Author Topic: MLA-30 active loop antenna  (Read 10840 times)

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Offline A.Z.

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MLA-30 active loop antenna
« on: December 09, 2020, 11:45:56 am »
I decided to pick up the newer MLA-30+ to see if there was any effective improvement over the MLA-30; judging from what I've seen/read, the main change is that now the performance at higher frequencies should have been improved; anyhow, I went on placing the order on Amazon and after 4 days got the antenna delivered.

Last weekend had a bit of time in my hands, so decided to give the MLA-30+ a quick test, just to ensure it was -at least- working, so I hooked up the antenna indoors placing the preamp box on a table "lazy susan" style and using a cardboard pipe to support the (very thin) steel wire of the loop, connected the preamp to the bias-t box and the latter to the receiver, plugged a 5V DC supply to the bias-t box and ran my SDR

Propagation wasn't so good, but signals came in, yet even the "+" model has the very same issue of the previous model, that is, the bias-t is very noisy; just to give you an idea, using the LLD (Linear Loaded Dipole) I've on my balcony, the noise floor is around -120 to -130 dB, plugging the MLA-30+ the floor goes up to -90 to -100 dB which, in my book, is quite a lot of noise; to confirm that the issue was due to the bias-t unit, I picked another bias-t used for a miniwhip antenna and used that one to power the MLA-30+ with a 12V DC supply, with the latter, the noise floor on the loop went down to -110 to -120 dB which, considering that the antenna was indoor, is a decent level

So I believe that modifying the bias-t to bypass the DC-DC converter and powering it directly through a 12V DC supply is a need and not an option (alternatively one may either use another bias-t unit or build one); the modification (didn't yet go on and make it) is shown in the attached picture



to perform it I'll open the bias-t box (whose circuit is luckily unpotted) and cut the traces maked in red in the picture, so excluding the (very noisy) voltage converter from the circuit while leaving the passive filtering components in place; at that point it will just be a matter of soldering two wires to pins #1 and #5 of the (now unused) USB port and using them to power the bias-t using a decent 12VDC power supply

Whith the above modification alone (and by the way, placing the antenna outside), the MLA-30+ should become a pretty decent performer; then I'm also planning to add a BNC connector to the antenna, the process, judging from this video



should be pretty easy, although, instead of using a single snap-on choke as the folk in the video did, I'll probably cut the coax a bit longer and use it to wind a "Guanella" choke which, for sure, will work far better, the modification will also allow me to use a longer coax ... and a better one (the one coming with the antenna isn't exactly "first class")

While with the above modifications, the MLA-30 will become a decent antenna, I believe that to really improve it some further modifications will probably be needed; if you look at the MLA-30 informations found on the G8JNJ web site and check the notes and the schematics, you'll see that another issue with the MLA-30 (and I suspect with the "+" too) is the very bad design of the input lowpass filter stage; such a filter expects to (at least) see the same (ok, a similar) impedance on both its ends, but in the MLA-30 case, the filter sees low impedance on the loop side and high impedance on the preamp input one; now, Martin (G8JNJ) had an unpotted circuit so he was able to try placing some resistors between the loop input and the board ground, but in my case, unpotting the circuit could result in a damaged board, so after some head scratching I decided that I'll try a different mod, that is placing a transformer between the loop and the preamplifier input, the transformer will be something like this  with the low impedance winding going to the loop and the high impedance one going to the preamp; given the very low impedance of the loop, I believe that a 1:16 transformer should work, so my idea is to wind 2 turns to the loop and 8 turns to the preamp (turns ratio 1:4 = impedance ratio 1:16) on a small binocular core (a FairRite #73 or similar should do)

To install the transformer I'll unscrew the two side screws with the winged nuts used to support the loop, fold the two rails connecting the screws to the preamp input down toward the (potted) board and solder the high impedance side of the transformer to them, then I'll reinsert the screws (nuts, washers, windged nuts) and connect the low impedance side of the transformer to them, this way I hope to (at least) improve a bit the preamp performances... although I'll probably have to fiddle a bit with the transformer trying different ratios to find the better working one

Ok, that's the plan, at least; will probably find time to start with the project next January or February and, at least, perform the first two modifications (bias-t and bnc+choke), posted the above just in case someone else, owning an MLA-30, wants to try the same mods

[edit]

Forgot; another planned mod is to replace the very thin steel wire used for the loop with some copper pipe or aluminium strip, that would be the easiest mod and may (hopefully) further improve the antenna performances



« Last Edit: December 09, 2020, 11:59:16 am by A.Z. »
 

Offline Co6aka

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Re: MLA-30 active loop antenna
« Reply #1 on: December 11, 2020, 03:19:22 am »
Basically, in a loop antenna you want to minimize loop inductance and resistance*, but a properly designed loop amplifier will be designed with its attached loop inductance and resistance in mind.  Typically, you design and construct a loop element to minimize inductance and resistance, then design the amplifier for it.  Anyway, you'll likely see improvement with anything "better" than that silly wire loop.

