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I have zero experience with antennas and little experience with electronics. I need to detect weak radio signals in the range ~100MHz - 1.3GHz and analyze them with an oscilloscope.

I bought a large, broad-band log-periodic dipole antenna. I also have an excellent oscilloscope that has more than sufficient bandwidth and sampling rate for such signals. The antenna is intended to be mounted on a roof and does not include a mast or any way to mount it (I guess the manufacturer assumes you to have these things.) I only want to use it indoors and would prefer to be able to move it out of the way often, since its quite bulky. Do I need amplification or additional grounding?

What I did so far:

The antenna has an N-type connector output. I used a BNC adapter and directly connected the antenna (which was just lying on a table) to the oscilloscope. This seemed to work: I saw some signal (microvolts) and could also identify some data packets from surrounding communication devices, indicating that the antenna is working. The low signal amplitude is not a huge problem for me, although I'm still thinking about amplifying it somehow.

However, after some time, I felt some static electricity shocks when touching the BNC connectors. This made me worried about damaging the oscilloscope.

Why does the antenna charge up?

The radio signals should not contribute to this, right? Do I need to ground the antenna somehow? Tutorials on installing TV antennas mandate to ground separately the mast to which the antenna is fixed.

However, this seems to be a precaution for lightning, which is not relevant in my case.

Is it OK to just connect the signal directly to my oscilloscope?

JRE
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Adomas Baliuka
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  • *I felt some static electricity shocks* I think it is more likely that there is capacitive coupling between the cable to the antenna and mains wiring in the building. You can measure that voltage with an AC voltmeter (multimeter). As the current cannot become that large, even if this voltage gets up to 100 V AC, it usually is not dangerous. – Bimpelrekkie Nov 24 '21 at 14:40
  • Yes it is OK to connect the antenna directly to the oscilloscope but you are taking a **risk**, if lightning hits it might induce charges into the antenna, you might damage the oscilloscope. Adding an amplifier might help with this. Then if you're lucky, only the amplifier gets damaged. I'm assuming the amplifier costs much less than the oscilloscope. – Bimpelrekkie Nov 24 '21 at 14:42
  • @Bimpelrekkie: The antenna is indoors. If lightning hits it there the whole house is likely to be damaged - including the operator, never mind the scope. – JRE Nov 24 '21 at 14:44
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    Realize that most oscilloscopes have by default an input impedance of 1 Mohm. Also realize that a long, coaxial antenna cable will have a **characterisitc impedance** of 75 or 50 ohms. For the most accurate (less signal reflections) you might need to **terminate** the cable at the oscilloscope with 50 Ohms or 75 Ohms. Some fancy oscilloscopes have a setting for this, if not, use a BNC terminator. – Bimpelrekkie Nov 24 '21 at 14:46
  • @Bimpelrekkie my oscilloscope does have a setting for this, which I am using (at 50 ohms, which is what is specified for the antenna). – Adomas Baliuka Nov 24 '21 at 15:15
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    I'm afraid you cannot extract much information of the time domain signal if there's many simultaneous signals caught at the same time by the oscilloscope plus some wide band noise. A specrum analyzer would be more useful. That's not the same as making Fourier transform to the signal which is caught by the oscilloscope into the memory - many oscilloscopes have such functionality. But a proper spectrum analyzer has more. It has filtering and amplifiers which together make possible to detect much weaker signals . –  Nov 24 '21 at 16:24
  • (Continued) About the electric shocks: Is it possible that your oscilloscope is intended to have a 3 pin grounded mains AC plug, but you use it without the protective ground? Many people have done so to avoid noise gathering ground loops via the protective ground. But then they offer to themselves and to all connected devices what's leaking through the EMC filtering from the mains AC to the signal ground of the oscilloscope - that's about 50% of the mains AC voltage through about 0,1uF capacitors. –  Nov 24 '21 at 16:35
  • @user287001 Thanks for your comments! My application needs to record and analyze the signal over a short time interval capturing a broad range of frequencies. So the oscilloscope is used to first check if everything looks fine and then to record traces of the signals with high resolution in both time and amplitude. The measured signals are analyzed off-line in software. /// The oscilloscope is powered by a standard 3-pin wall plug, including the protective ground, with no modifications. I don't fully understand your comment but what you describe should not be an issue. – Adomas Baliuka Nov 24 '21 at 17:00

3 Answers3

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Antennas are usually connected to Spectrum Analyzers. Looking at the RF signal in the time domain may not provide much information about the signal, like signal distortion. Distortion of a signal can be a very slight change in the wave shape which is hard to see without a reference waveform for comparison (good luck lining those up), whereas on a spectrum analyzer, the distortion would clearly show up as a spur, or some other aberration on the signal of interest.

As for static electricity, it was more likely built up on you from walking around and then discharged to the large metal of the antenna. Put on some anti-static gear and see if that problem goes away.

Aaron
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The oscilloscope should be grounded through its power cord. It will normally have a three pin plug - hot, neutral, safety ground. It should not be capable of being statically charged.

The antenna ground is connected to the oscilloscope ground and from there to the safety ground of the house. The antenna should not be capable of retaining a charge.

You, on the other hand, are not grounded. You move around and pick up a static charge.

When you touch the scope or the antenna, your charge discharges through the ground connection of the oscilloscope. That's the same as the "zap" you get when you touch a door knob in the winter. The same thing is happening with you and the scope and the antenna.

You need to be grounded. There are grounding straps made to connect you to the safety ground in your house. That might be a little inconvenient when trying to wave the antenna around. The ground straps for people tend to have wires around 1 meter (three feet) long.

Alternatively, you can try to prevent your body from charging up:

  • Wear cotton or linen clothing (blue jeans, cotton T-shirt, cotton underclothes, cotton socks.)
  • Do not wear shoes, or wear leather soled shoes.
  • Remove rugs or work in an area that isn't carpeted.
JRE
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You have two big problems to overcome with the setup you've proposed.

  1. The voltage levels into your O'scope are probably going to be much lower than what the O'scope can process. Have you done any sort of analysis to figure out the power levels your antenna will "see" and so the voltage those RF signals are going to induce on your antenna, and what the voltage will be into your O'scope?

  2. Without any sort of front-end filter (tuner), even if you overcome the voltage level problem, the 'scope will just see a whole bunch of different signals across the entire bandwidth of your antenna.

Like Aaron said, a Spectrum Analyzer is a much better tool for this than an O'scope.

SteveSh
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    A software defined radio is a cheap alternative to a spectrum analyzer, plus, demodulation and waterfall displays are available with SDRs. – qrk Nov 24 '21 at 20:13