Calibration of Bird Wattmeters
Roger Leone
I have seen several Bird slugs that have been drilled through the top,
presumably to tweak the pickup line for calibration purposes, then sealed
with silicone compound. This may be the only way to calibrate this type of
meter. It does decrease the resale value of the slugs, however.
Roger K6XQ
Richard Harrison
"Are these meters calibrated to read correctly at the factory or some
other standards lab?"
Yes. Bird published a paper on the calibration procedures used at the
Bird plant. I haven`t seen it. For field calibration, Bird says you need
a known standard along with its requirements and history.
Media which may need control are listed as continuous wave RF energy
(several frequencies), low-frequency and/or DC power, flowing air or
liquid, and combinations of these.
RF and low-frequency supplies must be well-filtered and clean. Heat
transfer must be known and accounted for. Calorimetry may be used for
calibration.
Bird has suggested diagrams containing multiple sources, low-pass
filters, coax switches, test meters, load resistors, etc.
I`d like to see Bird`s article on production testing and calibration.
Best regards, Richard Harrison, KB5WZI
Reg Edwards
Are these meters calibrated to read correctly at the factory
or some other standards lab ? If so what is the internal
impedance of the calibrating generator ? Or doesn't it
matter what the generator internal impedance is ? There may
be a masking pad of known impedance between generator and
meter being calibrated.
If anything more is done back at the shack before it is
used, what is it and how is it done ?
I am not familiar with what knobs or preset controls which
may be accessible to the user. Are there any ?
Thanks in anticipation
----
Reg
Bob Miller
>Question about Bird power meters.
There is some info at the Bird web site:
http://www.bird-electronic.com/home.htm
>
>Are these meters calibrated to read correctly at the factory
>or some other standards lab ?
They do have a "Repair & Calibration Department" at their factory, so
it would seem the meters are calibrated there.
> If so what is the internal
>impedance of the calibrating generator ? Or doesn't it
>matter what the generator internal impedance is ? There may
>be a masking pad of known impedance between generator and
>meter being calibrated.
>
>If anything more is done back at the shack before it is
>used, what is it and how is it done ?
>
>I am not familiar with what knobs or preset controls which
>may be accessible to the user. Are there any ?
There are no knob or controls on the externals of my Bird 43 case. I
haven't looked inside the case, though, as to what might be in there.
Bob
k5qwg
Richard Harrison
"If so, what is the internal impedance of the calibration generator?"
For field calibration, Bird suggests (4) 100 watt transmitters, one
each, 30 MHz, 100 MHz, 200 MHz, and a variable frequency 250 - 1000 MHz
transmitter as calibration generators. (5) low-pass filters are
specified to be used on the outputs of these transmitters. Their cut-off
frequencies are: 30 MHz, 160 MHz, 240 MHz, 500 MHz, and 1000 MHz. No
output impedance is specified for these calibration transmitters, but a
50-ohm impedance is specified as the load on these transmitters via the
metering circuits. Catalog listed elements are available in powers of
1-watt to 10000-watts. So, attenuators and amplifiers to calibrate some
less common elements would be handy. There are also elements available
as regular catalog items to 2300 MHz.
Reg Edwards
I am now a little wiser about high class instruments as
distinct from the boxes usually found in amateur shacks. I
also visited the website but there was nothing about
calibration procedures or insructions to users about
limitations which may be occur in other than standard 50-ohm
systems.
Its interesting the power meters do not indicate reflection
coefficient or swr relative to 50 ohms. On the surface, it
would have been so easy to include such meter scales. Those
values have to be obtained indirectly by calculating from
the indicated Forward and Reflected watts.
After all, the values of reflected and forward watts are of
no practical use to anybody except that their difference
should be equal to the acual power generated and radiated.
--
Reg, G4FGQ
Richard Harrison
"After all, the values of reflected and forward watts are of no
practical use to anybody except that their difference should be equal to
The Bird wattmeter is usually the only antenna system instrument most
commercial radio operators would use. The number of assigned frequencies
of interest to a particular operator would be small. The antenna or
matching network is selected for the frequency to be used. This load is
then adjusted for a match by minimizing the the reflected power. The
transmitter is then adjusted for a specified power by monitoring the
forward power indication.
The condition of the transmitter output, antenna, and transmission line
can be determined using only the wattmeter. Very few would consult the
graph supplied with the meter to determine the numerical value of the
SWR. The operator charged with making sure performance is up to par does
so with no reference to SWR.
