KB6NU's Ham Radio Blog

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Counting down to the new Ampere

Posted: 30 Aug 2016 10:24 AM PDT
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Many moons ago, Keithley came out with a meter that could measure
femtoamps. Being the electrical engineering geeks that we were, my
colleagues and I calculated how low a current that actually was and joked,
Hey, at that current, you could probably count the electrons as they flew
by. Well, what do you know? Thats kind ofÂ*how theyre going to define the
new Ampere. I dont think that you have to be a measurement geek like me to
appreciate this work.

This article is from the National Institute of Standards and Technology
Tech Beat...Dan


After it’s all over, your lights will be just as bright, and your
refrigerator just as cold. But very soon the ampere the SI base unit of
electrical current will take on an entirely new identity,* and NIST
scientists are at work on an innovative, quantum-based measurement system
that will be consistent with the impending change.

It won’t be a minute too soon. The ampere (A) has long been a sort of
metrological embarrassment. For one thing, its 70-year-old formal
definition, phrased as a hypothetical, cannot be physically realized as
written:

The ampere is that constant current which, if maintained in two straight
parallel conductors of infinite length, of negligible circular
cross-section, and placed 1 meter apart in vacuum, would produce between
these conductors a force equal to 2 x 10–7 newton per meter of length.

For another, the amp’s status as a base unit is problematic. It is the only
electrical unit among the seven SI base units. So you might logically
expect that all other electrical units, including the volt and the ohm,
will be derived from it. But that’s not the case. In fact, the only
practical way to realize the ampere to a suitable accuracy now is by
measuring the nominally “derived� volt and ohm using quantum electrical
standards and then calculating the ampere from those values.**

In 2018, however, the ampere is slated to be re-defined in terms of a
fundamental invariant of natu the elementary electrical charge (e).***
Direct ampere metrology will thus become a matter of counting the transit
of individual electrons over time.

One promising way to do so is with a nanoscale technique called
single-electron transport (SET) pumping. Specially adapted at NIST for this
application, it involves applying a gate voltage that prompts one electron
from a source to tunnel across a high-resistance junction barrier and onto
an “island� made from a microscopic quantum dot. (See animation below.)

The presence of this single extra electron on the dot electrically blocks
any other electron from tunneling across until a gate voltage induces the
first electron to move off the island, through another barrier, and into a
drain. When the voltage returns to its initial value, another electron is
allowed to tunnel onto the island; repeating this cycle generates a steady,
measurable current of single electrons.

There can be multiple islands in a very small space. The distance from
source to drain is a few micrometers, and the electron channels are a few
tens of nanometers wide and 200 nm to 300 nm long. And the energies
involved are so tiny that that device has to be cooled to about 10
millikelvin in order to control and detect them reliably.

Conventional, metallic SET devices, says NIST quantum-ampere project member
Michael Stewart, can move and count single electrons with an uncertainty of
a few parts in 108 in the uncertainty range of other electrical units at
a rate of tens of millions of cycles per second. “But the current in a
single SET pump is on the order of picoamperes [10-12 A],� he says, “and
that’s many orders of magnitude too low to serve as a practical standard.�






Researchers surround an open dilution refrigerator that cools the SET unit
to near absolute zero. Clockwise from left: Michael Stewart, Bahman Sarabi,
Neil Zimmerman. Click on image forÂ*view of the SET chip.




So Stewart, colleague Neil Zimmerman, and co-workers are experimenting with
ways to produce a current 10,000 times larger. By using all-silicon
components instead of conventional metal/oxide materials, they believe that
they will be able to increase the frequency at which the pump can be
switched into the gigahertz range. And by running 100 pumps in parallel and
combining their output, the researchers anticipate getting to a current of
about 10 nanoamperes (10-9 A). Another innovation under development may
allow them to reach a microampere (10-6 A), in the range that is needed to
develop a working current standard.

“At present, we are testing three device configurations of different
complexity,� Stewart says, “and we’re trying to balance the fabrication
difficulties with how accurate they can be.�

In addition to its use as an electrical current standard, a
low-uncertainty, high-throughput SET pump would have two other significant
benefits. The first is that it might be combined with ultra-miniature
quantum standards for voltage or resistance into a single, quantum-based
measurement suite that could be delivered to factory floors and
laboratories. The overall effort to provide such standards for all the SI
base units is known as “NIST-on-a-Chip,� and is an ongoing priority of
NIST’s Physical Measurement Laboratory.

