best active loop antenna circuit? - Page 1
Forum section: electronics / rf-microwave

author: coppercone2
author_rank: Super Contributor
timestamp: January 20, 2021, 06:16:18 am
content: So I bought a little portable tecsun radio and I just discovered my old active loop amplifier I built had been completely destroyed by corrosion, so I decided to remake it.Before I design a PCB for this, I was wondering if people found better or more advanced circuits to use, perhaps with an op-amp. I had used pretty much this beforehttps://www.qsl.net/m0ayf/active-loop-receiving-antenna.htmlI never got to play with it much though, and I was wondering if people have a preference. The antenna right now is a 1m loop, which I might also remake because its not bent very nicely and the aluminum weld in the middle is strong but has some nasty undercut. Having a quiet portable radio might actually make it fun to use this time.I think the corrosion might be some flux residue (maybe aluminum weld flux) that rinsed into the amplifier during brief outdoor use. Anyway this time I am looking to make it permanent rather then a dead bug prototype.Or just suggestions, maybe replace the neon with a fast GDT or something (I actually used a neon because it was in the parts box). This antenna will stay unscreened, I will make another screened one if necessary.Any does anyone have any thoughts on this:https://www.yumpu.com/en/document/read/43592064/wideband-active-loop-antenna-amplifier-with-passive-ik4hdqnet? I am NOT looking at building that because I am looking to reduce the number of magnetic elements not increase them, unless its off the shelf parts.

author: A.Z.
author_rank: Super Contributor
timestamp: January 20, 2021, 07:58:40 am
content: Hello CopperCone, let me start by saying that I'm not pretending that what follow are the "best" antennas, I'm just trying to throw some ideas on the table to attempt helping you to decide how to go on with your loop project, that being said, if you are (were) satisfied with the M0AYF activeloop preamplifier and are going to rebuild it, my first suggestion is to have a look at the design described in this paperhttps://www.larches-cottage.co.uk/rx_antenna/loops_rx/Evaluating%20a%20Broadband%20Active%20Loop%20Antenna.pdfthe antenna uses a slightly modified version of that preamplifier but with some twists and may be worth evaluating, not willing to go all the way and build a "custom coax" as the author did, one may just use a regular "fat" coax (RG8 or the like) and some PCV pipes or even a "hula hoop" to keep the coax loop in shape; the notes in the paper are somewhat sparse, so I suggest re-reading it multiple times to get a grip on how the whole thing is connected together, in short there's a "moebius" loop built using coax and connected like the "youloop" currently sold byAirSpy, the loop is then connected to a transformer which has a center tap on the antenna side to mantain the loop balance/pattern, the secondary of the transformer goes then to a modified version of the M0AYF preamp schematic, the resulting antenna gives good performance from LW up to above 30MHzIf you prefer to start simple and avoid soldering stuff, an alternative idea may be starting by building this simple (passive) loop antennahttp://www.kk5jy.net/rx-loop/the antenna is very easy to build and, not willing to use stiff wire as in the original version, it would suffice building a cross shaped support (using e.g. wood or PVC pipes) and then use some "fat" wire for the loop, the loop is fed using a simple transformer (see the link at the above page) and, as is, offers pretty good performances, then willing to turn it into an active loop, it would suffice connecting an off-the-shelf LNA (or whatever preamp) to the output of the transformerA third idea may be going for the Wenzel "loopifier" antenna described herehttp://techlib.com/electronics/antennas.htmlthis latter one uses a tuned preamp and going for it, I suggest building the version with the separate controlbox, this antenna uses a TL592 IC for the preamp but the design is pretty smart, since the IC is set for lowest gain and the tuning allows to reject off-band signals which could, on an untuned loop, cause IMD issues, the tuning isn't "narrow" so there's no need to constantly retune the loop when scanning a band, but just when hopping from a band to another one and the performance is pretty goodOk, here are my three suggestions, hope they may be of some helpForgot, first of all, here are the informations about another loop preamplifierhttp://www.lz1aq.signacor.com/docs/wsml/wideband-active-sm-loop-antenna.htmit's more complex than the M0AYF or Wenzel ones, but according to the informations found online it offers very good performance and may be worth checking, that preamplifier was used (for example) for this loop designhttps://sites.google.com/view/vk1hw/antennas/hermes-loopa total different idea for a (passive) loop is represented by the "Loop on Ground" (aka LoG), the design is from the same author of the passive loop I suggested above and can be seen herehttps://www.kk5jy.net/LoG/by the way, to set it up (or should I say down?) you'll need some backyard/garden space, but other than that, it's very simple to put together and offers good performances, here are the results of a test runhttps://swling.com/blog/2021/01/loop-on-ground-antenna-part-2-tom-upgrades-his-low-profile-low-noise-portable-dxing-antenna/the advantage of passive loops is that they do really "set the noise" and, at the same time, they won't overload the receiver input and this, with active loops may be a problem with some receivers, some preamps may boost the signal to the point that the receiver front-end gets overloaded with the result of de-sensitivizing the receiver, so I believe it may be important to keep this issue in mind when evaluating the various designs