Reg Edwards
> The condition of the transmitter output, antenna, and
transmission line
> can be determined using only the wattmeter. Very few would
consult the
> graph supplied with the meter to determine the numerical
value of the
> SWR. The operator charged with making sure performance is
up to par does
> so with no reference to SWR.
So the purpose of SWR meters, alias Forward & Reflected
Power meters, whatever scales may appear on the face of the
instrument, can be summarised as follows -
(1) To indicate that the load on the transmitter is a
resistive 50 ohms and not some other unknown value.
(2) When (1) has been satisfied, to indicate the
transmitter power output in watts.
This description seems to de-glamourise the instruments.
What with built-in automatic tuners and ALC, is the demise
of everybody's favourite instrument now in sight ?
----
Reg. G4FGQ
Richard Harrison
"What with built-in automatic tuners and ALC, is the demise of
everybody`s favourite instrument now in sight?"
The Bird wattmeter has been around for about half a century. Competition
has been slow to appear. There are a couple of U.S. clones. Bird warns
the cloned parts aren`t interchangeable with the genuine article. Why
anyone would try surprises me as the prices asked by the imitators don`t
seem to be discounted. My experience has been in using Bird slugs with
off-brand precision 50-ohm coaxial line sections and indicators. The
results seem identical to me.
There was a cold-war period when vendors said they weren`t allowed to
ship Bird wattmeters to foreign addresses. This export inhibition didn`t
help win new customers.
Sooner or later, Chinese pirates will flood the market with cheap copies
of the Bird wattmeter. That will likely just about end the market for
most other transmitter antenna and transmission line measuring devices.
CAM
> So the purpose of SWR meters, alias Forward & Reflected
> Power meters, whatever scales may appear on the face of the
> instrument, can be summarised as follows -
>
> (1) To indicate that the load on the transmitter is a
> resistive 50 ohms and not some other unknown value.
Actually, an SWR meter does indicate that the load on the
transmitter is not a resistive 50 ohms when the reflected
power is not zero. And when used on the 50 ohm coax between
the tuner and the antenna, gives the SWR and forward/reflected
power which allows one to calculate feedline losses due to
SWR. And the more one knows about transmission lines, the
more an SWR meter will be useful. Installing a 50 ohm SWR
meter at a current maximum point on the ladder-line will
yield all sorts of useful information.
As you know, I vary the length of my ladder-line until I get
minimum 50 ohm SWR. This happens at a current maximum point
on the ladder-line. Knowing the SWR gives me only two possible
values of resistive loading. A simple pick-up loop on the ladder-
line allows me to ascertain the relative current and therefore
the actual resistance at that point. Before I got my MFJ-259, I
used the pick-up loop and my SWR meter as an "antenna analyzer".
As that may seem unclear, here are the steps.
1. I use knife switches to vary the length of the ladder-line
until I bring the current maximum point to my 1:1 choke. This
occurs at a minimum SWR meter reading.
2. I have a ladder-line pick-up loop permanently attached to
the ladder-line on the ladder-line side of the 1:1 choke. It
has a small PC board with components that rectify the RF signal
so its relative magnitude can be read on a multimeter. (I have
a few of these PC boards with components left over from a
previous company)
3. Knowing the position of the current maximum point, the SWR,
and the relative magnitude of the current flowing past that point
allows me to calculate the actual resistive value seen looking into
the ladder-line.
4. Using a Smith Chart and specifications for the ladder-line,
I can calculate the feedpoint impedance of my antenna. No antenna
analyzer required.
--
http://www.mindspring.com/~w6rca
Richard Harrison
"So the purpose of SWR meters, alias Forward & Reflected Power meters,
whatever scales may appear on the face of the instrument, can be
I think the premise is mistaken. The Bird ignores SWR, thanks to its
directional coupler which really takes a sample of voltage and current
at a point on the line. It uses these to really indicate power. The
voltage may be high and the current may be low at a point on the line or
vice versa, and the powers can be the same at all points. Since the Bird
is ignoring SWR and indicating power it can be inserted any place in the
line. The forward and reflected powers are sensed and indicated one at a
time. These are assumed to have the same ratio everywhere in the line
until measurements show otherwise. If there is significant loss in the
line, SWR is highest at the reflection point and least at the
transmitter`s connection to the line.
Loss can be determined by power measurements at both ends of the
transmission line.
Wes Stewart, Radio N7WS
wrote:
>Thanks everybody for the info on this and other threads.
>
>I am now a little wiser about high class instruments as
>distinct from the boxes usually found in amateur shacks. I
>also visited the website but there was nothing about
>calibration procedures or insructions to users about
>limitations which may be occur in other than standard 50-ohm
>systems.