The other advantage is that an SET pump could be used in conjunction with
voltage and resistance standards to test Ohm’s Law. Dating from the 1820s,
it states that the amount of current (I) in a conductor is equal to the
voltage (V) divided by the resistance (R): I=V/R. This relationship has
been the basis for countless millions of electrical devices over the past
two centuries. But metrologists are interested in testing Ohm’s law with
components which rely on fundamental constants. An SET pump could provide
an all-quantum mechanical environment for doing so.

In a separate effort, scientists at NIST’s Boulder location are
experimenting with an alternative technology that determines current by
measuring the quantum “phase-slips� they engender while traveling through a
very narrow superconducting wire. That work will be the subject of a later
report.


* In 2018, the base units of the International System of Units (SI) are
scheduled to be re-defined in terms of physical constants, with major
changes in the kilogram, ampere, kelvin, and mole.

** Josephson voltage and quantum Hall effect resistance can be determined
via quantum constants to uncertainties of parts per billion or less.

*** The charge of a single electron will be fixed at a value of 1.60217X ×
10-19 ampere-second, where “X� will be specified at the time of the
redefinition. One ampere-second is the same as one coulomb.

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Blog.


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The proof of the pudding

Posted: 29 Aug 2016 11:43 AM PDT
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The old saying goes, The proof of the pudding is in the eating. Of course,
the same goes for amateur radio. We can argueÂ*about theory all day long,
but its on-air performance that really matters.

Last Saturday, I finally got a taste of some pudding. Last week, I put the
latest version of the 9:1 ununÂ*thatÂ*Ive been experimenting with into a box
(see below) and took it to the park to make some contacts.
The 9:1 unun I mounted in this box uses an FT82-61 ferrite core.

I first had to decide on a length of wire for the radiator. I chose 30-ft.
because thats the length that Thom, W8TAM, had such good success with a
couple of weeks ago. I also cut a 13-ft. counterpoise. Im not sure exactly
why I chose that length, but in the end, it seemed to work.

I shot the radiator up into a tree, connected the antenna and counterpoise
to the unun, and connected the ununÂ*to an antenna analyzer to make a few
measurements. On both 40m and 20m, the SWR that I measured was about 4:1,
well within the range of the antenna tuner in my KX3. One amusing thing
that I noted is that this measurement didnt seem to change at all on 20m
when I disconnected the counterpoise. It made a definite difference,
though, on 40m.

Just for kicks, I also tried to measure the SWR without the unun. In both
cases, the SWR was greater than 10:1.

I could have made more measurements, but I wanted to make some contacts. I
tuned up on 40m and found AA3EJ calling CQ. He heard me right away, and we
had a short QSO. By the way, I decided on working 40m as Rich, KA8BMA, had
also come to the park today to try out his end-fed antenna, and he was on
20m.

Next, since the band sounded like it was in decent shape, I thought Id give
SSB a try. Tuning around 7200 kHz, I found an NPOTA station in Kentucky
calllingÂ*CQ. After a couple of tries, he finally heard me. That was
actually my first SSB contact with the KX3.

I dont think I can draw any real conclusions from this operation, except to
say that the ununÂ*does indeed transform a high impedance to a lower one,
and that I was radiating some RF with the antenna. More experimentation is
definitely in order. I want to try a longer radiator and maybe different
lengths of counterpoise. I also want to try building a 64:1 ununÂ*and see
how the SWR measurements compareÂ*to the SWR measurementsÂ*made with a 9:1
unun.

In addition, I want to get some smaller gauge antenna wire. I was using
some 18 AWG wire that I had on the shelf. While that wire is great for
stationary dipoles, I think more flexible, smaller gauge wire is more
appropriate for field application.

Im also going to be modify the 9:1 ununÂ*assembly. Rick noted in watching
WG0AT videos that his ununÂ*used a panel-mount male BNC connector. That
allows him to plug the ununÂ*directly into the radio, so theres no need for
a jumper cable. Rick bought a bunch and gave me one on Saturday. Thanks,
Rick!

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