Many thanks to SWLing Post contributor, Paul, who shares the following story from the RTL-SDR.com blog:
Last month we posted a collection of reviews about the MLA-30 which is a budget magnetic loop antenna designed for receiving HF signals. The overall consensus from the reviews was that it worked decently for the price, but of course could never live up to the high end loops that cost hundreds of dollars.
Recently Martin (G8JNJ) reverse engineered the active circuit used on the loop from photos taken by M0LMK and has made some observations on it’s performance, noting that it’s design isn’t very good. First he notes that the amplifier chip is a Texas TL592B two stage video amplifier which isn’t that great for this application. His measurements show an OIP3 of 20dBm, a P1 saturation of -3dBm and a noise figure of 12dB.
Of interest, he explains that the creator of this loop has designed it poorly as the impedance match of the loop to low pass filter is very wrong, resulting in a very poor amplitude/frequency response. He shows how the response can be improved with a few termination resistors, but is still not great.[…]
Click here to read the full article at RTL-SDR.com.
The big difference between the Wenzel/Techlib loop (and clones e.g. the RH Electronics copy) and the MLA 30 is that the former are tuned loops, while the MLA-30 is untuned. That gives them a selectivity advantage, which also tends to minimise the effect of poor IIP3/OIP3 (the tuned circuit essentially reduces the level of the IM signal before it … well, intermodulates!).
Being a parallel-tuned resonant loop, the Wenzel/Techlib design also minimises the source impedance of the loop at resonance, which plays into & works OK with the video amp’s relatively low (but higher than the loop itself’s) ~1k input impedance. The MLA-30, by comparison, is mismatched all they way – I’m not sure exactly what they were trying to achieve with that, because it’s wrong from start to finish.
I’d disagree with Martin G8JNJ that it’s ‘based’ on the Wenzel/Techlib loop. There’s considerable differences between the two (not tuned, l/p filter/isolation/matching network, balanced o/p). In fact, it looks much more like a near-direct copy of one of the old video amp suggested designs or appnotes. Basically they’ve chucked a wideband amp on a non-resonant but highly-reactive balanced antenna, and called it a “wideband loop”
Hi Ron, agree with you about the fact that the MLA-30 is different from the Wenzel “loopifier”, the only thing which may somewhat recall the “loopifier” design is the use of the video amplifier chip, other than that, the design is different and, sincerely, as you correctly pointed out, the wideband design used in the MLA-30 is a really bad one; on the other hand, having built quite a number of samples of the “loopifier”, I can say that, being tuned and having the amplifier set for LOW gain, it works quite well. The only issue I ever found with the “loopifier” was the appearance of “ghost” signals and/or some IMD in the presence of nearby strong BCB or FM stations, but I managed to find the cause and solve it, basically the issue was caused by the wires used to carry the power and the tuning signals to the preamp, those picked up stray RF and re-injected it into the preamp; the solution was using an UTP (a network cable) to carry power and tuning, using a wire in each UTP pair for a given “signal” and grounding all the others, that cured the issue and allowed the “loopifier” to work really well, and that’s also why, since the “RH loop” seems a clone of the original “loopifier” and uses a tuned preamp, I suppose that it may offer better performance than that MLA-30 loop
Yeah, I’ve said before that I’ve got a soft spot for the Wenzel/Techlib loop – it was my first loop, I learned a lot from building & modifying different versions, & I still recommend it as a good, if somewhat noisy, active loop if that’s what you want.
One of the things it did teach me is that active antennas, and in particular tuned & amplified loops, are a bit too much of a compromise for my liking – tuned passive loops, great; wideband amplified loops, yeah, they work; together, there’s too many design compromises to make a good antenna for the kind of situations people mostly want them for (i.e. restricted urban/suburban use). Also, I can’t help but think that there’s better amp choices than the TL592 these days. I guess I should try one of them sometime… 😉
“think that there’s better amp choices than the TL592 these days”
Well, have a look at the AD605, the chips allows to adjust the gain from -14db to +34db, just add it a resistive attenuator (say -10db) and you’ll have a gain/attenuation control ranging from -24db to +24db, not bad I think 😉
I’ve actually looked at the AD605 for another purpose (ALC & fine level control for an RF signal generator) – distortion was a little high, and ultimately the bandwidth was too low for the direction I decided to go, but it’s not a bad chip & probably a step up from the ancient TL592.I’ve probably still got a couple in the unused parts bucket I could try.
There’s also plenty of other possibilities in the general area of video / line driver amps from both TI & AD (and, perhaps surprisingly, Maxim), as well as even normal high BW op-amps that could be used. Definitely worth a bit of a fiddle next time I build one of those loops.
I just put an MLA-30 on my roof yesterday. Admittedly, I don’t possess the technical knowledge to speak to the design flaws highlighted in this article, but I can tell you that in my particular setting, it performs far better than my PAR End Fed SWL ever did. I suppose it’s a matter of what you’re used to, but I’m quite pleased with MLA-30 so far. I bought some PVC pipe, some fittings, a couple clamps, spent under $50 total. Perhaps it’s not perfect, but right now it’s working well for my dollar.
