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All,

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???? Would a simple diode detector be a (relatively) accurate way to measure RF power?

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???? I suspect my o'scope is reading RF incorrectly.? I have 4 different x10 probes and with each one I get dramatically different results, up to a 30% difference.? So, I can't trust my scope!!

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???? Thus, measuring RF power presents a problem to me.? I don't have access to any calibrated power measuring devices, but am quite confident of the calibration of my DVM.?

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???? Or, if diode detectors are inaccurate, do any of you have any tried and true ways to measure RF power with a decent degree of accuracy...say within 10%?

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W2EB

Bill Lazure

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Bill

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I am surprised with scope probe inconsistency and don't know them well enough to comment. Maybe they are optimal at different frequencies.?

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I built a diode sensor per W7ZOI instructions. It was embedded in a QST Article on a transceiver. Maybe it's also online. Uses a silicon diode for few watt measurements, and if needed a as second Schottky diode for lower levels. I certainly think it's accurate to better than 10 percent mostly determined by diode forward voltage variation. Built with a coax connector for its input. Certainly good for HF.?

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For measuring very small RF power someone designed an accurate with a specific op amp. I sense you don't need it for this application. I recall this was Joe of njqrp.?

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Let me know via direct email ic you can't find it.?

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Curt

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Hi Bill,

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You didn't tell us what frequency range you were trying to measure, and what power range.? ?This can make a difference in how you can measure accurately.? ?Also, are you trying to measure the RF power that is already being terminated into a 50 ohm load?? ?If you were trying to measure with a 'scope you would have needed to do this as the scopes have high impedance inputs.

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You're attempt with your oscilloscope will work assuming that you're probes are working correctly, and you scope bandwidth is wide enough to measure the RF energy directly.? ?If you have a Rigol scope of something similar, you're scope might have a calibration point on it that you can use to test your scope probe for both amplitude and correct compensation.? ? On the Rigol 'scopes is to the right of the BNC input connectors.? ? It should not be that hard to determine which probes are working correctly.? ?This is a common problem, as probes take a lot of abuse!

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Your idea for using an RF probe is also a good one and allows you to measure the RF energy as a voltage.? ?One thing to keep in mind is that a diode detector provides a peak voltage, which needs to be converted to RMS voltage as part of the power calculation.? ?This is important if you're using a voltmeter (or oscilloscope)? to measure the detector output directly.? ? ?Curt mentioned a circuit by W7ZOI, who is always a great source of information.? ?HIs book Experiential Methods in RF design shows a simple QRP power and an RF? probe on page 1.16.? ? ?W7ZOI also published an accurate power meter in the June 2001 QST that uses a modern logarithmic detector IC for a wide range power meter.? ? I would recommend this article along with the EMRD information as he shows various methods for setting up a power meter into the RF Chain.??

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Let me know if you have trouble finding? any of these references.? ? ? ?

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73

Jim N8CH

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Hello Bill,

I almost did not want to respond because I consider myself a "novice" still around all these experts.? But I decided to write anyways.? Just consider what I say with a grain of salt.

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I used to get a lot of private emails from a big Elmer on here named Chuck Carpenter (SK).?? He was a big fan of the diode peak detectors and "RF taps" to measure RF power.? I wish I still had his emails.

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Take a look at the RF probe from qrpguys.com

And they have an attenuator at qrpguys.com as well approved by Chuck as well.? The 10/20/30dB 50ohm attenuator.

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kc9on had a smt version of a rf peak detector/dummy load for QRP.? i built it.? my first SMT project.?? Not sure why KC9ON stopped making this kit.

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You probably know this already, so just ignore me for the silly questions:

- Did you compensate your probes?

- Lose connections on the cable?? (has been issue for me in the past)

- Cat or dog bite your cable?? (has been issue for me in past as well)

- 1M Inputs??? (Most scopes do not have 50ohm inputs)

- Probes and Scope on correct setting of 1X vs 10X setting?

- If scope is 100 MHz, are you using 100MHz probes?

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What frequency are measuring?

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Unless, you have 50-ohm inputs, they can only take so much power, so you may need to attenuate via an attenuator.? Best to read the manual to know how much the oscilloscope can take.

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I know in the Electronics of Radio book for NorCal 40A CW Transciever Kit for the Questions.? It wanted to use instead of a probe, a BNC tee - one end to scope and the other to a 50-ohm termination (dummy load).

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73 Daniel KK4MRN

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All,

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??? I am making my measurements at 14.050 MHz.?

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???? I recognize this is a QRP site, but I'm trying to measure a wide range of power: mW to 100+W.?

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???? The measurements I've made so far have all been terminated with commercial dummy loads of 50 ohms, and all of the probes, operated on X10, have been properly compensated.? I made a "port" that allows me to clip the probes onto the coax center conductor while clipping the probe's ground to the shield of the coax between the transmitter and load.

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???? My scope displays Vrms and it agrees with the actual trace amplitude.? I plug that into the formula Vrms^2/50 to determine power.? When using the simple diode detector I made, I assume what I'm reading is very close to Vpeak, so I use the formula Vpeak^2/100 when using it.

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??? My gut says that the simple diode detector should provide some very accurate readings of Peak Voltage (within the limits imposed by the power level and diode selection) that can then be computed into power very easily.? Opinions:? would a 1N4007 be OK to detect/rectify 100W??

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???? However, I will try again to compensate all of the probes more carefully, and use the included Probe-to-BNC adapter I recently discovered to eliminate any impedance bumps I may be introducing by clipping the probes onto my homebrew "port".

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???? Given a choice, I'd rather use the scope for its wider voltage handling capability than a diode detector, but if I can't easily resolve the scope's inaccuracy, I'll have to rely on a couple of diode detectors using different diodes for the varying power levels.

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Bill

W2EB

East Syracuse, NY

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