[SI-LIST] Re: Open Termination
- From: "Sandor Daranyi" <sandord@xxxxxxxxxxxxx>
- To: si_monkey2@xxxxxxxxx
- Date: Thu, 16 Sep 2004 09:07:04 +1000
Hello Sitar,
Others have already given good information regarding your questions but sin=
ce you asked for something intuitive, maybe I can add something worthwhil=
e. The warning is that intuitive explanations can often be oversimplific=
ations.
> -----Original Message-----
> From: Sitar Moniker [mailto:si_monkey2@xxxxxxxxx]
> Sent: Wednesday, 15 September 2004 7:48
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Open Termination
>=20
> In SI and HSDD books, they assume that open termination offers infinite
> impedance. My understanding is open end line is exposed to air which has
> about 377 ohms.
>=20
> 1) How do you get infinite impedance for unterminated line?
The easiest way to see what happens is in terms of energy. (Looking at what=
happens to energy is a very useful technique when trying to understand s=
omething like this.) The driver pumps energy into the transmission line. =
The resulting energy wave travels down the line at close to the speed of =
light and eventually reaches the end. The energy COULD continue to travel=
in air but you would need a coupling mechanism between the transmission =
line and free air. If there is no coupling, it is irrelevant what would h=
appen in air, or indeed that it's 377ohms. If energy can't be transferred=
, then by definition the impedance must be infinite. If it weren't, there=
would be a finite impedance and energy could be transferred.
Nothing is perfect in the real world, so there can be some little coupling =
e.g. due to losses and radiation from the transmission line, so the menti=
oned infinite impedance is only an approximation, but for most practical =
cases, quite a valid one.
Of course, if you have an antenna at the end of the transmission line, sudd=
enly you do have a coupling mechanism and the story changes. Everything g=
ets a bit trickier though, because now you would have things like radiati=
on efficiency and the antenna itself would behave like an impedance trans=
former between the transmission line and free air.
> 2) Is there any intuitive way to show that voltage at the open end
> doubles- other than using math: ref. coeft. =3D +1?
From the previous answer, you may already see what happens here. When the f=
ace of the energy wavefront reaches the end of the transmission line and =
more energy still keeps arriving, it won't jump into air and it won't pil=
e up there, either. Instead, it will get reflected and the reflected wave=
will get superimposed on the arriving part of the wave. Why twice the vo=
ltage? Due to voltage superposition, which should sound familiar from bas=
ic electronic theory (a quick "voltage superposition" google search threw=
up: http://www.allaboutcircuits.com/vol_1/chpt_10/6.html).
Sandor
------
Sandor Daranyi
Snr Design Engineer
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List FAQ wiki page is located at:
http://si-list.org/wiki/wiki.pl?Si-List_FAQ
List technical documents are available at:
http://www.si-list.org
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
