The LMR family of coax (originally by Times Microwave but now everybody makes them) has a variant which has a DB suffix for Direct Burial. You could check to see if that variant can be continually immersed in salt water but my guess would be that it can be. Sean On Mon, Feb 4, 2013 at 2:42 PM, Richard Prosser wrote= : > Hi Joe, > > The PVC used for coax cable sheaths etc. is not totally watertight. Cable= s > designed for underground use normally use a polyethylene sheath and inclu= de > either a metallic water barrier or are grease filled (eg telephone twiste= d > pair cables) - or both. There are more exotic sheath materials that will = be > better than PVC or PE but all will allow some water (vapour) ingress. Ove= r > time this will degrade the VHF performance of a coax cable by oxidizing t= he > screen and the centre conductor (skin effect loss) and by degrading the > inner insulation (increasing dielectric loss). The effective permittivity > of the insulation may also be changed, changing the cable impedance. > > Coax cable designed for external use is available, but at a somewhat high= er > cost. And it's less flexible and harder to terminate. Often the outer > shield will be a metal barrier of some sort, rather than a braid. Or it m= ay > be a braided shielded cable inside an outer protection barrier of some > sort. > > How long the cable will last in a particular environment will depend on a > number of factors - not the least being how much operational margin you > started out with. > > You can run the cables inside conduit with ends sealed etc for longer lif= e, > or you can just resign yourself to replacing the cable at regular interva= ls. > > Richard P > > > On 5 February 2013 05:41, Joe McCauley wrote: > >> Thanks folks, you all confirm what I told the guys here. To answer a >> variety of questions, It did work satisfactorily, but I must say we are >> always having problems with the electronics in these boats. Last year I >> replaced the antenna cable in one of the boats. Might have been this one= , I >> can't remember. That one looked perfect until I got into the hull where = the >> cable passed the fuel tank. It was practically rotten in there over a me= ter >> length. I suspect fuel got spilled on it as we had issues with the fuel >> tank earlier in the year. The antenna feeds a simple j-pole antenna. The >> 'earth' will be via the radio ground to the motor. There is no provision= in >> the antenna mount for an earth there. >> >> I'm pretty sure I'll find cable/connection or maybe radio problems when = I >> get to look it over. >> >> Joe >> >> -----Original Message----- >> From: piclist-bounces@MIT.EDU [mailto:piclist-bounces@MIT.EDU] On Behalf >> Of Herbert Graf >> Sent: 04 February 2013 15:13 >> To: Microcontroller discussion list - Public. >> Subject: Re: [EE] Marine VHF radio antenna cable and seawater >> >> In a perfect situation you'd not notice any issues, a properly sealed an= d >> shielded cable won't mind whether it's awash or not. >> >> That said, if you are having problems, I've no doubt it could be the >> cable. A cable in the elements can degrade much more quickly then one wo= uld >> assume, especially in the presence of sea water. Is it possible to >> carefully inspect this cable over the whole run? Pay particular attentio= n >> to corners and where the cable mounts hold it. It's entirely likely that= a >> small crack or hole has formed letting water in. >> >> Also have a good look at the ends, look for signs of corrosion. I've had >> many cases where a properly weather sealed end looked perfectly fine on = the >> outside, but after years of exposure started to let water in. The >> freeze/thaw cycle can breach even the highest quality connectors. >> >> TTYL >> >> On Mon, 2013-02-04 at 14:21 +0000, Joe McCauley wrote: >> > This may not be the most appropriate forum for this, but there are a l= ot >> of knowledgeable people here.... >> > >> > Does anyone have any thoughts on what the effect on transmission >> > efficiency would be of the antenna feed cable for a marine VHF radio >> > being partly submerged in seawater. (f~160MHz) >> > >> > The cable on my scuba club's boat runs along the deck which is always >> partially awash. This is the way it was built. Apart from that reason, I >> don't think that the cable being immersed should cause significant issue= s. >> We are having radio transmission issues. The antenna is mounted ~1.5 >> meters above the deck. >> > >> > I've not had the opportunity to look at it yet & expect to find >> connection issues when I do, but it has been suggested that the immersio= n >> of the cable could be causing our problem. As I said, I don't believe th= at, >> but....... >> > >> > Thanks for any info, >> > >> > Joe >> > >> >> >> -- >> http://www.piclist.com PIC/SX FAQ & list archive View/change your >> membership options at http://mailman.mit.edu/mailman/listinfo/piclist >> >> >> >> -- >> http://www.piclist.com PIC/SX FAQ & list archive >> View/change your membership options at >> http://mailman.mit.edu/mailman/listinfo/piclist >> > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist --=20 http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist .