Electronics > RF, Microwave, Ham Radio

MLA-30 active loop antenna

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A.Z.:
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

Co6aka:
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.)

A.Z.:
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

vinlove:
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.

A.Z.:

--- Quote from: vinlove 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.

--- End quote ---

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.

--- End quote ---

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

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