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Old 03-02-2016, 10:32   #1
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extending Nasa Easy Navtex aerial cable

Last season, just before heading from Ireland to the Azores, I fitted a Nasa Easy Navtex, with the aerial on the pushpit of my Bowman ketch, in between the GPS aerial and the Hydrovane. On arrival in Spain I had received only incomplete messages, i.e. nothing useful. I decided that the problem was probably proximity to too many vertical pieces of metal (Mizzen mast, stays and Hydrovane) so I arranged for a Marine Electronics firm in A Coruña to move the aerial to the top of the mizzen mast, which involved extending the cable. I then received one complete message, followed by nothing intelligible for the rest of the trip. On returning to Ireland, I had the aerial and the set checked out, and they are apparently fine, the problem lies with connecting between two different types of coaxial cable. The original cable (3c-2vs 75 ohm) has a very thin multi-strand core and the extension cable run up the mast in Spain (marked Televes T-100 PLUS REF 214105 1513 M004) has a monofilament core about 1mm thick.
I have searched in various electronic parts suppliers, without finding a solution to joining these properly. some expert advice would be greatly appreciated.
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Old 03-02-2016, 17:35   #2
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Re: extending Nasa Easy Navtex aerial cable

First, I'd check with NASA to see what they recommend for extending the antenna. The Televes cable appears at first glance to be a slightly larger diameter cable, but the same 75 ohm impedance. The diameter of the T100 cable is specified as 6.6 mm, so you might try using a PL259 connector on the wire ends with a barrel connector between them. You'll probably need to different cable adapters UG 178/179 adapters to fit the cables. If you can't fit the T-100 cable through the adapter, there should be no problem drilling out the adapter slightly, or if there isn't enough metal left, just remove the jacket before installing the adapter.

This isn't strictly legitimate, but the cables have the same impedance so they can be joined together. The PL259 connectors are notorious for not being constant impedance so whatever slight modification you do is unlikely to have any effect at 518kHz frequencies.


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Old 04-02-2016, 12:02   #3
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Re: extending Nasa Easy Navtex aerial cable

Ketchcalimbo,
In addition to contacting NASA Marine, and getting their input....and/or also figuring out exactly what cable you have (as I'm not clear), in order to obtain the proper connectors / splice....here are a few things to note...


a) For receive only, usually there is NO issue at all in mixing coaxial cables, including using different sizes, impedances, etc....so it is very unlikely that this is an issue at all...


b) Connector issues are very common, especially when splicing cables, adding extra cable, etc....and even more so with older cables, but I see cable/connector, and splice issues a lot...


c) The cables you described seem to be a EU / Spanish version of a TV cable, 75-ohm, similar in size to the US standard RG-59 or RG-6....
(the "3c-2vs" might be a very flexible version of RG-59, with a stranded center conductor??? and the Televes T-100 looks like RG-6 CATV cable?? but I cannot be certain...)

And, if the center conductor of the cables were a solid-wire conductor, I'd highly recommend using weather-proof F-connectors (assuming the cable has a solid-conductor, center conductor....if it is stranded, then you will need to use BNC, UHF, etc. connectors...)


d) Placement of a receive antenna for the MF / 518khz NAVTEX signal is usually not critical, and if the antenna is at least a couple feet away from other long metallic objects parallel to the antenna, then the NAVTEX antenna should be fine...
And, I see no need at all to mount a NAVTEX antenna atop the mizzen mast....although it should work, there is no reason to do this!
(fyi, I use on of my lower shrouds as my HF WeFax and NAVTEX antenna....and it works great!)


BUT...


e) BUT, you've made no mention if you have any RFI issues on-board....do you have any MF RFI / Radio Noise on-board that may be reducing your ability to receive the NAVTEX broadcasts...
Many sailors new to MF/HF "shortwave" radio / SSB radio / NAVTEX / etc. forget that many of the modern consumer electronics devices, smart phones, chartplotters, misc. marine and consumer electronics and their associated chargers, and accessories, etc., all can radiate significant radio noise / Radio Frequency Interference (RFI), that can seriously effect their ability to use the radios!!
So, it is very possible that you have significant on-board RFI that is effecting your reception....please have a look at the "sticky" here at the top of the Marine Electronics page, and follow the links about RFI, etc., for lots of info and advice on clearing up RFI..
Marine SSB Stuff (how-to better use / proeprly-install SSB, & troubleshoot RFI, etc.)





---- Now, here is a tough part....NASA Marine...
In my experience (and what I've read of others experiences as well), NASA Marine doesn't have much of a "quality control" department, and some of their units come off assembly line working fine, and some not!!
{and, even if working as specified, their units tend to be at the low-end of the scale of performance and reliability..}

So, it just might be a combination of cable/connection/splice issues, along with some on-board RFI, and also some "NASA Marine quality" issues....all combining to cause these problems for you...
Since none of us are there, and since I don't have a precise grasp of what the exact cables you have actually are,nor exactly how your NAVTEX cables and antenna are routed and installed, etc., I cannot be sure of any recommendation other than what I've written above...


---- Also, a very important piece of information is:
NAVTEX is only designed to be used/received within approx. 200 - 250nm....(yes, many with Furuno NX-300's or NX-700's, typically get greater ranges....the fact is that NAVTEX is designed as a short-to-medium range system, covering areas up to 200nm - 250nm offshore, and that is it!!!)
So, with a mediocre receiver (NASA Marine) and a compromised antenna system (questionable cable/connectors, etc.), it is possible that the results you achieved are "normal" for that set-up...



