measuring RF in a circuit

[G0HZU]

I did a quick simulation to help you understand further how the meter is designed to work.

If we assume the meter movement is about 50uA then for 5W we need 25uA through the meter to get a halfway reading on the dial because this is where is it marked as 5W.

We know that 5W is 22.36V peak...

Also, the diode is a peak voltage detector so we should get 22.36V less 1 diode drop at the diode cathode = 22V approx with a Schottky diode. We then need to fit a calibration (current limiting) resistor inline such that this 22V will give 25uA indicated on the meter for 5W.

So the total calibration resistance (trimpot+ series resistor?)  needs to be about 860k ohm according to my quick and dirty simulation.
There are graphs for the RF AC voltage across the load and you can see the green DC voltage output from the peak detecting diode is just under the peak at 22V.

you can also see a graph for current through the meter movement at 25uA. But your meter might not have a 50uA movement so this is just a guide. If it had a 100uA movement then the 860k resistor would be a lot lower in value. But it may well use a regular 3 pin potentiometer here instead of an inline two pin trimmer anyway. But the idea is the same... to adjust the pot to produce enough current from the 22V to just drive enough microamps into the display meter to give a mid scale deflection for a 5W test signal.

 Hope this helps...

[Howardlong]

LOL it looks that way

I just replaced it with a BAV 21 as thats all I have
and now I get 18.77 VDC with cap d/c
and with cap origonal cap connected I get
19.30 and perfect SWR 

Will it be okay th leave the BAV 21 in there ?

Loving this thread!

Germaniums seem to be preferred in this application, lower voltage drop. That BAV21 will be OK for now, not sure how it'll behave at higher frequencies though.

[G0HZU]

IMPORTANT!  I wouldn't recommend trying to measure the meter movement current if it isn't stamped on it somewhere. This is because it will be very easy to fry the meter movement if you make a mistake. Then once the meter movement is damaged you will have nothing...

You don't really need to know the meter movement current to calibrate it. I would recommend the old school HP2800 diode here or the 1N5711. They are very good quality Schottky diodes with a 70V PIV rating and low capacitance and decent frequency response.

If the layout was improved and tightened up a bit you could calibrate this meter reasonably well with a DC power supply if you used one of the above diodes because their detection performance is very predictable. eg apply +22V DC to the diode cathode (check it for 22V DC with a DVM) and then adjust the high power range trimpot to show 5W on the dial. And that's the meter calibrated on the high power range without even needing an RF signal source.

For the lower power range apply 9.64V DC to the diode cathode and adjust the other trimpot for full scale reading of 1W because 10V peak is 1W but you have to allow maybe 0.36V for the diode drop.

But make sure you don't touch the direct connection to the meter dial with your power supply wires or you will fry the meter movement with the DC power supply.

[TimFox]

Yes, never apply a DC voltage or an ohmmeter directly to an unknown meter movement.
To check the full-scale, you need to apply a reasonable voltage (maybe only 1 volt) through fixed resistors, starting with a high value (maybe 100k) until you get full-scale deflection by adjusting the voltage.
Then, to determine the meter's resistance, keep the voltage and large resistor in place and put a variable resistor in parallel with the movement.  Start at high resistance, and reduce the resistance until the meter reads half-scale.  Remove the variable resistor and measure its resistance, which is very close to the meter resistance (so long as the series resistor above is much higher than both).

[Flump]

G0HZU Thank you for the explanation and Drawings 
I am going to leave this meter as it is now as its all working
as it should do, the BAV21 I put in has a voltage drop of .600mV
so I am using that in my calculations to calibrate the scale with.
I also changed that Cap for a 1nF ceramic .

Volts to watts
+ 0.600 (diode)
/ 1.414
square it
/ 50

watts to volts
x50
square root
x 1.414
-0600

And the scale seems nice and linear across its range.
If calibrated at 5W it is spot on anywhere from 1-20

I do have a nice enclosure with quite a large analog meter
if i can find it i want to build a new RF meter from scratch
Possibly following your design above if its suitable ?
My thoughts on the new one is to be similar to this one
but measure up to 100w, so SO239 with big internal Load
but also a BNC for a flylead (scope probe?) and to have a scale which
reads 1 - 10 and make 3 switchable ranges,
0-1 , 1-10 , 10-100 .

And I have ordered some of these to keep in stock anyway
http://www.ebay.co.uk/itm/121518325327?_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT

I also bought an RF probe and Blew it up within an hour of unpacking it,
apparently they dont like 100w on the pointy end
so I need to learn more about RMS voltages 

[G0HZU]

If you try and make a similar meter for 100W operation then I'd advise arranging the resistor network such that you can feed the diode detector from a tap point on the resistors.
This way the diode sees less RF voltage and so you could use the 1N5711 with its 70V PIV rating.

Obviously, this means the calibration theory will change a bit but this just means a bit more care is needing in working out the detector output for a given RF power.

Note that this type of power meter needs a clean sine wave to give reliable performance in terms of accuracy because of the detection system it uses. If there are any harmonics present then the uncertainty of the meter reading will go up a lot.

eg if you had a -30dBc harmonic the meter uncertainty could be +/-6% (approx) in the power reading. A -20dBc harmonic gives approx +/-20% uncertainty in power reading. Ideally, all harmonics should be something like -50dBc to make them insignificant with this type of meter if you want to get the most from it in terms of measurement uncertainty.

However, if you build this type of meter well then it should totally outclass something like a Bird 43 wattmeter in terms of accuracy across the HF bands. The Bird 43 is a very overrated power meter and I wouldn't want to own one. I'd be too embarrased to have one because they are so poor and unreliable both mechanically and technically.