|“GMDSS” LW / MW / SW
Magnetic Vertical Antenna
"MLB - MK2" Magnetic LongWire Balun Antenna ( 55 Foot Length)
(Seperate "Water Damaged" MLB information at bottom)
foot dutch made "RF Systems" model GMDSS Passive
Vertical Antenna (without the optional AK-1 Mounting Bracket)
We tested it along side a "RF Systems" MLB - MK2 (55 foot long wire balun antenna, see insert and chart below)
(Please Note : Discontinued product , posted here for archive information only.)
COUNTRY OF MANUFACTURE : Holland (the Netherlands)
Passive and Almost Elusive,
Two “6.5 foot “ Versions
When one first has a look at this antenna, you would think at this length it would have to have some kind of an electronic preamplifier to have any chance at all for even fair operation?
Well it’s does not and is completely passive in nature. If that was not enough the tested "Magnetic Transfer Antenna" (GMDSS version) is supposed to operate down to 100 kHz and up to 25 MHz.
It’s close cousin, the MT version, (not tested) starts at 500 kHz and coverage up to 30 MHz.
Of course using no electronics, which can add a cluster of noises all by itself, inter-modulation products are totally out of the question with this antenna.
This antenna is touted to feature a totally grounded element and a “Magnetic Transfer Balun” that is supposed to provide a lower background noise level. It’s counts on the receiver sensitivity to make up for the short element used. Performance may be more variable than usual with the receiver it's connected to ? We used the very sensitive WJ-8711A in our tests here. Any add on preamps were not used.
But they do tell you in no few words that the signal provided will be less than with full sized antenna’s. This warning should be heeded with most importance, as we will cover in this report.
First Looks Out Of the Mailing Tube
It stands at only 6.5 feet tall. This measurement was checked in testing, so it does not quite make the 7-foot specifications as from the manufacture. For the SWBC parts of the spectrum, either model specification is marked with the same performance and size.
It arrives in a very thin cardboard tube with plastic caps at each end. This will not cut it for shipping from the dealer to the owner by itself. The test sample came from the US dealer with provided proper support packaging around it including a piece of wood that ran the entire length of the tube. Otherwise it would have received damage (if not destroyed) for sure. The box the tube was in was received in very ruff condition and was only shipped 3 states away.
Antenna's outer shell is made with a heavy UV-resistant plastic (not
fiberglass) and an very heavy stainless steel bottom where the
mounting bracket attaches.
Internal helical "foil" element - fiberglass tube is filled with yellow foam to make the antenna waterproof even in a tropical climate.
RF Systems MLB - MK II Magnetic Long Wire Balun Antenna
the GMDSS vertical with this RF Systems "MLB - MK2"
Magnetic Long Wire Balun Antenna (at a slightly shorter 52 foot
length to fit my area). At one time they came packed in a
clear plastic tube (as shown in the picture). Later
production arrived in a larger plastic bag that allows
for the wire not to be so tightly coiled up. In any event
this antenna is also on the very pricey side. The MK1
version includes 41 feet of wire, where the MK2 is 66
feet for improved coverage of lower frequencies.
AK-1 Mounting Bracket,
What’s that Red Stuff For ?
The proper mounting location is outdoors and must be properly attached to something as high as one can get it. But as you pull the GMDSS out of the box there is no mounting bracket which makes you think someone forgot to include one at the factory?
It turns out that the mounting bracket model “AK-1” was an extra cost item. At a pretty steep price for a pressed piece of steel and 4 U-bolts seems a bit stiff ? However this is a heavy bracket, way above average. The bracket also allows for either a vertical or horizontal mounting surface to be used (say a balcony railing) which is a huge plus.
A small bag of red grease was found in the AK-1’s box. This is for placing on the threads and nuts of the u-bolts after installation to help disassembly down the road. However this makes for an awful mess too. I passed on this grease myself.
High as You Can Safely Go, Can Be Camouflaged
The real idea of the GMDSS antenna (or any outdoor antenna for that matter) is to get the receiving element as far as one can get away from household noises to create a improved signal to noise ratio. But at the same time it’s still somewhat stealthy in nature to keep sensitive nabors from raising the roof to any ugly antennas around. Matter of fact the specification sheet claims one can paint this antenna (using non-metallic paint) to hide it even further.
