MLA-30 active loop antenna

[Co6aka]

Antenna position/orientation, "gain" and the variability of RF noise makes such comparisons challenging, to say the least. Without an anechoic chamber, the basic way to measure internal noise is to substitute an inductor equal in value to the loop (inductance and resistance) and then enclose the amp in a RF-tight box, that may have to also be grounded, depending.

But there are other factors I discovered years ago... "Feedback noise" as I call it when the antenna hears it's amplifier's output, and IMD products resulting from strong out of band signals like the VHF-TV and FM-BCB soup here in NYC. Plus, there's my local "27MHz good buddy" on his "squawk box" with the obligatory "lean-year" "footwarmer" that actually does spew RF from DC to daylight, wall-to-wall treetop-tall rightatcha-c'monback, all of which modulates the entire HF spectrum, DC to daylight, but that's an extreme example. IMD aside, when the amp's headroom is used up, it's used up, even if the amp is quiet in a chamber or a box. PLUS... Something I hadn't considered before seeing SM5BSZ's videos covering audio frequency noise causing reciprocal mixing in MMIC amplifiers; gotta investigate that too.

The LM592/NE592 used in the MLA-30 is well documented and is easy enough to model and breadboard, and from there its limitations will become clear. The AD8129/AD8130 would most likely be better, but neither make a "good" loop amplifier. To do that, you need a very low noise figure with a very low input impedance plus very high linearity plus "juice" and the ability to pump it into a 50/75-Ohm coaxial cable. This is far from trivial, and far beyond the capabilities of a jellybean opamp.

[vinlove]

yeah, this is the part of my point.

I have 2x MLA30s positioned NS and EW directions.  A Wellbrook ALA1530 and longwire.  They all perform better at different times and days for different signals.
They all have times of quiet state, and get noisy and very noisy at times.

Even if the MLA30 which may feel noisy antenna due to the design or whatever, they get very quiet and work great, while the other antennas are noisy or deaf, and vice versa.

 I concluded that noise floor figure of MLA30 is not really critical factor for being good DX antenna. There are too many other factors involved in  this game. And yes in case any Active Mag Loops such as MLA30 and Wellbrook 1530, the directivity also kicks in. If it is pointing to wrong direction from the signal you are trying to listen, it will sound poor, and it can even NULL it out.

With all these factors playing on the actual RXing and DXing the bands, I thought it would be better to put another antenna such as a good vertical or sloper and switch inbetween different antennas for the DXing, rather than trying to mod the B-T of MLA30 imagining that it would enhance or improve hearing the faint signals, because it ain't going to.

[A.Z.]

Antenna position/orientation, "gain" and the variability of RF noise makes such comparisons challenging, to say the least. Without an anechoic chamber, the basic way to measure internal noise is to substitute an inductor equal in value to the loop (inductance and resistance) and then enclose the amp in a RF-tight box, that may have to also be grounded, depending.

Hi there, let me start by saying that, willing to properly characterize a preamp, the approach you describe is totally correct, on the other hand, for a coarse evaluation of the noise floor, using an SDR and an antenna switch, to compare a given preamplified antenna with a know one will at least give you an idea of the noise floor of the preamp, since the band conditions won't change in the seconds needed to switch between an antenna and another one

That said, while the TL592 used in the MLA-30 isn't exactly a quiet amplifier, it isn't the real issue with that antenna; most of the MLA-30 noise comes from the voltage multiplier IC used in the original BIAS-T circuit, by either modding it (or replacing the whole unit); as an example, I ran a simple test tuning to the Shannon VolMet and then swapping back and forth the standard BIAS-T and another one from a miniwhip, with the original BIAS-T, the VolMet signal got lost in the noise, with the other one, the VolMet signal was readable although with a bit more noise than the one I got using my "LLD" (linear loaded dipole) antenna, but that was expected since the MLA has a preamp (and then being wideband [untuned], it's also subject to IMD in case of strong nearby signals)

What I can say is that, by replacing the BIAS-T unit (or modding it), replacing the stock RG-174 coax with better one and adjusting (lowering) the preamp gain trimmer, the performance of the MLA-30 improves quite a bit, and while it won't become a skyloop or the like, the improvement is particularly noticeable with weaker signals, then by the way, there's little one can do with propagation, but at least, after the modifications, the antenna will contribute much less to the noise level  and, all in all, it's all about setting the S/N ratio, not about gain, that's also why I prefer using passive antennas, but I was curious to try the MLA-30 so here I am

[Co6aka]

I'd build a completely new DC injector from scratch, using a isolated shield BNC for at least the antenna side with a internal coaxial common mode choke attached. I've used RG-188 and four 75-mix beads that form the four sides of a square with the coax running a few turns through them - as many turns as would fit. (I'd like to experiment with alternating 75-mix and 73-mix beads to perhaps get a wider bandwidth choking effect.)

