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Design >> RF Design >> Does conjugate match elimite reflections?
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Message started by xxgeneral on Jan 25^th, 2012, 8:45pm

Title: Does conjugate match elimite reflections?
Post by xxgeneral on Jan 25^th, 2012, 8:45pm

Hi everyone,

I've learned the tranmission line theory. If the resistance of load is not matched with the resistance of tranmission line, it will cause reflecting waves.
Then, I also studyed that conjugate match make maximum power transmission.

There are some questions confusing me a lot.
1. Does the reflection not exist when source and load is so close that no transmission line is needed?
2. Does no reflection exist under conjugate match?

Thanks in advance!

Title: Re: Does conjugate match elimite reflections?
Post by aaron_do on Jan 29^th, 2012, 2:52am

Hi,

from my understanding,

1. You can still use transmission line theory, but you don't need to.

2. Yes, there is no reflection under a conjugate match.

regards,
Aaron

Title: Re: Does conjugate match elimite reflections?
Post by xxgeneral on Feb 8^th, 2012, 6:03am

Hi,
Thanks for your reply!

In my opinion, if the impedance is not consistent when the signal is traveling, then the signal will be reflected at the impedance-unconsistent spot. Conjugate match makes use of reflected signal, so that load can get maximum power. On the other hand, if there is no reflection, the load can't get maximum power. In a word, reflected signal folds on the incident signal, resulting a larger signal which may let the load get more power.

Hope for any opinion about this.

Thanks a lot!

Title: Re: Does conjugate match elimite reflections?
Post by aaron_do on Feb 8^th, 2012, 5:23pm

Hi,

Quote:

Conjugate match makes use of reflected signal, so that load can get maximum power. On the other hand, if there is no reflection, the load can't get maximum power.

This is a strange way of thinking about it. I'm not sure if its correct. Maybe you could explain what you mean further. In theory, if there's no reflection (and no loss), you will get maximum power transfer.

Anyway if you're interested in a little literature, I suggest you read "Power Waves and the Scattering Matrix" by K. Kurokawa, IEEE MTT 1965.

regards,
Aaron

Title: Re: Does conjugate match elimite reflections?
Post by RFICDUDE on Feb 8^th, 2012, 7:54pm

xxgeneral wrote on Feb 8^th, 2012, 6:03am:

Hi,
In a word, reflected signal folds on the incident signal, resulting a larger signal which may let the load get more power.

No, I do not think this is a proper way to think about the reflected wave.

First there is the forward wave and there cannot be anymore power than what the forward wave carries. Once the forward wave reaches the load then either all the forward power is absorbed by the load (perfect match) or some of it is reflected. The reflected wave represents the fraction of forward power that was not absorbed by the load.

The reflected wave adds in and out of phase with the forward wave to create a standing wave on the transmission line. But the real power is not more than what was delivered to the transmission line.

Title: Re: Does conjugate match elimite reflections?
Post by depend135 on Mar 11^th, 2012, 9:43pm

Hi, aaron:

reflection coefficient is defined as Γ=(ZL-Z0)/(ZL+Z0)
For conjugate match, ZL is not equal to Z0, why there is no reflection?

If reflection exists according to the equaltion, why maximum power transfer?

The conjugate matching and reflectionless matching really confuse me.

aaron_do wrote on Jan 29^th, 2012, 2:52am:

Hi,

from my understanding,

1. You can still use transmission line theory, but you don't need to.

2. Yes, there is no reflection under a conjugate match.

regards,
Aaron