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Simple HF Digital Wattmeter

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Jerry In Maine

New Member
I've seen digital panel meters like these for some time now:


3.5 DIGIT LCD PANEL METER, 0.2V SCALE
3.5 DIGIT LCD PANEL METER, 0.2V SCALE | AllElectronics.com


3.5 DIGIT LED PANEL METER, 200 MV
3.5 DIGIT LED PANEL METER, 200 MV | AllElectronics.com


I thought these might help me with my need for a RF Wattmeter for my HAM Radio shack. I though I'd place the transmitted RF across a 50Ω non-inductive resistor, then rectify the voltage across the resistors and apply it to the meter.

The schematic below shos the plan. Before I ordered parts I thought I put it put here to get comments to see if I've missed anything.

Any thoughts?

thanks
 

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RadioRon

Well-Known Member
One problem with this circuit lies with your use of an AC coupling cap to feed RF from the 50 ohm resistor to the diode. This will allow the DC level on the anode of the diode to change when RF is fed to the diode which will then change the duration of RF cycle that the diode is conducting. The resulting reading will be wrong in that case. It might be easier to simply not use the coupling cap and attach the diode directly to the 50 ohm resistor.

For HF detection, it might be best to use a schottky diode, but then you have to be careful not to exceed the peak reverse voltage of the diode. That depends on how much RF power you are planning to measure.

The DC voltmeter probably has a very high input resistance. In that case, the time constant of the RC filter after the diode may be too long and it might take a long time for a voltage reading to die off. Consider grounding the far end of the potentiometer and use a pot of some value around 5K or 10K. This puts the pot value as the main load for the filter and so the time constant will be quite reasonable.

Most RF wattmeters put a resistor attenuator between the 50 ohm load and the detector diode to help protect the diode from overvoltage, and also to allow several ranges of power to be switched in and out, by switching various attenuators in and out.
 

Leftyretro

New Member
Now a days one can built a much better, more accurate, wider range RF wattmeter utilizing the Analog Devices AD8307 log amp. I built a simple one that measures from -70dbm to +10dbm and seems to track within 1/2db or my HP 8601A sweep signal generators power output meter. With proper attenuation in front of the meter one can measure up to any practical power level they need to measure.

It really is a great device and worth the higher initial costs. I built many diode and other type RF power and voltmeters over the decades and none ever compared to the ease to build, ease to calibrate, rangeability and accuracy.

There are a ton of links using this chip, here is but one:

Digital Wattmeter with AD8307, PIC16F876, LCD Display

And for the device: http://www.electro-tech-online.com/custompdfs/2009/04/AD8307.pdf
Lefty
 
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