MTM Scientific AM Loop Antenna
MTM Scientific AM Loop Antenna
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If you’ve read any of my other antenna reviews you may have surmised by now that I am somewhat of an antenna fanatic. In addition to the large Wellbrook ALA330S which is a reference antenna for me I also own most of the small passive AM loops available (Select-A-Tenna, Terk, Degen, Tecsun) as well as a few C. Crane Twin Coil Ferrite antennas and a handful of other models from Sony, Degen and the like, and I have tried many more. I have experimented with various random and dipole wire antennas all my life as well. For me the excitement of trying out a new antenna is right up there with the acquisition of a new radio and I’m a firm believer that a good antenna is just as important, and sometimes more important than a good radio if you want the best possible reception. Indeed a passive loop antenna can sometimes transform a mediocre AM portable radio into a DXing machine and many people are astounded the first time they see a loop in action…they can frequently lift a feeble, barely audible trace of a signal right up to a fully listenable level, and the fact that they do it with no power and no direct connection to the radio is also impressive. I won’t go into antenna theory but in essence a passive loop antenna captures much more signal than the radio itself can with its tiny built in ferrite rod antenna, then radiates that signal into the radio’s internal antenna via induction. The only requirement is that your radio has a ferrite rod antenna as most do. If not you can connect these loops via a direct connection and I will discuss that below, but generally passive loops are best when used inductively.
The MTM Scientific AM Loop Antenna is quite a different animal from all of those however for a few reasons. First it is a large square wood frame loop measuring slightly over 17″ per side which makes it at least twice the size of the other passive AM loops I mentioned. This seemed like an exciting prospect as it seemed likely it would be able to capture more signal than the smaller loops.
Secondly the MTM Scientific will never convince anyone that it is a manufactured product…it looks decidedly home brew with its wooden frame, visible windings and exposed capacitor and connections, along with its traditional nut and bolt assembly method. On the other hand antenna lovers might like its appearance…you could easily claim you designed and built it yourself and it would be very believable.
Third and most intriguing, it is available as a kit. I have been building kits all my life and this looked like an easy and fun kit to build, and the cost savings is substantial. (The assembled antenna is $137.50 when available, the kit a cool $88.50. Additionally you can buy the plans only along with the hard to find variable capacitor…you supply your own wood, wire and hardware. That version costs only $24.75, although it is not always available).
My kit arrived very quickly and soon I had the parts, templates and instructions spread out on my workbench. It was then I realized that I didn’t possess some of the common woodworking tools needed to assemble the kit so a quick trip to the tool store was needed to buy a wood file, some drill bits and a countersink drill bit. I naively had expected that the wood parts would already have been cut to fit but instead there were a few pieces of wood stock and paper templates to use for cutting them out, drilling and filing to the proper dimensions. Don’t get me wrong…there is nothing difficult about this. It’s just that I am not a woodworker at all and although I have enough basic tools to tighten a loose doorknob and such I have never used a miter box before (luckily one was available to me although it not essential). Cutting the notches to mount the two arms perpendicular to each other was probably the most challenging aspect of it for me. I cut them a bit small then filed them to fit snugly together…not the best way to do it but finally they slid together and were at right angles to one another. Whew! You also have to cut small notches in the ends of the wooden cross arms to hold the windings in place with a hacksaw…that was easy. One step that could have been explained more clearly was the beveling required on the two support feet to allow room for the wires to pass…considerably more wood had to be removed than I realized and I ended going back to this step repeatedly before I could get them to fit properly. You will also be sanding and finishing the wood…again easy enough but it does take some time to do a nice job on it.