FWIW, I’m increasingly and strongly of the opinion that anyone who recommends random wires & longwires as the default go-to antenna in this day and ago – old hams, old SWLers, & retailers alike – should be ignored, or at least viewed with suspicion. They were the canonical AM or shortwave antenna 50 years or more ago, when the (electrically) noisiest things around the average house were fluro lights, a TV that was only on from 5pm to 10pm, and maybe a dodgy fridge or fan motor – but they’re noise magnets, picking up every bit of EMF in the area, and with the ubiquity of cheap/dodgy CFLs, LEDs, VFDs, and switchmode supplies in everything these days they really should be relegated to history.
There’s a reason why the proponents of them boast about their quiet rural antenna farms, or demonstrate them on the top of a mountain in national park or in a field outside the city – it’s because in city, urban, or suburban areas they mostly pick up as much or more noise as signal. A good, well-sited, balanced antenna – whether it be a dipole, loop, or whatever, tuned or wideband – will be at least somewhat immune to local hash & noise compared to a longwire, and the improved SNR that brings manifests itself as better performance /even if the antenna itself is considerably less efficient/.
Or, as you’ve discovered, even if the antenna is badly designed wth high noise & distortion…
You’re likely spot-on, Ron. The PAR End Fedz antenna was notably better than a random wire, but still very noisy in my environment. The loop offered a dramatic improvement.
The idea that the MLA 30 is not optimum is actually encouraging. I’d love to see how a well designed loop performs in my environment, but the obvious retail options (MFJ-1886, Wellbrook, W6LVP) are expensive by comparison. Knowing the performance I’m getting for $50, expectations from a $300+ loop would be fairly high.
It seems the biggest complaint with the MLA 30 is the amplifier. It would be great if a seller would offer an optimized amplifier. Could make for a great budget loop.
Willing to try a better loop, you may have a look at the “loopifier” found here
http://www.techlib.com/electronics/antennas.html
won’t cost you an arm and a leg and will offer quite good performance (and I believe far better than the MLA-30 one), alternatively you may consider this passive loop design covering the lower bands (from below 160 to 40 mt)
http://www.kk5jy.net/rx-loop/
then, if you have a bit of space (garden, backyard…) that is, at least 15 square ft, you may consider putting together this antenna
http://www.kk5jy.net/LoG/
having built and tested several samples of it, I can say that it’s really incredible, sure, it covers the lower bands (up to 40mt at 20mt the pattern starts “squirreling”) but on those bands it really rocks, and by the way it takes very little time, money and effort to set up, so if you’ve a bit of space… try it 🙂 !
I see Andrew’s suggested the Wenzel/Techlib “loopifier” loop already, which – see my other reply further up – is a fair choice, and much better than the MLA-30 appears to be. (In fact, if you’re up to desoldering & soldering SMD, you could probably mod the MLA-30 into a fair approximation of the loopifier loop).
But, if you’re up for a bit of part-hunting & mild DIY, let me also suggest a tuned passive (i.e. unamplified) loop e.g. as described in Chris Trask’s “A Simple Varactor-Tuned Loop Antenna Matching Network” or “A Varactor-Tuned Indoor Loop Antenna” papers (Google them). It’s almost one of my missions in life to convert people to the cult of tuned passive loop antennas 😉
Unfortunately, I don’t know of any pre-built equivalents for outdoor use, though Thomas has featured a couple of indoor versions e.g. https://swling.com/blog/2019/06/new-indoor-passive-loop-antenna-for-shortwave/
(Tip: For either the loopifier or Trask designs, you’ll be looking for AM varicaps/varactors which are getting rare these days. I’ve had good luck buying BB112’s, Sony SVC321’s, and Toshiba 1SV149’s on eBay.)
Got one and am enjoying listing to Short Wave radio with it on my Linux Box https://www.youtube.com/watch?v=JCI9yKdwVDM&list=PL_g5Ad_LMmeKxvibR_Ira0SjLF0gz4s_H
Sounds like there are quite a number of clones (ok attempted ones) of the “loopifier” designed by Charles Wenzel
http://www.techlib.com/electronics/antennas.html
but as far as I can tell (I built some samples of the ORIGINAL one) the clones aren’t up to par; as for Wenzel I contacted him recently about this other clone
https://rhelectronics.net/store/diy-kit-of-active-amplifier-magnetic-hf-antenna-3mhz-30mhz-for-hula-loop-shortwave-radio.html
and he just don’t cares; the designs are free to use and modify; as a note the rh loop seems better (didn’t try either) than that “MLA 30” loop; at a quick eyeball the circuit seems to almost match the original, although there’s an additional electrolytic capacitor and the gain resistor was replaced with a 5k trimmer which allows to set the gain; but at least the RH board keeps the tuning and bandswitch from the original circuit, so it may work well