author: coppercone2
author_rank: Super Contributor
timestamp: January 20, 2021, 09:06:55 pm
content: Shortened, too long and confusingDoes anyone have an opinion on what effect a steel project box would have on the loop if its directly under it? Will it cause field problems? The reason I ask is I have a pretty steel box I can use. The idea would be to make a plastic cube on top of the steel box that has the loop clamped to it with some wires or braid going into the steel box with batteries, preamp and local/remote and on/off switches. knowing this would let me know if my mechanical limits on the chassis design are known or unknown. The top plastic loop holder is aesthetically related to the electronics enclosure and kind of expensive to make so the idea of wasting building materials on building it is annoying. Let's just say my budget is blown on the 660 radio.Asthetically I want it to look like one of those lab loop antennashttps://www.tequipment.net/Com-Power/AL-130R/Antennas/?Source=googleshopping&gclid=CjwKCAiAxp-ABhALEiwAXm6IyRPpqslvgYBBKjGD-c_t3GX8OS30BDshWKWBHeUR-Yb8h41UHrUWIxoC-J4QAvD_BwEI might even bless this one with a LED lolSo, to put it bluntly,Does anyone see a circuit change or alternative circuit to the circuit chosen that has a very obvious benefit related to reception capabilities at the same form factor. From reading the other thread it seems that the options are1) signal quality (I guess distortion?, related to how pickup sounds)2) general sensitivityI chose that 660 radio in regards to the sensitivity of the blog review (the small sony claimed to have a better quality pickup but missed some signals), so does anyone think the choice of antenna preamplifier can 'weigh' the system in a particular direction to make it more useful? I fear if the radio is too sensitive and the antenna some how reduces sound quality at the cost of sensitivity I will have something that is annoying to listen to because of whine.

author: cdev
author_rank: Super Contributor
timestamp: January 21, 2021, 04:03:10 am
content: what about the malachite SDR radio?

author: coppercone2
author_rank: Super Contributor
timestamp: January 21, 2021, 05:05:24 am
content: that does seem useful because its portable and you might be able to quickly see differencesI got the one I got because honestly I wanted to see if I could find something to do while doing charcoal BBQread this about the malachute, it seems to have noise/signal problems according to the commentshttps://swling.com/blog/2019/11/the-new-malahit-dsp-a-portable-all-in-one-wideband-sdr-receiver/#comment-657213

author: A.Z.
author_rank: Super Contributor
timestamp: January 21, 2021, 07:57:23 am
content: Quote from: coppercone2 on January 20, 2021, 09:06:55 pmAssuming that the loop sits above the box (as for your description), if the loop is a shielded one, the effect of the metal box should be minimalQuoteSince you already stated with the M0AYF preamp, consider the modifications described here (already posted it)https://www.larches-cottage.co.uk/rx_antenna/loops_rx/Evaluating%20a%20Broadband%20Active%20Loop%20Antenna.pdfthe preamp schematic is the same, but with some optimizations and improvements, and the loop is a shielded one, built using coax cable, for details see the attached images and the PDF at the above URLQuotePickup SOUND ? An antenna picks up RF, not sounds, also, the antenna has no "sensitivity", the receiver doesQuoteYou're mixing different things here, dealing with loops, if properly built they'll give you the ability to null-out unwanted signals and/or improve the reception toward a given direction, the preamplifier helps a bit boosting weaker signals but then, if the receiver has already good sensitivity or a built-in preamp, the antenna preamp may overload the receiver input causing a number of issues (intermodulation, audio distortion and so on), to avoid those it's possible to add an attenuator and/or a gain control to the preamp

