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I'm trying to take a simple measurement of ~2.4 GHz (WiFi land) with an RF spectrum analyzer (Tektronix RSA607A) but have so far been unsuccessful. On the RF In port I have an N-BNC adapter connected to a meter-long BNC cable, which is connected to a BNC-SMA adapter and WiFi antenna. All items in the chain are spec'd at \$50 \Omega\$.

Setup

All I am able to measure, however, is noise. I tried using several different cables, reattaching all connections, and even plugging the whole shebang into a Tektronix MSO3054's RF port. I 'know' the spectrum analyzer is working -- I plugged a waveform generator into RF In by way of a BNC cable and said N-BNC adapter, and the analyzer successfully measured a 1 MHz sine wave.

I'm sure it's something trivial, but we don't have any RF-experience persons around to consult. Does this setup appear obviously incorrect somewhere?

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  • \$\begingroup\$ You might need a pre-amp to boost the signal level. Mini circuits is a good place to look for one. \$\endgroup\$ – EE_socal Nov 16 '18 at 20:15
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    \$\begingroup\$ BNC connectors aren't a great choice at microwave frequencies. What are you using to make the wifi transmit? Can you try some other radio source (even at a very different frequency) to make sure you are using the analyzer correctly? Has the analyzer been damaged, perhaps by someone wiring it to a source that has a DC offset? \$\endgroup\$ – Chris Stratton Nov 16 '18 at 20:58
  • \$\begingroup\$ @ChrisStratton Ah, forgot to add -- I tried it with an arbitrary waveform generator, too, with success. Edited my answer. I will try other sources -- currently I am just trying to measure ambient WiFi and a nearby IoT device (~1 m away). \$\endgroup\$ – calcium3000 Nov 16 '18 at 21:07
  • \$\begingroup\$ @ChrisStratton Also, I am going to purchase an N-SMA adapter to plug the antenna 'directly' into the spectrum analyzer to shorten the chain a bit. \$\endgroup\$ – calcium3000 Nov 16 '18 at 21:08
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    \$\begingroup\$ Is there anyone in your facility familiar with the usage of this gear? Seems like you have a lot of pricey toys, some of them easily damaged. Typically a place that buys these has people who know how to use them, both from a functional perspective and a not blowing up the front end one. \$\endgroup\$ – Chris Stratton Nov 16 '18 at 21:09
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The setup looks fine, IMO, however, I highly doubt you'll see ambient WiFi signals coming through without some sort of amplification, especially if the noise floor of your spectrum analyzer is not that great. You may have some luck if you put your WiFi source much closer to the antenna (and oriented correctly, which will depend on the far-field radiation patterns of the antennas), but even that may be problematic.

As mentioned in comments, BNC isn't an ideal connector for this, but I don't think it should be a real issue. Microwaves101 lists BNC as rated up to 4GHz.

I don't have any realistic numbers off the top of my head, but I'd guess you'd need a noise floor lower than say -80 or -90 dBm to distinguish any ambient WiFi signals from noise.

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