>
>Its interesting the power meters do not indicate reflection
>coefficient or swr relative to 50 ohms. On the surface, it
>would have been so easy to include such meter scales. Those
>values have to be obtained indirectly by calculating from
>the indicated Forward and Reflected watts.
If one could adjust the transmitter power to set the forward reading to just
full-scale, then the meter face could be calibrated in SWR. Because that is
usually not the case, such annotation would be generally useless.
>
>After all, the values of reflected and forward watts are of
>no practical use to anybody except that their difference
>should be equal to the acual power generated and radiated.
I have a Bird dual sensor line sampler and meter that I use in my 2 meter
station. I use a 2.5KW "forward" sensor ("slug") and a 250W "reverse" sensor.
These feed a dual pointer meter with appropriate power scales. By noting where
the pointers cross, an estimate of SWR is possible. The meter has green, yellow
and red areas that indicate 1.5 max, 2.0 max and >2.0:1 SWR, respectively. So a
quick glance at this analog(e) "computer" gives me the health of the antenna
system even while keying the transmitter.
Wes
>--
>Reg, G4FGQ
>
>
Reg Edwards
input impedance, R+jX of the the system, just measure it
directly with your MFJ 259 Analyser or equivalent. Your SWR
or reflected power meter only tells you half the story and
just gets in the way.
---
Reg
CAM
Actually, my method using the SWR meter is considerably more
accurate than the MFJ-259. A network analyzer would be nice.
--
http://www.mindspring.com/~w6rca
Reg Edwards
transmission line to indicate forward and reflected power,
and swr and refl.coeff, all operate on precisely the same
principle as the common or garden HF SWR meter which uses a
little ferrite torroid current transformer.
They are all resistance bridges with fixed ratio and 3 fixed
arms. There's nothing to choose between them except that
they can be used in different frequency ranges.
And none of them measure SWR. Its just a figment of the
user's imagination.
----
Reg
Reg Edwards
power meter will tell you immediately something has gone
wrong with the system. It can be arranged for the alarm bell
to ring if reflected power exceed 5 watts.
But it won't give the maintenance man the foggiest idea
about where to start looking for the source of the trouble.
He will need other measuring instruments.
--
------------------------------------------
Regards from Reg, G4FGQ
http://www.btinternet.com/~g4fgq.regp
------------------------------------------
CAM
> And none of them measure SWR. Its just a figment of the
> user's imagination.
Assuming the boundary conditions associated with a transmission
line, the addition of phasors proportional to the voltage and
current vectors results in a traveling wave proportional to
forward power. The subtraction of those phasors results in
a traveling wave proportional to reflected power. If you haven't
actually waded through the math, it is an interesting and
enlightening exercise.
--
http://www.mindspring.com/~w6rca
Dick Carroll
>
> Agreed an SWR or Refl.coeff., or forward power, or reflected
> power meter will tell you immediately something has gone
> wrong with the system. It can be arranged for the alarm bell
> to ring if reflected power exceed 5 watts.
>
> But it won't give the maintenance man the foggiest idea
> about where to start looking for the source of the trouble.
> He will need other measuring instruments.
Yes. SWR can be used as an indication of good antenna health in the
presence of proper working conditions. But if the transmitter and
receiver are proven by testing to be functioning properly, yet operating
conditions are abnormally poor, you'd better start suspecting problems
with the antenna or feedline.
I once had difficulty convincing the boss that there was something
wrong up on the tower that he needed to have the tower crew check on
simply because the SWR on the VHF antenna system was acceptably low. I
had thoroughly checked out the transciever and it functioned normally,
but couldn't be heard more than a fraction of its normal range and was
deaf to stations beyond several miles out. The antenna was 265 feet up,
and normally worked ranges to 100 miles. When the tower crew finally did
take down the antenna, it had lost some of its seals and was
waterlogged. But the SWR was very near its normal value.
Dick
Richard Harrison
"The antenna was 265 feet up, and normally worked ranges to 100 miles."
Typical, and when it doesn`t, and the radio is OK, there is an antenna
system problem.
If there are too many wavelengths of lossy cable between a defect at the
far end of the cable and a wattmeter, not much reflected power will be
indicated. Water invasion and retention can cause such a problem at the
top of the tower.
Dick Carroll wrote: "---if the transmitter and receiver are proven to be
functioning properly, yet operating conditions are abnormally poor,
you`d better start suspecting problems with the antenna or feedline."
