Magnetic Loop (RX) topologies confusion - Page 1
Forum section: electronics / rf-microwave

author: TinkeringSteve
author_rank: Frequent Contributor
timestamp: July 17, 2025, 05:07:07 pm
content: Hello!I have been looking at different things people do w.r.t. RX active broadband loop antennas. And also found some contradicting claims.For instance, that the shield in a shielded loop reduces near electrical noise - G.F.Mandel says, that's "folklore", and attributes reduction in noise to an effect due to symmetrical shield halves - with a shield gap in the "end" of the loop opposite to the feed point, and things cancelling each other out.If that's so, i.e. that the symmetry is necessary, my plans of making the shield gap *at* the feed point (i.e. connected to GND only the left side of the shield there), would be foiled.The possibility is something I wondered about, for construction wise advantages(1), and seemed to be confirmed by a venerable German book that has been around since the 1960s and continuously updated until recent years, perhaps ongoing: the "Rothammel". See attachment: showing two possibilities for the shield gap, top or bottom (feedpoint). Though this is more a practical than theoretical book and does not explain the why so much. It also doesn't mention noise at all, but improvement of direction finding characteristics.1) So, can she gap actually be at the feed point, or, what are the consequences of doing that?2) If I wanted to try switcheable inductances (N turns) & don't have one pipe to put multiple windings through, can I use coax cable turns, make a shield gap into each winding, connect the feedpoint shields all together, maybe even all the gaps mutually, electrically? Is this close to equivalent to having one metal pipe loop (with gap) with several internal windings of wire?3) Some HAM youtubers tested the performance of regular shielded loop vs. Moebius & seem to agree that the latter has a bit less noise. So would that actually be preferable to two loops inside a pipe (or my "laterally glued" version of that) for two windings? If the gap *has to* be at the top, switching between 1 turn shielded loop and Moebius would require 2 switches, as said, though - in that case that's also why I would prefer to just short windings with a switch in the base4) if I could indeed make the shield gap at the feed point, would the coax reaching into a little aluminum case, as a mechanical base (and shielded amplifier housing) at the feed point (which is connected to GND) spoil this gap in any way, or not - because one side of the coax ends reaching in will be electrically connected, whereas the other only has some capacitive coupling...?5) my initial idea was, in conjunction with not having the gap at the top, easily being able to also switch to *only* connecting the shield as antenna, to have a not necessarily magnetic loop, to, whlie the focus is still 80m...10m bands, allow reception of higher bands (say, up to 146MHz, using something like a 30cm diameter loop with 1..2 turns - or only the shield, and a broadband enough amplifier) .At my current level of confusion, I neither am sure whether this is even necessary (seeing claims that even the gap'd shield functions as higher freq antenna, coupling the signal onto the inner wire), *or* viable, lol! What say you?(1) esp. if one wants to use a e.g. rotary switch to switch between different configurations: say, moebius, regular shielded, unshielded loop, or perhaps use several windings and allow to short the inner conductor to one winding, when desired. Having to have two switches for that, with one at the other "end" of the loop, as opposed to only at the base, requires extra mechanical support at the top, etc.

author: Perrold
author_rank: Contributor
timestamp: July 17, 2025, 10:03:08 pm
content: Too much loop lore out there. Everyone also has a different application in mind and a different set of priorities even if it all seems the same at first glance. A unique operating and noise environment for each user further complicates matters. After playing around with loops for a while, the classic shielded loop with a gap at the top works well enough. It's an easy way to have a loop that shows noticeable shielding yet it's far from optimal. What I finally settled on is an unshielded loop coupled through a transformer that has the CT grounded. It seems to force a balance and reduce common mode noise better than anything else tested so far. A big complicating factor is that just because a loop is quieter than some alternative doesn't mean it can't be even more quiet. With a loop that is optimally balanced as best as I can perceive it to be, bringing my hand within a couple inches of each "hot" end increases noise levels by a significant amount. Even body capacitance from standing by the loop at arm's length is discernible. Clear null standing centered with the loop. Common mode balance and finding a noise null due to alleged loop directivity is also complicated. The nearfield noise bathing the loop isn't necessarily uniform. Rotating the loop for a null has just as much to do with finding a common mode balance due to what's induced into the loop from local e-field pickup.To actually partly answer some of your questions, I would not have a gap at the feedpoint and I'd look into a balanced unshielded loop instead. It'll probably work to a degree regardless but you'd be missing out. Multiturn loops are a waste of time unless tuned to resonance or used for really low frequency work. Using the shield or the loop itself as an e-field antenna works just as well as any other electrically short antenna. Getting true broadband performance out of a loop, actually covering 80-10m, is a compromise. It'll never be perfect but it can be really good in a noisy location.

