**From:** Chris Rokusek (*[email protected]*)

**Date:** Tue May 29 2001 - 11:41:51 PDT

**Next message:**Neeraj Pendse: "Re: [SI-LIST] : RMS Definition"**Previous message:**D. C. Sessions: "Re: [SI-LIST] : DDR SDRAM without parallel termination ?"**In reply to:**[email protected]: "Re: [SI-LIST] : Antenna Problem on the Board"**Next in thread:**Doug McKean: "Re: [SI-LIST] : Antenna Problem on the Board"**Reply:**Doug McKean: "Re: [SI-LIST] : Antenna Problem on the Board"

Richard,

One widely used first-order approximation for far field radiation from a

trace over a ground plane is:

k * I(f) * f^2 * A

E(f) = --------------------

r

where k is a constant,

I(f) if current at a given frequency

f is frequency,

A is loop area (sep distance times length)

r is antenna distance.

The above ignores phase effects, line resonances, and discontinuities which

are also all major players.

Using a closed form first-order approximation for calculation of Zo (use a

field solver if you like):

Trace Width = 8 mils

Metal Thickness = 1.4 mils

Height above Ground = 10 mils

Gives Zo(height=10) ~ 72.0

And Zo(height=5) ~ 50.0

This change in height and Zo will change I(f) and A in the formula. Given a

20 Ohms driver:

I(Zo=50) Vdriver / (50 + 20)

-------- = ------------------- = 1.31 (31% increase in current)

I(Zo=72) Vdriver / (72 + 20)

But the A was halved: A(Zo=50) / Z(Zo=72) = .50 resulting in

E(Zo=50)

-------- = 1.31 * .50 = 65% (35% decrease in radiation)

E(Zo=72)

The calculation of Zo is highly non-linear so your mileage may vary.

Also, even if the voltage on the TLine is kept constant between the two

cases then I(Zo=50) / I(Zo=72) = 72/50 = 1.44 and E delta is 1.44 * .5 = 72%

(28% reduction in radiation).

Anyone see any mistakes or disagree with these assumptions?

Best Regards,

Chris Rokusek

Innoveda

-----Original Message-----

From: [email protected]

[mailto:[email protected]]On Behalf Of [email protected]

Sent: Saturday, May 26, 2001 10:43 AM

To: [email protected]; [email protected];

[email protected]; [email protected]

Subject: Re: [SI-LIST] : Antenna Problem on the Board

Ron,

you have touched on a question that has been with me for several years, and

that question is how do I minimize transmission line radiation. I noticed

in

an earlier thread (different subject) that higher impedance lines radiated

more than lower impedance lines. I don't understand why this is true. It

seems to me that a lower Z line, if excited by the same voltage, would

radiate more due to having a higher current in the line. Also, by the same

reasoning, I thought that a source terminated line would radiate less that a

parallel (at the receiver) terminated line, since only half of the current

and voltage would be required to get a full signal swing at the receiver (+1

reflection coefficient). Your statements seem to indicate that reflections

caused by impedance mismatches are the primary causes of radiation--this

means that a parallel terminated line is much superior to a source

terminated

line where radiation is concerned. Please help me get a better

understanding

of these fundamentals. I would also like to know how you employ the trig.

Richard Ellison

972-569-8317

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**Next message:**Neeraj Pendse: "Re: [SI-LIST] : RMS Definition"**Previous message:**D. C. Sessions: "Re: [SI-LIST] : DDR SDRAM without parallel termination ?"**In reply to:**[email protected]: "Re: [SI-LIST] : Antenna Problem on the Board"**Next in thread:**Doug McKean: "Re: [SI-LIST] : Antenna Problem on the Board"**Reply:**Doug McKean: "Re: [SI-LIST] : Antenna Problem on the Board"

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