FWIW, for receiving purposes where loop CURRENT is very low, very low loop resistance isn't as critical as it is for transmitting, so aluminum is fine for the loop element. In my experiments with transmitting loops back in the early 80s, copper flash worked slightly better than aluminum flash, but of course it was much heavier so the frame had to be strengthened. 

* You also want to maximize area, while keeping the circumference (or diameter) within dimensions appropriate for the highest frequency of use.

(Another factor to consider for a loop amplifier is noise figure, the lower the better, which means eliminating resistors attached to the input. Another is linearity, of course.)
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Offline A.Z.

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Re: MLA-30 active loop antenna
« Reply #2 on: December 11, 2020, 08:10:24 am »
Co6aka, you're right, but the point here is trying to improve the existing MLA-30/MLA-30+ loop; now, the preamp circuit is fully potted and un-potting it would be quite a hassle and may result in damages to the board, so I decided it won't be worth going that way, what I'm planning is a series of modifications to improve the existing circuit/antenna without too much effort and, by the way, without pretending to obtain a "wellbrook" (or the like) at end  :)

As for the loop material, I agree about the fact that some copper tubing would be the best choice, but then I think I'll explore other (cheaper) materials too, again the target isn't to turn the MLA-30 into a "super antenna" but to apply it some easy/cheap mods which anyone could perform and which will improve the loop performances, nothing else
« Last Edit: December 11, 2020, 10:38:51 am by A.Z. »
 

Offline vinlove

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Re: MLA-30 active loop antenna
« Reply #3 on: December 11, 2020, 10:15:50 am »
Is Wellbrook a lot better performance than MLA30? The price is about 10 times higher.
My MLA30 was 25 euros including delivery.

I saw a youtube video, that says MLA30 is as good as any other active loop on the market.
But I don't know how the Wellbrooks like. I have never used it.
 

Offline A.Z.

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Re: MLA-30 active loop antenna
« Reply #4 on: December 11, 2020, 10:27:48 am »
Is Wellbrook a lot better performance than MLA30? The price is about 10 times higher.
My MLA30 was 25 euros including delivery.

Well, it's like comparing apples and oranges; the MLA-30 is a very cheap loop and has a number of design flaws, but it's cheap and the flaws can (to some extent) be corrected, the WellBrook loops are "top of range" but cost a lot more; then, willing to build a wellbrook "clone" yourself, you may find the informations and schematics here although, sincerely, if you're going for a build, I'd suggest you to consider this one

Quote
I saw a youtube video, that says MLA30 is as good as any other active loop on the market.
But I don't know how the Wellbrooks like. I have never used it.

Youtube videos aren't "the ultimate truth" 8)

As I wrote, I have an MLA-30 here and while I didn't fully test it, I can confirm that the bias-tee unit is VERY NOISY, that may be fixed, sure, but if one is going for an MLA-30 he'll need to be prepared to modify it or, at the very least, modify (or replace) the bias-tee unit

 

Offline vinlove

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Re: MLA-30 active loop antenna
« Reply #5 on: December 11, 2020, 01:39:21 pm »
Here is the link for the review comparing MLA30 vs. W6LVP.


The reviewer thinks MLA30 is better than W6LVP which costs 10 times more.
W6LVP is not Wellbrook, but I think they cost about the same.
 

Offline vinlove

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Re: MLA-30 active loop antenna
« Reply #6 on: December 11, 2020, 01:43:37 pm »
It is interesting that he thinks that MLA30 is quieter than W6LVP which costs 10 times more expensive.
He is an experienced SWL / BCL and knows what he is talking about, I thought.
 

Offline A.Z.

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Re: MLA-30 active loop antenna
« Reply #7 on: December 11, 2020, 01:48:02 pm »
It is interesting that he thinks that MLA30 is quieter than W6LVP which costs 10 times more expensive.
He is an experienced SWL / BCL and knows what he is talking about, I thought.

Lots of talk, no proof; again, having the MLA-30 (plus) in my hands and having other antennas (and having tried more in a past), the MLA-30 may be modified to become a pretty decent active loop, but even then, it won't be up to par with other loops (either built or bought); this does NOT mean that one should throw the MLA-30, just that the latter needs (and really needs) some modifications.

 

Offline vinlove

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Re: MLA-30 active loop antenna
« Reply #8 on: December 11, 2020, 02:48:29 pm »
It is interesting that he thinks that MLA30 is quieter than W6LVP which costs 10 times more expensive.
He is an experienced SWL / BCL and knows what he is talking about, I thought.

