Indoor antennas, including magnetic loops or any antenna that touts receiving the magnetic portion of the spectrum tend to be just as vulnerable to RFI as the telescoping whip on a portable or bare random wire to a tabletop receiver. Just the nature of the beast when we're swamped with electronic and electrical noisemakers from every direction.
With any amplified wide spectrum small loop antenna, the best we can hope for is to orient the nulls toward the worst sources of RFI. Sometimes that helps. Often it doesn't when we're surrounded by RFI from every direction.
If you try enough online receivers and SDRs around the globe, you'll encounter some Wellbrook loops and other highly prized loops that have raspy, buzzy RFI just as bad as anything we can get at home with a portable on the whip, or a bit of random wire strung along the ceiling or dangled outside a window. There's nothing magical about a Wellbrook or any loop in an environment plagued by RFI from every direction.
Same with the PA0RDT amplified mini-whips. Those are only as good as the location, and few online tuners have them mounted high enough to be relatively free of RFI. The popular Twente SDR is among the tiny handful that make good use of the design.
Plenty of online tuners and SDRs claim to be in quiet locations, but few actually are quiet. If they've strung up traditional wire antennas -- slopers, dipoles, doesn't matter -- reception is only as good as their nearest worst neighbor. The best, quietest SDRs tend to either be in remote locations, or very directional, such as aimed across the Atlantic or Pacific from either coast, with a few lobes off the backs that make domestic US reception possible. Those are often remarkably quiet because the designs tend to ignore the local RFI. The HFU accessible SDR with 250 foot V-beam aimed across the Atlantic is one such example, being among the quietest in the world of all the SDRs I've tried. But it depends on lobes off the back, small apertures of sensitivity toward US transmitters.
In 15 years in the same apartment battling RFI from neighbors' devices to faulty electrical power sources, to the nearby fire department, I've had the best results from homebrewed passive shielded loop antennas. See:
https://swling.com/blog/tag/pixel-shielded-magnetic-receive-only-loop/ and other articles.
None of my radios -- various portables including a Sony ICF-2010, and Palstar R30C sorta-tabletop/portable-ish hybrid -- needs an antenna amplifier. All of them have far more sensitivity than necessary, even on low/attenuated. That includes using the external antenna jack on the Sony, which is only slightly less sensitive than the whip.
Speaking of whips, through trial and error I discovered My Sony ICF-2010 portable is remarkably directional with the whip. Aim the tip of the whip antenna toward the worst local RFI and there's a null. Likewise, orienting the whip horizontally -- parallel with the ground -- is the quietest setting. Also the least sensitive, but it's often easier to actually copy the faint signal with reduced noise.
There are many designs for homebrewed loop antennas, including at least one that doesn't even look like a conventional loop -- the Villard antenna, which is just sheets of aluminum foil mounted on a large sheet of cardboard, plywood, foamcore or gatorboard. The Joe Carr receiving antenna book published in the 1990s includes most of these designs.
These can be tuned by adding a tuning capacitor. Some, like the Villard, and this ugly but functional doodad (
https://www.eham.net/article/40484), use overlapping sheets of aluminum foil with non-conductive spacers between the foil -- a sheet of paper, plastic, whatever you have available.
If you don't want to bother with a capacitor, or don't have one handy, too pricey, whatever, you can build the loop to dimensions tuned for the desired band. Optimal reception will be fairly narrow, with good reception possible at other bands scattered across the HF and MW spectrum. With trial and error you can build a very quiet, direction, functional passive shielded loop from nothing but old TV coaxial cable, an adaptor to suit the antenna jack on your receiver, and a place to hang or mount the ugly mess of coax. I used such a fugly loop for years, mounted on the inside of a closet door. The swinging door added directionality to null out the worst local RFI, and I could close the door to hide the antenna from snoopy landlords, maintenance crews and building inspectors who couldn't cite any valid reason not to have an indoor receive-only antenna, but thought there was something suspicious about it. Keep in mind that changing in communications devices have made "radio" a foreign object used only by misfit nerds and spies, as far as most folks are concerned.
Another version that worked even better -- although not an indoor loop -- was a stealth loop fastened to the inside of a wooden fence outside my ground floor apartment window. The loop was nothing but very thin magnet wire, in a roughly square shape, 8 feet along each side, following the dimensions of the fence (I used screws, nails or cup hooks, whatever was handy, to wrap the wire in a large "loop" shape). The wire ends were fastened to an old fashioned TV balun. I ran old TV coax, often scrounged from the dumpster, along the ground next to the building, tucked into the dirt. Then up the wall to a window with the sash cracked open just a bit (thumbscrew sash locks used to discourage burglars), and to the receiver. That loop nulled off the edges (opposite of most small loops, which null off the open faces), effectively nulling out the worst RFI from the parking lot and building lights. Worked great, until the apartment complex maintenance crews found it and tore it down. Which is why I built these from the cheapest materials available. I bought a bunch of magnet wire cheap from the local Radio Shack outlet store years ago. And I scrounged the TV coax and baluns from the dumpster after tenants moved, died or were evicted. So I didn't have more than a couple of dollars sunk into any outdoor passive loop. The only problem with that design now is TV baluns are hardly ever in demand or used anymore. I could use a proper homebrewed balun designed for HF, but the TV baluns worked despite not being optimized for HF.
The best performing versions of my loops were remarkably quiet, enough to reduce local RFI to just above the receiver's built-in noise floor, a sort of white noise low hiss. That enabled leaving the receiver on 24/7 with the volume just high enough to catch the faint sound of a pirate or other signal on 6925, 6955, etc. Rigged to an SDR we could just go by the visual cues onscreen.
The only downside was these passive loops picked up only fairly strong signals, and were more prone to fading. But there was a lot less tiresome RFI.
Topic: MLA-30 Loop antenna (Read 22925 times)