistors to form a 50-Ohm dummy load and a conventional detector to drive the meter.

I made up the instrument according to the clear instructions - with the exception that I substituted a 1N5711 for the 1N4148 specified in the words and music (as I intend to use the Sandford's detector in some comparisons with a digital meter I've been playing with).

You can see the thick-film power resistors and the (blue) 5711 diode here...

My diode substitution required me to make a little modification to the metering circuit to allow the system to be calibrated correctly - but everything was soon back on track. The instructions give a nice little wheeze for calibrating with a d.c. source - useful for those with limited test equipment.

I needed some application to test the new meter on - so thoughts turned to the recently-acquired Patriot...

I measured power output generated on key-down in CW mode across both 40 and 20m bands and show the results in the graphs below...

This is a neat little meter and a nice addition to the test bench - certainly easier than getting out a dummy load and hooking up to the scope (or choosing a dummy load with an integral detector and connecting to a multimeter) and doing the math.

Perhaps not so much fun, though!

...-.- de m0xpd

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