author: coppercone2
author_rank: Super Contributor
timestamp: January 21, 2021, 08:46:40 am
content: Yea I meant that the circuit has a focus on distortion vs noise floor? and that it might make things sound funny, then if the receiver is known to do something else that makes it sound funny, it might be crappy. Not sure if appreciable, I never played with RF turning into audio beforeNot sure if those parameters are even separable or directly related in the same direction. Can you have a circuit with decreased noise and increased THD (not totaling them as THD+N)? I know they usally measure with a tunable notch filter and then just measure RMS or power, but if the measurement also had a tunable band pass filter or perhaps performed with a PSD or if high power done with integrating a FFT.Ah with the update,Sorry, I glanced at it and missed figure 22, I thought it was suggesting something with a single transistor, I see it keeps the front end similar. Trifilar and a potentiometer I see. Did not read it yet but I recall something that a shielded loop has a decreased gain at the expense of protection from e-fields, from one of my old threads. Need to research that. IIRC the comparison with shields was some where herehttps://www.w8ji.com/magnetic_receiving_loops.htm*my memory might be of something related to resonance and multiple turns perhaps its not relevant to degrading the performance of a shielded single turn loopQuotehttps://users.monash.edu.au/~ralphk/shielded-loop-antenna.html

author: A.Z.
author_rank: Super Contributor
timestamp: January 21, 2021, 08:55:56 am
content: Quote from: coppercone2 on January 21, 2021, 08:46:40 amI'm at loss, can't understand what you mean by "focus ..." and "turning RF into audio", that's not the purpose of an antenna; at this point I believe you'd better go for the simpler passive loop approach, this onehttps://www.kk5jy.net/rx-loop/is very easy to build, just put together a cross shaped structure, wind a very simple (and cheap) transformer as shown herehttps://www.kk5jy.net/LoG/images/transformer-schematic.pngusing a binocular core (https://eu.mouser.com/ProductDetail/623-2873000202) and some enameled wire, place the transformer inside a small box, use some insulated wire to form the diamond loop, connect it to the transformer and then use a run of coax to connect the transformer to the receiver, done; being passive the loop won't overload your receiver frontend and will offer good performance, then if you want you may always add a preamp later

author: coppercone2
author_rank: Super Contributor
timestamp: January 21, 2021, 09:04:05 am
content: Not sure if those parameters are even separable or directly related in the same direction. Can you have a circuit with decreased noise and increased THD (not totaling them as THD+N)? I know they usally measure with a tunable notch filter and then just measure RMS or power, but if the measurement also had a tunable band pass filter or perhaps performed with a PSD or if high power done with integrating a FFT.

author: A.Z.
author_rank: Super Contributor
timestamp: January 21, 2021, 09:09:19 am
content: Quote from: coppercone2 on January 21, 2021, 08:46:40 amThe loop isn't the "standard" one, look at how the coax braid and center conductor are connected and you'll see it, also, take the time to fully read the PDF, you'll see that the author made some comparisons, using the M0AYF preamp, between a regular "wire" loop, a standard shielded loop and the "moebius" loop and the latter shown much better performances and nulling that the first two ones

author: coppercone2
author_rank: Super Contributor
timestamp: January 21, 2021, 09:51:46 am
content: I see a comparison between some kind of shielded loop and the unshielded loop talking about how quiet it is, then he just procedes to evaluate the new amplifier with two shielded antennas but the only frequency plots are comparing shielded loops ,Is there a picture of a weak signal taken with the shielded loop and unshielded loop ? the new design is certainly more of a hassle with the magnetic changeshere is my concern, it is based on things like thishttps://forums.radioreference.com/threads/magnetic-loop-antenna-questions-please.403661/#post-3274313*i assume this forum poster means weak communications signals that are useful not random bullshit on a spectrum analyzer

author: A.Z.
author_rank: Super Contributor
timestamp: January 21, 2021, 10:25:57 am
content: Quote from: coppercone2 on January 21, 2021, 09:51:46 amPlease see the "preliminary findings" at bottom of page #13 and also the section #4 detailing the modifications and the final resultsQuoteThe above discussion deals with the "standard" shielded loop, now look at figures #3 and #4 in the PDF, the selected configuration is #4 and it makes the difference (notice that while not shown in the picture - but detailed in text - the shield in figure #4 is connected to the center tap of the coupling transformer)I was wrong, they are discussing about the youloop, now, the youloop uses the same upper "twist" but the feedpoint connection is different (and imVHo not optimal) since the youloop doesn't use a tapped transformer and the shield of the loop is directly connected to the feedline, plus the youloop is totally passive while the "moebius" in the PDF is active and uses a better feedpoint design[edit #2]A question, is your choice of a loop antenna due to the fact that you live in a noisy RF area ? Because if that isn't the case, that is if you don't have so much/high man made RF noise around, there are other antennas which you may try and which are very simple to put together, for example, this onehttp://techlib.com/electronics/antennas.html#Improved%20Active%20Antennais really simple to put together and works pretty well from LW up to around 100 MHz, sure it isn't a loop, so doesn't have nulling capabilities, but may be worth a thought if your RF environment isn't very noisy

author: coppercone2
author_rank: Super Contributor
timestamp: January 21, 2021, 04:49:23 pm
content: I see other graphs with issues on the mobius design on forum posts, i.e. problems starting above 1.6MHz.https://forums.qrz.com/index.php?threads/moebius-loop-antenna.704219/page-5#post-5478889not sure if its an upgrade to HF , or at least it has more passband variationmay be because of the amplifier, seems to be a gamble

author: A.Z.
author_rank: Super Contributor
timestamp: January 21, 2021, 05:18:04 pm
content: that's one of the issues the author of that pdf solved, please and again, take the time to fully read it and you'll see how he obtained a good antenna going from LW up to 30 MHzjust in case, the trick is to minimize the inductance, that way the antenna works well at the higher frequencies

author: StuartA
author_rank: Regular Contributor
timestamp: January 23, 2021, 10:56:26 pm
content: For a long time, the mag loops made by Wellbrook were rated to be among the best; details of how to build a clone of their original model are given herehttps://www.george-smart.co.uk/projects/wellgood_loop/.I have one - very happy with it though haven't done any comparative tests. There is some discussion of what transistor to use in the clone; I used 2N3866 in mine. If you use 2N2222, be very sure you know what the pin outs are on your particular devices.

author: coppercone2
author_rank: Super Contributor
timestamp: January 23, 2021, 11:40:38 pm
content: that redesign of the amplifier I posted in the first thread looks like the wellbrook loop, it looks like the author may have modified it..Any does anyone have any thoughts on this:https://www.yumpu.com/en/document/read/43592064/wideband-active-loop-antenna-amplifier-with-passive-ik4hdqnet? I am NOT looking at building that because I am looking to reduce the number of magnetic elements not increase them, unless its off the shelf parts.

author: Co6aka
author_rank: Unknown
timestamp: January 24, 2021, 06:05:21 am
content: Quote from: coppercone2 on January 23, 2021, 11:40:38 pmThis is a much more professional design, and don't be frightened of the transformers. You could go with active augmentation to eliminate the two augmentation transformers, and use JFETs instead of BJTs. (JFETs simplify biasing, which simplifies temperature compensation -- something that MUST be considered for an outdoor amp/antenna.) To "upgrade" the design use several J310 in parallel, or for a high end "mil spec" design use CP666 JFETs at 28V. (CP640 JFETs at 15V work OK too.) Crystalonics has the CP# JFETS in SMD, or at least they did in the past. The CP# JFETS have their gate attached to the case/pad -- they're optimized for common gate applications.For a BJT design, older designs employed 2N5179, 2N3866, etc. There were some newer designs using SMD MRF# parts. The low input impedance of common base and common gate amps is easier to match to the lower impedance of the loop element.My last homebrew amp, from 2008/9 or so, used two CP666 in balanced common gate configuration powered at 28V. I didn't have a chance to experiment with augmentation though. Been dreaming of implementing that ever since. Maybe someday.

