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Gain and bandwidth - RF Cafe Forums

RF Cafe Forums closed its virtual doors in 2010 mainly due to other social media platforms dominating public commenting venues. RF Cafe Forums began sometime around August of 2003 and was quite well-attended for many years. By 2010, Facebook and Twitter were overwhelmingly dominating online personal interaction, and RF Cafe Forums activity dropped off precipitously. Regardless, there are still lots of great posts in the archive that ware worth looking at. Below are the old forum threads, including responses to the original posts.

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Post subject: Gain and bandwidth

Unread postPosted: Thu Apr 28, 2005 1:11 am



Joined: Sat Apr 23, 2005 2:09 pm

Posts: 17

Location: Tampa, FL

Hi all,

I was just reading the Pozar book and on p.553 it mentions that "it is preferable to design for less than the maximum obtainable gain to improve bandwidth."

Is there a formal derivation to this statement? I'm curious why.





Post subject:

Unread postPosted: Thu Apr 28, 2005 1:25 am



User avatar

Joined: Tue Mar 15, 2005 11:43 pm

Posts: 65

Location: Singapore


I think it's related to this condition "Gain-Bandwidth is equal to constant", which derived from Bode-Fano Theorem.

Therefore, by designing for less than the maximum obtainable gain, wider bandwidth will be obtained.

Thanks and Regards,





Post subject:

Unread postPosted: Mon May 02, 2005 1:30 pm

The last post is absolutly correct. Fanos limit shows that for wider bandwidth, the gain will be reduced.



Post subject:

Unread postPosted: Tue May 03, 2005 8:49 am

Aint Bode-Fano's criterion showing the VSWR-bandwidth product that a matching network can have for a given quality factor mostly for passive devices shuch as antenas?

I always had the impession that remplacing VSWR by GAIN was an oversimplification. In my mind it only work as stated if the device in unilateral.

Second, what is this GAIN-Bandwitdh tradeoff? What gain? A flat gain or the Maximum available gain over frequency, whatever value or rolloff it shows?

Very often people will design matching network to autocompensate the gain rolloff and obtain a flat response, by sacrificing the VSWR.

I see Bode-Fano as a good starting point to sea if a matching is feasable.



Post subject:

Unread postPosted: Tue May 03, 2005 11:43 am

You are correct , it is a criterion for passive networks. The matching networks are passive, therfore, the wider the bandwidth the higher the loss, hence lower gain.

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