[Elecraft] OT: Bird watt meter?

Wed Jul 2 13:34:00 2003

Bird meters are well regarded because they are rugged and accurate and
versatile.=20

Yes, they are still manufactured, See http://www.bird-electronic.com/

The Bird uses a "directional coupler" to sample the RF. This sample is
rectified and displayed on an analogue meter on the older instruments =
most
Hams have or on a digital display on the newer versions.  The limiting
factor in ANY RF power meter is accuracy over a range of powers and
frequencies. Bird handled this by manufacturing directional couplers
designed to cover certain power levels and frequency ranges with the
greatest possible accuracy. The couplers are small cylindrical units =
that
"plug in" to the front of the instruments. They have been nick-named =
"slugs"
over the years. So you have one basic "meter" and buy the slugs you need =
for
the frequencies and power ranges you need.

The cylindrical slug can be rotated 180 degrees. That allows measuring =
the
power in each "direction" on the feed line, providing the ability to
evaluate the SWR on the feed line with the same instrument.=20

Modern Birds go down to 1 watt or less. I believe that the older "slugs" =
go
down to 5 watts full scale. Working from memory, I believe that the Bird
accuracy is rated at 5% of full scale.=20

The OHR is probably 1/4 or less the price of a Bird, even a used Bird in
good condition. It is not as rugged. It won't measure power in the =
gigahertz
range. It is an excellent example of a meter designed for a particular =
need
- those of the HF Ham operator running low power.=20

When considering accuracy, you want to realize that the specifications =
are
for FULL SCALE readings on almost any meter. The OHR, for example, =
claims 5%
accuracy at full scale. That means that if the meter is properly =
calibrated
and used on the 10 watt scale, the reading with 10 watts of RF applied =
might
be off by as much as %5 or  0.5 watts. At 5 watts the error might still =
be
0.5 watts for a 20% "accuracy". That's pretty typical for most Ham (and =
many
commercial) applications.=20

If you try to measure 1 watt with such an instrument on the 10 watt =
range,
your error might be 50% since the reading might be off by as much as 1/2
watt. You will notice that OHR offers a 1 watt full scale range. That's =
why.
Now you can switch to a lower range and read with good accuracy again.=20

That's also why you need to have a slug with a full scale reading as =
close
as possible to your desired power level when using a Bird too.

Bird has a wonderful reputation. I first used one in the 1950's at =
Lockheed
Aircraft. They were highly regarded for their ruggedness and accuracy =
then.
In those days there really wasn't much else one could use to measure RF
power with any accuracy at all, especially into the VHF range.=20

Frankly, for HF, most commercial "Ham" wattmeters in the $100 to $200 =
range
are as accurate as a Bird. They are certainly accurate enough for any
"serious" Ham work.=20

If you are looking at a used one, be aware that any instrument like that
deserves regularly calibration. Is it has been very gently treated over =
the
years and no one has tinkered with it, you might have an accurate meter. =
But
it's strictly a chance that you take unless you are willing to send it =
off
to a calibration lab for a checkup. On Bird's behalf, I have noticed =
that
the Bird meters I've used professionally over the years seem to hold =
their
calibration very well IF they are treated with respect. Very rarely have =
I
seen a Bird obviously "off", and it was always the fault of a damaged
"slug".=20

I would recommend something like the OHR for any new ham looking for a =
good
QRP power meter as a better "bet" since it is a LOT cheaper, plenty =
accurate
for any Ham needs, and can be recalibrated without taking out a mortgage =
on
the house.

Personally, I use a couple of homebrew meters and one or two built for =
"Ham"
use.=20

Ron AC7AC
K2 # 1289

-----Original Message-----
Hi folks -
Could someone tell me:
1 - Why are Bird meters so well regarded?
2 - Are they still manufactured?
3 - What is a slug?
4 - Will they measure QRP levels? If so, better than OHR meter? 5 - What
would be the recommended slugs, specific meter (if there is more than =
one
model), and accessories for measuring milliwatts up to 100 watts. 6 - =
What
would be a reasonable price range to pay for the above? 7 - What would =
be
the most common problem to look out for when shopping for a used meter?

Thank you - probably should reply direct - but other Elecraft users may =
be
interested in learning from some of the pros :-) Thanks folks. 73/NZ7C

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