Any noise generator should eliminated, and that DC/DC converter scheme they used is just plain double-facepalm RF design. "Linear" sourced DC-in to a good common mode choke, followed by good VLF-HF filtering. Maybe embed a low noise 12V linear regulator after that. (Maybe a steel box would be better than a aluminum box for the DC injector - less EMI coupling to the DC-injection circuitry? Or use mu-metal?)

For DC injection, use at least two shielded (wire completely enclosed in ferrite) axial chokes in series, with a lower inductance connected to the antenna-side followed by a higher inductance to the DC source. Wideband choking impedance of the combination can be swept with a spectrum analyzer or sig-gen and scope; make essentially a RLR Pi-network and sweep it. I did that for my Wellbrook and it made a big improvement from AM-BCB down to 10kHz VLF.

As for coax... I switched over to good quality 75-Ohm braid-over-foil foam dielectric copper core (center conductor) coax; currently using RG-11 from the DC injector to the antenna, and RG-59 from the injector to the radio (because it's more flexible than RG-6.) Before changing the coax from RG-58, I de-powered the antenna and tuned around 10kHz-30MHz to get an idea of how much signal ingress there was. After changing the coax I was amazed by how much less ingress I heard. (The lower capacitance of the 75-Ohm coax might improve the amplifier's output capability.)

I also added 31-mix ferrites over the coax in two places, just outside the shack and just before the antenna. Likely I also need to do this on other feedlines to nearby antennas. Ferrites over the coax between the injector and radio don't seem to make any difference.

Beyond that, work would have to be done on the loop amplifier. If you're not in a potent RF-soup environment, making the loop element (diameter) bigger might help higher frequencies as the loop impedance will increase and be a better match to the amplifier's input.

On a somewhat related note, if anyone wants to have some fun, go to a auto junkyard and get some of those sharkfin antennas and experiment with the E-field. Supposedly they use a ASIC, which makes me wonder if it would be possible to do the same for a loop amp.

[A.Z.]

well didn' t go as far as totally rebuilding the injector (although it won't take much time), I had a RA0SMS miniwhip bias tee unit at hand, so I just used it, and it worked, the original unit isn't bad ... but that voltage multiplier totally screws the design

A thing which I didn't yet try is adding a (say) 16:1 or the like transformer between the loop and the preamp input, the idea is to get a better impedance match over the frequency range

as for the coax, the "RG174" which comes with the antenna is something one would never use, it's ok to wind a choke (18 turns around an FT140-43 core) but... nothing more, a run of decent coax, even RG58 (or 59) or some tv/sat cable will work MUCH better

Overall, for its price, the MLA-30 once properly modded/adjusted isn't a bad antenna, in particolar for space constrained installattions, but one should keep in mind that, to have something working decently, ut needs some tweaking...

and, btw, that's the fun

[Co6aka]

Of possible interest... This morning the following message was posted to some of the GROUPS.IO mailing lists:

If any one is interested I just finished building 5 more LZ1AQ loop amplifier boards with bias tees. These will be the last I build for awhile. The performance of this Improved version on the LZ1AQ is much better than the original design. For info and prices you can contact me off list, at everettsharp@aol.com..

I use this loop amplifier here at my QTH for my SDR receiver for coverage from 100kHz to 30MHZ and it is my best RX antenna.

Everett N4CY

I have no connection with that poster, just passing it along because the LZ1AQ loop amp is a much better alternative. I have one and have experimented with it quite a bit, though I'm not using it now.

[A.Z.]

Thank you "sobaka"

Yes, the LZ1AQ and the WellGood (Wellbrook clone) are both good

http://www.lz1aq.signacor.com/docs/wsml/wideband-active-sm-loop-antenna.htm

https://www.george-smart.co.uk/projects/wellgood_loop/

but, see, the purpose for this discussion (set aside the sidetracking attempts) was/is to "hack" the cheap MLA-30 and improve its performances w/o spending too much, sure, the final result won't probably be up to par with other circuits, yet, I believe that, with some easy/cheap (that's the unwritten rule of this "game") mods, the MLA-30 may become a good pick for people willing to start with SWLing and not willing to invest an arm and a leg

[etkal]

I have a great solution to allow you to feed 12v into the Bias Tee and bypass the noisy micro USB 5v step-up.

