Diode for passive field strength meter - Page 1
Forum section: electronics / rf-microwave

author: RoGeorge
author_rank: Super Contributor
timestamp: April 18, 2023, 11:58:07 am
content: How to choose a more sensitive Ge diode out of the ones they happen to be in the scrap boxes?Most are unmarked, or with colored bands or dots, which I don't know how to decode. I've tried selecting those with minimum Vf when measured with a DMM, but it doesn't seem to be a direct relation between the Vf and detection sensitivity.Testing them with a DMM on DC mV (10uV resolution), by clipping the diode between the DMM's probes, and letting it sit on the workbench, while pressing the buttons of a 40MHz remote from a cheap RC toy that is less than 1 meter away. The setup is very sensitive to wires and body position.Is there a way other then try them all and see which one performs better?

author: A.Z.
author_rank: Super Contributor
timestamp: April 18, 2023, 12:15:15 pm
content: you may try the third circuit herehttps://www.homemade-circuits.com/field-strength-meter-circuit/and substitute the diode to find the best performing one (highest measurement) just replace the buzzer with a DMMAlternatively, you could follow the directions herehttps://www.petervis.com/Radios/making-a-crystal-radio/germanium-diode-test.html

author: RoGeorge
author_rank: Super Contributor
timestamp: April 18, 2023, 12:35:00 pm
content: In practice, the test results are not consistent. The detected RF level varies a lot with my position near the remote, and the results are very sensitive to the position of the DMM probes, too.Vf seems unrelated with the performance as a detector. I guess the ones I have with Vf small are not good enough at higher frequencies (40MHz). The thumb rule, that smaller Vf should make a better RF detection, is not true for my setup.

author: MasterT
author_rank: Frequent Contributor
timestamp: April 18, 2023, 03:03:37 pm
content: 40 MHz translates to 7.5 meters of wavelenght. Right antenna is about 1/4 or 1.9 meters. Likely probes of the dmm are shorter.Right way is attenuate signal from oscillator and make dirrect connection to the diode via dc blocking cap.I find out that 1N4148 / 1N916 is about 1.5 times less efficient for rectification 100 MHz sine wave than 1N5711.

author: iMo
author_rank: Super Contributor
timestamp: April 18, 2023, 04:27:23 pm
content: 1N5711 is the best RF detection diode you can get today easily and cheap.It works here at 70cm happily in my power meter (1.8-470MHz).There are some trick how to increase its sensitivity (ie a small forward current, like 10uA).

author: RoGeorge
author_rank: Super Contributor
timestamp: April 18, 2023, 04:41:52 pm
content: My comparison of Vf was between various models of Ge diodes only, and only from the ones that I have access to. On the DMM's diode-test, their Vf measure between 0.17V ... 0.48V at 0.8mA, depending of the model. The ones with smallest Vf were not the best detectors.Will test eventually by direct contact, through a small C, thanks.The only Schottky I have are in SMD format, soldered in various sizes of SMPS. I've read about Zero Bias Schottky Detector Diodes, too, but don't have any of these AFAIK:http://www.hp.woodshot.com/hprfhelp/4_downld/lit/diodelit/an969.pdf

author: David Hess
author_rank: Super Contributor
timestamp: April 18, 2023, 04:51:54 pm
content: I just test them in the circuit with an RF signal source and variable attenuator.Quote from: iMo on April 18, 2023, 04:27:23 pmI have had excellent results with the 1N270 germanium switching diode to 470 MHz and higher, but I have a couple thousand available.QuoteI like using a pair of matched diodes in a bridge configuration with RF fed to one side of the bridge. Then the bridge current is adjusted for maximum sensitivity. Taking a differential output allows the second diode to temperature compensate the first diode. The bridge should be laid out for best symmetry with the same coupling circuit on both sides and the input on the non-RF side grounded.

author: jonpaul
author_rank: Super Contributor
timestamp: April 18, 2023, 05:17:04 pm
content: depending on the freq range a PIN or Schokkty may be better.Most Ge are in axial TH pkg that limits the HF BWj

author: LaserSteve
author_rank: Super Contributor
timestamp: April 18, 2023, 09:20:54 pm
content: I once contacted Dr. Wenzel for a calibration curve for a 1n914... He was kind enough to run a quick curve. Way before I could afford nice gear..Please enjoy the results:http://techlib.com/electronics/detect.htmSteve

author: RoGeorge
author_rank: Super Contributor
timestamp: April 21, 2023, 09:19:16 am
content: Quote from: David Hess on April 18, 2023, 04:51:54 pm1. - Not sure I picture the exact schematic, does that means 2 diodes total, or 4 diodes?I was testing with the attached schematic, where there is no ground, but it still works because of the stray capacitance everywhere around the wires and parts. No DC filtering capacitor either, expecting the inductance of the uA-meter to average the current (don't know its inductance value, only R, 600\$\Omega\$). I guess the mechanical inertia will be a much bigger integrator anyway than the uA-meter's inductance.2. - Letting aside the DC bias option, am I testing this the right way (with no ground and no capacitor)?3. - Do I still need matched diodes if I don't need a calibrated output?I've only tested indoors, and the indication varies wildly with the surroundings. This makes me think any diode matching will be futile for my setup. My hope is to use the field strength meter to get a visual indication from the air, while tuning a Tx antenna nearby. A didactic exercise, to observe the match between reflections in the Tx antenna and the field in the air.