author: coppercone2
author_rank: Super Contributor
timestamp: January 24, 2021, 06:46:57 am
content: It does look like an interesting circuit, I may build it with 2N5109 and the transformers. Those transformers linked there are cheap and windable. Might as well try 1 'magnetic version' of this amplifier. I made some q-dope so it should be ok making those home made transformers decent.Not sure on the CP666(D) version because of the name alone, I have seen problems seriously reported to be related to the devil/possession on unit #666 of equipment before. Also defiantly not doing a quote buy lol

author: Co6aka
author_rank: Unknown
timestamp: January 24, 2021, 03:11:51 pm
content: The devilish 666 part number reflects its power and endurance -- it's a hulluva JFET!You can get a couple of watts out of 'em!I think I ordered a half-dozen and they were about $5 each, but that was back in 2008. About five years ago I came across about as many CM640 (CM have the military temp spec) in sealed military packages at a hamfest. The Crystalonics JFETs have relatively high capacitance due to the die size, but then paralleling J310s produces the same results.FWIW, for transimpedance applications, like for an active whip, the capacitance is an issue to consider. I was able to mitigate with bootstrapping, and I experimented with cascode configurations, but never settled on a "solution" other than increasing the capacitance of the whip via physical construction - it looked like an eggbeater.(As long as the whip doesn't look like a wifebeater it'll work.)Also-FWIW, for TO-5 and TO-18 packages, try to use those funky heatsink caps with the integral BeO insulating wafer and threaded stud (Wakefield 260-series I think?) so the amp housing can become the heatsink.

author: cdev
author_rank: Super Contributor
timestamp: January 24, 2021, 04:17:04 pm
content: A broadband loop is going to pick up lot of junk if you use a preamp and use AC power. Whatever you do, I'd really try to avoid that.If you do use a broadband preamp, try to power it with a battery.Personally, I prefer a balanced magnetic loop. Not a no tuning loop.Keep it away from the ac power ground, in particular. Even shielded loops have this problem sometimes. If you use a ground, use an outdoor ground rod or similar.Even they are not so good for me, personally. I suspect that may be because I live in an area of igneous rock (basalt, a long-extinct volcano) and the ground is not so good.

author: A.Z.
author_rank: Super Contributor
timestamp: January 24, 2021, 04:26:22 pm
content: Quote from: cdev on January 24, 2021, 04:17:04 pmwell, I partially agree, while the performance of a tuned loop will indeed be better than the one from a wideband one, and while using battery power is a good idea, using a good and well filtered power supply (e.g. a filtered/stabilized and NON SWITCHING power supply to feed the preamp using a well designed bias-t) will work fine too, then, by the way untuned loops have the disadvantage of being wideband, so for example, if you're listening the 80m band and have a nearby ham transmitting on (say) the 20m one, a tuned loop won't have too much problems, while a wideband one would get overloaded

author: cdev
author_rank: Super Contributor
timestamp: January 24, 2021, 04:30:47 pm
content: Exactly.I suppose I should be grateful because this strange ground situation has forced me to learn and do things I'd otherwise never done, to try to overcome it.I live in an area that's got a lot of traprock, a specific geological situation where the rock is very very hard and looks burnt, as if the volcano was just yesterday. (They crush it up to use for railroad gravel).There is even a place not far from here where there are the classic hexagonal columns typical to basalt.Looking at Wikipedia I see that there is a similar area near Melbourne, called Organ Pipes National Monument.

author: A.Z.
author_rank: Super Contributor
timestamp: January 24, 2021, 05:19:25 pm
content: Quote from: cdev on January 24, 2021, 04:30:47 pmI suspect that, using some ground radials and/or some "chicken wire" (metallic grid) laid down on ground under your antenna(s) may improve the radiation pattern and, in turn, performance; did you ever try ?

