The "Stockton" directional coupler has been getting a bit of press
lately on the list. I just wanted to point out that David Stockton,
GM4ZNX, did not invent that particular directional coupler himself,
as he himself readily admits, and that it's been around for quite a
few years and is well documented in the ham and professional press.
However, he and his SPRAT article are responsible for stirring up
renewed interest in it. (When I say "Stockton directional coupler",
I am referring to the circuit consisting of a pair of interconnected
toroidal coils with two pieces of coaxial cable, arranged to form a 4
port hybrid; it forms the heart of the "Stockton wattmeter" in SPRAT
#61, and is also used in the OHR WM-1 wattmeter.) Here are some
extracts from an unpublished article I wrote a couple years ago--
"This circuit may look vaguely familiar to some of you. It appeared
in the January 1987 issue of QST, in an article by John Grebenkemper,
KI6WX, titled "The Tandem Match--An Accurate Directional Wattmeter."
The same coupler circuit also appeared in the August 1979 issue of
Ham Radio, in an article by Hank Perras, K1ZDI, titled "Broadband
Power-Tracking VSWR Bridge." (The QST article incorrectly referenced
the May 1979 issue of HR.) In addition to that, it was also in the
November 1984 issue of the Hewlett-Packard Journal, "A Broadband
Two-Port S-Parameter Test Set" by W. M. Spaulding. Finally, it
appears as Figure 10 in the application for US Patent 3,426,298,
"Broadband Directional Coupler," patented on 4 February, 1969. (My
thanks to KI6WX for copies of the last two references, which he
included with his letter to me.)
"Issue #61 (Winter 1989/90) of SPRAT, journal of the GQRP Club,
contained an interesting article on a directional QRP wattmeter. It
was written by David Stockton, GM4ZNX, and has since come to be known
to many as the Stockton wattmeter, although the basic circuit has
been around for quite some time (see notes at end). This is a neat
little circuit, and his version uses two meters, one each for forward
and reverse power. A kit of parts is available from Kanga Kits in
the UK as well as Kanga US, in Ohio. The Stockton was reviewed by
WB8VGE in his QRP column in the October 1992 issue of 73 Amateur
Radio Today, as well as W5HKA in the January 1992 QRP Quarterly.
"The circuit is quite simple, consisting of little more than a
directional coupler, some load resistors, a couple detector diodes,
capacitors and a pair of meters. The coupler is beautifully simple,
very broadbanded, and is little more than a pair of toroidal
transformers arranged as a 4-port hybrid.
"A four-port hybrid is an interesting device. You pump power into
one port (A) and get almost all of it out on the other side (B). The
remaining two ports (C and D) are terminated with whatever impedance
you want to work with. A portion of that power from A to B is
coupled out to port C (hence the slight loss on the thru-side), where
it can be monitored with a scope, power meter or diode detector.
(Ideally, none of the forward power appears at port D, although in
actual practice there will probably be a bit.)
"If power is fed in reverse through the hybrid, from B to A, a
portion of that reverse power is coupled out of the other terminated
port (D). In other words, we have a directional coupler which can be
used to measure forward and reverse power, and calculate SWR from
those if we wish. The interesting part is that we can flip the
circuit from left to right or upside down and it still acts the same
way....."
After I wrote the article, I later stumbled on another reference
showing the circuit, this time with a good description of how it
works. Grab a copy of Introduction to Radio Frequency Design by Wes
Hayward, W7ZOI, recently reprinted by the ARRL, and look on page 157,
figure 4.37. There it is again. The description starts on the
bottom of page 156.
A couple years ago I did some extensive experiments with the
directional coupler circuit. I built up 9 (nine!) of them, each on a
separate piece of unetched PCB material with 4 BNC connectors, so I
could swap input/output and terminations, and do instant A/B/C
comparisons between any and all. (Yes, that ate up a full three
dozen BNC jacks, but it was only a dent in my collection :-) .)
Some of the variations were different numbers of turns, different
cores, different types of coax. I also experimented with the shield
of the coaxes--they say to ground one end, and one end only, of each
piece. Does it make a difference--what if you ground both ends?
Yes, it makes a difference, and I proved it (response suffers). What
type of core material is best? At HF, you can use type 43 or 61 with
no problems. Can you push performance to two meters? Easily, with
proper construction. Some day I might get around to polishing off
the article and publishing it, but it's already 9 pages on the
computer, which translates to 4 1/2 on the printed page, and that
doesn't include diagrams.
It's a nice, simple and elegant directional coupler circuit. My
thanks to GM4ZNX for bringing it to our attention again with his
SPRAT article several years ago.
73 and Queue Our Pea DE WA8MCQ
-- Mike Czuhajewski, user of the UniBoard System @ wb3ffv.ampr.org E-Mail: Mike.Czuhajewski@hambbs.wb3ffv.ampr.org The WB3FFV Amateur Radio BBS - Located in Baltimore, Maryland USA Supporting the Amateur Radio Hobby, and TCP/IP InterNetworking
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