Looking at my board, it doesn't quite match the circuit, in that the last component prior to the DC choke is the LED and its resistor.  Cutting the trace just before that (to the right), separates the voltage boost from the output.  You can then add a 12v input across the DC choke pins 2 and 3.  I used a 5.5 x 2.1mm DC cord and plug.  This keeps the LED and the choke in the circuit.

In order to restore the micro USB capability, I added a 12v jack to the side, and jumpered the 12v+ contact on that to the other side of the cut trace; there is an unused pad that works nicely.  Nothing is necessary for the negative contact.  Plugging the 12v plug into the jack then basically shunts from DC choke pin 3 back across the trace to complete the original circuit.

  73, Erik W1QED

[A.Z.]

Hi Erik, that's almost what I did too, then I also lowered the preamp gain using the unpotted trimmer found on the preamp board and adjusted the trimmer inside the bias-t unit for no attenuation (lowers noise a bit too); at the moment I'm using the preamp with a 18m perimeter wire loop with pretty good results, I also wound an isolation transformer (1:4 impedance ratio) placed between the loop and the preamp, that helped improving things a bit on some bands (like the 15m one), all in all it isn't so bad given its cost, then ok the noise floor isn't as low as the one from other units, but given the cost ...

[clytle374]

Hi Erik, that's almost what I did too, then I also lowered the preamp gain using the unpotted trimmer found on the preamp board and adjusted the trimmer inside the bias-t unit for no attenuation (lowers noise a bit too); at the moment I'm using the preamp with a 18m perimeter wire loop with pretty good results, I also wound an isolation transformer (1:4 impedance ratio) placed between the loop and the preamp, that helped improving things a bit on some bands (like the 15m one), all in all it isn't so bad given its cost, then ok the noise floor isn't as low as the one from other units, but given the cost ...

Thank you for this information, you saved me from wasting my time trying to get more gain out of this amplifier to compensate for a weak receiver.  Also the bias T is definitely a poor design.  Thanks for the heads up about using the sdrplay's internal bias T and it's lower voltage.  I'd also like to nominate you for the most calm and patient user award

[A.Z.]

Thank you for this information, you saved me from wasting my time trying to get more gain out of this amplifier to compensate for a weak receiver. 
Also the bias T is definitely a poor design.

You're welcome, again, I don't pretend that the MLA-30 could "magically" become some Wellbrook or the like, yet given its low cost, it's possible to somewhat "remediate" to some of the design flaws and squeeze a bit better performance out of that loop; to recap

1. Modify the bias-t unit to exclude the voltage multiplier, feed the unit with 12V from a good PSU and set the attenuator trimmer inside the bias-t to "zero attenuation"

2. Add a BNC connector to the preamp unit and replace the coax with good quality one, keep the original coax (inside the box, from the preamp to the connector) a bit longer and use it to wind a CMC choke on an FT140-43

3. adjust (lower) the gain of the preamp to obtain good signal but keep the noise floor as low as possible

4. add a 1:x transformer (experiment with the ratio, 1 to the loop, x to the preamp input) between the loop and the preamp, the transformer will be hosted inside the preamp box

Thanks for the heads up about using the sdrplay's internal bias T and it's lower voltage.

Using the internal BIAS-T of some SDR units will work, but performance will degrade since the reduced voltage (about 5V) will also reduce the dynamic range

I'd also like to nominate you for the most calm and patient user award

LOL !! Nothing special, one may, sometimes, loose patience in real life; but it makes no sense to me loosing patience in a forum discussion

[clytle374]

Horrible and quick hack to remove most of the switching noise from the power supply ending up in the antenna lead.  Nearly offensive practice, but it is temporary and helped a lot.

[A.Z.]

Horrible and quick hack to remove most of the switching noise from the power supply ending up in the antenna lead.  Nearly offensive practice, but it is temporary and helped a lot.

