EMRFD Message Archive 1707

Message Date From Subject
1707 2008-06-14 19:02:20 rcbuckiii Advanced VHF Wattmeter
Bob (K3NHI) will probably have the answer to this but others may be
interested. I just completed the power meter Bob described in the
May/June 2002 QEX. It is working well except I need to solve a
problem with the analog meter. I built the unit in the 3x4x5 case
suggested. However after cutting out the hole for the analog meter I
discovered the meter that I have has a 50ua movement. I haven't
located a suitable 1ma movement yet. I do have a larger one attached
outside the case temporarily that is working.

I'm thinking that I should be able to put a resistor in series with
the 50ua meter where it connects to the junction of R26 and R19 and
still use the meter I have. I would have to determine the value of
resistor required to produce a full scale reading with 1 ma flowing
through a test circuit. I think a resistor in parallel with the meter
would work but it would also probably upset the scaling factor of the

Has anyone built the power meter using a meter with a sensitivity
other than 1 ma? Or does anyone know where to obtain a 1-5/8 x 1-5/8
meter with a 1 ma rating? The Jameco one is no longer available.

1709 2008-06-15 07:34:04 bkopski Re: Advanced VHF Wattmeter
Hi Ray, et al -

I think the first thing I'd try with the 50 uA movement is to make it
look like a 1 mA meter by paralleling a resistor with the meter

If you know the meter resistance or if you know the meter voltage
drop at full scale deflection you can calculate the resistance.

If you know neither value, you can by trial and error find a resistor
value which when paralleled with the meter would cause (about) full
deflection when 1 mA is passed thru the combination. Either
approach should work.

The only remaining issue would then be how much voltage appears
across the meter/resistor terminals at full deflection. It has to be
low enough so the pwr. mtr. circuit values as published will still
drive the meter full scale, i.e., so the the meter drop does not "use
up" the available voltage from the circuit. My guess is that all
will work out OK.

If you still have difficulties please get in touch and I'll try to
help out further - which means possibly having to change some
resistor values in the pwr mtr circuit.

As an aside, there was a follow-up article to the May/June 2002 one -
namely in QEX for Sept/Oct 2003. This article describes a very
simple mod to the first versi
1710 2008-06-15 14:46:09 rcbuckiii Re: Advanced VHF Wattmeter

Thanks for the reply. I know the internal resistance of the meter so
I will calculate the resistor needed and give it a try.

I built the RF Power Calibrator from QEX Jan/Feb 2004. I also need to
find/build an accurate 0 dBm signal source so I can locate 0 dB on
the meter scale. Or maybe temporarily remove the 20 dB pad from the
calibrator and set the output to the correct voltage for 0 dBm. I
could then feed 0 dB to the meter through my step attenuator and
locate the other calibration marks.

I added the enhancement from the Sept/Oct 2003 issue when I built the
Power Meter. I figured it would come in handy if I ever get around to
finishing the W7ZOI/K7TAU spectrum analyzer I started last year. I
have all the boards and parts but haven't had the time.

I actually designed and made a PC board when I built the Power Meter
so I wouldn't have to hand wire everything. I usually take the time
to design a board for everything except the simplest projects. I did
however use the "ugly" construction method for the calibrator.


1711 2008-06-15 20:02:49 rcbuckiii Re: Advanced VHF Wattmeter
Update. I got the correct value resistor put across the 50ua meter
and it behaves properly. I just have to find the correct data points
and make a faceplate for it.

I also ran some tests
1712 2008-06-16 05:01:59 bkopski Re: Advanced VHF Wattmeter
Hi Ray and all -

As in message 1676, I am still willing to assist with the faceplate
task if you are interested. Chances are our two meters are the same
except for the electrical aspect - which you have now fixed. I just
measured the external dimensions of the plastic housing of mine and
it is 1 47/64" square - you can compare. If you'd like, just give me
your address and I'll send a copy of the same faceplate as in mine -
ready to put into place.