Lots of talk, no proof; again, having the MLA-30 (plus) in my hands and having other antennas (and having tried more in a past), the MLA-30 may be modified to become a pretty decent active loop, but even then, it won't be up to par with other loops (either built or bought); this does NOT mean that one should throw the MLA-30, just that the latter needs (and really needs) some modifications.

yup, maybe it would need side by side comparison on the same signal and receiver.
I am sure he has some videos on that testing = comparing MLA30 vs. other active loop antennas.
 

Offline A.Z.

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Re: MLA-30 active loop antenna
« Reply #9 on: December 11, 2020, 02:51:18 pm »
yup, maybe it would need side by side comparison on the same signal and receiver.
I am sure he has some videos on that testing = comparing MLA30 vs. other active loop antennas.

One, amongst a bunch of others (just a matter of searching a bit)

https://swling.com/blog/2019/09/david-reviews-and-compares-the-mla-30-magnetic-loop-antenna/

also see

« Last Edit: December 11, 2020, 03:01:50 pm by A.Z. »
 

Offline Lord of nothing

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Re: MLA-30 active loop antenna
« Reply #10 on: December 11, 2020, 03:56:09 pm »
When the Temperature allow I put my MLA-30 outside and the Cable goes truth the open door and I use the stock Bias-T. In my case the work good for my perspective. I could not see anything who produce a lot of noise. I would like try some better Bias-T. Is there someone who is recommend?
I dont have any Solder Iron or Power Supply or anything elese. So it must be fool proof.
Thanks
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Offline A.Z.

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Re: MLA-30 active loop antenna
« Reply #11 on: December 11, 2020, 03:59:42 pm »
When the Temperature allow I put my MLA-30 outside and the Cable goes truth the open door and I use the stock Bias-T. In my case the work good for my perspective. I could not see anything who produce a lot of noise. I would like try some better Bias-T. Is there someone who is recommend?
I dont have any Solder Iron or Power Supply or anything elese. So it must be fool proof.
Thanks

Are you using it with an SDR ? And if so, which one ? I'm asking since some SDR units offer a builtin bias-tee option, in such a case you may connect the coax coming from the loop directly to the SDR SMA (keeping out of the combo the bias-tee box and the additional coax) and power the MLA-30 preamp using the SDR bias-tee voltage, that will be 5V, so lower than the original 12V, so gain will be lower, but the preamp will still work and noise floor will be lower

[edit]

also, in the spite of "better than nothing", consider adding to the MLA-30 coax some chokes, those may either be wound on a toroid using the coax or you may buy some snap-on chokes and add them to the coax, in such a second case, group the chokes together (a group will be at less 6 chokes, more is better) and use some tape to keep them tight together; those will help taming the noise picked up by the "not so good" coax which comes with the MLA-30, then you may also consider replacing the original thin wire loop with a fatter one (or even a piece of copper pipe)

« Last Edit: December 11, 2020, 04:45:57 pm by A.Z. »
 

Offline vinlove

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Re: MLA-30 active loop antenna
« Reply #12 on: December 11, 2020, 05:05:07 pm »
I am feeding the MLA30+ with a rechargeable phone battery 5V DC, instead of wall wart psu, and it is already making big improvement in noise.
Problem is that, the battery must be recharged when runs out. And when the voltage drops, the reception goes abnormal.
 

Offline A.Z.

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Re: MLA-30 active loop antenna
« Reply #13 on: December 11, 2020, 05:21:34 pm »
a battery is for sure better than a noisy power supply or phone charger, but the voltage multiplier inside the bias tee will stll be introducing noise
 

Offline Lord of nothing

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Re: MLA-30 active loop antenna
« Reply #14 on: December 11, 2020, 06:31:13 pm »
How about a bigger PSU like I use for CB Radio?
Like such: https://www.conrad.at/de/p/voltcraft-fsp-1122-labornetzgeraet-festspannung-12-v-dc-2-a-25-w-anzahl-ausgaenge-1-x-511410.html
I dont know what the internally use and how good the are but for my CB Radio it work good.
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Offline A.Z.

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Re: MLA-30 active loop antenna
« Reply #15 on: December 11, 2020, 06:53:22 pm »
that seems to be a stabilized (non switching) unit, and may fit well, but you will need to modify the bias tee unit as I wrote
 

Offline vinlove

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Re: MLA-30 active loop antenna
« Reply #16 on: December 12, 2020, 08:36:20 pm »
Blimey, just found that my SDRPlay RSP1A has a BIA-T function built-in. It can be switched on the SDRUno software.
When connected with MLA30, the BIAS-T of RSP1A works, and it works very well.  Noise is well down, and signal is up.

