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Old 30 December 2010, 06:24   #1
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Waterproofing a Metz Manta antenna

The new RIB raider on the Vipermax does not seem to work properly - something wrong with the connection in the bottom I think as it does not seem to screw in properly (the first one got broken in the UK, that one screws right onto the coax fitting fine, but the replacement does not).

So I thought I would bite the bullet and get a Metz Manta as they seem to be well thought of.

Problem I have is that the bottom of the antenna is not waterproof for use on an A frame installation.

Photo of the antenna is here: http://www.saltyjohn.co.uk/metzmanta...ennaaerial.htm

The fat part of the base has an O ring (you can see it in the photo) so would seal onto the top of the plate on the A frame OK, but the fitting into the bottom is just a double ended SO259 connector which screws into a hole in the base of the antenna. Normally I use PTFE tape to waterproof threads but obviously that's no good on an antenna because it will insulate the connection ... likewise I'm not really keen on using sealant for the same reason.

Any thoughts on the best way to waterproof this properly? The plug onto the SO259 is no problem because I got a roll of the Bandit tape with the aerial so will wrap that up as directed, but it's not at all waterproof where it screws into the bottom of the antenna. The best idea I can come up with is to just squidge a load of silicone sealant around the base when assembling it onto the A frame but I'm not that convinced by that.

A separate problem is that the antenna is secured by a nut on the barrel of the SO259 connector. The antenna plate on the A frame is much thicker than the normal mounting bracket and in order to get the nut to engage on the threads I have to back the SO259 out so far there are only a couple of turns of thread still left in the antenna base, I haven't figured out how to sort this yet, what I really need is a double ended SO259 that is threaded all the way down the barrel but I haven't found one locally yet.

Does anybody see any problem with cutting the plug off the end of the RIB Raider antenna and using the same coax? I'd rather not run a new coax because it looks like being a PITA due to the amount of other stuff in the trunking...
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Old 30 December 2010, 06:59   #2
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So the male 259 screws into the antenna as well as on to the plug? I' waiting for delivery of my Metz Manta at the mo, will be fitting it direct to the a-frame same as you So I guess I'll be asking myself the same questions soon.

I would probably just use a waterproof grease on the thread into the base of the antenna ie Tef-gel or similar, won't take much to fill the gaps in the thread making it moisture proof, even vaseline would prob do.

I'll have a better idea on mounting etc when I've got mine in front of me
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Old 30 December 2010, 07:09   #3
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Check it all works first and then Sikaflex the whole connector. It's the only way to be sure it's protected. It'll also hold it all together and prevent the connector coming loose.
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Old 30 December 2010, 07:50   #4
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Don't use silicon sealer around open electrical stuff or electronics. The sealers commonly use acetic acid and the vapour will be trapped withing the electrical stuff and cause corrosion. I'd be opening up the Vitronics piece of shit and doing a proper job of connecting the co-ax then resealing it. Use Sika or Tigerseal as Andy suggested for a permanent seal but be sure to clean the surface of the pvc co-ax outer cos it'll have traces of contamination on its surface to prevent sticktion of the Sika.

Personally I think it's crap that these marine components aren't given much more thought to their possible applications and fitments and designed accordingly. Having said that, I've got a shortish Pacific Arials antenna for my AIS which has lasted a good few years and still seems fine. It doesn't appear to be any better built that the rest but it's still there. I did dismantle it and do a proper sealing job when fitting it though. Just for info, internally it's a simple J antenna but, interestingly, the co-ax core is soldered to the impedence matching side and the screen it soldered to the feeder. Puzzled about that but it's receiving AIS info from 15 miles away so I doubt it's a mistake.
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Old 30 December 2010, 08:02   #5
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Just looked at the Metz antenna, I wouldn't be buying that, the connector with eventually give you problems, the stainless whip doesn't appear tapered so it may stress out at the bottom and the typical application appears to be mast head and so not bumpy-thumpy as a rib is. Could be another 50kwid down the drain.

