Let's mod the MLA-30 loop.
The MLA-30 single turn loop right out of the box isn't a bad antenna. You could do worse buying unknown loops off eBay or from a foreign supplier. I have the DmgicPro branded MLA-30+ bought right off of the Amazon site. It was reasonably priced at about $45. You might find them cheaper on sale. Some advertised as plus versions are responsive down to about 100 kHz, but not all. Be sure before you buy if you need longwave coverage.
As usual, click any image for the bigger picture.
 |
| The MLA-30+ with cover removed. |
You can spend literally hundreds of dollars on sophisticated, ultra low noise, premium loops. In most cases they will perform wonders. Another step up would be to buy (or build) a premium preamplifier module and roll your own. These modules can be had for about $150 and up.
If money is tight, or maybe you're a beginning experimenter, we go with what we can afford.
The MLA-30 comes with a nice preamp mounted in a sturdy, water resistant ABS plastic box and 30 ft. of coax feedline. A single turn loop, 26 inches in diameter, made of stiff spring steel piano-wire comes with it as well. Rounding off our kit is a bias-T and a short coax jumper feedline for our receiver.
I'll use the MLA-30 brand name from here on to generically include the MLA-30+ as well.
Some Defects for Sure
The stock MLA-30 design falls down in two areas.
One, it comes with 30 feet of some sort of cheap, skinny coax, similar to but nothing like the quality of RG-402, RG-316, or RG-174. It is hardwired to the pre-amp output right through a hole in bottom of the pre-amp box. I've skinned this coax and it has only a bare amount of thin spiral-wrapped braided wire trying to be a shield. This has to go.
Second, the bias-T box is overly complicated and uses a buck converter method to convert 5 volts up to 12 volts to power the pre-amp. In and of itself it's a fairly clean converter. But to make matters worse, they provide a micro-USB connector to connect to a 5 volt USB source, itself likely to harbor all kinds of incoming trash because the average consumer will probably plug this into a chopper charger module. This has to go too.
 |
| The preamp box with modified SMA connector. |
People complain about the MLA-30's signal amplifier because it's a VHF TL592B chip. I find that hardly an item to complain about. The noise figure of the pre-amp has been measured by others at about 12 dB - not the best, but not the worst either. At shortwave frequencies, compare that to the published noise figure of these SDR receivers: the SDRPlay RSP1B & RSPdx (~15-20 dB) and the low noise Airspy HF+ Discovery (~7 dB). The MLA-30's pre-amp also seems to show a sharp peak at about 7 MHz, so be aware of that. It does not show linear gain across the shortwave bands, 3-30 MHz, varying from about 10 to 30 dB.
All that said, I don't believe the MLA-30 to be a bad loop, and certainly at $40 to $50 it will get the job done in many cases and you won't be out $200 to $500 dollars for a commercial masterpiece, either. Here at RADIO-TIMETRAVELLER we are all about practicality, so we will go with the MLA-30 and see what we can do with it.
But a Bonus Too
There is a bonus found here, as well. The MLA-30 has a potentiometer gain control on its pre-amp board. This is wonderful news because at its approximately 30 dB gain (full out) the MLA-30 can be a bit over amplified in certain situations. Once we get rid of the 30 ft. of coax and change out the bias-T and supply some well filtered DC, we will have a nice single turn amplified loop for MW and SW.
Let's Start
First we'll get rid of that coax and install a connector at the bottom of the preamp box. Consider a panel mount SMA female connector or a panel mount BNC connector, both inexpensively available via Amazon. Open up the pre-amp box by unscrewing the four cover screws. Remove the bead of silicon that holds the coax stationary, coming out of the bottom hole. Slip the coax up through the hole a little farther, then cut it off, leaving about 6 inches of length. Leave enough so that you can strip the end of the coax.
SMA panel mount connectors:
https://www.amazon.com/dp/B078H4F8R6
Next we will drill our new hole, enlarging the existing hole. On mine I've chosen to use the panel mount SMA female connector. The panel mount BNC connector will need a bigger hole. Choosing the properly sized drill bit, drill a newly-sized hole using the existing hole the coax came out of. Be very careful here, as when the drill bit finally punches through the case it will grab and want to pull the drill through suddenly which will destroy the circuit board if it comes into contact with it. Hold on to that drill like your life depends on it.
Mounting the Connector
Mount the SMA or BNC connector in the new hole. Be sure to tighten the nut securely.
Warm up your soldering iron. Strip the coax end back about an inch or a little more, so the center conductor and shield are available, then attach both to the connector and solder. The shield is ground of course.
 |
| Detail of the connector mod. |
Replacing the Bias-T
Now to replace the bias-T. I bought a nice, simple bias-T from Amazon, which works clear down into the longwave band. It gives good results down to 200 kHz or a little lower and has only 0.1 dB attenuation in the MW range. It was only $13.
Bias-T:
https://www.amazon.com/dp/B0BR2WCH5D
 |
| The new bias-T. |
Better Power
Now we need power for the bias-T. In my junk box I had a nice Jameco linear wall adapter, #2328175, which produces 12 volts filtered DC at up to 1500 ma. I connected that to my new bias-T. You must use a linear adapter. A linear one will have some heft to it because it has a big transformer inside. For the total purist, it is possible to power the bias-T with a battery. The MLA-30 runs nicely on a 9 volt battery, and with a little less preamp gain too. The square 9 volt battery will supply about 3 days of continuous service. Used a couple of hours a day, it will likely last two or three weeks.
Jameco 12 Volt DC Linear Wall Adapter
Also possible, and perhaps preferable, is to use a common three terminal voltage regulator module board to provide 9 volts, regulated DC. It works with either 12 volts AC or 12 volts DC input. Run your Jameco linear transformer, above, into it, and connect the 9 volt output to the bias-T connector. Two wires in and two wires out. Simple.
9 volt regulator:
https://www.amazon.com/dp/B0816B34FR
Be sure to pick the LM7809 module (9 volts). The MLA-30 preamp will work very well on 9 volts, and offer slightly less gain as well.
 |
| Jameco power supply and 9 volt regulator. |
Let's Try it Out
You'll need a short length of good quality patch coax to get from the pre-amp box to the bias-T. Then run some RG58 or even RG6 from the bias-T to your receiver. Be sure to recheck the connections at the bias-T before you power everything up! You don't want to fry your receiver by reversing the connections, sending 12 volts into the receiver front end!
6 inch SMA RG-316 coax patch cables:
https://www.amazon.com/dp/B07NCLZWHH
OK, with everything ready, power up the bias-T and your receiver. If you have an SDR, great, you will not only hear, but see the difference immediately.
Adjusting the Warp Drive (apologies to Mr. Scott)
"I'm pushin' 'er as hard as I can, Captain. She can't last much longer!"
Now for the last step, we'll adjust the pre-amp gain control in the pre-amp box. This potentiometer, or "pot", turns oppositely to what you know as a volume control. That is, by rotating the control clockwise you will be decreasing the signal level. Start with the control fully clockwise and we will turn in the opposite direction, counter-clockwise.
On your receiver you're more likely to experience more signal overload on the MW band. I have a big problem here with two close by 5 KW transmitters. Increase the pre-amp level by turning the pot counter-clockwise slowly. Keep increasing gain (counter-clockwise still) until signal overload develops. You may be lucky and encounter none even at full gain. If you do encounter overload, back the pot off in the other direction until the overload stops. Leave the control there and fasten the box's cover back on. You are done with the mods.
Wrap Up
That wasn't so bad. You now have a nicely-modified, workable MLA-30 loop!