author: TinkeringSteve
author_rank: Frequent Contributor
timestamp: July 18, 2025, 04:10:56 pm
content: Quote from: Perrold on July 17, 2025, 10:03:08 pmWhat's very low to you and what diameter did you have in mind?The simple formula given in my book makes it look like N of turns compensates for smaller area i.e. diameter, and after my go-to loop for SW listening, which is a 1 x 1 m² square of RG58 with one transistor behind it, I tried with far smaller diameters of 1-turn loops and more amplification, which so far all stink compared to the 1m² one on the HF bands.I'm basically experimenting how well I can get away with small diameters like 20..30cm.But I guess the higher self capacitance is what makes it inherently less broadband? (I am using amplifiers with stated low input impedance in order to press the Q more flat, if I udnrestood correctly)Btw, so far I am always powering my loop amplifiers from a battery, to, at least for now, make things not more complicated by introducing yet another source of noise. The receiver I'm listening on is likewise battery-powered.Though my 1955 tube radio with 8 AM stages, on a power strip with inbuilt noise filtering, my 1m² _indoors_ loop sometimes overwhelms the thing so I have to attenuate the signal. It works very well, too.

author: Perrold
author_rank: Contributor
timestamp: July 19, 2025, 05:00:08 pm
content: Quote from: TinkeringSteve on July 18, 2025, 04:10:56 pmVery low as in 100kHz or lower. Diameter roughly 1m. Have had great results with tuned multiturn loops for VLF work. Can pull in WWVB or the submarine stations better than anything else in terms of raw signal level. Can be a very viable strategy up through the BCB. Tuned loops for HF are very much a thing but typically you're limited to a single turn unless it's the lower bands.In practice the ambient SNR at lower frequencies is so high that increasing signal levels does nothing helpful and is often detrimental. Additional turns compensate for lack of signal grabbing with smaller loops but loop inductance goes up as n^2 and even with a 1m loop the SRF usually falls below 10m. Increasing diameter trades more favorably than increasing the number of turns provided you have the room for it.A completely gut feel answer is that a 20-30cm loop will be worthless above 5-10MHz, probably closer to 5MHz. Too little signal being captured to overcome pre-amp noise. The allowable pre-amp NF at lower frequencies can be absolutely disgusting due to high ambient noise. If multiturn then you'll probably hit a SRF around 5-10MHz with only a few turns. The prevailing wisdom of using a low input impedance pre-amp is generally sound but it limits how broadband you can make the loop. The moment the loop begins to have appreciable inductive reactance a low impedance pre-amp is the wrong tool.

author: TinkeringSteve
author_rank: Frequent Contributor
timestamp: July 19, 2025, 06:01:58 pm
content: Oooh, on a tangent, could you elaborate on the submarine stuff? E.g. point to a good resource where I can learn about it? I did have the vague idea that they send stuff at very low frequencies to penetrate the water, but that's where it ends. I'd be interested in basic things like, when/where can I even expect to pick that up, and is there anything that's not encrypted, that I could interpret, etc...My smaller loop diameter experiments are to have practical experience with what's possible, to also allow acquaintances with vague interest in listening in to the waves out of curiosity (armed with e.g. an affordable SDR stick, but only some short telescope antenna...), in small German apartments and, to their fault, without my carelessness about how an apartment with ugly large antennas looks.They do sell some commercial loops of 20cm diameter advertised as 50k...30MHz or so, but I have no experience with those to compare with my stuff here.