No Coax Included, No BNC Connectors Used
After the bracket is obtained and mounted the owner can then connect the coax from the antenna to the receiver. This is also a user provided part of the antenna, as none is included. It accepts the more standard PL-259 plug (well standard for HF use anyway). So unlike the Wellbrook antennas there are no BNC connectors to be found. This will make life easier for those who care not to assemble the more difficult to work with BNC connectors on a cable or have a hunt for a pre-assembled cable.
Built Like A Tank
Properly mounted, this antenna should take just about anything mother nature can dish out (well almost as we cover later). There are no ground radials to break off either (either by trees, wind , birds or other varmints). This is totally opposite of the Wellbrook loop antenna’s that appear to be much less forgiving with extreme weather conditions.
Performance with Such a Short “Passive” Antenna
Here is where the men and the boys part ways. Signals are indeed at a lower level when compared with a long wire or a tuned dipole. Very weak signals are swept into nothing, where the long wire still produces a very usable signal (provided no local noises are present).
OFF AIR SIGNALS
Receiver : WJ-8711A
|RF Systems GMDSS
(30 foot height, metal mast)
|RF Systems MLB
(25 foot height, 52 foot length)
|Frequency in Khz||In dbm (approx. s-units)||In dbm (approx. s-units)|
|350 (local beacon)||-95 (5)||-90 (5.5)|
|540 (equal)||-95 (5)||-95 (5)|
|1000||-80 (7.5)||-70 (9)|
|1230||-40 (+30)||-30 (+40)|
|3210||-70 (9)||-55 (+15)|
|4915||-95 (5.5)||-85 (6.5)|
|5085||-85 (6.5)||-80 (7.5)|
|5960||-80 (7.5)||-70 (9)|
|6190||-75 (8.5)||-70 (9)|
|7225||-100 (4.5)||-80 (7.5)|
|7415||-85 (6.5)||-70 (9)|
|9475||-80 (7.5)||-75 (8)|
|9990 (equal)||-70 (9)||-70 (9)|
|11565||-75 (8.5)||-65 (9.5)|
|12095||-100 (4.5)||-85 (6.5)|
|13610||-100 (4.5)||-80 (7.5)|
|13830||-110 (2.5)||-95 (5)|
|15110||-65 (9.5)||-50 (+20)|
|15760||-95 (5)||-80 (7.5)|
|17680||-100 (4.5)||-85 (6.5)|
|17860||-85 (6.5)||-70 (9)|
|21520||-100 (4.5)||-80 (7.5)|
|21670||-100 (4.5)||-85 (6.5)|
31 Meter Band Sometimes
As the chart shows above (antenna's mounted outdoors of course), sometimes (more so at the top end of the band near 10 MHz) in the 31-meter band signals were almost equal between the MLB long wire and the GMDSS. This varies a bit depending with the station. Other than the 31-MB, LW and lower end of the MW band, it was always a weaker showing.
Cut off Starting Around 17 MHz
Specifications on the GMDSS version (and the graph with the included paperwork) indicate that that coverage should be at least OK to near 25 MHz?
Well in the testing period it did hold up through 19 meters quite well. But starting from about the 16-meter band, the performance starts to decrease a bit more than elsewhere. The antenna had a hard time reaching the 25 MHz specification.
Additionally signals received with the GMDSS in the 13-meter band appeared with more background noise where the MLB long wire was quieter.
On the plus side long wave and mid-lower end of the medium wave band were a real treat with this GMDSS version. Almost equal signal strength to the long wire again.
The standard MT version (not tested) is supposed to reach a bit higher with frequency coverage (30 MHz). But of course the lower frequencies should suffer with this model.
Did it help With the Signal to Noise Ratio?
I very much liked the “no noise” factor being this is a passive antenna.
For most of the undesired interference at the test location that was being picked up by the lower mounted (MLB) long wire antenna (TV set buzz, light dimmers etc), the answer is yes with help taming the local noise. The amount of improvement varied with the frequency, and many times it is only a modest decrease or none at all in some cases.
When a buzz is irking from a nearby TV set can be reduced, while allowing you to still catch a signal is a huge treat for the ears and ones patience.
But as indicated above, very weak signals on the long wire will be a no go with the GMDSS. On the 31-meter band (9 MHz) you might have a better chance as for some reason at the test location it worked almost equal with a good number of signals (but not all).