Well, that's pretty extreme ! Didn't the modifications shown here

https://www.eevblog.com/forum/rf-microwave/mla-30-active-loop-antenna/?action=dlattach;attach=1126610;image

suffice ? Those mods exclude the voltage multiplier IC from the circuit, at that point you'll power the unit using 12V DC from a clean supply connected to pin 1 (+VDC) and 5/6 (GND) of the USB socket (which won't be used anymore and could be removed and replaced with a different connector - or either you may pass a red/black wire out from the connector hole)

As a note, I've been using for almost one year now the MLA-30 preamp connected to a horizontal wire loop (about 18m total) and it's working well, all I did was adjusting the preamp gain by using the trimmer found inside the preamp box to avoid overloading (by the way the preamp box was modified by adding a BNC connector and a choke inside the box), the whole thing is fed using a run of "RG6" (tv/sat) double shield coax

[clytle374]

Yes, but that required my digging through boxes to find a  12V wallwart.  This took 15 minutes and still runs off USB.  I'm building a new loop and amplifier anyway, and this honestly felt more vengeful.  Lol

[A.Z.]

Yes, but that required my digging through boxes to find a  12V wallwart.  This took 15 minutes and still runs off USB.  I'm building a new loop and amplifier anyway, and this honestly felt more vengeful.  Lol

As for the loop, if you didn't yet choose a design, have a look at this one

https://www.larches-cottage.co.uk/rx_antenna/loops_rx/Evaluating%20a%20Broadband%20Active%20Loop%20Antenna.pdf

then, for the preamp, you may also consider a remotely tuned one, like the one found here

http://www.arrl.org/files/file/QEX_Next_Issue/Jan-Feb2018/Steber.pdf

[clytle374]

As for the loop, if you didn't yet choose a design, have a look at this one

https://www.larches-cottage.co.uk/rx_antenna/loops_rx/Evaluating%20a%20Broadband%20Active%20Loop%20Antenna.pdf

then, for the preamp, you may also consider a remotely tuned one, like the one found here

http://www.arrl.org/files/file/QEX_Next_Issue/Jan-Feb2018/Steber.pdf

Thanks, I bookmarked and will read tomorrow, been soldering 402 components on my limesdr for the last 3 hours modding the input circuit for HF.  My nerves are shot, and I managed to lose a part to other other antenna port, soldering through a jeweler's loupe.  I'm getting decent performance now, that and a DIY LPF to keep the FM broadcast out of it.

I was following a webpage https://www.lz1aq.signacor.com/docs/wsml/wideband-active-sm-loop-antenna.htm  I think it was recommened here, but not sure anymore.  RF is out of my comfort zone, but if you know what you're doing, you're not learning anything is a motto of mine.  Is this forum tolerant of stupid questions?  I asked on some FB groups and people don't know. Probably start a thread here and see how it goes.

Thanks, Cory

[A.Z.]

As for the loop, if you didn't yet choose a design, have a look at this one

https://www.larches-cottage.co.uk/rx_antenna/loops_rx/Evaluating%20a%20Broadband%20Active%20Loop%20Antenna.pdf

then, for the preamp, you may also consider a remotely tuned one, like the one found here

http://www.arrl.org/files/file/QEX_Next_Issue/Jan-Feb2018/Steber.pdf

Thanks, I bookmarked and will read tomorrow, been soldering 402 components on my limesdr for the last 3 hours modding the input circuit for HF.  My nerves are shot, and I managed to lose a part to other other antenna port, soldering through a jeweler's loupe.  I'm getting decent performance now, that and a DIY LPF to keep the FM broadcast out of it.

I was following a webpage https://www.lz1aq.signacor.com/docs/wsml/wideband-active-sm-loop-antenna.htm  I think it was recommened here, but not sure anymore.  RF is out of my comfort zone, but if you know what you're doing, you're not learning anything is a motto of mine.  Is this forum tolerant of stupid questions?  I asked on some FB groups and people don't know. Probably start a thread here and see how it goes.

Thanks, Cory

Hey Cory, the LZ1AQ is a very good preamp, so if you're building it... go for that, I thought you didn't yet start the build, so I offered those links to give you some ideas, but the LZ1AQ is definitely a good preamp

[clytle374]

Hey Cory, the LZ1AQ is a very good preamp, so if you're building it... go for that, I thought you didn't yet start the build, so I offered those links to give you some ideas, but the LZ1AQ is definitely a good preamp

I'll go ahead an venture off topic a bit. Between that amplifier and a video by a mikrowave1 youtube channel. I decided to go with the LZ1AQ amplifier and loop.  The issue I have is this SDR's sensitivity drops off pretty bad at the low end of HF.  The mod I just completed helps a lot, but still pretty bad below 10Mhz. The LPF I built keeps the FM broadcast out of it since it's very sensitive up there.  I'm currently actually getting the AM broadcast band and 160M okay.  I did do the opposite of the good thing and turned the gain up on the MLA-30.   So I decided to slightly mod the input of LZ1AQ's amplifier with a first order LPF trying to counter the slope of the SDR slope, and the output for more gain.

