complexe conjugate vs "bridging" impedance matchin

Hi everyone,

I was wondering if someone has a good explanation or article about when we should use complexe conjugate or bridging impedance matching (bridging is used for low output impedance seeing a high impedance to maximize voltage transfer).

Regards
Ghost

Re: complexe conjugate vs "bridging" impedance mat

Yeah, I am interested in this area too. Normally you want to do an impedance match. Say you have an antenna, a 30 foot cable, and a receiver--if the antenna is matched to 50 ohms, the cable is 50 ohm, and the receiver presents a 50 ohm input impedance, then you can connect them without any weird effects. That is a signal travelling down the transmission line will not significantly bounce back off of the receiver input. Such a bounce back would cause a standing wave on the transmission line, which can cause a very large gain ripple as the frequency is moved.

Similarly, if you want to transmit 100 watts with antenna, you pretty much want to transmit the maximum power possible into free space. In this case, theory says that you need to match the transmit antenna impedance to the source impedance!

But you pay some penalties for all of this. Your antennas need to be "matched". If you also want a physically small antenna, that means you need to match out the antennas reactance with a large tuning reactance. The bigger this reactance, the narrower the bandwidth will be where you can maintain a match.

In some cases, you might want a short antenna, but also want to use a broad band. In that case you might pick a short monopole, and make your receiver have a very high input impedance (mosfet, etc). So you maximize the voltage coming out of the antenna, and do not load the antenna voltage with any sort of low impedance receiver input. Obviously, in such an arrangement, you can not use a long length of transmission line between the antenna and receiver, or you might have 30 dB of gain ripple!

So I guess a "bridge" match is useful where you are using "short" antennas and do not need to have a long transmission line interconnect.

There are examples of the bridge match working successfully. In the audio industry, most connections are bridge matched. That is the source is a low impedance, and the load is a very high impedance (at least 10 times higher than the source's). In your car's AM radio, the antenna is very "short", so the AM receiver front end presents a high impedance load to the antenna, and does not try to do an imedance match.

Re: complexe conjugate vs "bridging" impedance mat

Yes biff I do agree with you, I'm trying to get some more input on the subject. I think a general rule of thumb may be that when the distance between components is short as compared to the signal wavelength, we can switch from conjugate matching to bridging. Now it is still sounds strange to me that the bridging is a very bad match powerwise but good for voltage when power and voltage are directly proportional!!!

Re: complexe conjugate vs "bridging" impedance mat

Much of microwave engineering is counter intuitive. It is the job of the microwave student to learn how the electrons dance around!

Check this out: