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Author Topic: Shielded Loop (Come On , Feel The Noize)  (Read 264 times)

Offline NQC

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Shielded Loop (Come On , Feel The Noize)
« on: April 08, 2021, 1654 UTC »
Hey All,

I am a new guy here and figured I'd run this by folks.

I have seen shielded  loops in my travels over the years, but was always kind of puzzled on exactly how they are supposed to work.The "usual" description is that the loop  "responds" only  to the magnetic aspect of a "signal" and is therefore less noisy. Hmmmmmm.

Maybe my understanding of RF physics is WAY off . I THOUGHT that a "signal" had both a magnetic aspect and a voltage aspect and that they were basically  a "representation" of the same thing, at the same energy level ,inexorably bound ,but 90 deg out of plane to each other.

SO- any "signal " recovered ,whether by magnetism or voltage, should be equal in energy (or so I  would THINK). But (apparently/obviously) this isn't true or no one would ever bother to shield a loop, right ?

Concept :A shielded loop has a gap in the shielding so the turns aren't completely "shorted". That I  get.

The part that throws me off is how "undesired" or "desired" "signal" can be "cherry picked" from it's  voltage vs magnetism to optimize "results".

Is "undesired" signal (re shielded loops)  taken to mean arc noise, such as a local leaky power line insulator or (at a stretch lightning crash "arcs") ? Perhaps "device" noise, such as plasma TV's , switching power supplies, dimmers, etc, etc are not in the "arc" class of noise and thereby irrelevant re noise reduction discussion of a shielded loop.Don't know.

Also the  TYPE of loop construction  seems to be important. The ones I have seen seem to be  shipboard LF DF loops with a BNC female unbalanced (50/75 ohms ?) on the base box. Epoxy seems to fill the shield top  gap. Not sure about the  internal winding, but I would THINK they are scramble wound / non spaced / untuned low Q affairs (correct ??). Basically patch your receiver's coax in and "deal with it." Maybe this is  OK at 200Kc, but at the  AM Broadcast "transition point" (above 1000 or 1200 KC) maybe another story.

 My recent flop (in the apt.)was to try and aluminum foil wrap (with a gap)  onto my 24" X 24" (scramble wound) Low Frequency Engineers LF- 600S (for 7 KC "Whistler" use), just for chuckles. If anyone desires to hear raging loud 60 Hz noise, this is the antenna for you  ! It does work awesome in quiet QTH's though. Basically, an abject failure with no 60 Hz reduction at all. Maybe it was  simply in  too strong a 60 Hz field ?

So, circling back,   (after 53 years of "playing around" with the radio hobby)   I am stumped on how  shielded loop "deep" theory actually works.

To make matters WORSE, I have 13.8 Kv at 20 ft on one side and 13.8 at 35 ft (with pole pig) on the other. I live on a corner and am clobbered from two different 90 degree compass headings (bad). These lines leak fairly bad at times, especially when wet.Got a run around from power co, even I know I realize they are "supposed" to respond.  I also have device noise (plasma, et al) from the other apartments.

To FURTHER complicate things, I only have ferrite receivers right now, the 50 ohm Icom stuff ( R-70, R-71a, IC-735, etc, etc) is all long gone. I have no options for roof antennas anymore (I USED to have a fairly quiet  40 M dipole, a 85 ft long wire, a HiFer  tx dipole,a 6 M SSB  dipole, VHF/UHF ham  verticals ie various Diamonds ,QRPP HF/VHF/UHF,etc, etc). No more roof access.

All the loops I have made (so far) were single winding for direct couple to Ferrite receivers. Nothing   has been built (so far) with  a stand alone tuned primary/secondary pick up loop arrangement for a 50 ohm unbalanced coax lead to a  rig.At  former low noise QTH's  I had preferred that my loops (if possible) can be tuned to peak and not just unspaced low Q scramble wound just  for the sake of being able to be shielded.That was before I moved to noiseland though.

Buying another 50 ohm rig for BCB / LW is not totally out of the question , but I am not sure if there is any way other  to go.

There is also the factor (FWIW) that my Fe receivers may also be  picking up undesireds directly (to a degree) by their internal Fe rod, and a loop may only "boost" this problem.