Note that you do not need a 0 dBm source. The -20 supplied by the
calibrator will do just fine. The calibrator and a known attenuator
can be used to set the correct displayed value and change in value of
both the DVM and analog readouts. Starting with the DVM part, follow
the adjustment procedure for a reading of "-20", then using a pad (eg
10 dB) continue the adjustments to read "-30". This is an
interactive adjustment pair so some back and forth between the trim
pots will likely be needed. Same for the analog display.

(Note that playing with the pad in the calibrator is NOT


1713 2008-06-16 08:25:35 Bill Wright Re: Advanced VHF Wattmeter
How did you find the internal resistance of the 50ua meter?  I have a 100ma meter that I need to use in a 200ma circuit.  I have played around with a 10 k pot across the
meter terminals to control the full scale reading but no joy.
73 Bill

Update. I got the correct value resistor put across the 50ua meter
> and it behaves properly. I just have to find the correct data
> and make a faceplate for it.
> I also ran some tests on the RF Power Calibrator. My idea was to
> jumper out R1(620 ohm) and have the oscillator put out +20 dBm into
> the 20 db pad, which would have given me 0 db into the Power Meter.
> Unfortunately since it is CMOS it will only put out .710 volts into
> the pad with R1 jumpered out. Back to the drawing board for my 0
> source.
> Ray

[Non-text portions of this message have been removed]
1715 2008-06-16 08:57:35 chuck adams Internal Meter Resistance
On Monday 16 June 2008 07:13:31 Bill Wright wrote:
> Ray
> How did you find the internal resistance of the 50ua meter?

...snip snip...

> 73 Bill
> KD5yyk

Any current meter should have a very low resistance, otherwise
they significantly affect the circuit they are in.

What I usually do is take a one volt battery (NiMH usually) and
measure its voltage. Using Ohms law I calculate what resistor value
will give me the full scale current reading.

Place the battery, resistor, and meter in series and see how close
you get to full scale deflection. Then measure the voltage across
the meter with a DVM (which usually has a 10M ohm internal
resistance itself) that doesn't effect the circuit much.

Using Ohms law, use the voltage across the meter and the
current reading to determine its internal resistance.

You'll have to get as close as you can to the needed resistance
from your junk box. If in doubt, go a little higher in resistance
so you don't damage the meter.

Take extra care in doing all the math. A decimal point in the
wrong place can cause you to do damage to the meter.

Hope this helps.

dit dit


chuck adams, k7qo
1716 2008-06-16 11:10:18 rcbuckiii Re: Advanced VHF Wattmeter

Thanks for the offer. I will send you my address. I'm sure it is the
same sized meter. I'm not at home right now but the 1-5/8 square
quick measurement I made a couple of days ago is within the range you

I was going to use 0 db for the range above -20 db. But that is 1 mw
of power, outside the range of the oscillator. By using your
faceplate I won't have to worry about it. And the digital meter gives
me a more accurate reading anyway.


1717 2008-06-16 11:10:25 rcbuckiii Re: Advanced VHF Wattmeter

I just noticed, your email is private. The address shown for me in
QRZ.com, ARRL, and FCC database is correct. Or I can send you an
SASE. Just let me know.


1718 2008-06-16 11:10:27 rcbuckiii Re: Internal Meter Resistance

That is the way I determined mine. As you say, you have to be careful
with the math.

Bill, another way to do it is here:


Note: At the link above, when the say set R2 to zero, they really
mean to have the decade box in the open positi
1721 2008-06-17 10:33:49 Bill Wright Re: Internal Meter Resistance
Thanks guys.  I will check out the decimal point issue when I get home this evening.
73 Bill
1723 2008-06-18 04:16:21 Alan Melia Internal Meter Resistance
An even easier way is to set up the Full Scale current then shunt the meter
with a variable resistance until it reads half scale ....then the resistance
value equals the meter resistance. Easy if you have a resistor sub-box.

Make a "custom" resistor by picking the next highest prefered value and
paralleling higher values....though this doesnt work for high current where
the meter resistance is in the low ohms range, but you need Kelvin
connections (4 terminal resistor) as well then.

Alan G3NYK