I hate to pick nits but there were a few other steps that could have been clearer in the assembly instructions, notably exactly how many turns of wire are supposed to be done and exactly where you stop. Following the instructions explicitly my wire did not “end up” where it was supposed to. In other words, if you complete exactly the specified number of turns of wire the loose end will be on the opposite side of the antenna from where it needs to be to attach to the variable capacitor. You either have to undo a partial turn or add a partial turn, which is what I did. I also performed a quick check first to be sure the antenna would still resonate and tune from 520- 1710 (the limits of my “test” radio) and it did so I was not concerned about it. When building one I recommend you do this quick check for band coverage before finalizing your assembly. I also glued my hardware in place to help keep it from loosening as the wood seemed soft enough that I couldn’t snug them down as much as I might have otherwise. Finally, it is not clearly stated that it is necessary to sand or scrape insulation off of the ends of the magnet wire in order to achieve a connection…several readers have emailed me that their loops didn’t work and this was always the cause. But this is all part of this homespun feel of this unusual kit. The reward is in the performance of the MTM Scientific Loop which in a word is HOT!!!
MTM’s website also gives instructions for making a simple pickup coil which allows a direct connection to a radio requiring one, generally due to the lack of a built in ferrite rod antenna. It is simple enough…a single turn of wire placed on the wood frame and connected via a cable to the radio. I have not yet tested this setup – MTM does provide quite a bit of additional magnet wire which can be used for this. With my other passive loops I have generally found a much lower signal level using a direct connection than induction, perhaps due to the fact that some world band radios have attenuation at AM frequencies through their antenna input jacks to prevent AM signals from spilling over into the SW bands. Other radios have additional gain for their internal antennas compared with those coming through the antenna input jack – also to prevent overload. Then there is the question of impedance matching…radios can have high, low or both impedance antenna inputs and passive loop antenna manufacturers don’t seem to ever quote the ideal impedance for their loops. Generally I prefer to use my passive loops via induction and for radios requiring a direct connection I stick to amplified designs. I do need to test the MTM with both high and low impedance inputs however and for times when a direct connection is needed I thank MTM for making it possible.
BUT THE PROOF IS IN THE PUDDING: HOW WELL DOES IT WORK?
In a word – Superbly! I tested the MTM Loop’s performance by comparing it against my other well-known loops on several radios which had analog signal strength meters to make relative comparisons easiest. I used a Grundig Satellit 800, A Sony 6800, a Panasonic RF-2200 and the just-released Grundig Satellit 750. I confirmed my results on several other radios ranging from the Eton E5 to a Sony 2010. On all of these radios the MTM Scientific Loop performed amazingly well. Some signals were only marginally stronger than the smaller loops but some were much more potent with meter readings several db higher than the smaller loops. Listening quality was also sometimes improved more than signal strength comparisons alone would indicate. There were some tough, noisy signals with which the MTM was able to extract much cleaner, clearer reception than the other loops could. Not all signals…not even most of them…but several. If there was a difference it was always clearly in favor of the MTM. Because of this it was also able to help some very sensitive radios such as the Panasonic RF-2200 and GE SRII more than the smaller loops could. Conventional wisdom is that the more sensitive the radio is, the less it is helped by a passive loop and I have found this to be true. But the MTM so improves reception on some signals that even with my best AM portables it often transformed weak, barely audible signals to ones I could actually listen to and enjoy.
One of our readers has forwarded to me some very comprehensive test results he achieved pairing his MTM Loop with his C.Crane Twin Coil Ferrite Antenna (itself an excellent AM antenna and one of my main stays). He describes (and his audio files seem to prove) that he has achieved not only increased gain but some incredible “steering” accomplishments, with the ability to peak one signal and null another with extreme precision. He uses the MTM to capture the signal. Radiates it into the Twin Coil’s Ferrite head end, then connects the Twin Coil to his radio. This may not be easy for the average listener to achieve but it is a testament to what can be done with a little ingenuity and inventiveness of thought, and it is precisely that kind of person who is likely to most appreciate the MTM Scientific Loop.
AM RADIO LOOP ANTENNA(Catalog #AMDX1000)…$137.50 (Assembled version listed as “Sold Out” as of this writing – contact them for details)
LOOP ANTENNA STARTER KIT (Catalog # LPKIT)…$24.75 Includes variable capacitor
DELUXE LOOP ANTENNA KIT (Catalog # DXKIT)…$58.75
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