Recently, there has been a lot of buzz around the MLA-30 magnetic loop antenna from China because it is being marketed as an active (preamplified) 100 kHz – 30 MHz broadband antenna and only costs about $43 from various sources on the internet.

MLA-30 and Wellbrook ALA1530LF comparison

That is a fraction of the cost of receiving loop antennas from UK’s Wellbrook and W6LVP that can go for as much as $350.

Wow! $43 is really inexpensive and having read some good qualitative reports on the antenna, I decided to get one and see how it would stack up against my Wellbrook ALA1530LF loop. So, off an order went on eBay. While waiting for the antenna to arrive, I started thinking about how I would design the comparison experiment.

MLA-30 and Wellbrook ALA1530LF comparison

To perform the comparison between the antennas, I oriented both the MLA-30 and the Wellbrook to have maximum gain in the East/West direction (same as the ground loop). Then starting at 720 kHz, I worked my way up the RF spectrum to 15.730 MHz.   When I found a station,

I would listen to it using all three antennas and record the noise floor, peak signal, and difference between these two measures in dBm from the spectrum analyzer on the SDR Uno software.  For each antenna, the RF gain was set at the point just below ADC overload.

In the test, one interesting finding was that the noise floor of the Wellbrook remained relatively constant throughout the entire RF spectrum studied. The noise floor of that antenna remained between -115 to -127 throughout the entire RF spectrum.  I did not expect that to occur, but the people at Wellbrook have had a lot of experience in mitigating noise.

The story was a bit different with the MLA-30 and the ground loop. The MLA-30 had a noise floor that was about the same as the Wellbrook loop at -115 to – 125 dBm only the AM broadcast range.

Once above the AM broadcast frequencies, the noise floor for the MLA-30 went up to between 100-110 dBm.  As for the ground loop, it stayed between 100-115 dBm throughout the entire RF spectrum studied.

SO, I think it is safe to say that the MLA-30 preamplifier has been optimized with the AM broadcast listener in mind. Once out of the AM broadcast band, as far as noise floor is concerned, the Wellbrook was clearly superior to the other two antennas tested. This does not mean that the MLA-30 was not useful above 1.7 MHz.

It just means that the listening experience was a bit noisier than the Wellbrook loop. Additionally, the following image shows the spectral difference in signals received in the AM band from the three antennas studied.

Because my interest is in frequencies below the AM broadcast band, I decided to drop down to a local airport beacon at 379 kHz and see how the antennas compared. At that frequency the noise floor for the MLA-30 was a bit better than the Wellbrook (-132 vs 127), but that was because the MLA-30 was simply not hearing as well at this lower frequency.

The Wellbrook produced a signal peak at -55 dBm while the MLA-30 was at -80 dBm and the ground loop at -60 dBm (but quite noisy at a noise floor of -112 dBm). Since the beacon at 379 kHz is quite loud, I decided to try and copy some amateur digital stations at 476 kHz (630M) with all three antennas. As can be seen from the picture of the WSJT-X waterfall below, the MLA-30 was nearly deaf on the weak signals produced by these stations.

As my experimental comparison of the three antennas continued from the  AM broadcast frequencies up, I decided to record the strength of each signal heard above the noise floor. As can bee seen in the chart below, the Wellbrook antenna consistently outperformed both the MLA-30 and the ground loop antenna with only a couple of exceptions in the area of 3.5 MHz.

Given that my Wellbrook antenna has been optimized for use on frequencies below the AM broadcast band, and documentation at Wellbrook that alerts the user of the low frequency antenna that signals will be a bit attenuated in the higher frequencies, these findings were a bit surprising.

Again, this suggests that even though the MLA-30 can hear signals up to 30 MHz, the preamplifier in this antenna appears to have been optimized for the AM broadcast listener.

 

Difference between Peak Signal and Noise Floor (dBm)

* = best signal to noise

Frequency (kHz)          Wellbrook          MLA-30          Ground Loop (preamp off)

660                                  45                       38                    50*

720                                  57*                     51                    50

1110                                37*                     25                    30

1450                                64*                     45                    56

1600                                73*                     49                    60

3220                                15                      32*                   10

3358                                18                      33*                     0

4005                                33*                     20                    15

4985                                45                      39                    48*

5330                                30*                     25                    25

7210                                43*                     36                    34

7310                                38*                     32                    31

7780                                53*                     51                    43

9395                                64*                     50                    63

9565                                47*                     33                    41

11775                              54                      53                    57*

12030                              43*                     39                    36

15550                              40*                     21                    38

15730                              30                      19                    38*

 

Best Signal to Noise

Wellbrook ALA1530-LF        13/19    (68%)

MLA-30                                     2/19    (11%)

Ground Loop                            4/19    (21%)

As a final experiment, I decided to qualitatively assess each antenna for its ability to null out a noise source.  I used several broadcast stations in the AM band and the 40M band for this test. Here I found something very interesting.

Qualitatively, it appears that the MLA-30 has a much narrower and deeper null than the Wellbrook which I assume to be caused by the difference in diameters of the tested antennas (Wellbrook = thick aluminum tube, MLA-30 = thin wire). While it was usually impossible to completely null out  transmissions with the  Wellbrook – particularly in the AM band, the MLA-30 could be rotated to completely silence all but the strongest local broadcasting stations.

I suspect that occurs simply because it is harder for the small wire diameter to see a signal than a larger diameter loop, thus making it easier to completely null a noise source with the MLA-30.

Another observation I made between the Wellbrook and MLA-30 antennas was that the Werllbrook had nearly zero perceptible intermodular distortion.  This is something that Wellbrook has spent a lot of time and effort in the design of their proprietary preamplifier.

However, the MLA-30 had a very serious issue with IMD and in some cases, stations that were clearly heard with both the Wellbrook and the ground loop were totally obliterated by IMD when switching to the MLA-30. Those stations were excluded from the above table.

Across the RF spectrum studied, I found about a dozen stations that could not be heard by the MLA-30 due to IMD and all of these stations were easily heard by the Wellbrook and ground loops.

 

So, what is the verdict on the MLA-30?

Even though the claim is that the antenna is broadband, I believe this antenna has been response optimized for the casual AM broadcast listener.  That is not to say it does not work outside of the AM band.

It will certainly work for receiving signals up to 15 MHz (the point where I stopped the study), but the signals will not be as easy to hear because of the high noise floor and sometimes signals will not be at all intelligible due to intermodular distortion.  Below the AM band, the MLA-30 simply does not work unless the signal is from a very strong station as was the case with my airport beacon test.

If you are just looking for an antenna to listen to local AM broadcasts, at $43, the MLA-30 is hard to beat from the perspective of price. The antenna’s ability to null out local noise sources is also a positive for those interested in AM listening.

However, if pulling weak signals out of a noisy RF band is what you are looking for, the MLA-30 falls a bit short of the mark.  In that case, the more expensive antennas like the Wellbrook and  are the way to go.

Original by Thoughts, observations, experiments, and ramblings  in Amateur Radio by N1DAY and KC4SIT.

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