Reflection-less matching for antennas?

[EngrN]

Hi all,

We know that for maximum power transfer Zs = ZL* , this is known as maximum power transfer matching. But there is this reflection less matching which says Zs = ZL. Now if the Z has reactive part then there would be a difference. In antenna feed matching network design which method should we consider and why?

Reference: https://en.wikipedia.org/wiki/Impedance_matching

Kind regards,

 

[tony_lth]

Who said "But there is this reflection less matching which says Zs = ZL. "?

In maximum power transfer Zs = ZL*, reflection less matching

 

[mtwieg]

This is a common point of confusion. Zs=ZL gives no "reflections" in terms of travelling waves, while Zs=ZL* gives no reflections in terms of power waves (which is what we care about for power transfer). See **broken link removed** for more details.

 

[EngrN]

Thank you for the reference - got the point that they look at the matching from different perspective but the matching condition remains the same that is real part(i) = real part(j) and reactive (i) = reactive * (j) - to be conjugate of the other .

So for an antenna "It is a well-known fact that there exists a power reciprocal relation between the transmitting and receiving antennas. However, it seems to be less well understood that it is between the exchangeable power from the source and the actual power to the load that the reciprocal theorem generally holds. Unless the matching conditions for both antennas are satisfied, the reciprocal theorem does not necessarily hold between the actual powers nor between the exchangeable powers."

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If we design a matching circuit for the antenna for maximum power transfer/ reflection-less matching , it should be effectively equally whether the antenna is transmitting or receiving? I think it would.

 

[mtwieg]

If we design a matching circuit for the antenna for maximum power transfer/ reflection-less matching , it should be effectively equally whether the antenna is transmitting or receiving? I think it would.

Correct, assuming everything is linear and reciprocal.

 

[biff44]

I guess the answer is, who cares about the reflectionless condition? I can put a 10 dB attenuator between my antenna and source, and get no reflections...but it would not help the system link budget!

 

[EngrN]

There are many scenarios where you have to make sure that there are no reflections for instance generating few watts gets expensive as the frequency goes up so you do not want to waste that power by reflections or in radars these reflection might end up at the receiver causing false alarms, or in a beacon working on batteries, reflections would result in draining the battery earlier.