Why are you choosing the short type of antenna, have you a height restriction or is it a durability issue?
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Old 30 December 2010, 08:23   #6
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Quote:
Originally Posted by jwalker View Post
Just looked at the Metz antenna, I wouldn't be buying that,
It's good enough to be spec'd on all USCG vessels and comes with a lifetime warranty

Wouldn't be my 1st choice for a primary vhf antenna though either I'd agree. I'm using an 8ft Comrod for my main vhf but using the Metz with my 2nd radio which is primarily for handling the AIS and as the emergency back up.

I don't think you'd get much water finding it's way vertically up from the underside of your a-frame and past all the threads. Even a squirt of that Wurth electrical contact spray would be enough to prevent corrosion inside the base.

If not send it back and get a new one
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Old 30 December 2010, 09:32   #7
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Quote:
Originally Posted by martini View Post
It's good enough to be spec'd on all USCG vessels and comes with a lifetime warranty
That was my thinking. The RIB raider on my old boat always worked really well, I think the threads in the base of this one have been cut wrong or something because the coax fitting goes into the old (broken) one fine and easily screws right in but the new one tightens up half way in - nothing visually obvious though.

I'm going to try and find a completely threaded double ended connector and then seal the antenna base onto the A frame, wrap up the plug and coax with the Bandit tape and then sealant over the top of that and around where it goes through the A frame, should be OK I think. Noted on the corrosion, thanks, hadn't thought of that. You can't buy Sikaflex here and all my tubes have dried out but I have some other waterproof (non silicone) clear sealant that I used to seal the radio etc in properly when I got the boat, so I'll use that.

There is a height restriction (low bridge) at one of the main launch sites so I'm just trying to keep it down.
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Old 30 December 2010, 10:43   #8
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Well I now know what I need - it's called a bulkhead connector, like this:

http://www.dxengineering.com/Parts.a...No=DXE-363-SST

whether I can find one anywhere around here may be a different matter...
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Old 30 December 2010, 10:46   #9
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Problem with that is that is is effectivley 2 connections and as a rule of thumb a connetion can loose 1db and affect VSWR if not done properly!!
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Old 30 December 2010, 11:09   #10
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I think the lifetime warranty is a bonus to some folk but I feel they rely on inertia, after all and ignoring all the hassle of removing, returning and refitting, if the first one fails are you gonna want to rely on the replacement and hope it doesn't fail too?
Quote:
Originally Posted by BogMonster View Post
There is a height restriction (low bridge) at one of the main launch sites so I'm just trying to keep it down.
I asked because I use a different approach and have done this without failure for about fifteen years. I use a long high gain antenna on a nylon mount (not polythene or stainless). The nylon allows a bit of flexing to limit the flex load on the antenna base section but I mount the antenna base low enough so that it has a support arm further up the mast. The support needs to be non-metallic where it's close to the antenna. I've used two different methods of attaching the antenna to the support on each different installation, both very simple and worked fine, because I need to fold the antenna down each time after using my boat. The first setup used a rubber bonnet catch, the ones you'll be familiar with as car accessories, my present setup which has been in use for the past five years, is to use a couple of turns of double sided velcro around the antenna and a fixed stub of tube. You might imagine the velcro wouldn't hold but it does and the mast is two metres tall. The tube is slightly offset out of line with the base and has a short piece of thick walled rubber tube pushed on to it to give the neccessary stand-off from the antenna to prevent it effecting its performance. It's actually got other features and a bit more versatility than I've explained but that's for a different reason. And before the pedantic come along, I did check it out with an SWR meter during installation and it was spookily close to the antenna specification.

Here's a linky to what appears to me to be a mechanically well thought out design, the mount is wider at the base and tapering and the antenna element also wider at the base and tapering to reduce inertia loads. I'm not recommenting it because I haven't any experience of it but it would be where I put my money if I needed that type of product. All IMO of course.
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