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Lots of good ways to measure QRP power.? I like to sweep a load resistor at the desired frequency to make sure it's a pure 50 ohms resistive and then I use a scope with a carefully compensated probe.? With a nice sine wave, RMS power is a simple calculation.? Another way is to put a 50 ohm load resistor in a thermally insulated box, poke a small hole in the side of the box and measure the temp rise of the resistor with an IR temp gun.? Calibrate the measurement with a DC current thru the resistor.? Again, the resistor has to be pure with little or no reactance at the measurement frequency.? And paint the resistor black to get the emissivity near one.

Dave, kx3dx

You don't want to be connecting a VNA to your rig, ever, only to an aerial without the rig. Nearly all SWR meters measure power too. The ones for CB use a strip line sampler, so above 27 MHz they take too much signal (though you can cut it in 1/2 for 50 MHz or use about 1/3rd for 144 MHz. They can be adapted to measure power only on any band. The HF SWR meters use a toriod transformer with the signal using 1 turn. They may not always be good for 160m or 10m. The CB type meters are OK on 10m, and may be OK 21 MHz to 29 MHz.

If you are measuring power to a dummy load you don't need the SWR circuitry, simply a resistor divider and pair of 1N60 diodes as a peak detector. That then needs calibrated.

An Ardunio or other mpu is only needed for an automatic SWR meter where you don't have to set forward power reference level. All simple SWR meters will also measure forward power as they need that for the cal setting and sensitivity knob, though some may have poor calibration and many only work over a particular range. There were expensive easily damaged wideband HP power meters that could measure almost any frequency and waveform as they used a thermistor. The really expensive Bird meters are more robust and much cheaper!

No cheap VNA is any use to measure power. Mostly the signal generator is no use for anything other than built-in measurements on cheap ones.

One old method was to use two lamps behind a screen that were approximately 50 Ohm at the desired power (tungsten filament lamps can vary 1:10 between cold and hot). Then you connected one to device under test and the other to a PSU. You adjusted them the same appearance/brightness and then the DC PSU readings are your power. Still can be handy above 1 GHz as a rough check.

You can use a cheap CB meter on 28 MHz on the FT817 with a dummy load, if you have a method to calibrate / check it (most have gain pots). That then tells you the FT817 and its inbuilt metering is working (or not), Then assume the FT817 metering is a rough guide. A quality 100 MHz scope may be good enough for HF to see peak to peak on a 50 Ohms dummy load and then you can calculate the power = (peak to peak / 2.828a) squared / 50

Mike

Excellent post. You must be an RF engineer.

No, I was not recommending connecing a VNA to the output of a rig directly.

I was referring for the need to use one (properly) to determine for example
if a PA was sprogging first, in order to verify a fundamental power out issue
first, before assuming that a broadband? additive power meter is telling you
the truth.

I was trying trying to highlight the difference between selective frequency
power meters and non selective, and that in *some* situations you may be
fooled by them.

One superb recent classic example of this is of those dumb users that
are modding things like Quansheng HT's of late for broadband use.

Custom firmware allows 18-1300Mhz instead of the usual 2m/70cm
bands.

Then you get the CB'ers claing 5w on 27Mhz with them.

Oh no you're not.

These fools are using trashy wideband additive power meters that show
5w when the rig is keyed, but 4.999w is coming out somewhere daft
at about 120Mhz - not good at all.

Now there's a real life example to illustrate the point.



Brilliant HT's when used within the usual 137-176Mhz or 400-470Mhz range
on TX where the filters do their job, whilst offering some amazing experimental
features and a useful wideband multimode RX from 18-1300Mhz, but
deffo not for TX outside the above two ranges mentioned above.

Anyway, this is more of a general purpose RF post rather than a specific
FT817 on, my apologies for some topic drift.

73 de Andy

> 1N60 diodes as a peak detector.

Germainium diode voltage drop drifts with temperature, that's why
we don't use them anymore.

Use a decent Shottky diode with ultra low capacitance like a BAS70
with only 1pf.

They will be good for measuring down to 1mw with a simple power meter.

For sub 1mw levels you can use add some forward DC bias to the device
and recalibrate.

You'll then get a stable 0.35v drop like a Germanium that is more constant
with temperature.

Anyone remember the old AM broadcast receivers from the 60's/70's ?

On a hot day the those old Germanium trannys wilted and lost the plot.

- Andy -

Two excellent field tools to establish that your radio actually transmits are:

• See how fast a resistor heats up using a calibrated ouchometer - thanks VK3YE
• Have a torch light bulb dummy load.

To make an accurate measurement, the best, cheapest and most accurate way is a simple dummy load and Schottky diode.