author: coppercone2
author_rank: Super Contributor
timestamp: January 24, 2021, 07:43:09 pm
content: oh yea this looks like a job for 8-12 AA's, that is my plan

author: cdev
author_rank: Super Contributor
timestamp: January 24, 2021, 07:52:53 pm
content: Yes, I've used several radials and ground rod+ antennas combinations. Its better than anything connected to AC.But its my understanding that the entire area is just gravel and earth that's accumulated over this igneous rock which is utterly non-conductive.I don't know if that's why, but the AC has all sorts of junk on it.Quote from: A.Z. on January 24, 2021, 05:19:25 pm

author: coppercone2
author_rank: Super Contributor
timestamp: January 24, 2021, 08:24:09 pm
content: do those rock hexagons have a flat top? I looked at pictures online of basalt and that is crazy.I ask because it seems like the perfect place to put shady equipment, flat surface, 0 cost, won't start a firealmost picture perfect there to have natures made rack mount shelving system with some seismic or radio equipment or whatever else there is, also probobly a decent reflector so you can cook on those edges if you give it a fan maybe, also lasersif i had the money i think id buy a piece of land with those on it

author: coppercone2
author_rank: Super Contributor
timestamp: January 24, 2021, 09:30:10 pm
content: Did anyone here every have a specific problem with common mode noise? The reason being I wanted the amplifier to have a battery and non battery external power switch. Not sure how common the RF-line-power control is, for protection, so I suppose you need to isolate it but see if the signal is present to control the relays (whoo added complexity).I see he just puts some chokes and capacitors on it, but if you supply power from Vdd/Ground, I suppose you can put common mode chokes on it thereConsider LT3045 with soft start so you can usetantalum capacitors with low ESRon the rail filter, maybe. esr not low enoughAnd remote enable so you can still use a +12V on the coaxial to activate it, but you are not transferring power on the coaxial connector *shudder*, just a voltage signal to turn the enable on and off. The power good can be connected to a transistor for the units LED, to give you a real nice indicator of battery capacity with minimum components. Size the battery pack with NiMH so that the low voltage region pisses the LDO off, I suppose you need a short-bypass switch on it if you want to make it easily compatible with Alkaline and lithium primary to vary the power.If you wanna do lithium secondary.. then I am not gonna work that out, have fun dealing with that shit lol

author: A.Z.
author_rank: Super Contributor
timestamp: January 25, 2021, 04:54:34 pm
content: Quote from: coppercone2 on January 24, 2021, 09:30:10 pmWhy don't you simplify it ? I mean, power the preamp over coax using the usual BIAS-T injector/splitter circuit, then use the power coming over the coax to both power the preamp and the relay, when in TX the power will just be cut-off so powering off the preamp AND disconnecting the loop, at this point you may use a battery or a power supply unit to feed DC to the preamp, the feed box (bias-t injector) may have a relay to control power through the TX PTT a led and a power connector allowing to use batteries or a PSU as desired/neededQuoteThe best common mode choke you can build for HF is made by winding 18 turns of thin coax on a #43 material toroid, one end of the choke to the antenna (preamp output in this case), the other one to the coax feedline connector, the choke may be hosted directly inside the same box hosting the preamp

author: coppercone2
author_rank: Super Contributor
timestamp: January 25, 2021, 09:04:04 pm
content: I don't like the idea of putting power over coaxial, thats the original design I had made, I thought it was kinda lame that you need a weird adapter box in the RF chain.

author: Co6aka
author_rank: Unknown
timestamp: January 26, 2021, 02:22:07 am
content: Another angle to consider re delivering power over coax is that DC bias affects the magnetic properties of a RF transformer, so for optimal performance you'd need a "DC injector" on both ends or any RF transformer would need to be constructed such that opposing equal currents flowed in its windings. Actually, DC will affect the magnetic properties of the inductors in your DC injector as well. And, keep in mind that ferrite core materials in RF transformers will add some intermod. For VLF-HF it's not practical to use air core transformers though. I found that larger cores were less prone to adding intermod. DON'T use those tiny MiniCircuits and such transformers!Toroids may be better than binoculars, but for large impedance ratios I like to use the construction techniques applied in RF power amplifiers so it's binoculars first-choice.To simplify experimenting, I ran DC power to my testbed via a foiled twisted pair; actually it was two foiled twisted pairs with a common drain wire. On the remote outdoor testbed I had two linear regulators in their own shielded and filtered box, followed by a "capacitance multiplier" circuit (which I concluded was overkill and unnecessary.) RF and common mode filters on each end of the twisted pair, and ferrite beads covered with that hot-snot filled heatshrink over the cable jacket in two places. (Same for the coax.) I used two DC supply pairs and regulators for +/- supply capability.A ground screen will help if your ground isn't so good, especially on a rooftop, but for balanced loops I found that less noise was picked up when the amp housing wasn't grounded. For lightning safety I used several of those gas discharge tubes in parallel, and that's what I'm using now with the Wellbrook-on-a-rotor installation. I had to move the loop from my postage stamp size back yard to my roof because it was too close to civilization's cacophony of RFI generators, so a direct ground rod is out. My #00-wire rooftop grounding system acts like part of the antenna if I connect to it, and it's pretty bad just connected via several gas tubes. Likely my other antennas are re-radiating lots of RF-crud too.Actually, I should build a tuned loop for 60Hz and use it the other way around - prolly save a fortune on my electric bill.(Hey, maybe use that scheme to charge batteries in a active antenna?)

