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I have a loop antenna.

enter image description here

When I connect the probe from my oscilloscope, does the probe also act as an antenna? If so, how should I fix that? I read about feeding technique, but I still do not understand it clearly.

Is it okay that I cover my antenna with scotch tape? Is this a correct way of "shielding antenna"?

I want to test the magnetic direction finding method and the source of the signal is an electric spark. I read that loop antenna is suitable for this. I find a loop antenna in my lab and this is the specification I figured

  • Turn : 10 turns
  • Loop diameter : 11.6 cm
  • wire diameter : around 0.08 mm
  • length of winding : 0.9 cm
  • frequency : ?

I dont know how to calculate the inductance and capacitance for this circular loop antenna, ir how to measure it with tools in my lab, so I don't know its resonance frequency yet. But, I already test it with dc spark generator (pictured) and yes it detects something when the spark occurred. But, I am not sure about the frequency shown on the oscilloscope (around 130 kHz), and also about my question above.

So moving from this, I decided that I want to make my own square loop antenna, mainly because I already found the legit equation/formula on the book and internet for the inductance and capacitance, so I can calculate the resonance frequency.

About feeding the antenna: is it critical for any regular loop? Or at least in my case? I read that feeding antenna technique is connecting and transforming the radio signal into electrical signal (or vice versa); since I use an oscilloscope, is it balanced already? I only made the end of the loop on the bottom, so I suppose it is symmetric. and I cover all of the surface with scotch tape on top of it.

Note: Even if this experiment fails, I still need something to write about in the project report.

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    $\begingroup$ Hi, can you tell us: What is the antenna for? What frequency and application? How many turns? Will it stay connected to the scope, or be connected to something else? $\endgroup$
    – tomnexus
    Dec 20, 2020 at 16:32
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    $\begingroup$ What have you already read about feeding technique and what don't you understand? $\endgroup$
    – Phil Frost - W8II
    Dec 20, 2020 at 16:58
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    $\begingroup$ @PhilFrost-W8II is it critical for any antenna? would you kindly check my edit, sir? $\endgroup$
    – Ramanda Nvl
    Dec 20, 2020 at 18:06
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    $\begingroup$ I like this photo! 1. Can we see a close-up of the spark gap assembly on the left? I am wondering if the either the spark gap or the loop needs to be turned 90° and/or moved a little further away (thoughts, anyone?). 2. Why do you think it needs to be shielded? I don't believe that it does. 3. You can cover your antenna with plastic tape, but it will not provide any shielding effect whatsoever. $\endgroup$
    – Mike Waters
    Dec 20, 2020 at 19:51
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    $\begingroup$ 4. Is the black vertical cylinder to the left your impulse generator/power supply? It may contain a transformer that is coupling to your loop more than the spark gap is. If so, it may need to be shielded by covering it with metal foil. $\endgroup$
    – Mike Waters
    Dec 20, 2020 at 20:01

2 Answers 2

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When I connect the probe from my oscilloscope, does the probe also act as an antenna?

As it appears in your photo, yes. In some applications this may not matter, but in your case it sounds like your experiment relies on the loop having a predictable pattern for direction finding. In this case, having the probe (and also the scope chassis, and its power cord) as part of the antenna is going to significantly alter the directionality of the antenna.

Do address this, you require some kind of balun. One way to do this is to obtain a magnetic field probe for your scope and place it inside and coplanar with the antenna. The leads of the main loop where you have the scope probe now are then either shorted together or perhaps connected to a capacitor to make the loop resonant on the desired frequency. The antenna and the magnetic field probe are then coupled through their shared magnetic flux as a transformer.

There are other ways to do it of course, but it's a big enough topic for a question of its own.

You might try measuring the common-mode current to determine if your balun is effective. If handling or moving the scope probe changes your measurement, that's a sure indication that your feed method is ineffective, and what you are seeing on the scope isn't coming just from the loop.

Is it okay that I cover my antenna with scotch tape? Is this a correct way of "shielding antenna"?

Covering the antenna with tape is fine, but it's not any kind of "shielding". Perhaps you are referring to a shielded loop, which when properly designed is similar to a magnetic field probe. A shielded loop may be nice for your application as it contains a balun, but I'm not sure how one could be made with multiple turns. I don't know if that's problematic or not for your application.

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  • $\begingroup$ Thank you, sir! one more thing, let say i have bought magnetic probe. and put it inside and coplanar with my main loop. does the probe only receive signal coupled from my main loop? doesnt the probe also act as a loop antenna itself? $\endgroup$
    – Ramanda Nvl
    Dec 23, 2020 at 16:24
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    $\begingroup$ @RamandaNvl The probe and the antenna measure the magnetic flux through the area they encircle. With the probe inside the antenna, what magnetic flux could flow through the probe without also flowing through the antenna? By placing the probe inside the antenna you're making two circuits with a shared magnetic flux: a transformer. $\endgroup$
    – Phil Frost - W8II
    Dec 23, 2020 at 16:35
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Partial answer to part 1 of your question. Yes, the loop created by the open space between the ground lead of your scope probe and the probe tip acts as a small (parasitic) loop antenna. But mostly for wavelengths much shorter than 1/10th the diameter of the loop (potentially caused by the rising and falling edges of a spark). If you are just testing directionality, you might orient any parasitic loops to be in the same plane as that of the loop antenna under test.

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  • $\begingroup$ Thanks, sir. How about the oscilloscope impedance? will it also affect the frequency resonance calculation? $\endgroup$
    – Ramanda Nvl
    Dec 20, 2020 at 21:13
  • $\begingroup$ +1. What do you think about mounting a nearly identical loop as the transmitting antenna? Blocking capacitors between both sides of of the gap and the loop. $\endgroup$
    – Mike Waters
    Dec 20, 2020 at 23:59
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    $\begingroup$ More than just a "small (parasitic) loop", as long as there's no balun, all the common-mode current picked up by the scope's power cord, chassis, and the lead to the probe will be received at the feedpoint as well. $\endgroup$
    – Phil Frost - W8II
    Dec 21, 2020 at 13:16

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