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
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:
Originally Posted by Ketchcalimbo
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!!!