Yes, and the Bird wattmeter is a good instrument to confirm transmitter
capability, even when the antenna system performs as a dummy load. A
Measurements Model 80 or equivalent signal generator is a good
instrument to confirm receiver capability.
Richard Clark
On Sat, 21 Oct 2000 15:24:31 -0500, CAM <cam...@bigfoot.com> wrote:
>Actually, my method using the SWR meter is considerably more
>accurate than the MFJ-259. A network analyzer would be nice.
Dear Readers,
I would like to express how Cecil has above offered a superb example of how precision and
and accuracy are not unheard of outside the Standards Lab.
The method described and the results he found are very real examples of how to achieve
high accuracy without having to do any more than pay attention to first priciples. There
are certain operations, such as using the Smith Chart, that require special knowledge; but
even here, these computations can be performed by the computer.
73's
Richard Clark, KB7QHC
George, W5YR
my faithful Bird 43 is significantly in error on 20 meters. It reads 115
watts when the correct reading should be 100 watts. As far as I know,
VHF and UHF operation is normal and readings are correct.
I have cleaned all the contacts and connections in the Bird that usually
cause problems so I believe that the metering circuit is working
properly.
I can only conclude that the 250-watt HF element is at fault.
Question: can these be calibrated in "the field" i.e., my workbench? I
have never seen anything on this procedure and supposed that it was a
factory-only affair. I do have an accurate r-f power source to use in a
calibration procedure.
So, if anyone has any information on this, I would appreciate hearing
from them. Lacking any inputs from this source, I will contact Bird
although I am confident that their response will be "return it the
factory."
72/73, George W5YR - the Yellow Rose of Texas NETXQRP 6
Fairview, TX 30 mi NE Dallas in Collin county QRP-L 1373
Amateur Radio W5YR, in the 55th year and it just keeps getting better!
Icom IC-756 PRO #02121 (9/00) Kachina #91900556 (12/99) IC-765 (6/90)
Richard Harrison
"So, if anyone has any information on this, I would appreciate hearing
from them."
Bird says to consider what + or - 5% accuracy means. Checking the 500
watt scale at a level of 10 watts allows + or - 25 watts variation while
the 100 watt full-scale tolerance allows only + or - 5 watts
variation.
Bird says that replacing the crystal rectifier should yield + or - 3% of
the full-scale reading of the factory calibrated unit.
Bird says inaccuracies may result from spurious signals outside the
design range, effects of the deviation from purely sinusoidal wave
shape, effects of ambient and instrument temperature, meter movement
accuracy, and the history of the instrument`s maintenance.
To calibrate, Bird says, requires concern with the standards used, the
media employed, and the operator performing the calibration.
George is surely right. Bird will very likely say, "return it to the
factory".
Richard Harrison
"Can these (Bird slugs) be calibrated in "the field" i.e., my
workbench?"
About 40 years ago I sucessfully recalibrated a Sierra wattmeter which
uses the same principle to indicate power, but physically is quite
different. It is a big clunker which uses a rotating barrel in place of
plug-in elements.
Some years later I saw an article in some ham magazine, I am sorry I
don`t remember the details, on how to repair and recalibrate Bird
elements.
Return to the factory for recalibration is likely to restore confidence.
Reg Edwards
Cecil,
When an SWR meter is connected immediately at the output of
a transmitter (it nearly always is) it indicates the SWR on
a transmission line which does not exist. Who wants to know
that ?
But at least it does correctly indicate the magnitude of the
reflection coefficient of the load relative to 50 ohms.
Unfortunately it does NOT indicate the angle of the
reflection coefficient. So half of the information which
might be useful to the station mainenance people is not
available.
---
Reg.
Gary Coffman
>I can only conclude that the 250-watt HF element is at fault.
>
>Question: can these be calibrated in "the field" i.e., my workbench? I
>have never seen anything on this procedure and supposed that it was a
>factory-only affair. I do have an accurate r-f power source to use in a
>calibration procedure.
George, if you peel back the label on the slug you'll find a screwdriver
adjustable pot. That will adjust the slug's calibration. But unless you
really have the proper setup to do the calibration, I'd send the slug back
to Bird. You don't want to twiddle this unless you're 100% absolutely
sure that it is the problem.
Gary
Gary Coffman KE4ZV | You make it |mail to ke...@bellsouth.net
534 Shannon Way | We break it |
Lawrenceville, GA | Guaranteed |
CAM
> When an SWR meter is connected immediately at the output of
> a transmitter (it nearly always is) it indicates the SWR on
> a transmission line which does not exist. Who wants to know
> that ?
