Dampening of a 2.4 Ghz signal

[steinar96]

Greetings.

I'm making a signal strength gauge for 3G transmitters (3G dongles and phones). I have a 35dBi directional 1990---2170 Mhz Yagi antenna and most transmitters seem to be transmitting max 36dBm. I have located a few rf signal strength ICs but very few of them measure as high as 36 dBm. I'm having a hard time finding a proper way to dampen RF signals at these frequencies so that the most powerfull transmitters don't saturate the design . A resistor divider at the input will surely effect the input matching of the antenna to the IC and there seems to be a short selection operational amplifiers working at these frequencies.

Suggestions are appreciated.

 

[davenn]

This is why a reasonable receiver system has AGC ( Automatic Gain Control).
If you do anything to permanently attenuate the strong signals, that attenuation will also be applied to the weak signals meaning they may not be receivable

Dave

 

[steinar96]

Thank you for your response. In my case though shifting the dynamic range upwards is alright since this is a near sensing gadget (in the same house/building as the transmitter). I don't mind not being able to pick up very weak signals.

 

[jiripolivka]

Thank you for your response. In my case though shifting the dynamic range upwards is alright since this is a near sensing gadget (in the same house/building as the transmitter). I don't mind not being able to pick up very weak signals.

If you need to estimate transmitter power, you must have a calibrated step attenuator in the receiver, or use a logarithmic converter in it. Your "35 dBi" antenna sounds very strange- with that gain at 2.45 GHz it must be a 2 m parabolic dish, or larger!

 

[biff44]

there are a number of ways of doing it.
1) use a sweeping heterodyne receiver with a narrow IF band to measure power
2) use switched banpass filters to record power in a narrow band
3) use varactor tunable filter (either banpass or bandstop)
4) measure broadband power in a dsp, and digitally filter
5) use frequency selective ferrite limiter
there are probably 10 or more other ways to do it

 

[jiripolivka]

there are a number of ways of doing it.
1) use a sweeping heterodyne receiver with a narrow IF band to measure power
2) use switched banpass filters to record power in a narrow band
3) use varactor tunable filter (either banpass or bandstop)
4) measure broadband power in a dsp, and digitally filter
5) use frequency selective ferrite limiter
there are probably 10 or more other ways to do it

From I can see now, you wrote about 2.4 GHz but really you want to check cell phone base station emission over 1.9-2.2 GHz. No yagi can have "35 dBi" at all!

You can use a simple detector with a 30-dB step attenuator with 10-dB steps Calibrate the detector from a signal generator, then add the attenuator value to read the power level.
As an antenna you can use a simple dipole with 1.6 dBi gain or a real Yagi, with 6 elements you can have ~6 dBi and a suitable pattern to get direction to a source from hand.

 

[steinar96]

Greetings.

The intent is not to measure the base station emission. But the emission of cell phones and 3G dongles and other transmitters which usually broadcast to a base station. The 35dBi is a number given for the prototype antenna which is actually off ebay so it may very well be way off. The Yagi was chosen for directionality and locating purposes. Don't like the idea of doing my own antenna.

How are T and Pi attenuators for this purpose, are there any problems with using simple resistor networks to attenuate the signal before it goes into a IC with a 50 ohm impedance, I have been considering using a IC of this kind or similar

http://cds.linear.com/docs/en/datasheet/5505f.pdf

It goes up to 18dBm so i need to attenuate the signal if the strongest devices are broadcasting with a strength of 36dBm. Basicly i need to shift the dynamic range of the input down.

Thank you for the responses so far.

 

[jiripolivka]

Greetings.

The intent is not to measure the base station emission. But the emission of cell phones and 3G dongles and other transmitters which usually broadcast to a base station. The 35dBi is a number given for the prototype antenna which is actually off ebay so it may very well be way off. The Yagi was chosen for directionality and locating purposes. Don't like the idea of doing my own antenna.

How are T and Pi attenuators for this purpose, are there any problems with using simple resistor networks to attenuate the signal before it goes into a IC with a 50 ohm impedance, I have been considering using a IC of this kind or similar

http://cds.linear.com/docs/en/datasheet/5505f.pdf

It goes up to 18dBm so i need to attenuate the signal if the strongest devices are broadcasting with a strength of 36dBm. Basicly i need to shift the dynamic range of the input down.

Thank you for the responses so far.

I would only repeat that a Yagi antenna cannot have a gain >16 dB except extremely long narrow-band ones. A handheld Yagi with ~10 elements can have 6-9 dB when wideband.
I would not dare to use such antenna for MEASURING purposes. To point to a source, yes.

What you need ahead of a detector (calibrated between -10 to 0...10 dBm, BAR12 diode for example) is a switchable attenuator, 0-30 dB in 10-dB steps. Do not attempt to use an IC or digitally controlled IC attenuator, such devices cannot handle >18 dBm. Check mechanically switched attenuators, specified for +20 dBm (JFW, Wenschel, and others). At 2 GHz, do not try making such attenuator from resistors, it will not work as a calibrated device.