I didn't know that before, because I got the RSP1A without any documentation, because it was cheap used one.
Thanks to this thread, I found it has the facility and I don't have to modify or upgrade the MLA30 noisy BIAS-T.  :-+ :phew:

If you use SDR, then check if it supports BIAS-T function built in. If it does, then you save time and money and effort for modding noisy BIAS-T of MLA30.
« Last Edit: December 12, 2020, 08:38:09 pm by vinlove »
 

Offline A.Z.

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Re: MLA-30 active loop antenna
« Reply #17 on: December 12, 2020, 09:13:56 pm »
five volts
 

Offline vinlove

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Re: MLA-30 active loop antenna
« Reply #18 on: December 12, 2020, 09:27:58 pm »
yup. but seems work.
When BIAS-T is switched off from SDRUno, there is no reception.
When switched on, MLA30 is operational with good signals.

 

Offline A.Z.

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Re: MLA-30 active loop antenna
« Reply #19 on: December 12, 2020, 09:50:58 pm »
it works, sure, but gain is lower and IMD worse
 

Offline vinlove

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Re: MLA-30 active loop antenna
« Reply #20 on: December 12, 2020, 09:55:42 pm »
Did side by side testing ,and there is no noticeable difference on HF, but on MW, there is, yes.
So, if you are MW DXer, the 5V bias-t is no use for MLA30.
But if you just listen to HF only, 5V seems OK.  It is definitely quieter and same signal strength on HF.
 

Offline Co6aka

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Re: MLA-30 active loop antenna
« Reply #21 on: December 12, 2020, 11:22:18 pm »
FWIW, I use a Wellbrook ALA1530LN as a benchmark reference for my own designs -- gotta know if mine are at least up to par! 8)

"Inside the MLA-30 Active Loop Antenna"
https://www.m0lmk.co.uk/2019/09/12/inside-the-mla-30-active-loop-antenna/

1. The MLA's DC injector contains a DC/DC converter :palm: so bypassing it and using a QUIET power source would be a good first step. Also, there's a lot of room for improvement in these injectors, in terms of power filtering, coupling DC to the antenna feedline, and receiver protection.

2. The MLA's loop amp appears to use a differential OpAmp...



...so it's certainly limited in its ability to handle strong signals (2nd and 3rd order intercept) and its noise figure (at least 3dB due to R1 and R2.) What's the Vcc for the OpAmp? Use a higher Vcc if possible.

Noise figure and 2nd order intercept are the two primary factors for your loop amplifier, and low inductance is the primary electrical factor for your loop element (also because you don't want any resonances in your loop element.) SM5BSZ has done a lot of research into low noise amplifiers for the HF range, so consult his website for ideas in this area.

3. Conducted noise on the feedline (noise from your shack to your antenna, conducted by the coax braid) is often a nasty problem, so adding two feedline chokes, one just outside your shack and one just below your antenna, is usually a good thing. I made mine with twelve jumbo 31-mix beads, arranged in a square with three on a side, sealed in a plastic utility box, with connectors on opposite sides. My loops are roof-mounted because ground level noise around here is much worse. If your antenna and coax run is ground level, it could be more effective to use metal enclosures grounded to their own rods (and then you can add lightning protection as well.)  Probably a buried coax run would be good too.
« Last Edit: December 12, 2020, 11:23:55 pm by Co6aka »
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Offline A.Z.

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Re: MLA-30 active loop antenna
« Reply #22 on: December 13, 2020, 10:49:22 am »
co6aka; I think I covered all your points in my initial post, the bias tee unit needs to be either modified to exclude the noisy voltage converter or totally replaced, as for the preamp, the lowpass filter at its input has a very bad input/output impedance mismatch, and as for IMD, it isn't a big issue as long as you adjust the gain trimmer for minimal gain (just the bit needed to compensate losses), then replacing the coax, adding a good choke and replacing the thin loop with a better one should further improve that cheap antenna performances, no miracles, by the way

[edit]

forgot, while "maxwell" type chokes are ok, they require quite a lot of ferrites, I prefer "guanella" chokes wound in toroids using coax, those in my experience offer better results


« Last Edit: December 13, 2020, 11:10:14 am by A.Z. »
 

Offline vinlove

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Re: MLA-30 active loop antenna
« Reply #23 on: December 13, 2020, 12:07:17 pm »
coupling DC to the antenna feedline, and receiver protection.



Two questions.

1. Receiver protection from what? Do you mean possible damage from over voltage or current to the antenna input from MLA30? Or would it be protection from thunder and storm damage? Or  Harmful RFI damage possibilties?

2. Can Wellbrooks hear weak DX signals, MLA30 cannot hear? Any examples, if yes?

 

Offline A.Z.

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Re: MLA-30 active loop antenna
« Reply #24 on: December 13, 2020, 12:45:20 pm »
1. static and nearby tx

2. yes

it's a matter of horses for courses
 


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