Again, the SWR meter reacts to voltage and current exactly *as if*
it were installed in a 50 ohm coax line and if one understands
that, length doesn't matter (except for losses). If the SWR meter
is installed between a transmitter designed to drive 50 ohms and
the 50 ohm Z0-match provided by a tuner, all is well. And when we
adjust the SWR to 1:1, we know the exact magnitude and phase of
the reflection coefficient and impedance seen by the transmitter. :-)
--
http://www.mindspring.com/~w6rca
Reg Edwards
Cecil, you used the word "if" three times.
That would hardly correspond to a positive and determined
approach to fault-finding problems in the station mainenance
handbook. ;o)
Incidentally, during WW2 the maintenance handbooks which
accompanied USA-manufactured radar equipment excelled at
describing unambiguously, in a crystal-clear manner, exactly
what and how to do things. If you could read and understand
English, and were not physically handicapped, you were as
good as the radio and electronic engineers at MIT.
----
Reg.
Richard Harrison
"You don`t want to twiddle this (calibration adjustment) unless you`re
100% absolutely sure that it is the problem."
Yes. The Bird wattmeter is a very reliable instrument which tolerates
many excesses, but not all. It can be overloaded beyond its yield point.
It may also have some late appearing defect. So, readjustment may make
it read right at one point only to be off the mark at other levels.
Best regards, Richard Harrisomn, KB5WZI
Richard Harrison
"This cover does not want to come off "readily" but I nave not really
been aggressive in trying to remove one."
Bird intended to discourage tinkering. I`d return my slug to Bird for
checking and calibration.
Ralph Mowery
getting it off the slug (probably why they recommend returning it to the
factory). I think it must be glued on. I have seen a few at hamfests
that have had small holes over the screw. They were probably made by
very carefully probing with a sharp object and finding where to enlarge
a pinhole.
"George, W5YR" wrote:
>
> Folks, I hate to appear totally "dumb," but *where* is this calibration
> adjustment that everyone mentions?
>
> I can find nothing on the case proper to adjust other than the meter
> zero-adjustment screw. The only other possibility that I can see would
> be the thin metal cover over the front of each slug. It appears to have
> been seated with a blow with a centerpunch (to account for that mark in
> the center of the arrow). This cover does not want to come off "readily"
> but I have not really been very aggressive in trying to remove one.
>
>
George, W5YR
adjustment that everyone mentions?
I can find nothing on the case proper to adjust other than the meter
zero-adjustment screw. The only other possibility that I can see would
be the thin metal cover over the front of each slug. It appears to have
been seated with a blow with a centerpunch (to account for that mark in
the center of the arrow). This cover does not want to come off "readily"
but I have not really been very aggressive in trying to remove one.
So, any help will be appreciated.
72/73, George W5YR - the Yellow Rose of Texas NETXQRP 6
Fairview, TX 30 mi NE Dallas in Collin county QRP-L 1373
Amateur Radio W5YR, in the 55th year and it just keeps getting better!
Icom IC-756 PRO #02121 (9/00) Kachina #91900556 (12/99) IC-765 (6/90)
--
Dr. David Kirkby
>
> Question about Bird power meters.
>
As a student I used to work for a government calibration lab that checked the
calibration of Bird meters. They were specified to an accuracy of 5% of FSD.
(Most were the Bird 43's). A site would send one in, with a selection of
elements to have the calibration checked. We did not adjust them, only indicated
pass/fail.
So few passed the +/- 5% of FSD spec, that many places sent them in with a note
that +/- 10% of FSD would be considered acceptable.
I'm not able to recall every detail of the testing procedure, but know that
power could be measured using a water caliometer. High power attenuators were
available, which were calibrated (I'm not sure exactly how). However, meters
were not tested using water calorimeters on a day-to day basis. It was too time
consuming to check a meter of limited accuracy.
The day-to-day calibration of the meters used a transmitter, low pass filter,
the Bird meter, a high power attenuator (plus possibly smaller ones) and a
Marconi (or perhaps HP, I forget) low-power meter, measuring mW or possibly uW
(I can't recall) exiting from the attenuators. The first high power attenuator
was 30 dB. At some frequencies, a directional coupler and load was used instead
of the attenuator. The attenuator or directional coupler and marconi power meter
were all calibrated in house, ultimately against precision standards (water
calorimeter etc.). A computer controlled the transmitter and read the data from
the Marconi (or HP ??) power meter, while storing the calibration data for the
attenuator or coupler.
