Someone has asked my magazine column where the name "hybrid coupler"
came from; and why do some people reserve the name "hybrid" for only
certain types of couplers?

That isn't as easy a question as it first seems. Everybody seems to talk
about "hybrids" but few authors are prepared to define exactly what it
means - much less explain why.

The following is a draft outline for my magazine article. It looks much
more authoritative than I actually feel about the subject - especially
about the origins of the term "hybrid", so I would welcome any
comments...


Hybrid is originally a biological term for a "cross-breed" that inherits
different characteristics from two different kinds of parents. The word
has then taken on a much wider sense - for example, we talk about
"hybrid road vehicles" or "hybrid analogue/digital systems".

However, a "hybrid coupler" is a much more specific term, which came
from the early days of landline telephony. Originally, a two-conductor
line could only be used in one direction at a time; but with a hybrid
coupler [Figure] at both ends, that single line can be used for full
duplex communication in both directions. An important feature is that
the coupler provides isolation between the links in opposite directions
[insert more description here, including bidirectional amplifiers].

So why was this device specifically called a "hybrid" coupler? There's
nothing particularly hybrid about the coupler, so my best guess is that
the word originally applied to the communication link - the
bidirectional offspring of two unidirectional parents. But in the
strange way that technical language evolves, "duplex" has become the
common word for a bidirectional link, while "a hybrid" has come to mean
the coupler itself. The same word has then been applied to RF couplers
that share some of the characteristics of the original telephone hybrid.

The best descriptions of RF hybrid couplers I've been able to find on
the web are in:
http://en.wikipedia.org/wiki/Power_d...ional_couplers
and:
http://www.rfcafe.com/references/ele...%20coupler.htm

In RF engineering the name "hybrid" has become attached to the 4-port
coupler in [Figure]. When ports 2, 3 and 4 are matched, the input power
at port 1 is divided equally between ports 2 and 3, with a phase shift
of 90deg between the two output ports. Since half the input power
appears at each output port, the transmission loss to each individual
port is 3dB, so this device is sometimes called 3dB coupler. The reason
why this device is called a hybrid is because of the port-to-port
isolation it provides. In the matched condition, ports 2 and 3 are
isolated from each other, and port 4 is isolated from port 1. [Add more
details of the 3dB/ 90deg/quadrature coupler configuration.]

However, the Wikipedia author(s) also point out that nowadays "any
matched 4-port with isolated arms and equal power division is called a
hybrid or hybrid coupler." This wider definition also includes 0deg
hybrids and 180deg hybrids, in addition to the original 90deg variety.
Some 3-port devices are also called hybrids because they provide
isolation between the output ports and have a 'virtual' internal 4th
port, an example being the Wilkinson 0deg power divider.

Of course, some devices that do not meet every part of the above
definition may still be called "hybrids" in practice. And a device that
offers port-to-port isolation is likely to have other names as well, eg
it may be called a "bridge". Are we confused enough yet?

Originally all these definitions were also restricted to passive linear
devices, but as op-amps get faster and faster, and move up into the RF
domain, it becomes possible to create the same port-to-port behaviour
using active devices. It is now possible to build active hybrids that
can operate up to at least 100MHz, and their advantage over
transformer-based hybrids is that they have no lower-frequency limit.
However, any hybrid still has to be a linear device.

[More to add...]



How does that look, as a first draft? I would particularly appreciate
comments on the probable history of the word "hybrid" in telephony and
electronics.





--

73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek