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