author: biastee
author_rank: Regular Contributor
timestamp: April 23, 2023, 05:30:32 am
content: Quote from: RoGeorge on April 18, 2023, 11:58:07 amYou didn't mention frequency range & application, though these factors will heavily influence the selection of diode and test setup.The detection sensitivity doesn't depend on Vf alone, as the diode capacitance also plays a role, especially at higher frequencies. At even higher freqs, the package parasitics also influence the sensitivity.You can compare the diodes' sensitivity by either simulation or experimentally. The former requires knowledge of the diodes' spice parameters. You can find the spice parameters for common Ge diodes in KJ Smith's Crystal Radio Handbook . On the other hand, experimental comparison requires a fixture to match the source / generator impedance (typ. 50 ohm) to the diodes' small signal impedance (fig. 1-2). At HF, a sloppy LC network can be used as the matching. Above UHF, then use a 3-stub tuner for the input match. The output load RL depends on your application. If your application connects the detector output to a DMM, then RL = 10 MOhm. On the other hand, in power harvesting applications, the RL is much lower to simulate battery resistance.Both simulation and experiment will yield a voltage sensitivity value, γ3 in mV / uW at a specific frequency. Use this γ3 number to compare different diodes.QuoteThe referenced zero-biased Schottky (HSMS-285x) is different from the rest because it is fabricated on p-type silicon , whereas normal Schottky diodes are n-type. Using p-type Si, increases the saturated current Is to ~ 3 uA (vs. n-type's pA range). Hence, the Is is equal to the external bias current applied to the n-type Schottky.QuoteDiode detectors are hard to match because their impedances are in the kOHm range. However, using 2 of the referenced diodes in a voltage doubling Greinacher configurationconveniently halves the input impedance, thus making impedance matching easier - see table 1 in this link.Referenceshttp://www.lessmiths.com/crystal/dtest.shtml B. L. Smith and E. H. Rhoderick, “Schottky barriers on p-type silicon,” Solid-State Electronics, vol. 14, no. 1, pp. 71–75, Jan. 1971

author: RoGeorge
author_rank: Super Contributor
timestamp: April 23, 2023, 11:31:35 am
content: Thanks, will keep reading the linked references.Quote from: biastee on April 23, 2023, 05:30:32 amThe hope is to use this field strength meter to get a visual indication from the air, while tuning a Tx antenna nearby, because it's the first time trying to match a Tx antenna. I know the basics about transmission lines, impedance matching, Smith charts, etc., but never used any of that in practice.The transmitter is just an AM modulator in the MW band (~1MHz or so), something like this (though the exact schematic is not yet decided):Posted in:https://www.eevblog.com/forum/rf-microwave/multi-stranded-lc-restore-from-an-am-pocket-radio/msg4816427/#msg4816427For coils I was using the ferrite rod antenna of a former AM pocket radio. No idea if such a small oscillator can be detected with a passive field meter. The signal is so low that an AM radio can not receive the modulator from further than 1 meter away (the Tx has no external antenna now, it emits through the ferrite rod antenna of the oscillator).Even if the signal from this particular oscillator is too small and the field meter can't detect it, would still a nice tool to have around for the lab. I will probably change that AM modulator schematic anyways, because it outputs a mostly square wave carrier.Briefly said, the frequency range and application are not well decided, something under 100-200MHz for now. 200MHz limit is arbitrary, because that's what I can scope/generate with the lab instruments I have.The field detector is just as the AM modulator, for the fun of it, plus a little hands on experience with RF.Didn't considered the input impedance of the detector yet, mostly because I don't know what would be the expected impedance of a telescopic antenna. I don't know much about antennas.Have found a telescopic antenna stick in the scrap boxes. It has about 75cm when fully extended, and thought of using it as the Rx antenna for the field meter.I've read last weekhttp://qrp.gr/ultimatextalset/index.htmwhere between others, the impedance of a simple, then voltage doubling detectors are considered. I'm not sure if considering the single diode detector as a "transformer" with ratio 1:\$\sqrt{2}\$, then deducing the input impedance in terms of transformer ratio squared is correct, or just a coincidence in results (Vdet in the next quote fromhttp://qrp.gr/ultimatextalset/index.htmis Vantenna entering the diode):Quote

author: RoGeorge
author_rank: Super Contributor
timestamp: April 24, 2023, 12:00:12 pm
content: One diode is different from all the others.The one on the right hand side has a flat contact with an S shaped golden metal band, while all the others have a needle contact point.LATER EDITThe diode with a metal band is not a Ge "flat contact" diode, as I wrote in the pics filenames, but a Schottky diode, sorry.

