[SI-LIST] Re: Open Termination
- From: Sitar Moniker <si_monkey2@xxxxxxxxx>
- To: sandord@xxxxxxxxxxxxx
- Date: Thu, 16 Sep 2004 12:04:02 -0700 (PDT)
Thank you. It definitely helps to understand in terms of energy. As you
mentioned, due to voltage superposition, the voltage at the open end gets
doubled. Extending this idea, will there be an occasion (ideal or otherwise)
when the open end voltage becomes 3X or 4X. If so, under what conditions? If
not, what limits this from happening?
Sandor Daranyi <sandord@xxxxxxxxxxxxx> wrote:
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
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