I understand that these are not the answers you were hoping for, but I do hope this helps!

Fair winds..

John
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Old 05-02-2016, 04:46   #4
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Re: extending Nasa Easy Navtex aerial cable

Many thanks for your very informative response John. I don't think I should have serious RFI problems, as I have limited amounts of electronics on board and no SSB or HF radio. When the aerial was in its original position on the pushpit, pretty much everything but the Navtex was switched off for long periods, including my Raymarine plotter and VHF radio, but still no signals. The domestic system is 24 Volt, with a step down transformer for the Navtex and the plotter, could the transformer be problematic?
I have been in contact with Nasa, and one of their suggestions is to replace the aerial with a H Vector mushroom antenna, which is sensitive to the magnetic component of the Navtex signal rather than the electrical. This would cost £60, which is not too extravagant. Calimbo's stern is quite narrow, so the pushpit is rather crowded with Dan Buoy, Ladder, Horsehoe Buoy, Hydrovane and GPS aerial, not to mention running back stays and lower shrouds close by. I am contemplating ordering a H Vector aerial with 12m of cable, so that if it does not work on the pushpit, I will have along enough cable to run it from the masthead without joints.
Thanks again,
Jonathan
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Old 05-02-2016, 07:18   #5
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Re: extending Nasa Easy Navtex aerial cable

First off, good that you are in contact with NASA Marine....

Secondly, while you may not have any serious RFI, you should be aware that RFI is around us everyday....and it has nothing to do with SSB radios / HF radios....
It is just that the most frustrating problems RFI causes are with trying to use radios to receive signals (whether they be MF, as NAVTEX......HF, as most long-range marine comms....or even VHF, as most short range marine comms)

Please understand that there are many threads here-abouts, discussing RFI to VHF radios and AIS receivers (and some RFI to MF and HF radios, as well), caused by LED lights (actually caused by the LED light controllers, built-into the light / light fixture)

Further there is RFI caused by refrigeration units, digital panel meters, chargers/inverters,battery monitors, cell phone hot spots, Wi-Fi routers, cell phone chargers, tool battery chargers, automatic bilge pump sensors, electric motors of all types, even small circulating fans, solar charge controller, wind generators, towed-water generators, diesel fuel pumps, gensets, diesel engine tachometers, fresh-water pressure pumps, and even sometimes some other marine electronics, especially autopilots, etc. etc. etc....plus many more items...
And, note that most of these can also produce RFI even if "switched off", but still have some power to them...

You'll note that I made no mention of chartplotters or other radios, as these are typically NOT causes of RFI....but are the things that RFI interferes with!!

On my boat, in addition to the some very minor RFI from my shore-power battery charger....the frig and my new variable-speed fresh water pressure pump, are thankfully all I have to contend with as RFI culprits!



Now having written all of the above, the single most telling point is what you wrote here:
Quote:
Originally Posted by Ketchcalimbo View Post
The domestic system is 24 Volt, with a step down transformer for the Navtex and the plotter, could the transformer be problematic?
"Transformers" work on AC not DC, so you either have a "voltage regulator" or a "DC-to-DC converter", and yes, either of those could be a direct cause of significant RFI...

In actuality, "DC-to-DC converters" are notorious for the amount of RFI they produce!!!
They are small DC-to-AC inverters, with small AC-to-DC switching-power-supplies, that output the correct DC voltage for your equipment, but many of them are RF Noise generators as well!!!

First thing to do:
a) Get a small MF/MW radio, or even a portable SW radio, tune it to as near 518khz as you can....
b) Switch on its BFO, or "SSB switch" or "CW switch" (whatever that radio calls it)
c) Listen to the radio, noise, static, etc. with it nearby your "24v to 12v transformer" and/or up on-deck....
d) Completely disconnect both the 24vdc input to your "transformer" as well as the 12vdc output....
e) See if there is any difference in the radio noise....

Note that doing this in AM mode, might not show any/much difference....so, you should really have a portable radio with a BFO / SSB / CW switch, and switch it ON...


Again, you may not have any significant RFI on-board, but with what you tell me in your last message, it seems like an almost sure bet that you do!!!
Do the above tests, and report back....and we'll get you going!

I would not advise spending money on anything until you determine what the problem is....
I realize this "guess-n-replace-stuff, until the damn thing works" approach is very common these days with electronics, but for those of us who have been doing this for 40 years, it really seems like even the manufacturers and so-called "professionals" have given up on actual testing / troubleshooting (as it costs more $$$$ than just shinning-on a customer, and telling him to "buy something else")
But, if you do your own testing/troubleshooting it costs nothing....so go for it!
(If you haven't a portable radio, find a local ham operator or two....they're sure to have one laying around, and most would happily volunteer to help!!!)


Also, please take note of my earlier info, regarding the fact that antenna placement for a 518khz NAVTEX signal is not critical....but also remember that you may be asking a lot of the NAVTEX system to get you weather reports well outside of its designed coverage area....
So, in that situation, everything becomes more critical!
But, there is no need to use a "masthead antenna" for a 518khz MF NAVTEX signal!!!


Fair winds..

John
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