How a Microwave Receiver Works: Voltage & Current Outputs

[SataSata]

How does a microwave receiver works? I can connect it to a multimeter and when a transmitter beam microwave to it, I received voltage and current output. When I put them closer, the output increase. How did the receiver convert intensity of the microwave to voltage and current? I assume the intensity is proportional to current, but what about voltage? And which one, voltage or current is more accurate in measurements considering environment factors. For example when I measure in current, the output is unstable and changing.

Here's a picture of how the receiver looks: http://www.pascocanada.com/images/products/wa/WA9800A_330_32633.jpg

 

[CWatters]

Is this a homework question?

Hints: Are microwaves AC or DC and what does a meter need to drive the moving coil?

 

[SataSata]

No, this isn't exactly a homework question. I'm experimenting on microwaves interference for my previous lab and this is just my own follow up. The reason is that I found many similar experiments online and that they all measured the intensity with current, but when I'm doing the experiment, the professor insisted that I measure in voltage. So I'm just curious about the difference.

So microwaves are AC and the receiver actually only have 2 cable slots to connect to a digital multimeter instead of what's shown on the picture with a moving coil meter.(I picked the picture because I can't find any other picture that look exactly like my receiver)

Normally if I were to measure the current and voltage of a circuit, the multimeter need to be connected 2 different ways, one for current and one for voltage. But in this case all I did is to plug the cables to the receiver and I can get both current and voltage output. So I assume one of it isn't directly proportional to the intensity. And further more the receiver stated output as current so I guess the output is connected in series so if I measure in voltage, won't the readings be wrong?

So knowing the inner circuits of the receiver is one thing, I would also like to know exactly how EM waves are converted to electrical circuit. Example through photoelectric, magnetic inductance or?

 

[davenn]

that receiver is just a microwave diode in a cavity and the meter is connected across the diode

depending on the frequency, and that feed horn looks to be around 10GHz ( you can confirm)
if it is around 10GHz the diode is commonly a 1N23, 1N415E or similar. The diode rectifies the incoming microwave signal and generates a DC current to the meter

cool stuff to play with ... I play on many microwave amateur radio bands from 2GHz to 24GHz

in the pic below is a simple transmitter / receiver cavity for 10 GHz from my collection

on the left is the tiny GUNN diode for TX and on the right the larger white barrelled 1N23 receive diode

Dave

 

[SataSata]

Thank you Dave. But I still don't understand how the diode convert microwave signal to DC current. What type of diode is it? Isn't there supposed to be something like a dipole antenna inside and the diode then will rectify the AC current to DC? Won't only one diode produce a half-wave output?

 

[davenn]

What type of diode is it?

They are usually point contact or Schottky barrier diodes

here's a little on Schottky Barrier diodes ...

A Schottky barrier diode (SBD) consists of a rectifying metal-semiconductor barrier typically formed by deposition of a metal layer on a semiconductor. The SBD functions in a similar manner to the antiquated point contact diode and the slower-response pn-junction diode, and is used for signal mixing and detection. The point contact diode consists of a metal whisker in contact with a semiconductor, forming a rectifying junction. The SBD is more rugged and reliable than the point contact diode. The SBD's main advantage over pn diodes is the absence of minority carriers, which limit the response speed in switching applications and the high-frequency performance in mixing and detection applications. SBDs are zero-bias detectors. Frequencies to 40 GHz are available with silicon SBDs, and GaAs SBDs are used for higher-frequency applications.

...there supposed to be something like a dipole antenna inside and the diode then will rectify the AC current to DC?

no, a dipole isn't used, in some cases a monopole antenna is used but that isn't even needed in this situation

I'm not up enough on semiconductor theory to speak about the interaction of the EM wave and the diode and how a current is produced, someone else may be able to answer that ... I did do some goggling after I posted my first response but didn't find any decent explanations Schottky barrier diodes