I had also have  heard this very old vague   saw that "noise" in an RF amplifier has a tendency to get get amplified more than "desired" signal (baloney ?).

FWIW, I do almost NO HF listening now, I  pretty much roam from around  1720 KC down to 7 Kc.

I DO get a fair amount of DX, ie  IL,TN,KY, NC nearly every night, WSB (usually fair or worse) some  nights , R. Progresso Cuba 640- fair maybe one or twice month, especially in winter. Also recently snagged WQFG689 1710 NJ ,along with some new X banders.

So it could be worse , but would like to improve my limited situation if possible .

QTH Boston MA (noisy) apartment. Currently using Panasonic 840 or Sangean ATS 803A  with the  basic unamp'ed /no output jack Select-A Tenna. Currently a small open wire /directly coupled single winding  loop is under construction , two others planned (a medium and a very large). A number of discrete  circuit board projects planned ( maybe a Q multiplier, transistor  receivers, crystal sets etc).  Used to be into VERY high Q crystal set Dx, when  I had a long wire.

Also a  VFR private pilot , worked for NWS as volunteer at climate  research station for 10 years  and into ANYTHING on or  near Earth ,"far" stuff too- but not as much.

Anyhoo, that's my sad story and I'm sticking to it.

ANY help or input is GREATLY appreciated.

de NQC
« Last Edit: April 08, 2021, 2108 UTC by NQC »
Main station receiver : Bed springs to  blue razor blade to 2000 ohm cans to radiator

Online Ray Lalleu

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Re: Shielded Loop (Come On , Feel The Noize)
« Reply #1 on: April 08, 2021, 1917 UTC »
Just a beginning of a reply.

Near the end of an antenna arm/wire, the RF current is low, the voltage is high.
'near' is to be measured in wavelengths, say less than 10% of a wl.
So in LW/MW/low SW, your are likely to be 'near' the noise 'antenna'  made by the power wires in your house. The man-made RF noise is mostly electrical, not magnetic.
(When a wave goes farther from the radiating antenna, progressively the proportion between electric and magnetic goes to a fixed value. That value is measured in ohms and is 377 ohms for a wave in free space)
A wave coming from a station far away has already been modified to that low value. But the noise from the power wire of your home is at a much higher impedance, meaning mainly electrical fields and less magnetic field.

So a receiving antenna - made sensitive to magnetic field only - (a small loop) is the antenna to use in a place with high man-made RF noise.

Then, all the problem is how to ensure that the antenna is really sensitive only to the magnetic field, and that's not so easy, Also, you would want to place the loop in a good place, but that means you'll  have to use a line, and that line becomes part of the antenna, and may carry noise from the room to the real antenna. Try to find texts about common mode vs differential mode.

Enough for a first reply...
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Offline NJQA

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Re: Shielded Loop (Come On , Feel The Noize)
« Reply #2 on: April 09, 2021, 0949 UTC »
Being in an apartment this may not be an option for you, but...the best LF/VLF antenna I have used was a Loop On The Ground (LoG).  It went a long way towards taming local noise.  Signal levels were low, but so was the local noise.  The resulting SNR was quite acceptable.

The second half of the equation was in blocking common mode signals on my feedline from the LoG.  This required common mode chokes at both the antenna feed point and at the receiver.

W1VLF has a Youtube video where he shows the difference in LF performance of an active whip antenna with and without common mode chokes.  The difference is impressive.

Offline NQC

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Re: Shielded Loop (Come On , Feel The Noize)
« Reply #3 on: April 09, 2021, 1225 UTC »
Hey All.

Thanks  for the replies.

Ray : A good deal of the info that you presented either newly educated me or helped me recall things that I had read about /experienced, but couldn't recall because they were so long ago.

Many (many)  years ago , I read some info about Common Mode phenomena . I believe it was about noise reduction techniques re wire antennas (not re loops). Differential Mode does not sound as familiar, although in my future  research it might come back  to me (maybe).

FWIW , I am partially of Franco American ancestry and years ago spent some  time in Bitche ,as well as in  Paris. I also enjoyed a number of QSO's with French amateur radio operators back when I was on HF.

Merci.

NJQA: Your LoG sounds cool. I have read some really cool stuff about ON Earth antennas, buried antennas, Earth AS antenna , various types of counterpoise , through  Earth comms systems (especially WW I spark systems), single wire telegraph ,etc, etc, etc.

Some radio operators concentrate more on the "well  ABOVE Earth" aspects of    radio, and less re ON or below it . There is a LOT that can be done  "low". Much of this (IMHO) seems to apply more to below frequencies  10 Mhz ,especially those   below 2 Mhz and really so in to LF/VLF/ULF.

I will check out the W1VLF video.

Thanks.

de NQC
Main station receiver : Bed springs to  blue razor blade to 2000 ohm cans to radiator

Offline ChrisSmolinski

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Re: Shielded Loop (Come On , Feel The Noize)
« Reply #4 on: April 09, 2021, 1346 UTC »
An excellent question. Shielded loop manufacturers like to say their loops only respond to the magnetic component of a signal. Or sometimes also make claims that the antennas only pick up the desired signal, and not noise/RFI. 

This is hogwash.

W8JI has a good page describing how they actually work here, I'd strongly recommend reading it, but the short answer is "the shield is the actual antenna": https://www.w8ji.com/magnetic_receiving_loops.htm
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Online Ray Lalleu

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Re: Shielded Loop (Come On , Feel The Noize)
« Reply #5 on: April 09, 2021, 2145 UTC »
That page from W8JI makes me dubious.

He is speaking about loops not small enough (only the left part <10% should be considered).
Then he never speaks about the RF noise in a real place.

And he chooses to show us badly designed loops, and never goes up to really balanced designs.

In the last, only paper work.
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Offline NQC

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Re: Shielded Loop (Come On , Feel The Noize)
« Reply #6 on: April 09, 2021, 2200 UTC »
Hey All.

Ray: I read up on common vs  differential and picked up some good points and interesting info from that pursuit.Interesting on how a  "signal" behaves at distance.I did not know that.

Chris: First of all - LOVIN' the website !  I read W8JI and found it to be quite good.

The one thing I keep seeing everywhere ( W1VLF, etc) is coax feed, which while I GET and certainly find  advantageous, doesn't apply to my current situation of isolated  loop to  receiver ferrite coupling.

It seems to me that if you run "wide open" (no coax), you are going to get your "clock cleaned" by noise.

The symmetry and elegance of the  construction  needed lends itself much more to an unbalanced feed scenario than open ferrite  and an open air loop.I don't think there is enough aluminum  foil on a  roll to bail me out of this ( Hi).

I suppose I could wrap the loop and an enclosed VC and try to run with it. But I also have concerns (maybe unfounded) that the open and spaced  windings might take a Q hit by being in proximity to Al foil, even if it IS non ferrous . It sort of seems to be barking up the wrong tree to   quiet down  a air coupled   air loop down by shielding. But even if the concept may be a non starter, I learned a boat load that I didn't know already.

I DO have two (count 'em two) QF1a audio filters that I can piggy back in series. I realize that an "audio" approach to electromagnetic QRN may not be the best way to go, but what the hey - worth a shot. The only thing I DON'T like is that they are 120 VAC, even IF I install a chassis ground to L2/G . I really hate to run any receiver  off of  house current if I can avoid it. Oh, BTW, good luck trying to get it to operate on 12 VDC. Autek made it sound so easy,but it was a failed nightmare. I was lucky to get it back to 120VAC function again. Anyhoo,   exploration of the entire issue  has been really fun ,even if it may be the wrong way to go. My noise is variable and per the DX summary above, my QTH is still "usable" but certainly NOT optimal.

I am still going to "soldier on" and build this   medium sized  (bedside) loop anyways, shield or no shield. I have been saving two small spools of 660/46 for a special occasion and may pop that it in for the  air  turns.

The other smaller loop is already partially finished and will be a test bed for active devices, etc.

More to follow,

NQC
Main station receiver : Bed springs to  blue razor blade to 2000 ohm cans to radiator

Offline syfr

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Re: Shielded Loop (Come On , Feel The Noize)
« Reply #7 on: April 11, 2021, 1325 UTC »
Thanks for the link to W8JI's page....good information from a professional RF design engineer with decades of practical experience.
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