[Amps] Bird wattmeter

Gary Schafer garyschafer at comcast.net
Mon May 7 17:41:36 EDT 2007

Hi Tom,

In this case with the bird 43 meter all of the meter movements are 30
microamp movements and I would guess that all of the slugs are designed to
put out 30 microamps at full scale into the standard resistance of the meter
movement which is said to be 1400 ohms. I measured 1399 on one and 1406 ohms
on another meter.

All slugs have to be designed to operate on the same amount of current
regardless of their power rating as all of the different power scales are on
the same relative points on the meter scale. That would mean that the same
amount of current must flow in the diode of all slugs or the scales would be

So it would seem that by applying 30 microamps of current to the meter
movements should result in full scale reading on all bird 43 meters in order
for all 43 type slugs to be compatible.

I am finding a 10% low reading on one meter and a 5% low reading on another
meter that I have when 30 microamps is applied to the movement.

My original question was if bird actually marked full scale at exactly the
30 microamp point or at some point close by and should I expect to see full
scale reading with 30 microamps applied or something else.

I do agree that power is very difficult to measure accurately. Even trying
to measure the attenuation of an attenuator accurately is difficult at rf.
Source and load impedances have to be very close.
I have an hp microwatt meter, a good Boonton rf millivoltmeter, hp 8405
vector voltmeter, an hp 3586C with .01 db readout. But when you add up all
the uncertainties in each piece of equipment the error gets pretty large.

I have a good bird 30 db 200 watt attenuator that I have been trying to see
how close to 30 db it actually is in hopes of using it for power
measurements but I am having little luck in getting any two instruments to
read close enough to be of much use. A couple tenths of a db error makes for
several percent error.

Back to the bird meter.. :>)

Gary  K4FMX

> Someone else posted:
> > [snip]
> > All slugs will give 30uA at indicated power.>>
> Which is, as Hal pointed out, not true. More important it is
> the wrong concept of what the meter measures. The meter
> movement and slug has to be looked at as voltage and
> resistance, current and resistance, voltage and current, or
> even by power and resistance. You have to know and calibrate
> with TWO known parameters, not one.
> We can read 30uA and the meter still be off (which would be
> the way to build a meter to read voltages much higher than
> the meter movement FS voltage), or we can read 42 mV and be
> off (which would be the normal way to build a current meter
> that has a current sensitivity many times less tthan the
> current measured in the shunt).
> The slug is designed and calibrated into a certain
> resistance and current, or a certain voltage and and
> current, or certain voltage and resistance. So when  we
> check the slug ONLY for current we are not really verifying
> anything except current. The slug is not an infinite series
> impedance current source that would output the same current
> regardless of load resistance.
> The normal aging in a meter is in the magnet, although
> things can stick. Hairsprings are not generally a time
> problem, but they can be a temperature problem.
> All that said, one of the most difficult things to measure
> (other than Q)  is RF power. The most accurate way to
> measure it is with heat. Fortunately knowing the absolute
> power is one of the least necessary things.
> 73 Tom
> > The slugs have a small variable potmeter that can be
> > adjusted for
> > calibration. Then you need a good reference and gain
> > access to the pot.
> > I check Bird meters as you explained with a 12V source
> > connected to the
> > Slug connection via a 470k pot and a multimeter reading
> > microamps.
> > I have many slugs where most are purchased seconhand. It
> > is not unusual
> > that readings are off by as much as +-10%, comparing slugs
> > or measuring
> > with a Narda coupler and HP435 powermeter. With a meter
> > that reads full
> > scale with 30uA and new slugs, power is accurate within
> > +-5%. The Bird
> > 43 is a nice instrument that last for decades if it's
> > cared for.
> > [snip]

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