author: A.Z.
author_rank: Super Contributor
timestamp: April 24, 2023, 03:58:44 pm
content: forgive me, but if you're pruning an antenna, why don't you just use a gdo, a fsm doesn't seem the tool of trade to me, but then maybe I'm wrong

author: RoGeorge
author_rank: Super Contributor
timestamp: April 24, 2023, 10:55:43 pm
content: Don't have a grid-dip-meter, might build one once I'll get a good air variable capacitor.

author: biastee
author_rank: Regular Contributor
timestamp: April 24, 2023, 11:35:28 pm
content: Quote from: RoGeorge on April 23, 2023, 11:31:35 am:-) U want to experience the pitfalls of AM modulating the oscillator! These pitfalls are avoided by modulating the subsequent stages, e.g. driver & final amp, or, where no further amplification is desired, a variable attenuator.QuoteA long time ago, I witnessed a commercial shield-room being tested for shielding effectiveness.The test used a pair of telescopic antennas configured as dipoles. By adjusting the length of the antenna, the dipole could be brought into resonance, hence 75 ohm, at various frequencies in the VHF range. A balun is needed to connect the telescopic dipole to a coax cable. A no-adjustment solution is the biconical antenna - EMI labs commonly use this antenna over 25-300 MHz. Both antennas will ensure your detector sees a 75 ohm source impedance.The odd diode with the S-contact is a Schottky diode! See the 3rd photo here:https://en.wikipedia.org/wiki/Schottky_diode

author: A.Z.
author_rank: Super Contributor
timestamp: April 25, 2023, 07:00:17 am
content: Quote from: RoGeorge on April 24, 2023, 10:55:43 pmOh, you may just use a polyvariconhttp://sm0vpo.altervista.org/use/gdo2.htmhttps://youtu.be/-H2lxcZQNAE

author: iMo
author_rank: Super Contributor
timestamp: April 25, 2023, 08:13:05 am
content: With such a detector and a short piece of wire wired to the diode (as the antenna) you would see some larger meter deflection at VHF/UHF frequencies only. At 1MHz you would need large power to see something. That is how it works, I had/have such a detector (w/ 1xGa201, 2xGa201 and 1x1N5711) with 40cm long antenna a couple meters off my balcony antennas (X-50 for 2m/70cm and an EFHW antenna for HF bands) working with 30-100W. No way to see a signal off your 1MHz ferrite rod TX. You would need a larger frame antenna with the input LC tuned to the 1MHz to see "something" off any diode detector (couple of milliVolts perhaps)..PS: for your intended purpose a single transistor FM VHF TX (ie. 80-108MHz band) with a sub mW power would work just fine..

author: RoGeorge
author_rank: Super Contributor
timestamp: April 25, 2023, 08:30:26 am
content: Quote from: biastee on April 24, 2023, 11:35:28 pmIndeed, it all started by trying to improve this schematic (not mine):Fromhttps://www.eevblog.com/forum/rf-microwave/how-can-i-improve-this-cheap-am-transmitter/msg4785965/#msg4785965That seems like a Peltz oscillator (AKA differential oscillator) which I happened to tinker with some time ago (in audio). One thing led to another, and now having fun with all kind of RF and radio related topics.Agree that a nice sinusoidal oscillator followed by a differential pair as a modulator, followed by an eventual power amplifier and a matched antenna will make a proper AM modulator.Thanks for pointing to the Schottky diode picture, I didn't know some are in glass.Took that as a Ge diode, after reading somewhere Ge diodes might be other than needle-type contact (or maybe I am mixing things here, I think that webpage was about Ge diodes, don't remember where exactly, some diode used in a guitar effects circuit, can not find that page again).Later edit:Found it, from one of your links:http://www.lessmiths.com/crystal/dtest.shtmlQuoteI might have mixed "gold bonded" with "golden S-shaped metal band" seen in my diode, plus the diode was in the same envelope with the other Ge diodes.I wonder if there are any flat-contact Ge diodes?

author: David Hess
author_rank: Super Contributor
timestamp: April 26, 2023, 01:51:31 pm
content: Quote from: RoGeorge on April 21, 2023, 09:19:16 amThe bridge configuration I used was two diodes going to ground and two resistors going from the positive supply to the diodes to forward bias them slightly. The bias supply is adjusted for maximum sensitivity. The averaged DC signal is taken differentially with a high gain differential input amplifier.This configuration is more handy for removing the forward voltage drop of a silicon diode, so 1N4148s or whatever could be used as detectors with good sensitivity, but it works with any diodes. Next time I use it, I will try 2N3904 base-emitter junctions which should be really fast.

author: biastee
author_rank: Regular Contributor
timestamp: April 26, 2023, 02:19:44 pm
content: Like in the attached images?

author: David Hess
author_rank: Super Contributor
timestamp: April 26, 2023, 03:59:30 pm
content: Quote from: biastee on April 26, 2023, 02:19:44 pmThat is a full-wave version of the same thing, so same idea. That may be even better than what I did.