Gives marvalous reception in a noisy environment.
An expensive solution. (Ian Smith)
Mixed results. (Andrew Clegg)
Just a con? (Dave Kenny)
No, it's not a con. (Brian Underdown)
Works great on lower frequencies.(Wian Sianstian)
Misleading ads - and a very expensive solution. (Don Moore)

Here's another view.
A matching transformer (and the MLB) mounted outside on the end of a wire provide, I believe, three features:
1. A ratio dividing the impedance mismatch. (Of course, where there was a match the ratio will make things worse, but the worst case will be reduced.)
2. A DC path to earth which reduces (at least) local static noise.
3. A shielded signal feed through the dwelling's noise field.
Since it's difficult to separate these gains, I can't tell you which gives the biggest benefit. The sum total *is* a distinct reduction in noise and some signal strength gain compared to the same wire, sans matcher/co-ax.
I've used a home-brewed matcher based on the suggestion in the Lowe booklet and the RF Systems MLB. The MLB is just as good as the home made one and it's weather proof. It's also expensive, not a balun and I find it hard to believe that it adjusts it's matching ratio with frequency. I can't say that it doesn't, but let's say that I'm a sceptic.
I don't know what sort of receiver you use, but if you can hear signals/RFI without any antenna you could get a better one. I know boatanchors are a sod to silence because of the ventilation requirements, but modern solid-state receivers can and should be totally silent. Even if your receiver is shielded, the power lead may need attention. If you can't hear signals, but can hear RFI without any antenna, I'd suggest that the receiver is generating it's own RFI.

I have a little experience with the magnetic balun. I bought a magnetic longwire balun (I think from Palomar, I'm not sure) that's designed for coupling a long wire to 50 ohm coax. It claims to cut down on noise pick up. I have a 40 ft wire connected to the balun, and then 50 ft of RG-8 coax (I went first class... see below) leading to my rig (a Yaesu FRG-100). The wire is up about 20 feet from the ground. I also have a wire about 15' off the ground right below (parallel to the first wire), and that one is connected directly to RG-58 coax (no balun), and the shield of the RG-58 is connected to a 4' radio shack ground rod. My observations are:
(1) The antenna with the magnetic balun picks up more noise from my computer than the one without the balun. The computer is just a few feet from my radio, so it's the coax that's picking up the noise. I thought this was exactly the problem the balun is supposed to cure!
(2) On the other hand, the signal strengths received with the balun antenna are substantially higher than the non-balun antenna. E.g., a signal that is 20 dB over S9 on the non-balun antenna is 40 dB or so over S9 with the balun antenna. Is this due to the balun or the fact that the balun antenna is 5 feet higher than the non-balun antenna? I don't know.
(3) I tried grounding the shield of the RG-8 coax to the ground rod, jus like for the non-balun antenna. It did not seem to affect noise pick-up one way or the other.
So, the results are mixed! I'd be interested in hearing anybody else's experiences.
About the coax... RG-8 vs RG-58. I bought the MFJ antenna analyzer (handy thing to have!). I used the analyzer to measure the attenuation of the two types of coax, both 50' long. I found that neither coax had significant attenuation of 30 MHz signals across the entire 50'. Attenuation at lower frequencies will be even less. So, if you need coax in reasonable lengths, may I suggest not wasting your money like I did on RG-8? Another benefit of RG-58 is that it's softer and more flexible. When I hook my RG-8 up to a small antenna matcher unit, I can't get the unit to sit straight because the darn stiff coax likes to torque it into a particular position.

I have come to the conclusion that magnetic long wire baluns may be just a con, at least the one I bought from Barton Communications here in the UK (19.95 pounds as advertised in Shortwave Magazine etc) did not appear to work and I ended up returning it for a refund. I tested it on several diffrerent receivers and aerials, but inserting the balun always resulted in a LOSS of signal of between 6 and 20dB...

One point I always make about RF Systems MLB is that it shows good performance across VLF/LF/SW range for recieving. Prior to using a MLB our antenna system for recievers was a dipole fed into a a.t.u(global at1000). That particular system was poor below 3 MHz; of interest to us was the ability to recieve 132.5 kHz fax station in Europe, and very rarely was it able to recieve reliably.
Some may say, why not use a wire design for the vlf/lf? No point when this product produces good results without the need of extra add ons. Using the MLB, this proved to be more effective and still able to produce acceptable performance across range up to 30 MHz. It's not a con, otherwise I would have something like a 100 returned used MLB's if this was the case.

Dave Kenny wrote: I have come to the conclusion that magnetic long wire baluns may be just a con.
Hmm, that surprises me. I have been using the (original) RF-Systems MLB for quite a while now, and I am quite pleased with it. The coaxial inlead does help to conquer a lot of the locally generated computer hash.
More important, however, is that it shows a consistent improvement in reception on the lower frequencies, when compared to connecting the same aerial to the high impedance antenna connector on my Lowe HF-225.
On frequencies above 6 MHz or so, the improvement is not so obvious, but the advantage of having a coaxial inlead remains there.
BTW, I should point out again that I use the original RF-Systems MLB (a Dutch product :), and that I don't trust the other MLB's. I just don't think they are able to make goods of the same quality RF-Systems' products have, also because the expertise RF-Systems has built up over the years.

Following is a direct quote from the article in the 1994-95 edition of Fine Tuning's PROCEEDINGS. The article is "Impedance Matching Devices for Simple Wire Antennas" and it is by John Bryant and Bill Bowers. I do not have the technical background to explain this anymore myself, however the authors are well-respected for their knowledge of the technical aspects of the hobby, and I certainly have no reason to doubt their conclusions. The CAPS are those of the authors, not me. I have sustituted *asteriks* in place of their underlining.
"...the manufacturer makes the claim (in several places) that the 'transformation ratio changes with frequency.' At the end of the tests, the authors cut the Magnetic Balun apart. The housing is rather cleverly asssembled from a short section of plastic plumbing pipe and two end caps. Inside the case we found what appears to be a normal ferrite-based transformer further encapsulated in plastic 'potting' material. Rather than cutting further, Bill put the device on his sophisticated test bench. Sure enough, DX Systems' claim that the 'transformation ratios change with frequency' is simply NOT TRUE and is, at best, very misleading advertising and a clear misrepresentation. The manufacturer also states that this 'balun' provides 'static protection'. This is also not true in the accepted technical sense of the term since there is no external grounding stud on the unit. The only path for static charges to take to ground is down the coaxial shield to the receiver chasis. You should also note that the parts and material costs of this device, if purchased RETAIL on the open market, cannot exceed $6.00, less than 1/10th of the purchase price. Finally this devise is NOT a balun. The normally accepted definition of 'balun' is a transformer which converts from *bal*anced to *un*balanced configuration. This unit is an impedance matching device designed to attach an *un*balanced feed line to an *un*balanced antenna. Although the foregoing description is harsh, the 'Magnetic Longwire Balun' is misrepresented by the manufacturer and is VERY unfairly priced."
PROCEEDINGS is an excellant read if you want to get into some really intelligent and deep material for serious radio hobbyists. This is absolutely NOT a beginners' book, however! The 1994-95 edition is $20.50 plus $4.00 shipping to North American addresses from
Fine Tuning Special Publications
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USA
Overseas prices are available upon request.