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Field strength meter -- diode

Started by mkr5000 December 31, 2021
Nothing could be simpler, an antenna, a coil and a 1N4148 diode but I'm seeing some schematics that have 2 diodes with one replacing the coil and the cathode connecting to the antenna.

Now that I'm old and stupid can someone tell me what that diode is doing please?

Thanks.
On Friday, December 31, 2021 at 7:20:03 PM UTC, mkr5000 wrote:
> Nothing could be simpler, an antenna, a coil and a 1N4148 diode but I'm seeing some schematics that have 2 diodes with one replacing the coil and the cathode connecting to the antenna. > > Now that I'm old and stupid can someone tell me what that diode is doing please? > > Thanks.
No worries, "old and stupid" is everyone's fate (if we're lucky). I assume this is the circuit?: https://coolcircuits.blogspot.com/2011/03/field-strength-meter.html the circuit needs a closed DC path in order for current to flow. A RF coil would do that, but has very low impedance at low signal frequencies - and so not much output to drive the meter. Using a diode in its place allows the circuit to work down to a very low-frequency cutoff (set by that 50n capacitor and the other components in the circuit). how practical this circuit really is, I'm not sure... cheers, RS
On Friday, December 31, 2021 at 2:33:46 PM UTC-5, Rich S wrote:
> On Friday, December 31, 2021 at 7:20:03 PM UTC, mkr5000 wrote: > > Nothing could be simpler, an antenna, a coil and a 1N4148 diode but I'm seeing some schematics that have 2 diodes with one replacing the coil and the cathode connecting to the antenna. > > > > Now that I'm old and stupid can someone tell me what that diode is doing please? > > > > Thanks. > No worries, "old and stupid" is everyone's fate (if we're lucky). > > I assume this is the circuit?: > https://coolcircuits.blogspot.com/2011/03/field-strength-meter.html > > the circuit needs a closed DC path in order for current to flow. > A RF coil would do that, but has very low impedance at low > signal frequencies - and so not much output to drive the meter. Using > a diode in its place allows the circuit to work down to a very > low-frequency cutoff (set by that 50n capacitor and the other > components in the circuit). > > how practical this circuit really is, I'm not sure... > cheers, RS
Gotcha. I spaced right through it needing a path. Would like to improve on it to possibly be able to drive an on/off circuit with a 5v pull up/down or something. Would there be a simple transistor amp (or an op amp possibly) that would be sensitive enough to react to the microvolt output of something like this? -- thanks
> Gotcha. I spaced right through it needing a path. Would like to improve on it > to possibly be able to drive an on/off circuit with a 5v pull up/down or something. > Would there be a simple transistor amp (or an op amp possibly) that would be sensitive enough to react > to the microvolt output of something like this? -- thanks
If you just want to see "something is there" and accuracy is not important, then the simple circuits, using transistors are fine. But if you want the reading to mean something real (electric field strength in V/m, etc.), then to get better linearity and range, I'd use an opamp. Any low-power op-amp would be OK, as long as it works at your battery's voltage. (If the sensed field strengths are high, then the amp stage may not be needed - the output from the diode can drive a sensitive meter directly.) I'd pull out my ARRL Handbook at this point. to give you more details. (My copy is in my office, far away...) Maybe we can find something online... =RS
On Friday, December 31, 2021 at 9:23:42 PM UTC, Rich S wrote:
> > Gotcha. I spaced right through it needing a path. Would like to improve on it > > to possibly be able to drive an on/off circuit with a 5v pull up/down or something. > > Would there be a simple transistor amp (or an op amp possibly) that would be sensitive enough to react > > to the microvolt output of something like this? -- thanks > If you just want to see "something is there" > and accuracy is not important, then the simple > circuits, using transistors are fine. But > if you want the reading to mean something > real (electric field strength in V/m, etc.), then > to get better linearity and range, I'd use an opamp. > Any low-power op-amp would be OK, as > long as it works at your battery's voltage. > > (If the sensed field strengths are high, then > the amp stage may not be needed - the > output from the diode can drive a sensitive > meter directly.) > > I'd pull out my ARRL Handbook at this point. > to give you more details. > (My copy is in my office, far away...) > Maybe we can find something online... > > =RS
this looks like a good starting design with a wide signal level range.. http://kf3g25cw.altervista.org/Links/ARRL_QST_FDM_Schematic_Corrections/Images/Corrected-202107-QST-Schematic.png (Schematic #2 from ... http://kf3g25cw.altervista.org/Links/ARRL_QST_FDM_Schematic_Corrections/ARRL_QST_FDM_Schematic_Corrections.html ) Though we could do much better, a modern choice, for the opamp, instead of an LM3900. Like, MCP6041-I/P MCP6043-I/P MCP606-I/P MCP6141-I/P MCP6143-I/P NJU7001D NJU7021D NJU7031D NJU7051D NJU7061D OPA705PA RE46C311E8F TLV2760IP TLV2761IP = RS
> this looks like a good starting design > with a wide signal level range.. > http://kf3g25cw.altervista.org/Links/ARRL_QST_FDM_Schematic_Corrections/Images/Corrected-202107-QST-Schematic.png > (Schematic #2 from ... > http://kf3g25cw.altervista.org/Links/ARRL_QST_FDM_Schematic_Corrections/ARRL_QST_FDM_Schematic_Corrections.html > ) > Though we could do much better, a modern choice, > for the opamp, instead of an LM3900. Like,
And Let's narrow down that list to just those with Rail to Rail Inputs MCP6041 MCP6043 MCP6141 MCP6143 OPA705PA RE46C311 = RS
On Fri, 31 Dec 2021 11:19:59 -0800 (PST), mkr5000 <mikerbgr@gmail.com>
wrote:

