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Design >> RF Design >> Impedance matching RX/TX: 3dB power loss?
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Message started by Visjnoe on Oct 26^th, 2009, 9:00am

Title: Impedance matching RX/TX: 3dB power loss?
Post by Visjnoe on Oct 26^th, 2009, 9:00am

I know this is kinda basic, but I found out I do not fully understand this, so here goes...

Suppose you have a wireless TX delivering a certain amount of power P into a 50 Ohm antenna load. Assume there is no path loss.
The wireless RX is well matched to a 50 Ohm receiving antenna.
How much power is now delivered to the LNA of the wireless RX?

I always thought the 'Thevenin' model for the antenna + wireless RX was a voltage source with a 50 Ohm source resistance, loaded with the 50 Ohm input resistance of the wireless RX. In such a configuration, you loose however 3dB of power (optimum power match).

Therefore, if you transmit e.g. +10dBm into the air at the TX side, do you only see +7dBm at your LNA? Or rather +10dBm? If you still have +10dBm, then where has the 3dB power loss over the optimum power match gone?

Regards

Title: Re: Impedance matching RX/TX: 3dB power loss?
Post by loose-electron on Oct 26^th, 2009, 11:46am

What happened to
- the radiation pattern of your transmit antenna
- the sensitivity pattern of your receive antenna
- the distance between receive & transmit antennas
- Losses of the ether medium :)
- rotational orientations of antennas and their polarizations.

Those are the ones that quickly come to mind.

There is a little more to the "Can you hear me now?" question than whats going into the antenna at the transmitter.

Look up - Transmitter-receiver link budgets and antennas and propagation as a start point to your research.

Title: Re: Impedance matching RX/TX: 3dB power loss?
Post by Visjnoe on Oct 26^th, 2009, 11:59am

Thanks for the pointer to further literature.

I am aware however that the issue is much more complicated than I presented it.
I am however in doubt even on a nearly-ideal system. So I need to understand this
basic issue first, before I start adding things that make it more real and complex.

So any brief explanation on the -3dB power loss (if existing) is more than welcome

Regards

Title: Re: Impedance matching RX/TX: 3dB power loss?
Post by loose-electron on Oct 27^th, 2009, 8:47am

Maybe this set of articles would be useful:

http://www.ets-lindgren.com/pdf/768ce.pdf

Title: Re: Impedance matching RX/TX: 3dB power loss?
Post by RFICDUDE on Nov 1^st, 2009, 4:00am

The "missing link" here is that the terminal resistance of the antenna is not due to thermal losses in the antenna; rather, it is due to the coupling of the antenna to freespace. The "available power" the antenna can deliver is related to how well the antenna is coupled to freespace at the frequency of interest. If it is well coupled, then almost all of the EM power incident to the antenna is collected and made available at the terminals of the antenna. There isn't a 3dB difference for this case because 50ohm internal resistance of the antenna is due to the coupling (and the form factor of the antenna) to freespace and not a thermal resistance.

Now when dealing with circuit matching the story is different because you are matching between thermal resistances (lossy sources and loads). When matching lossy sources, you are matching between two resistors, so 3dB of signal power is going to be dissapated in the losses of the source. This is one reason why power amplifiers are typically not conjugately matched (results in less power efficiency).

Hope this helps a little.
You can dig around in antenna books (see Balanis) for some better explainations of antenna impedance (what is it and where does it come from) and antenna efficiency.

Title: Re: Impedance matching RX/TX: 3dB power loss?
Post by Visjnoe on Nov 2^nd, 2009, 10:27am

Thank you all for your inputs and explanations.
I will try to catch up on the antenna literature...

Regards