While working at the lab, I purchased my own element to cover the 70 cm amateur
band. I checked the new element at work, only to find it was not within spec,
even when brand new. (The meter was not then new, but I had bought it new and
never abused it). I returned the element to Aspen Electronics (the UK
distributor) and it was calibrated at 432 MHz for me while I waited (I did not
care too much for its accuracy outside the amateur band). I then returned it to
work, to find that indeed it was then within spec at the spot frequency.
My one thought on these meters is that they are fine for tuning a TX up to
maximum, but don't even consider them as precision instruments. They are not,
nor will they ever be. Few meet the published spec after use, and while I can't
say how many do when new, the only element I personally purchased was out of
spec when new. Hence in my opinion, they are not really worth calibrating. They
are undoutably, in amateur circles, a very over-rated piece of equipment. If you
want to see if your amp is working they are good, but if you want to *measure*
power, with any degree of certainty, I would buy a used low-power meter, and
somehow get someone to calibrate an attenuator for you using a calibrated
spectrum analyser. That will give far better accuracy than a Bird ever will.
Dr. David Kirkby Ph.D,
Senior Research Fellow,
Department of Medical Physics, University College London,
11-20 Capper St, London, WC1E 6JA.
Tel: 020 7209 6406 Fax: 020 7209 6269
email: da...@medphys.ucl.ac.uk web page: http://www.medphys.ucl.ac.uk/~davek
Amateur radio callsign: G8WRB
Reg Edwards
experience with RF wattmeters. My own experience does not
extend above 30 MHz and, insofar as precision measurements
are concerned, 0.001 dB, only below 5 MHz. But I have no
reason to doubt your findings by extrapolating my own
experience to higher frequencies.
My interest is in measurements in general. I enquired about
Bird meters only because they appear to be of the best US
quality and from the frequency of being mentioned in this
newsgroup a fairly large number of amateurs have practical
experience of them. Whereas I have none.
It has been said (I forget by who) Radio is the most inexact
of all the sciences. This is not surprising when one
considers the propagation fade depths experienced at all
frequencies. Perhaps it is the unpredictability inherent in
radio propagation which amateurs have found so intriguing
for the best part of 100 years.
--
------------------------------------------
Regards from Reg, G4FGQ
For free radio design software go to:
http://www.btinternet.com/~g4fgq.regp
------------------------------------------
Dr. David Kirkby <da...@medphys.ucl.ac.uk> wrote in message
news:39F6506F...@medphys.ucl.ac.uk...
Richard Harrison
"Hence in my opinion, they (Bird wattmeters) are not really worth
calibrating. They are undoubtably, in amateur circles, a very over-rated
piece of equipment."
Dr. Kirkby is too pessimistic. I spent over a quarter century with a
company which operated over a thousand radios which all got regular
checks with Bird wattmeters. Logs were kept which recorded the readings.
Many different Birds were used and they agreed closely, short term, or
long term, barring changes in the power measured. These measurements
were repeatable and different meters agreed. Moreover the measurements
were usually exactly what was expected and conformed to the
specifications of the equipment being checked. Poor instruments don`t
give consistently good results. All of the Bird wattmeters gave
consistently good results, and were interchangeable. Dr. Kirkby`s
experience is very different from mine. Since my experience is with
dozens of Bird wattmeters making thousands of measurements, ever since
1960, and counting, I still use a Bird almost every day, I think Dr.
Kirkby`s experience is atypical.
+ or -5% is not claimed as laboratory accuracy. But, compared to any
other commonly available field measurement RF wattmeter, the Bird
deserves the reputation it has with amateurs and professionals alike.
Richard Clark
Hi Dr. Dave,
Finally! Corroborating testimony from another who has worked at the
bench with these instruments.
The 5% specification was always a fantasy and as you indicate even for
BRAND NEW.
Richard Clark
(Richard Harrison) wrote:
> I still use a Bird almost every day, I think Dr.
>Kirkby`s experience is atypical.
Hi Richard,
To quote another Doctor, Samuel Johnson LLD
"The triumph of hope over experience."
Richard, these assessments from Metrologists (now two here in this
board) do not denigrate the robust design and general applicability,
they simply point out that the determination of RF Power is extremely
difficult if your objective is accuracy. Bird meters do not qualify
as accurate except for the most lax meaning.
On the other hand accuracy is not necessary for day to day operations,
whereas repeatability and ease of use are. The robustness is probably
the only asset to be found.
Where accuracy counts (for the FAA or FCC), the law mandates that such
instruments be calibrated and seals affixed and certificates made to
indicate the exact nature of usage. This is neither the common
practice nor the common experience of the Ham OR EVEN PROFESSIONAL.