Works For the Right People, MUST Be Outside and High to Be Worth It
This antenna was a huge surprise, providing very useful signals with a decrease in noise picked up by local sources. But that’s only if the owner can mount this antenna a distance away from the house or noise sources. Otherwise it makes little sense.
Testing was done closer to the structure and house buzzes and did NOT provide any benefit for noise reduction. One can get the same or better performance say using just a length of wire across the room, with the household noise just as bad of course.
This is not a DX antenna, so for any real weak signal work this is not a good choice.
A prospective owner should choose the version with care. If you need the higher or complete SWBC coverage, it’s best to go with the MT version. If MW and the middle and lower parts of the SW spectrum (19 meters and below) are the main bands of choice, then the tested GMDSS version is the one to go with.
“Expensive” is the word here. The antenna with the AK-1 bracket (which is a required accessory) makes the already high price into the gray unreasonable area. This is a very pricey “stick” for the performance given.
A third shorter version was available
If this antenna is not short enough for you, RF Systems did offer a 4-foot model called the GMDSS-2 (not tested). Coverage is more limited from 500 kHz to 18 MHz. Cannot say here if the performance would be even less? But cutting off almost 3 feet is certainly not going to help the cause. Uses the same AK-1 mounting bracket.
For a passive design, the pricey 6.5 foot RF Systems GMDSS was almost the smallest “pole” antenna around and is very useful for noisy listening posts that also have covenant restrictions. If you have room and location for a good outdoor passive long wire or dipole, this is NOT the antenna for you. But for what it is, it can be a very useful alternative in many limited situations provided your bank account will not hiccup AND you are able to mount it up in the air at some height outdoors.
Final Word : Mother Nature and the GMDSS
In June of 2008 we had a very nasty thunderstorm with excessive lightning. The GMDSS took a semi-direct hit that blew it to pieces, in fact no part of it was left in the AK-1 mount. Pieces of the antenna were found as far as 4 backyards away. KA-BOOM was right. As you can see in the picture below of some of the remains below, the antenna uses a coil made out of FOIL wrapped on a fiberglass form (it's not a wire coil as you might think). A very small balun was found at the base at the coax connector. The foam they inject in the tube to help keep water out, was fried so badly to think it was never included in construction (all was all chared black). The foil antenna element was just about entirely vaporized. This should tell you to ground metal antenna mounting masts with a proper thick ground wire and one or two 8 foot ground rods into the earth , and even then it may not matter with direct or semi-direct lightning hit ? Note: I did not replace the antenna due to the high cost involved. Thank goodness the lightning did not touch the horizontal MLB long wire antenna at all.
(Please Note : Discontinued product, posted here for archive information only.)
2008 a NASTY lightning strike turned this expensive GMDSS antenna
into a pile of garbage.
(Not all of it is shown here, but was a good part of what was left. Never recovered all of the pieces from neighbors.)
RF Systems MLB (Magnetic Long Wire Balun) - "Water Logged"
As covered in the
above review we used a "RF Systems" MLB (Magnetic Long
Wire Balun) antenna for comparsion and still in use here at HQ.
20 years with no problems even if the outer plastic looks a bit
ruff. However another owner has not had quite the same luck with
his 17 year old weather beaten RF Systems MLB. It pretty much
died and the reason was water got in it and ruined the device.
NOTE : Later production featured a tiny drain hole on the bottom. Our 20 year sample did not have this drain hole and we never had any water issues.
Check out the pictures below for the details. Our thanks to Joel N9SHJ for the information and pictures. But not too bad for 17 years.
Tip : Perhaps not a bad idea to spray the entire balun with clear lacquer / clear enamel (even 2 or 3 coats) once installed and connector taped . Be sure and check the main stud tightness with a pair of real pliers (nice and tight) before you do this as well .
RF Systems MLB (Magnetic Long Wire Balun) in the elements for 17
(Photo : Joel T.)
Time to see
why this Balun died ?? Using a "Dremel Tool" to cut it
(Photo : Joel T.)
cut open (in a vise) a nice puddle of water poured out.
(Photo : Joel T.)
As you can
see it's a corroded disaster inside. No wonder it stopped working.
Water had been in this one for some time
(Photo : Joel T.)
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