NASA Navtex aerial

[chasroberts]

Just stripped down the aerial housing to find a lot of water in there and the components presumably stuffed?
Reception has been poor for a while so this may have been the problem.
I have a nice backstay nearby that I am considering connecting the aerial to. The aerial cable has a red and a blue wire in it and unfortunately it's not obvious to me which would be the equivalent of the core in a coax cable which I understand from previous posts on this subject is the one that needs connecting to the backstay.
All thoughts and advice would be appreciated.
As always many thanks

Chas

 

[TonyS]

The core of the coax lead, if I remember correctly, has a positive voltage for the active aerial. I connected this directly to the backstay and have had excellent reception for a few months, then it deteriorated! I had a look at the back stay anchor point and all around it had rusted, losing the electrical contact. When cleaned up the good reception was restored. I think the secret is to put a low capacity capacitor between the two to stop the rusting.

 

[captainslarty]

Hi Chas
I presume its the old style antenna, small box with whip antenna ?
If so, you may be better off getting it replaced - I have just ordered another from Nasa - 35 squids, for the old navtex prop, not plus model.
The antenna and amp is part of the overall receiver input stage. Try it on the back stay, but, you must not allow the 5v feed to contact the backstay - from centre antenna pin. Even the newer series 2 navtex antenna will not work with this set, and they are also active. So, I went for the a new antenna. If you decide to replace the whole navtex, let me have first refusal on the old receiver.
Hope you get it sorted.
Joe

 

[chasroberts]

Thanks guys. The navtex is the PRO-plus version so no chance of getting rid of it just yet! The aerial cable is not coax it's just like ordinary electric cable with a red and a blue wire. The aerial is the standard NASA tube like thingy with a flat plate inside with electrical stuff but as far as I'm aware it's not active and has no seperate electrical feed as does the active antenna with the Target SSB next to it.
Keep the thoughts coming please.
Thanks again
Chas

 

[hubert]

If the cable is red and blue screened then you have an early version of the receiver. That is a 518khz receiver and you get the 490khz channel by throwing the switch at the bottom of the antenna cable. This tells the electronics in the antenna to convert the 490 signal to 518 before sending it down the cable. If you use a wire antenna then you will loose the 490 option. Also the receiver may not perform well without the selective amplifier in the antenna. But then again it might.

 

[billskip]

IMHO dont connect the aerial to any part of your rigging unless you use the specialy designed ceramic insulators de-coupling the section of backstay.

As said by TonyS he found rust appearing at the base of his backstay, I hope he also inspects the mast head connection as well because this may also be corroded.

You could try insulating the aerial ( not directly making a fixed connection) and taping it to the back stay, you may then get a better signal through inductance.

AS said all IMHO

 

[chasroberts]

Thanks for that Bill

There is a proper insulated section of the backstay but it's out of reach at present. In the meantime if I use the induction method you mentioned which of the two cables should i wrap - the red or the blue (reminds me of the bomb disposal scene in Lethal Weapon 3)

Thanks

Chas

 

[Jonny_H]

If you go for a new aerial, rather than risk it outside again, you can mount it inside. Ours is mounted under the deck behind the instrument panel and gets good reception.

 

[chasroberts]

Thanks John

Think you've got the right idea.

Chas

 

[thalassa]

[ QUOTE ]
If you go for a new aerial, rather than risk it outside again, you can mount it inside

[/ QUOTE ]
Had two boats with this arrangement. No problem. I'll second this...

 

[billskip]

Chas

IMHO try all three combinations ie both then red then blue and see which gives you best reception. (if it works )

The reason I say if it works is that maybe the original aerial is made to a specific size and length as nonmally I believe they are "tuned" to the device.

look out for bombs in the bog /forums/images/graemlins/grin.gif

 

[doug748]

Great idea. Mine goes inside this winter. Less chance of people swinging themselves aboard by grabbing it, as well.

 

[whiteoaks7]

my antenna was the tubey thing also and it also got full of water. Dried it out carefully with a hairdrier then sprayed the whole thing with circuit board lacquer and it all worked fine.

The connection however, is a 50ohm co-ax which also carries the d.c. for the active antenna. I used RG58 but put a 75ohm TV plug at the receiver so I'm a little amazed it works!!!! but it does.

 

[zeemeeuw]

I've a navtex target receiver without an antenna. May I use a other antenna. What's inside the original active antenna from nasa ? Are there diagrams to find from the circuit inside the antenna? regards

Marc (Belgium)