Microstrip line wider than quarter wavelength.

[shmily0447]

Hi,

Under the condition that the length of a microstrip line is much larger than the width:

Does anyone know why the microstrip line is always designed narrower than a quarter guided wavelength?

What if the microstrip line is wider than guided wavelength?

Thanks.

 

[reidintransit]

The width is generally chosen to acheive a specific input impedance. Maximum power transfer occurs when the input impedance matches the load impedance. Wider traces take up more space. TXLine is a free microstrip calculator that has proven itself to be quite accurate.

 

[shmily0447]

Hi,

Thanks for the reply.

I mean what if the characteristic impedance is the same for two different MS lines, but on of the them is much larger than the other. Of course, the substrate thickness and permittivity may differ.

 

[reidintransit]

your structure will stop behaving like a microstrip line and allow other modes, increased radiation losses. increased noise.

 

[vfone]

For very wide microstrip lines, the fields are almost all in the substrate, while narrower lines have more field energy in air.
The insertion loss of the narrower microstrip lines is reduced because, air essentially has no dielectric loss compared to standard circuit board substrates.
Meantime, wider microstrip lines have lower current density, and thus, lower ohmic loss.

 

[reidintransit]

http://www.prof-robertson.com/index_files/MICROSTRIP.pdf see page 8

 

[shmily0447]

Hi,

Thanks. It helps.

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Hi, Thanks.

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Hi,

On page 8, it warns against the width of MS line as half guided wavelength rather than a quarter wavelength.

I am wondering what if the line width is a quarter wavelength. 'Cause I always see a warning in TXLine when then line width is greater than a quarter wavelength. Any idea about that?

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Hi,

Is that right to say, the wider microstrip the lower Ohmic loss but the higher substrate loss. The narrower microstrip the higher Ohmic loss but the lower substrate loss.

In that case, which loss dominates?

I mean the total loss a microstrip line.

 

[reidintransit]

**broken link removed** has a great explaination of this and many other basic topics in microwave engineering.
Transmission line attenuation Rule of Thumb #79
Different loss mechanisms have different behaviors over frequency. Metal loss is proportional to square-root frequency. Dielectric loss is proportion to frequency. Dielectric conduction loss is constant over frequency.
metal loss (what you are calling ohmic loss) dominates lower frequencies and dielectric loss dominates at higher frequency.

 

[shmily0447]

Hi Reidintransit,

Tks for the reference. Do you have any ideas of why we are warned against using the quarter wavelength wide microstrip line？

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Hi R.,

Do you have any ideas or experience of the Leaky Modes and spurious oscillations, as mentioned in 'The danger of high-frequency spurious effects on wide microstrip line' by D.R.Jackson?