Have i used the wrong cables on my navtex??

[tel1]

Being the cheapskate i am would i be wrong in saying that a t.v extension cable from asda is not good enough for my navtex?? I bought the new aerial and fitted it, i then used a female t.v connector and then a t.v extension that goes to my navtex, yet all im getting is a complete load of rubbish on my navtex that only non human people can read!!! ive attached a couple of pictures to show you the connector and the rubbish on my navtex screen. I fear the worst i that i have to buy some special cable?? Or my new aerial is duff??? Or my old navtex needs to walk the plank??

Thanks in advance!

 

[Bilgediver]

Being the cheapskate i am would i be wrong in saying that a t.v extension cable from asda is not good enough for my navtex?? I bought the new aerial and fitted it, i then used a female t.v connector and then a t.v extension that goes to my navtex, yet all im getting is a complete load of rubbish on my navtex that only non human people can read!!! ive attached a couple of pictures to show you the connector and the rubbish on my navtex screen. I fear the worst i that i have to buy some special cable?? Or my new aerial is duff??? Or my old navtex needs to walk the plank??

Thanks in advance!

Oh you old cheapskate!!!!!!!!!!!!!!

It is probable that the antenna coax on the Navtex is 50 ohm cable whereas the TV lead is 75. To compound this you have used the worst possible connectors These tee vee connectors are fine for VHF TV signals if assembled with care however they are a sort of push fit assembly usually so it is hot or miss as to whether you get a good connection via the conductor pins.

I think you need to ask SWIMBO for permission to borrow the plastic and go to Maplins and get 50 OHM coax and some suitable 50 ohm BNC connectors.

The navtex signals are pretty weak and so you need to do as much as possible to minimise losses.

 

[wotayottie]

a t v extension cable is the wrong impedance for your use - its 70 ohm not 50, and the push fit connectors arent really ideal. but i can't see that causing your problem. suggest you have a word with nasa - they are very helpful,

 

[elton]

I'd have thought the Navtex error detection would have rejected a weak/noisy signal, rather than trying to decode and display it.

 

[Bilgediver]

I'd have thought the Navtex error detection would have rejected a weak/noisy signal, rather than trying to decode and display it.

Whoopsie!!!!!!!

We all seem to have forgotten that this probably uses an active antenna. This is fed about 5 volts up the antenna feed from the navtex. It is highly possible this supply via the tv connectors is not being sustained so anything could be happening

 

[William_H]

NASA antenna cable

I can't find any data on the NASA antenna except that it receives on about 500khz and it is an active antenna. It would be wrong to assume that the cable should be 50 ohms (maybe it is) but certainly for a non active antenna at that frequency a low capacitance high impedance coax cable would be more appropriate as in a car radio for LW MW. So my point is you need to find out what the nasa navtex needs for an extension cable.
However cable impedance really only shows itself over longer distances at high frequencies. So there will be little difference between 75 ohm and 50 ohm cable over a few metres at 500khz.

OP best bet is to remove the antenna or the receiver to a location where the antenna can connect to the receiver without extension and see if it works then. If not then there is a problem. He presumably cut the cable to fit the TV connectors so he may have simply introduced a problem of open circuit of the centre or outer or a short from outer to inner. A DVM on ohms should soon check that out.

IMHO TV connectors and cable are OK they may not last so long in salt air but should be fine in the short term. They are commonly used for depth sounder transducer for instance. As wotayottie says.

To sum up I don't think the problem is simply 75 ohm wrong 50 ohm right for cable. good luck olewill

 

[john_morris_uk]

To sum up I don't think the problem is simply 75 ohm wrong 50 ohm right for cable. good luck olewill

I agree totally - the difference in impedance at 500 Khz is not going to make any difference. The problem is more likely to be a duff connector not feeding the 5v correctly up the cable to the active antenna.

Try it without the extension to prove the system - as someone else has already suggested.

 

[tel1]

Emailed Nasa

Well some conflicting advice here, What i have done is emailed nasa with the problem, and next weekend i shall be removing the aerial so i can test it directly to the navtex. Hopefully its just me being a cheapskate and nothing more than that!

 

[robertj]

Just had exactly same problem. I rang NASA and they informed me that the cable is ordinary tv coaxial, which I have used
the problem for me was the NASA fitting which goes in the navtex itself had the centre pin missing.
NASA sent me another connector and guess what it's was a normal tv one not like the one supplied.

 

[tel1]

Just had exactly same problem. I rang NASA and they informed me that the cable is ordinary tv coaxial, which I have used
the problem for me was the NASA fitting which goes in the navtex itself had the centre pin missing.
NASA sent me another connector and guess what it's was a normal tv one not like the one supplied.

Trouble is my navtex was working fine before, I lost the old aerial in rough seas and up until then i was receiving ok.

 

[robmcg]

NASA instructions say to extend with ordinary coax cable 75ohms. Did the same job this weekend but haven't tested yet. Used black satellite 75 ohm coax as it was only stuff available. Probably find mine is b***erd now too!!

 

[BruceDanforth]

I've seen one do that on it's display and it needed all 3 buttons pressed at once to reset it. Type 'nasa navtex manual' into google and you will unsuprisingly get the manual which tells you on an about page 2 that the coax is 75Ohm and describes the reset proceedure.

 

[tel1]

I've seen one do that on it's display and it needed all 3 buttons pressed at once to reset it. Type 'nasa navtex manual' into google and you will unsuprisingly get the manual which tells you on an about page 2 that the coax is 75Ohm and describes the reset proceedure.

Nasa got back to me and this is what they said:-

Dear Terry,

The first is easy, there is a contrast control on the back of the receiver, use this to darken or lighten the display. It is temperature sensitive so if it gets hot it will get darker and colder lighter.

Re the random data, this is old data in the memory and can be cleared, press all 3 keys together to access the welcome screen, from here you can press the up and down keys together twice to clear the old data.

Re the cable, it should be okay but bear in mind it is a live antenna with 5V coming from the receiver, any connections you have should ideally be soldered, why not as a test basis just use the original antenna wire to ensure all is well with the receiver/antenna without the new cable. If this works as expected it would point to the new cable/connections being suspect.

Regards,

NASA Marine

So the first thing to do is clear the old data and then see if i get anything, if i get nothing overnite then the next job to do is put the navtex with the aerial without the extension. Hopefully i acn do all this at the weekend so i shall let you know how i get on......

 

[john_morris_uk]

NASA instructions say to extend with ordinary coax cable 75ohms. Did the same job this weekend but haven't tested yet. Used black satellite 75 ohm coax as it was only stuff available. Probably find mine is b***erd now too!!

Please believe my first post on this topic - the difference in impedance between 50 Ohms and 75 Ohms at this frequency on receive won't make one jot of difference.

Besides which, the device is designed for 75 ohm impedance coax according to NASA....

Furthermore, you can't break it by having a faulty antenna connection - it just won't work. Its not like a transmitter into the wrong load - its receive only and it just won't work (or work poorly) if the antenna is wrong.