author: A.Z.
author_rank: Super Contributor
timestamp: January 26, 2021, 07:51:36 am
content: Quote from: coppercone2 on January 25, 2021, 09:04:04 pmWell, I won't call it "lame", all in all, if you can't (or don't want) to run a separate cable to carry DC power, the power-over-coax is the only solution, some people even uses the same approach to control a remote antenna switch, here's an examplehttp://n1al.net/ham/ant_switch.htmusing only the coax to control the remote switch, that circuit allows to select between four antennas, again, if one has the "luxury" of running a separate cable it would probably be better, but if that isn't possible then.... better that than nothing

author: A.Z.
author_rank: Super Contributor
timestamp: January 27, 2021, 05:20:29 pm
content: As for loop antennas, this paper carries some pretty interesting informationshttp://www.lz1aq.signacor.com/docs/experimental-comparison-v10.pdfand may be worth reading

author: cdev
author_rank: Super Contributor
timestamp: February 27, 2021, 11:43:08 pm
content: I like wenzels or Trask's varactor loop designs. I have built something much like thaat using four MVAM -109's and a multiturn vernier dial equipped pot as the voltage divider to adjust the back bias in the varicap things. It kicked ass. . I just bought some copper tubing to re-impement it. Done right, it ends up being quite selective. Ideal for use with the cheap direct sampling RTLSDRs. That combination works really well. Both receiver and antenna have shortcomings which are overcome by the resonance loops. The loop acts as a pre-selector, or narrow bandpass filter. Making the peak quite sharp.. just a few KHz. So called active nontuned loops have never seemed interestng to me. On the other hand, tuned loops never stop surprising me.I have a very noisy QTH, maybe some quirk of my QTH makes it this way.

author: A.Z.
author_rank: Super Contributor
timestamp: February 28, 2021, 09:25:35 am
content: here's an idea, let's start from the same approach used in the Wenzel's tuned loop, that is, a loop connected to a varactor tuned stage feeding a preamp, but let's change it a bit (note:didn't build it, just "thinking aloud")let's connect two grounded gate JFETs (e.g. a pair of J310) to the ends of the loop, they won't offer much preamplification but just serve as impedance matching buffers, the output of the JFETs will then go to the varactor tuned stage and, from there, to the inputs of an AD605 preamp, the AD605 offers a gain adjustable between -14 to +34 dB, so we'll add a simple 10dB resistive attenuator bringing the range to -24 to +24 dBthe result would be an active, tuned loop with adjustable gain (and attenuation) which could easily withstand strong signals witout overloading, plus, using a led+lcr to control the gain, the preamp tune/gain could be easily remotizedWhat do you think ?another (crazy ?) idea may be implementing an AGC circuit in the preamp, in such a case the AGC would pick up the tuned signal at the output of the tuned stage and use it to adjust the preamp gain/attenuation, then it will still be possible to have remote gain control, although it would then just act on the max gain value, while the AGC would deal with gain (up to the max adjusted level) and attenuation[edit #2]AD605https://www.analog.com/en/products/ad605.html#product-overview

