Technical Topics > Equipment
MLA-30 Loop antenna
Pigmeat:
I've built a number of passive loops over the years, starting w/ a MW loop from a schematic from Radio Nederlands. I screwed that one up royally, but it worked on 90 and 40/41 meters with the Walkman tuning cap I used with it. I was able to dx Papua New Guinea, something I wasn't able to do with my wire antenna's and get rough headings on my fellow pirates and found most of them were full of crap about where they were tx'ing from. So was I, so there were no hard feelings, just laughs.
Passive loops of that size, one meter, on HF work about as well as a 60 ft. outdoor wire, but they're directional and null interfering stations to an extent, so there's plenty to listen to. You can simply set them by the window and have fun. I used to put them on a picnic table in the backyard in warm weather, but the neighbors always asked a lot of questions. (I used nylon bags full of gravel to tighten the wires on my dipoles. They thought I was practicing Voodoo, and the bags were to keep bad luck away. Lol.)
I've never tried the MLA-30, but I have listened to online SDR's using it. They seem to work well and are quiet, but I don't know if the owners have modified them? Most radio heads like to tweak their antenna's, and I suspect many of MLA-30's I've encountered have been tweaked. I know for a fact one of them has, as the owner is an old friend. It was sounding good earlier tonight. Have fun playing with them.
RobRich:
From one of my posts in the MW thread....
--- Quote from: RobRich ---Lowered voltage at the bias tee I was using to feed the MLA-30 preamp down to 4.5v last night. Add some voltage drop due to 75' of RG-6 feedline. Gain barely dropped at lower-HF bands and MW, while SNR likely improved. I still need to check voltage versus gain on upper-HF bands.
Positive supply voltage for the TL592B differential amp is listed at 3v to 8v. That tracks with my results. Gain at 3v (minus line loss) dropped considerably. The preamp at 1.5v dropped out.
12vDC as supplied by the stock USB->12vDC boost regulator in the bundled bias tee is likely overdriving the amp IC and potentially affecting SNR. YMMV, of course.
--- End quote ---
Dropping voltage might improve SNR with the inexpensive video preamp IC used in the MLA-30 design. I assume the MLA-30+ uses the same or similar chip.
alpard:
I sold all my radio stuff to buy electric guitars and amp, and have been away from radio hobby for couple of years. I am recently back to the radio.
From my memory MLA-30+ was OK, but for serious DX stuff, it lacked a bit. On LW it was non working but from MW to SW, it was OK.
DXing for the low power AM stations at nights, the Wellbrook ML was far better. But Wellbrook was about 10 times more expensive.
Teotwaki:
--- Quote from: RobRich on January 14, 2023, 2238 UTC ---From one of my posts in the MW thread....
--- Quote from: RobRich ---Lowered voltage at the bias tee I was using to feed the MLA-30 preamp down to 4.5v last night. Add some voltage drop due to 75' of RG-6 feedline. Gain barely dropped at lower-HF bands and MW, while SNR likely improved. I still need to check voltage versus gain on upper-HF bands.
Positive supply voltage for the TL592B differential amp is listed at 3v to 8v. That tracks with my results. Gain at 3v (minus line loss) dropped considerably. The preamp at 1.5v dropped out.
12vDC as supplied by the stock USB->12vDC boost regulator in the bundled bias tee is likely overdriving the amp IC and potentially affecting SNR. YMMV, of course.
--- End quote ---
Dropping voltage might improve SNR with the inexpensive video preamp IC used in the MLA-30 design. I assume the MLA-30+ uses the same or similar chip.
--- End quote ---
Rob
Thanks for bringing up the suggestion for the lower supply voltage.
The reverse engineered schematics do not show any on board voltage regulator so I agree that a +12v supply is too high, assuming the Chinese used a chip specified the same as the TL592B. I've dropped the voltage into the bias Tee to +6vdc and the loop's amplifier gain seems the same. I'll try to make some signal strength measurements later this week.
Jim
Teotwaki:
Here is a schematic of the AL1530 Wellbrook loop
The writeup is very interesting too.
https://www.george-smart.co.uk/projects/wellgood-loop/wellgood-loop-history/
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