author: Co6aka
author_rank: Unknown
timestamp: July 19, 2025, 09:20:43 pm
content: Only have a couple of minutes to post so I'll have to be a lot less general at a later time/date. I messed around with loops a lot when I was a youthful player, and then in to 80s when I time to enjoy life. In general what I learned was:"Shielded" loops for "nose reduction" are nonsense, like the literature says. In general they're deafer than "unshielded" loops. Essentially, the shield provides capacitance so at a specific frequency it's self-resonant. A cap/caps can be placed across the "inner loop" to tune the loop to a lower SRF. If you need a rugged idiot-proof plug-and-pray loop for a idiot operator this could fit the bill.Tuned loops work best but the tuning mechanism, antenna impedance and feedline must be carefully considered. Best overall results obtained with either a transformer or active BALANCED unity gain buffer.For untuned loops, you want the lowest possible/practical loop inductance, and for the active transimpedance amplifier you want input impedance to match loop impedance with a low noise figure. The best amp circuits I've fiddled with use several parallel JFET on each side (balanced) feeding a solid low-IMD line driver. And don't send power up the signal coax -- think about it. Outdoor UV-resistant direct bury STP (shielded) CAT5/6 cable with quality connectors is my favorite. (Two pairs for power, one pair for signal, and one pair for control/monitoring.)Loops are all about balance. Nulling and any anti-noise benefit depends entirely upon balance. This is where loops and their designers fall apart.Common mode feedline choking at the antenna AND just outside the shack are often needed because the feedline shield is a great conductor of noise, and an antenna as well.L8R!

author: asis
author_rank: Frequent Contributor
timestamp: July 20, 2025, 07:20:27 am
content: Hi,Everything is clear and convincing.Look:

author: TinkeringSteve
author_rank: Frequent Contributor
timestamp: July 20, 2025, 04:16:10 pm
content: Tuned antennas, for RX, are not what I'm currently interested in, though. I get the benefits of extra selectivity, but am not interested in paying the price of terrible practical usability for the use case of scanning across bands with SDR.I am building a tuned loop for 11m, though. Just to test whether I can actually get outside my apartment, if positioned inside the large ceiling-to-floor window corner. While CB is mostly dead, I do sometimes hear people with just a wire in the room. If I can get out to them, I might reevaluate whether I want to spend the time to get a HAM license and do a little bit on 10m and 2m (where I get a few FM repeaters, through those very windows) ;-)

author: TinkeringSteve
author_rank: Frequent Contributor
timestamp: July 21, 2025, 06:08:47 pm
content: As for the coupling loop of a TX antenna: Does it have to be exactly at the opposite end from the gap in the loop where the capacitor is mounted?I mean especially, does it have to be on the line between the gap and opposite end, or can it be a bit rotated to one side off that vertical center line (assuming the gap is at the top).I have seen people changing the coupling or SWR by moving it away a bit from touching the outer loop, but I haven't seen it off center from that line (which would give me some mounting & wire management advantages, to have the vertical center line as the tangent on one side of the inner loop, in my cheap, provisorial setup)

author: Perrold
author_rank: Contributor
timestamp: July 22, 2025, 04:07:27 am
content: Quote from: TinkeringSteve on July 19, 2025, 06:01:58 pmNot much to tell. I'm only familiar with what the US does. It's about three stations (Hawaii, Washington, and Maine?) at about 20kHz. Transmit power is about a megawatt going into the antenna with the actual radiated power being a good bit less. Signal levels are typically very high at the receive end with ambient noise being the limiting factor. It's all very much encrypted so the novelty wears off fast. Reception is 24/7 thought better at night.

author: mike449
author_rank: Contributor
timestamp: July 31, 2025, 04:02:50 am
content: Here is a 2-turn shielded loop with a "gap" at the feedpoint.Some people claim this can not work because of the shielding, but it does. I used it with an LZ1AQ clone preamp that has low input impedance and decent CMRR. It works very well on MW (and maybe below, I don't listen there), but not so well on HF because of low self resonance frequency.I used 2 pieces of RG-6Q cable, held on a PVC pipe cross next to each other in a somewhat circular shape. Diameter about 1m.

author: A.Z.
author_rank: Super Contributor
timestamp: July 31, 2025, 03:27:22 pm
content: Quote from: TinkeringSteve on July 20, 2025, 04:16:10 pmwhat's the size of that window (width, height) ?I'm thinking that you may use some tape to stick a wire loop like this onehttps://qrper.com/2023/11/a-portable-oblong-antenna-for-2-meters/to the window; just recalculate the loop for the desired center frequency.Freq = 145.000 (MHz)Lambda = 300 / FreqPerimeter = Lambda * 1.0678Short = Perimeter * 0.333Long = Perimeter - ShortShort Side = Short/2Long side = Long/2all in metric units, use a balun to feed the loop, a W2DU will work for V/U, an "ugly" one will be ok on HFHTHat 145MHz, placing the lower side at 83cm from ground results in a gain of 8.54dBi at 22° (5.27dBi at 10°) , not bad at all for a simple piece of wire