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High power RF measurement setup - RF Cafe Forums

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 Post subject: High power RF measurement setup
Posted: Thu Mar 08, 2007 10:57 am 
 
Captain
User avatar

Joined: Thu Mar 08, 2007 10:29 am
Posts: 10
Hello,

I am hoping i can get some information and advice about taking measurements at high power.

i am thinking of a kind of RF system that is like this:

a klystron delivering about 150kW power to a cavity via waveguides.
in between klystron and a circulator, i connect a directional coupler to couple power to measure the delivered power, and then another directional coupler between the circulator and cavity to couple power to measure the reflected power.

to measure reflected power from a cavity, maybe 1 to 3kW, would i connect an attenuator in between the cavity and reflectometer please? or are there reflectometers that take inputs at high power please?

I look forward to hear of any advice or info i could use to understand more about high power measurements, and make my setup more realistic.

thank you for your kind attention.

regards,
tmntRF


 
   
 
 Post subject:
Posted: Mon Mar 12, 2007 4:31 am 
 
Captain
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Joined: Thu Jan 11, 2007 12:59 pm
Posts: 16
Hi,

normally you get this attenuation in the coupler - ie if you have a coupling factor of let say 60dB you will have a signal 60dB lower at your measuring output. In your example 150kW is the same as about 82dBm, with 60dB coupling you will have 22dBm to your powermeter. If this is to high you can easily attenuate it with a small attenuator - or choose a higher coupling factor.
Remember that you could be interested of a lower coupling factor for reflected power measurement since the reflected power, under normal conditions, is much lower than forward power.

Regards
Jens


 
   
 
 Post subject:
Posted: Mon Mar 12, 2007 6:56 am 
 
Captain
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Joined: Thu Mar 08, 2007 10:29 am
Posts: 10
sm7ovk wrote:
Hi,

normally you get this attenuation in the coupler - ie if you have a coupling factor of let say 60dB you will have a signal 60dB lower at your measuring output. In your example 150kW is the same as about 82dBm, with 60dB coupling you will have 22dBm to your powermeter. If this is to high you can easily attenuate it with a small attenuator - or choose a higher coupling factor.
Remember that you could be interested of a lower coupling factor for reflected power measurement since the reflected power, under normal conditions, is much lower than forward power.

Regards
Jens


Hi Jens,

Thank you for your reply and information.

May i ask, this coupler that you mentioned, it should be built-in inside commercially available power meters??

Regards,
Yi


 
   
 
 Post subject:
Posted: Mon Mar 12, 2007 2:18 pm 
 
Captain
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Joined: Thu Jan 11, 2007 12:59 pm
Posts: 16
Hi,

no the coupler is a piece of waveguide that you have to place in line with your existing waveguide. You then connect your powermeter-sensor to the coupled output of the coupler.
As you mention the coupler for measuring output power could be placed before the circulator and the coupler for reflected power after the circulator - seen from the transmitter.
You don't mention the type of waveguide or frequency, but I'm sure that whatever the type there are couplers to buy.

Regards
Jens


 
   
 
 Post subject:
Posted: Mon Apr 02, 2007 11:21 am 
 
Captain
User avatar

Joined: Thu Mar 08, 2007 10:29 am
Posts: 10
Hi Jens, how are you?

May i further ask, does the attenuator, while attenuating the power level to a reasonable level for monitoring, effect the frequency of interest at all please? would a practical attenuator introduce reactances to the RF signal and then effects the frequency of the signal i would like to measure please?


 
   
 
 Post subject:
Posted: Mon Apr 16, 2007 2:21 pm 
 
Captain
User avatar

Joined: Thu Jan 11, 2007 12:59 pm
Posts: 16
Hi tmntRF,

no a real attenuator made for high frequencies is purely resistive, (or at least in theory), it'll never affect your frequency.

You will have some mismatch, ie reactance, but most often this is so low you will not notice - if you use an attenuator made for RF frequencies. If you build your attenuator by yourself, it could be different of course.


Regards
Jens




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