I need more gain.  The mikrowave1 video "loop amplifier #3" also used the 180 ohm 'swamping resistors' to lower the output impedance of the output stage.  I don't know how to calculate the output impedance of a common emitter into a transformer. I did google it to death. But mikrowave1 said a 9:1 impedance would be better than the resistors.  So I decided they both we close to the same, even though mikrowave1's were only common base circuits.  LZ1AQ didn't specify a toroid material. 

So I threw the semi-educated guess at it and used a T50-2 core, twisted 7 windings, and got 19 turns before it was full.  I needed 20 turns to get LZ1AQ's inductance specs iirc. I put 6 windings in series with a CT, and a single winding for the output.  Testing 1 turn to 1 turn gave me this.
Picture 1
I then built it on a breadboard, just to see what I would get.  I didn't build the lowpass on the breadboard.  And I guessed how to couple 50 ohm output of my SA TG to the low impedance of the amplifier.  Used 6 turns on a t50-2 core with the input passed through the core.  2.5:1 turns sounded right, but didn't work much.   
Picture 2
I got this, only want to got to 50Mhz, and this is a breadboard so I have not faith in the numbers.
Picture 3
So I'm confused about the impedance.  The 9:1 output seems to be a voltage loss, when I need a gain.  But all the current is passing through the transformer this way.  Any advice? Should I start a thread here? Or will I get ripped apart?  Fastest way to swim is jump in the deep end.  If this project isn't doomed due to my ignorance, I'm going to make a PCB tomorrow. 
Thanks, Cory

[A.Z.]

regarding the LZ1AQ preamp, check here

https://easyeda.com/3ym3ym/LZ1AQ-loop-preamp-SMD_copy

as for the remainder, go on and open a new thread, add full details and clearly specify your questions and doubts

[A.Z.]

Also, I'm a big fan of the "step by step" approach and also of the "don't add stuff if it isn't needed" one so, in your case, I'd start with a plain vanilla PASSIVE loop as the one described here

http://www.kk5jy.net/rx-loop/

and build the loop and the needed impedance transformer, done that, in case you'll want to add a preamp, doing so will be easy since ANY preamp accepting an impedance around 50 Ohm will fit; more, adding that "tuned preamp" (see my previous post), would result in a tuned preamplifier (which thanks to the varactor tuning may be remotely controlled) which not only will boost the signals from the loop but may also help avoiding the various "bandstop" filters

[clytle374]

as for the remainder, go on and open a new thread, add full details and clearly specify your questions and doubts

Thanks again.  I'm going to read the links you sent. And do a little more research and I'll start a post.  A passive loop will leave me completely deaf below 8mhz.  Varactor options had crossed my mind.  THe plan is for this to be my monitor when I finally get a transmitting mag loop built.  And either learn CW and use the HW-16 I just rebuilt, or rebuild the HW-101.  On the plus side the amplifier on the breadboard with a 2 foot loop was picking up 20 meters in the basement. 

[7danny]

at the moment I'm using the preamp with a 18m perimeter wire loop with pretty good results

Am I reading this right? That's almost 60 feet?

[A.Z.]

at the moment I'm using the preamp with a 18m perimeter wire loop with pretty good results

Am I reading this right? That's almost 60 feet?

yes, laid horizontally in a triangular shape at about 9m from ground, works pretty well

[A.Z.]

Beyond that, work would have to be done on the loop amplifier. If you're not in a potent RF-soup environment, making the loop element (diameter) bigger might help higher frequencies as the loop impedance will increase and be a better match to the amplifier's input.

revamping this old thread due to some recent "experiments"; if you have an MLA-30 around, try the following:

build a cross shaped structure using non conductive material (e.g. PVC pipes, wood sticks...)

use the cross to support a diamond shaped loop, made using 2.5 mm wire and having 76 cm sides

connect the MLA preamp at one of the side corners of the antenna, routing the coax along the horizontal arm and then down along the vertical support

at the opposite side corner place a 530 Ohm resistor

install the antenna at about 3m from ground (or ground plane), possibly, for testing, place it in a garden or yard

aim the preamp box corner at the desired receiving direction and start listening

turn the antenna 180° and check the effect

note: willing to build a passive version or to use a different preamp, place a 9:1 galvanic transformer at the feedpoint and connect the coax or whatever preamp after it (the 50 Ohm match after the 9:1 will be almost perfect)