author: cdev
author_rank: Super Contributor
timestamp: February 28, 2021, 03:52:57 pm
content: In my experience any strong signal often overloads the RTLSDR, sometimes ridiculously so, making a variable attenuator really necessary. Otherwise one strong signal will bleed across the whole HF bands.Radio stations in the souythern US used by bible thumpers like "overcomer ministry' and also Radio Havana rebroadcasting China Radio International develop very high signals where I live sometimes and they do this. I would like to measure the voltage with my o-scope. Its significantIt may be over the 0.5 volts or so it would need to light an LED. I.e. "free energy"But, being a diode an LED may add interference into the mix. not so not desirable in a radio setting. As it would be exactly what we dont want, a source of nonlinearity.But if your point is to add a VERY low IMD preamp with continuously adjustable gain. Sure, it might be helpful. Ive made similar very high ratio matching transformers (unun) to see what I could accomplish on HF with just them and a long wire. I guess that is what youre doing with an antenna tuner, typically..

author: A.Z.
author_rank: Super Contributor
timestamp: March 01, 2021, 10:12:51 am
content: Quote from: cdev on February 28, 2021, 03:52:57 pmThat's why I thought that using an AD605 configured to offer both amplification and attenuation (-24 to +24 dB with a simple resistor) could be a good ideaQuoteWell, the idea (again, didn't build it, it's just off the top of my head) is that a pair of grounded-gate JFET would allow avoiding a transformer since they would offer pretty low input impedance (and pretty high output one), then the AD605 placed after the varactor tuned circuit, thanks to the adjustable gain/attenuation would help taming strong signals, plus the AD605, compared to the TL592 used in the Wenzel loop, has lower noise figure, so the resulting tuned preamp may probably work pretty well; at that point the resulting preamp unit would have two connection, one to the coax carrying the RF signal and a second one to a (say) CAT5 cable used to carry the DC and to remotely control the tuning and gain (the latter using a LED+LDR so that we'll just use a control voltage to adjust the LDR resistance by varying the LED voltage) so, as for the requests from others, the unit won't use transformers or bias-t and would have a pretty low components count while allowing total control over the preamp tuning/gain, not bad I think

author: cdev
author_rank: Super Contributor
timestamp: March 01, 2021, 03:33:41 pm
content: I can't use an RTLSDR on HF with any kind of decent antenna without a continuously variable attenuator. I have an upconverter which embeds a fairly decent LNA, and it is overloaded if certain kinds of dirty signals are anywhere near it. They splatter right across the bands.Here is the datasheet for the ad605. A dual amp? I have to mull this over a bit. I have frequently contemplated dual polarization antennas, and a dual RF amplifier may be useful in that setting.https://www.analog.com/media/en/technical-documentation/data-sheets/AD605.pdf

author: A.Z.
author_rank: Super Contributor
timestamp: March 01, 2021, 03:44:15 pm
content: Quote from: cdev on March 01, 2021, 03:33:41 pmWell, that may indicate that the upconverter frontend (or the LNA) are overloaded by the signal, a preamp with a gain/attenuation control may help, then by the way, a tuned antenna and/or a preselector may help as well

author: cdev
author_rank: Super Contributor
timestamp: March 01, 2021, 11:01:25 pm
content: I am wondering how the erformance is above 30 MHz, because of the problems with the mirics clone that @radiolistener has described.Poeple who want the better firmware basically have to pay $50 more, what does that buy and who/what does it go to and for?Quote from: coppercone2 on January 21, 2021, 05:05:24 amHonestly, @coppercone2, you could use almost any small metal box. For the thick wire/loop part I would consider using good quality banana plugs and copper tubing. You only need a metal box if there is an amp in there. You could also use a plastic box, like an electrical conduit box.

author: cdev
author_rank: Super Contributor
timestamp: March 01, 2021, 11:03:47 pm
content: I love huskies, malamutes, etc. Wouldn't "malamute" be a good stupid name for an SDR? It's better than "Fifi" ...Malachite is a (green, copper-bearing,) mineral.

author: A.Z.
author_rank: Super Contributor
timestamp: March 04, 2021, 07:48:39 pm
content: Malahit is (to cite words from a respected/knowledgeable friend) the "clone of a clone", now by the way, anyone is free to make an educated choice

author: cdev
author_rank: Super Contributor
timestamp: March 27, 2021, 09:35:26 pm
content: I think I'll pass if it uses the MSI chip as it's frontend. "internally generated noise" is indeed its big problem. it's very unstable.