Connect an antenna directly to oscilloscope

[Si14]

Hi,

Is it possible to direct connect an antenna working below 100MHz to an oscilloscope with coaxial cable and see the radiated signals?

Thanks.

 

[waht]

If you are close to a transmitter you might see a waveform that stands out from the noise, but in general you are not going to pick up anything meaningful from the air, except maybe a giant 60 Hz fuzzy sine wave from power lines. That's because all of the radiowaves within the bandwidth of the scope are super imposed on top of one another. What you see is a net effect of all radiowaves which is unintelligible.

A spectrum analyzer can resolve radio waves from the antenna.

 

[Si14]

Thanks. I see...
So the antenna can be connected to a spectrum analyzer directly?
Since typically a receiver is placed after the antenna and the role of that is to amplify and filter out the signal for the desired bandwidth. So the analyzer can act like a receiver?

 

[berkeman]

Thanks. I see...
So the antenna can be connected to a spectrum analyzer directly?
Since typically a receiver is placed after the antenna and the role of that is to amplify and filter out the signal for the desired bandwidth. So the analyzer can act like a receiver?

Yes, a spectrum analyzer has way more gain than an oscilloscope, and it is frequency selective. For weaker radio signals, though, you would still need a preamp between the antenna and the spectrum analyzer.

BTW, many spectrum analyzers have a "zero span" feature, where you can zoom in (in frequency) on a received signal like an AM radio channel, and then turn on zero span and drive a speaker with the audio output. Pretty handy when identifying landmark radio signals to help you get your bearings...

 

[Si14]

Thanks.
Is it safe to connect antenna directly to oscilloscope or spectrum analyzer?

 

[berkeman]

Thanks.
Is it safe to connect antenna directly to oscilloscope or spectrum analyzer?

Depends what you mean by safe. If you're outside in a lightning storm, then no. If you are inside, and are careful not to accidentally zap the antenna with electrostatic discharge (ESD) as you shuffle across the carpet, then yes, it's safe. You are talking about a receive-only antenna setup, right?

 

[Si14]

Yes, it is a receiver antenna. It is used indoors, inside a lab, so there's no carpet! Transmitter is placed about some meters away and it's power shouldn't be that much. Do I need to worry about the electrostatic discharges as I am handling the antenna?

 

[berkeman]

Yes, it is a receiver antenna. It is used indoors, inside a lab, so there's no carpet! Transmitter is placed about some meters away and it's power shouldn't be that much. Do I need to worry about the electrostatic discharges as I am handling the antenna?

You need to be reasonably careful not to ESD zap the inner conductor on the coax imput to the spectrum analyzer. It has a little ESD protection on it typically (depending on the manufacturer), but not a lot. Treat that inner conductor on the coax input jack to the spectrum analyzer (and anything conductive connected to it) with care.

 

[Si14]

Thank you very much for your prompt replies. Is it possible to get rid of the ES charges induced to the coax with some kind of shortening? For example connecting a grounded wire to that inner conductor?

 

[berkeman]

Thank you very much for your prompt replies. Is it possible to get rid of the ES charges induced to the coax with some kind of shortening? For example connecting a grounded wire to that inner conductor?

Before you plug your coax into the spectrum analyzer input coax connector, it is good practice to touch both the outer and inner conductors of the coaxial cable (or whatever feedline you are using from the antenna), and touch the grouded case of the SA. This discharges you, the coax, and the antenna.

Then be careful when you have the antenna exposed and connected to the SA -- just use usual care to avoid charging yourself up and zapping the bare antenna metal. Even without carpets, if you have chairs in the lab, you can acquire a significant static charge when sliding off of the chair.

 

[Si14]

Thanks. That was very informative.

 

[sophiecentaur]

There is nothing inherently different between an antenna and any old piece of metal. Many receiving antennae are of 'tuned length' but an office chair or a coathanger can also happen to resonate at a certain frequency. When this happens, more volts may appear at some point on it. You need to be very near to a high power source for this to worry you, however. Wire fences near high power broadcast transmitters can sometimes spark!
Remember, though, that the effect is not "static"; it is AC. Receivers always have some input resistance and may be 'matched' approximately, to optimise the received signal power so they are connected to a resistive load.

Spectrum analysers have much more gain than oscilloscopes, which incorporate wide band amplifiers. A spectrum analyser has very high gain but narrow band filtering and will select signals around one particular frequency and amplify them by many tens of dB - it is, in effect, a 'swept' radio receiver, whereas an oscilloscope is, essentially, a voltmeter with a graphical display showing how the total volts vary with time.

 

[dlgoff]

Even without carpets, if you have chairs in the lab, you can acquire a significant static charge when sliding off of the chair.

Even if you sitting and think you are discharged, you can generate several thousand volts just by lifting your feet off the floor. So I always keep my feet still after discharging myself by touching a ground source.

 

[sophiecentaur]

Thank you very much for your prompt replies. Is it possible to get rid of the ES charges induced to the coax with some kind of shortening? For example connecting a grounded wire to that inner conductor?

Use a loop antenna - that will short the inner to the outer and avoid any electrostatic charge.