navtex

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gtuson

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i have been given a navtex but without active ariel. Its not a Nasa product. I have been trying long ariel- uninsulated wire. I recieve signals from Niton - but unreliably. Some clear and complete and some not at all. It is programmed properly. Am i wasting time with this - or if i persevere with ariel(longer/ located higher etc) might i eventually get decent reception?
 
What sort of Navtex is it? ie paper or electronic display If its paper you could try this website http://www.dsearleuk.force9.co.uk/ I have a paper one and found it very useful.You could try searching the net for the makers website which is how I found the above.If all else fails you could try an insulated flex attached to a shroud?
 
Try it with a bog standard car aerial, I think you will find it works. Don't buy one especially just try an old one first.

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If Keltman's solution doesn't work (aerial needs to be 20'+ twin-core & strip one core & ground it to boat ground) you might try Nasa active aerial - cost £35 or so and should plug straight in. Byron's car aerial might work (eg Navtex is close to Radio 5 frequency although is SSB not AM) but a longer aerial would be better imho.

Reception for Navtex can be hit & miss (it's a v. low power signal) depending on where you are eg if in port with lots of interference about might not be so good.
 
Your trouble could be one of two.
The aerial feeder you use should be co-axial to prevent cross talk from other systems. The antenna should have a specific impedance, and you bit of stripped wire will not necessarily meet this parameter. If you want to try the stripped wire make sure that it is stripped to one quarter wave length for best reception. Best to find out what antenna it was designed for and then get the correct antenna. Height in the boat do not really matter at the NAVTEX frequencies the pushpit rail will do just as well and it is easier to get at.
Second possible problem is where are you trying from, some areas have awful reception for NAVTEX broadcasts, the Medina being one.


Chris Stannard
 
Tricky stripping the wire to quarter-wavelength at Navtex frequencies - equates to about 150metres, I think!

At these kind of frequencies random lengths of wire are as likely to work as "proper" antennae, unless it is an "active antenna" which contains some kind of tuned amplifier to boost the signal, or if there is considerable interference from other equipment such as engine alternator, echo sounder, in which case screened cable when in the vicinity may help. A car aerial may be a convenient way to do things, but there is nothing special about it. It is a metal stick on the end of a screened cable. Suck it and see is as good a way to proceed as any!
 
You can determine if the unit SHOULD have an active aerial by connecting a voltmeter between the centre pin and outer of the aerial connector. If you find a voltage, almost anything readable, then the unit is intended to work with an active aerial and is unlikely to work at all well without.

If there is no voltage then a old car aerial is as good as anything but as the others say you may well find that just moving a few 100' will make all the difference especially away from a marina.

If there is a voltage then it just might do something if you can find the transmitter and park next door with your bit of wire ..... otherwise you just HAVE to have an active aerial. Fitting a Nasa aerial when it is not a Nasa unit may or may not do anything. It might stop it working at all.
Roger
 
NASA active HF is "passive" at it's output - at least, I assume so as you can interchange both single wire and active antennae without detriment.
 
A slight problem with this could be the fact that a quarter wavelength at 500kc/s is around 550 ft.

That is one of the main reasons that most aerials are active.
 
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