>Nothing could be simpler, an antenna, a coil and a 1N4148 diode but I'm seeing some schematics that have 2 diodes with one replacing the coil and the cathode connecting to the antenna. > >Now that I'm old and stupid can someone tell me what that diode is doing please? > >Thanks.
It's basically a 2-diode half-wave voltage doubler. A low barrier schottky diode might work better than the germanium antique, especially at high frequencies. -- If a man will begin with certainties, he shall end with doubts, but if he will be content to begin with doubts he shall end in certainties. Francis Bacon
Rich S wrote:
> >> this looks like a good starting design >> with a wide signal level range.. >> http://kf3g25cw.altervista.org/Links/ARRL_QST_FDM_Schematic_Corrections/Images/Corrected-202107-QST-Schematic.png >> (Schematic #2 from ... >> http://kf3g25cw.altervista.org/Links/ARRL_QST_FDM_Schematic_Corrections/ARRL_QST_FDM_Schematic_Corrections.html >> ) >> Though we could do much better, a modern choice, >> for the opamp, instead of an LM3900. Like, > > And Let's narrow down that list to just those with Rail to Rail Inputs > MCP6041 > MCP6043 > MCP6141 > MCP6143 > OPA705PA > RE46C311 > > = RS >
_Anything_ is better than an LM3900! Cheers Phil Hobbs -- Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com
On Friday, December 31, 2021 at 12:47:23 PM UTC-8, mkr5000 wrote:
> On Friday, December 31, 2021 at 2:33:46 PM UTC-5, Rich S wrote:
> > https://coolcircuits.blogspot.com/2011/03/field-strength-meter.html > > > > the circuit needs a closed DC path in order for current to flow.
> Would there be a simple transistor amp (or an op amp possibly) that would be sensitive enough to react > to the microvolt output of something like this? -- thanks
<https://usa.banggood.com/-30-0-30A-Galvanometer-Scientific-Current-Sensor-Sensitive-Ammeter-Electric-Current-Detector-Analog-Display-p-1441415.html> A moving-needle meter for 30 uA with 100 ohms internal resistance can cover the 3mV range, with 30 uV easily resolved. Add a battery for collector bias, and use a low-noise transistor B-E instead of one diode, and you're good for lower signals. So low, in fact, that some frequency tuning is a wise addition.
On Fri, 31 Dec 2021 16:19:49 -0800, John Larkin
<jlarkin@highland_atwork_technology.com> wrote:

>On Fri, 31 Dec 2021 11:19:59 -0800 (PST), mkr5000 <mikerbgr@gmail.com> >wrote: >>Nothing could be simpler, an antenna, a coil and a 1N4148 diode but I'm seeing some schematics that have 2 diodes with one replacing the coil and the cathode connecting to the antenna. >>Now that I'm old and stupid can someone tell me what that diode is doing please?
>It's basically a 2-diode half-wave voltage doubler.
Yep. More on how it works including an LTSpice model output (without the actual model): <http://k6jca.blogspot.com/2020/11/understanding-basic-rf-field-strength.html>
>A low barrier schottky diode might work better than the germanium >antique, especially at high frequencies.
Yep. "RF and microwave power detection with Schottky diodes" <https://www.infineon.com/dgdl/Infineon-AN_1807_PL32_1808_132434_RF%20and%20microwave%20power%20detection%20-AN-v01_00-EN.pdf?fileId=5546d46265f064ff0166440727be1055> The voltage doubler arrangement is Fig 15 (Pg 10). -- Jeff Liebermann jeffl@cruzio.com PO Box 272 http://www.LearnByDestroying.com Ben Lomond CA 95005-0272 Skype: JeffLiebermann AE6KS 831-336-2558