[SI-LIST] Re: Open Termination
- From: Sitar Moniker <si_monkey2@xxxxxxxxx>
- To: gstokes@xxxxxxxxx
- Date: Wed, 15 Sep 2004 09:24:07 -0700 (PDT)
Thank you, Geoff. To be specific, let us consider a microstrip line the load
end of which is left open. There are two impedances. One is the impedance of
the line (V/I) and the other, beyond the point where the line ends, is the
impedance of free space (E/H) . The 377 Ohms I mentioned earlier corresponds to
this (E/H). Am I right so far?
If the open termination is modelled with fringing capacitance and possibly
radiation loss, what is the magnitude of this impedance so we could approximate
it with infinity?
V and I on the line are due to propagating E and H waves. Then, what is the
equivalent of a (V/I)=50 Ohms in terms of corresponding (E/H)? What is the
equivalent of (E/H)=377 Ohms in terms of (V/I)?
If ideal open circuit approx (Vi=Vr) is good especially at lower frequencies,
why do we routinely use it at higher frequencies as well? Is there a somewhat
realistic model for open termination?
Geoff Stokes <gstokes@xxxxxxxxx> wrote:
Sitar
In addition to your modern engineering books, you also need a classical book
on electricity and magnetism or microwave/rf engineering. This will show
that:
1. Impedance of a wave in free medium (e.g. air) is not the impedance of a
transmission line. One is E/H, the other is V/I.
2. Open termination is not infinite impedance and is sometimes modelled
with fringing capacitance and possibly radiation loss.
3. For the ideal open circuit (which does not exactly exist as I just said,
but sometimes it's good enough for an approximate analysis, especially at
lower frequencies), the incident wave voltage, Vi amplitude is equal to the
reflected wave amplitude, and in phase with it. So the total at the open is
2Vi. At some other point on the line, the phases are different, and so you
will plot a standing wave.
Good luck
Geoff
> -----Original Message-----
> From: Sitar Moniker [mailto:si_monkey2@xxxxxxxxx]
> Sent: 14 September 2004 22:48
> To: si-list@xxxxxxxxxxxxx
> Subject: [SI-LIST] Open Termination
>
>
> 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.
>
> 1) How do you get infinite impedance for unterminated line?
>
> 2) Is there any intuitive way to show that voltage at the
> open end doubles- other than using math: ref. coeft. = +1?
>
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