It does not alter that it is the common hope.
George, W5YR
I think that I will just let things alone and rely upon other better
calibrated power meters for detailed work.
I guess it just "gets me" that the comparatively low-tech metering
circuit in my MFJ 989-C tuner can track the output of my Kachina from
100 watts down to a few watts and be right on the money to within a
needle width at all power levels. The Kachina output power is regulated
digitally to within a small fraction of a dB. It also tracks the meter
and digital/panel indications of the IC-756PRO quite closely as well.
72/73, George W5YR - the Yellow Rose of Texas NETXQRP 6
Fairview, TX 30 mi NE Dallas in Collin county QRP-L 1373
Amateur Radio W5YR, in the 55th year and it just keeps getting better!
Icom IC-756 PRO #02121 (9/00) Kachina #91900556 (12/99) IC-765 (6/90)
Richard Harrison
"Bird meters do not qualify as accurate except for the most lax
meaning."
We are fairly lax in the U.S.. The FCC inspectors on visiting the
stations where I worked would accept the thermoammeter common-point
reading along with our declarations of measured impedance at that point
as definitive of the transmitter power output. They would at times also
make fieldstrength measurements at our designated monitoring points, for
power verification. antenna efficiency, and compliance with null
impositions. The FCC didn`t require any seals, stickers, or traceability
to standards. They did approve frequency standards and modulation
monitors, as I recall. We also employed a frequency monitoring service
to wire us a report every month to compare with our logged reading at
the service`s measurement time.
Every two-way radio shop I ever saw, including those operated by radio
manufacturers as company stores, used the Bird wattmeter as a power
measurement standard. For microwaves they would use the appropriate H-P
test set.
The radio manufacturer wouldn`t tolerate a power meter that read low on
power any more than an auto manufacturer would tolerate speedometers
that gave low indications.
On the other hand, a radio manufacturer wouldn`t want a wattmeter that
read high because customers would ask why such a powerful radio didn`t
talk farther.
Manufacturers and users also have maximum FCC licensed power
restrictions that demand compliance. The inspector`s power meter had
better be scaled the same as the licensee`s.
My experience has been, that if Link, RCA, GE, Motorola, or whoever said
the radio would put out 500 watts, it sure did with a little extra in
reserve. Anybody`s trusty Bird wattmeter would give very nearly the same
reading in measuring that output in that instance.
I think Bird is truthful in their accuracy claim. There are many reasons
other than a poor sample being measured to find inaccuracies. Bird gives
quite a list starting with the standard against which the sample is
being compared. There is the signal being measured which must be a clean
sinusoid with no spurious signals attached. Temperatures must be
controlled. The right media must be properly used, and it all will fail
if the operator doesn`t know exactly what he is doing.
We have two metering experts testifying that they`ve found the Bird
wattmeter very short of the claimed accuracy. I don`t know how many
samples they`ve tested. I`ve seen thousands of measurements with no
reason to question the results. Questionable slugs have been rare
indeed.
But, it happens. You can`t put 10000 watts throough a 1/10-watt slug
without the possibility of damage.
I own no Bird stock and I don`t know anyone who works for Bird. I`m not
loyal. I`d buy something else in a heartbeat if it were cheaper and
better. I never thought I`d be defending something I thought was common
knowledge. The Bird isn`t just something that works well, it`s about the
only thing that conveniently measures power in the field accurately
while the equipment being measured is operating, (normally?)
Richard Clark
(Richard Harrison) wrote:
>
>I think Bird is truthful in their accuracy claim. There are many reasons
>other than a poor sample being measured to find inaccuracies. Bird gives
>quite a list starting with the standard against which the sample is
>being compared. There is the signal being measured which must be a clean
>sinusoid with no spurious signals attached. Temperatures must be
>controlled. The right media must be properly used, and it all will fail
>if the operator doesn`t know exactly what he is doing.
>
>We have two metering experts testifying that they`ve found the Bird
>wattmeter very short of the claimed accuracy. I don`t know how many
>samples they`ve tested. I`ve seen thousands of measurements with no
>reason to question the results. Questionable slugs have been rare
>indeed.
Hi Richard,
I've seen the same thousand, and qualified (the less encompassing
version of calibrated) all of them. Absolutely every Bird that came
through the door failed to meet standards. There is no way on god's
green earth that any casual observation will reveal accuracy or
inaccuracy in any reading.
