Loop antennas used for long and medium wave reception are based on the principle of a large loop whose inductance is tuned out with an air variable capacitor or varactor in series with loop. It does just what Doug describes: allows a lot of current to flow at a very sharply tuned resonant frequency.
Ken Javor Phone: (256) 650-5261 From: Douglas Smith <[email protected]> Reply-To: Douglas Smith <[email protected]> Date: Mon, 29 Jan 2018 22:55:06 -0800 To: <[email protected]> Subject: Re: [PSES] [SI-LIST] Re: Measurement Dilema - THE EXPLANATION! Hi Istvan, If I connect the oscillator directly to the scope chassis, I get similar readings at different frequencies. I generally get less amplitude as well. The small capacitance between the scope and oscillator is capable of tuning the whole system to a low input impedance on the probe end. I have seen this in many scenarios that ended up loading the oscillator to a near short circuit! The whole system is resonant. I only get enough current to get large probe voltages at discrete frequencies. I had to tune around to get the results you saw. These is a lot more information when I do the demo live for a class that are not practical to put in a video (would last way too long). I think amateur radio operators can identify with the amazing characteristics of resonant systems. A loop is not nearly dangerous as a loop terminated in a small capacitance! Tesla coils resonate using stray capacitance. I built a 600 Watt coherent 300 kHz RF driven Tesla Coil in the 9th grade, but that is another story...... Doug K4OAP Doug Smith Sent from my iPhone IPhone: 408-858-4528 Office: 702-570-6108 Email: [email protected] Website: http://dsmith.org On Mon, Jan 29, 2018 at 22:24, Istvan Novak <[email protected]> wrote: > Doug, > > I see your point, but I think we would need a little more convincing > argument for the amount of current you assume in this example. > > I agree with the approximate calculation of the L*dI/dt voltage drop, > where the ground-lead inductance is assumed to be 100nH and I assume you > measured the rise/fall times of the HC240 to be around 2ns. If we > assume 40mA current delta, we approximately get the voltages you see on > the screen. I am just not convinced that the current is really in the > order of 40mA. If I am not mistaken, you say (and the video suggests > the same) that the oscillator's return (its 'ground') is left floating. > In this case it matters much less what is the inductance of the > ground-lead loop (you say it is 100nH, not disputed), but for current to > flow through the oscillator output, it has to find its way back to its > return by going through the parallel of the two probes' braid impedance > (we can call it common-mode impedance) and eventually it has to close > back to the floating return of the oscillator. This last portion of the > current loop, closing back from the oscilloscope chassis/ground to the > oscillator return would need to be in the order of 50 pF or higher in > capacitive coupling to not limit to current to lower values. Judging > crudely from the video, the 'stray' capacitance of the oscillator might > very well be much lower. However, here is another possible contributor > to the waveforms on the oscilloscope screen: the oscillator imposes a > common-mode voltage onto the scope probes, which creates current in the > shields and these being passive probes, through the finite surface > transfer impedance of the cable braid, voltage is induced at the probe > connections. To prove/disprove the existence and amount of this > potential contributor, you could redo the test by eliminating the > ground-lead loop (for instance by using coax receptacles for the probe > tips), leaving everything else the same. If the displayed voltage drops > significantly, I would consider as a proof that in fact the dominant > source of the waveform is related to the ground-lead inductance. If the > displayed waveforms would not change considerably, it still would not > prove that the finite surface transfer impedance is the man cause, but > it would prove that there should be another major contributor beyond the > ground-lead inductance. In this letter case further tests could be > devised to nail down the major contributor. > > What do you think? > > Regards, > > Istvan Novak > > Oracle > > > > On 1/29/2018 3:13 PM, Doug Smith wrote: >> > OK Everyone, here is the explanation: >> > >> > The box is superfluous, it just hides what is actually going on. The box >> contains two heavy gauge wires. One shorts the two tips sig and gnd together >> and the other connects the center point of the first wire to the center pin >> of the BNC connector. This is equivalent to removing the box and putting a >> stiff wire into the BNC center pin on the generator and connecting both >> probe tips and both ground leads to this stiff wire. >> > >> > Doing so causes the generator to push a current out the prob e ground leads >> creating a voltage across the inductance of the leads that create a loop at >> the front of the probe. The current flowing through the ground leads creates >> magnetic fields that for the most part are captured by the loop formed by the >> ground leads and probe delivering the induced voltage to the probes. >> > >> > The probes only respond to voltage between their tips and ground lead >> attachment point, and the ground lead induced voltage is delivered to each >> probe by loop they form. >> > >> > Since the probes are lying apart from each other, one on the plastic table >> with a loop in its cable, and the other over a highly conducting (center >> layer) ESD mat, their common mode impedance will be different, varying at >> different frequencies. That results in different common mode currents on the >> probes and the ground lead induced voltages are therefore different and that >> is what is displayed on the scope. It is interesting that the current output >> of an HC240 Octal Inverting Buffer can induce volts across the ground leads >> when its output is only 5 V P_P, but entirely understandable if you calculate >> e = Ldi/dt = 100 nH*.040 A/2ns) as an approximation. >> > >> > For the generator to push a current onto the probe cables, it must form an >> image current somewhere and that is displacement current to the nearby scope >> chassis and some radiation from the oscillator itself into the "ether," >> although it is on the small side to radiate itself efficiently at 40 MHz. >> > >> > I suspect the total current being pushed onto the probes is about 40 mA, >> the short circuit current of the HC240 IC being used. At some frequencies, >> each probe cable will resonate with the capacitance back to the scope from >> the oscillator. By changing the frequency, I can make either probe register a >> larger signal than the other probe. >> > >> > I love experiments like this. I have been doing this one for over 20 years >> for my classes and have have tons more demos to challenge engineering minds. >> Most of my demos have an unexpected result and the discussion that follows >> elucidates some engineering principle or addresses a myth. >> > >> > Doug >> > University of Oxford, Course Tutor >> > Department for Continuing Education >> > Oxford, Oxfordshire, United Kingdom >> > -------------------------------------------------- >> > Doug Smith >> > P.O. Box 60941 >> > Boulder City, NV 89006-0941 >> > TEL/FAX: 702-570-6108/570-6013 >> > Mobile: 408-858-4528 >> > Email: [email protected] >> > Web: http://www.dsmith.org >> > -------------------------------------------------- >> > >> > >> > >> > On Mon, 29 Jan 2018 14:40:17 -0800, "Tom Dagostino" wrote: >> > >> > Why is everybody looking for tricks or complex answers. This is really a >> > very simple, basic issue. If you look closely you will see the difference. >> > >> > Tom Dagostino >> > 971-279-5325 >> > [email protected] >> > >> > Teraspeed Labs >> > 9999 SW Wilshire Street >> > Suite 102 >> > Portland, OR 97225 >> > >> > >> > -----Original Message----- >> > From: [email protected] [mailto:[email protected]] On >> > Behalf Of Austin Mack >> > Sent: Monday, January 29, 2018 2:28 PM >> > To: [email protected]; 'si-list' >> > Subject: [SI-LIST] Re: ***UNCHECKED*** Measurement Dilema >> > >> > Hi All, >> > >> > It looks to me like the oscillator and its power pack (with very long >> leads) >> > are in close proximity to the CH2 probe cable. Since the oscillator output >> > is tied to GND at the probes, it and the floating power pack will be >> > radiating like crazy at 40MHz and electromagnetically and capacitively >> > coupling to the CH2 shield which BTW is close to a quarter wavelength. >> > >> > Austin >> > >> > -----Original Message----- >> > From: [email protected] [mailto:[email protected]] On >> > Behalf Of Doug Smith >> > Sent: Friday, January 26, 2018 2:53 PM >> > To: si-list >> > Subject: [SI-LIST] ***UNCHECKED*** Measurement Dilema >> > >> > >> > >> > >> > Hi All, >> > >> > Can you explain the result in this video I just made? Scope plots of the >> > same two nodes are completely different. Probes and scope are operating >> > normally, no problem with the equipment itself. >> > >> > If you have been to my seminars you know the answer, please do not post the >> > answer unless you have not seen this experiment until now. >> > >> > Hint 1: There are no EM fields radiating from the shielded box affecting >> the >> > probes. >> > Hint 2: There are no active components inside the box. >> > >> > https://youtu.be/qj-HBFMEJiY >> > >> > I do a lot of experiments in my classes that give surprising results. Each >> > one addresses a design or troubleshooting problem that engineers do not >> > realize their troubleshooting efforts. Next >> > one:http://emcesd.com/bcsem_hfmeas.htm on March 13-16. >> > >> > Doug >> > >> > >> > >> > >> > >> > >> > >> > >> > >> > ------------------------------------------------------------------ >> > To unsubscribe from si-list: >> > [email protected] with 'unsubscribe' in the Subject field >> > >> > or to administer your membership from a web page, go to: >> > http://www.freelists.org/webpage/si-list >> > >> > For help: >> > [email protected] with 'help' in the Subject field >> > >> > >> > List forum is accessible at: >> > http://tech.groups.yahoo.com/group/si-list >> > >> > List archives are viewable at: >> > http://www.freelists.org/archives/si-list >> > >> > Old (prior to June 6, 2001) list archives are viewable at: >> > http://www.qsl.net/wb6tpu >> > >> > >> > ------------------------------------------------------------------ >> > To unsubscribe from si-list: >> > [email protected] with 'unsubscribe' in the Subject field >> > >> > or to administer your membership from a web page, go to: >> > http://www.freelists.org/webpage/si-list >> > >> > For help: >> > [email protected] with 'help' in the Subject field >> > >> > >> > List forum is accessible at: >> > http://tech.groups.yahoo.com/group/si-list >> > >> > List archives are viewable at: >> > http://www.freelists.org/archives/si-list >> > >> > Old (prior to June 6, 2001) list archives are viewable at: >> > http://www.qsl.net/wb6tpu >> > ------------------------------------------------------------------ >> > To unsubscribe from si-list: >> > [email protected] with 'unsubscribe' in the Subject field >> > >> > or to administer your membership from a web page, go to: >> > http://www.freelists.org/webpage/si-list >> > >> > For help: >> > [email protected] with 'help' in the Subject field >> > >> > >> > List forum is accessible at: >> > http://tech.groups.yahoo.com/group/si-list >> > >> > List archives are viewable at: >> > http://www.freelists.org/archives/si-list >> > >> > Old (prior to June 6, 2001) list archives are viewable at: >> > http://www.qsl.net/wb6tpu >> > >> > >> > > > ------------------------------------------------------------------ > To unsubscribe from si-list: > [email protected] with 'unsubscribe' in the Subject field > > or to administer your membership from a web page, go to: > http://www.freelists.org/webpage/si-list > > For help: > [email protected] with 'help' in the Subject field > > > List forum is accessible at: > http://tech.groups.yahoo.com/group/si-list > > List archives are viewable at: > http://www.freelists.org/archives/si-list > > Old (prior to June 6, 2001) list archives are viewable at: > http://www.qsl.net/wb6tpu > > - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe) <http://www.ieee-pses.org/list.html> List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher <[email protected]> David Heald <[email protected]> - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <[email protected]> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe) List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <[email protected]> Mike Cantwell <[email protected]> For policy questions, send mail to: Jim Bacher: <[email protected]> David Heald: <[email protected]>