But really, this is of no importance. Most measurements of power are
not concerned with absolute accuracy, just relative accuracy. Taking
the same out of spec Bird and measuring forward and reverse power
suffers equally and after a fashion balances out. Birds are robust
and good service monitors, but are not absolutely accurate.
And George, your Kachina probably exceeds the accuracy of the Bird,
but using a Bird to prove it is rather getting the horse before the
cart. Now when I say accurate, as he stated that the settings offered
correlated good readings, this only proves the scale linearity of both
instruments, but says absolutely nothing about absolute accuracy. The
one relative to the other offers very good evidence of relative
accuracy, which for his purposes is more than adequate. However,
observing a new out-of-the-ordinary reading is suggestive, but lacking
a third (non-Bird) standard, it is simply a coin toss as to where to
point the finger at the offending device. All should note that this
out-of-the-ordinary reading is still within the so-called 5%
specification from Bird.
Richard Harrison
"Absolutely every Bird that came through he door failed to meet
standards."
I could fix `em like that too. Or maybe they just sent Richard the bad
ones.
We had Termaline meters in addition to the Thruline meters. The
Termaline meter is a Dummy load with an R-F voltmeter calibrated in
watts across the dummy load. Our Thrulines in series with the Termalines
would would usually read very nearly the same. There is more than
coincidence involved here.
Dr. David Kirkby
> Dr. Kirkby is too pessimistic. I spent over a quarter century with a
> company which operated over a thousand radios which all got regular
All I can say is that we were a calibration lab, with temperature and humidity
accurately controlled. We were a government lab, with standards that were
internationally traceable. This was a whole different level of accuracy compared
to transmitters whose output power possibly depended on temperature, mains
voltage etc.
> + or -5% is not claimed as laboratory accuracy. But, compared to any
> other commonly available field measurement RF wattmeter, the Bird
> deserves the reputation it has with amateurs and professionals alike.
I'm not saying Bird claim +/- 5% is laboratory accuracy - what I am saying is
that few meters meet that 5% spec. Most meet +/- 10% of FSD, but few meet 5%.
The number failing to meet +/- 10% was not small, but I can't recall exact
numbers.
For what it is worth, I've met other professionals who share the same opinion as
me. I bought a Bird 43 as a ham and when a much older ham told me they were not
very good, I did not believe him. Perhaps I did not want too, as I was at school
at the time and a Bird 43 was quite a financial investment for me. The chap who
told me this had worked for the Ministry of Defense too.
I'm not active now, but if I did become active again, I would probably buy a
Bird again. They have an accurate 50 Ohm section of line (unlike most ham
meters) and are accurate enough for most amateur use, but accurate to +/- 5% of
FSD they are not.
I think we will have to agree to differ.
--
William E. Sabin
Richard Harrison wrote:
> We had Termaline meters in addition to the Thruline meters. The
> Termaline meter is a Dummy load with an R-F voltmeter calibrated in
> watts across the dummy load. Our Thrulines in series with the Termalines
> would would usually read very nearly the same. There is more than
> coincidence involved here.
A Bird 43 running *near full scale* that has 5% of full scale accuracy can
in principle be off by 0.2 dB. Isn't this plenty good enough for most
practical purposes?
One other thing. If a mfr guarantees 5% of full scale as it leaves the
factory, it had better be substantially better than that during assembly.
Else at the factory inspection station the number of rejects will be as much
as 50 percent. This would be intolerable for any mfr. The tail of the
probability curve should be quite small beyond the 5% level. This makes
the "expected" accuracy more like 3% of full scale.
It's my understanding also that wattmeters with digital readout are more
accurate and that a lot of the error rating is budgeted to the D'Arsonval
meter.
Bill W0IYH
Richard Harrison
"What I am saying is that few meters meet that 5% spec."
I had a thought while conversing with George, W5YR. Our technicians
always carried their Bird wattmeters with them. I leave mine in my coax.
I disconnect the coax from my radio during frequent thunderstorms.
That`s not much of a problem in Great Britain, but it is here. I don`t
remove my Bird Wattmeter, so, to some extent the wattmeter is still in
harm`s way. Unless I mend my ways, my meter may soon join Dr. Kirkby`s
majority.
Best regards, Richard Harrison, KB5WZi
Bill Nelson
: Finally! Corroborating testimony from another who has worked at the
: bench with these instruments.
: The 5% specification was always a fantasy and as you indicate even for
: BRAND NEW.
Not surprising - as they contain analog meters, and most meters are not
accurate to better than 10% FSD.
--
Bill Nelson (bi...@peak.org)