Author Topic: VHF aerial and high VSWR  (Read 552 times)

MagicalArmchair

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VHF aerial and high VSWR
« on: January 28 2020, 16:51 »
I posted this on PBOs forum as it was a more general, non Bavaria specific, question - however, it appears to have grown into one...

https://forums.ybw.com/index.php?threads/ais-700-vswr-fault.533851/#post-7033155

The AIS 700 reports high VSWR. The final poster asks "Just a thought, do you have a deck connector? These can get water in and this would effect SWR as it changes impedance" - I am assuming the VHF aerial is all factory fit, I once again kick myself for not photographing every square foot of the boat last time I was on her (and I won't be back there for three weeks now, boo). Is there a deck connector? Anyone else had this issue and had to replace their VHF aerial? I sense a trip up the mast in the offing......


Salty

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Re: VHF aerial and high VSWR
« Reply #1 on: January 28 2020, 20:52 »
I have a masthead vhf transmitting and receiving aerial which used to be shared with a class “B” AIS transmitting system via an aerial splitter. Periodically the AIS system would show a High VSWR” warning notice on screen, but poking a button or two on the Icom AIS instrument usually resulted in the warning disappearing off the screen. I don’t know what caused it, but having had  a stern “A” frame fitted a couple of years after installing the AIS, I decided to fit a separate transmitting aerial for the AIS. That aerial is mounted on top of the “A” frame where it complies with the Icom recommendations regarding height above head and distance away from other transmitting or receiving aerials and human bodies and distance away from other instruments etc., and is connected directly to the transmitting output from the AIS. The splitter was taken out of the system such that the main vhf radio continues to use the masthead aerial while the AIS now has its own dedicated transmitting aerial.  The High VSWR warning previously seen has not returned.

Symphony

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Re: VHF aerial and high VSWR
« Reply #2 on: January 28 2020, 22:38 »
Neither of my Bavarias have had deck connections for the VHF, but in line with good practice the cable goes through a deck gland and the connection is a plug and socket inside the boat. Remove the panel in the deck head, usually around where the mast post is and you will find all the connections for all the mast cables. However if there is a leak through the gland water can of course get into the interior connections, but such leaks are usually pretty obvious as the water drips onto the panel and then into the boat!

It is of course possible that someone in the past has replaced the internal connection with a connecting deck plug, but think it most unlikely.

MagicalArmchair

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Re: VHF aerial and high VSWR
« Reply #3 on: January 31 2020, 20:42 »
I want to take a replacement PL 259 plug down to the boat, so I can eliminate the possibility of it being a bad plug/badly fitted PL 259. The RG cable, I remember, seemed quite thick to the VHF aerial (far thicker than the supplied patch from the VHF radio to the AIS 700). Is it RG 213 cable as standard factory fit? - It certainly seems too thick to be RG 6.

MagicalArmchair

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Re: VHF aerial and high VSWR
« Reply #4 on: June 27 2020, 14:23 »
I have found the culprit. As Sympony said, the cables travel through the deck via a gland. That gland leaks like a sieve, meaning the union between the RG8 connected directly to the VHF aerial and the RG213 cable that connects to the AIS unit was corroded to hell.

The offending deck gland:



The corroded union:



Added to that, the plug on the RG8 side just fell apart in my hands when I undid it from the union. The core had never been soldered into the plug. I went to refit the plug, only to find the the braid just fell apart in my hands. I cut a few inches off, and the coax improved a little. It was still very corroded though.

Corroded coax:



When I refitted the union and the PL259 to the RG8 cable, the VSWR was still too high, so its going to be a replace VHF/Coax job. Some questions for the sage panel of Bavaria experts:

  • This is almost certainly the factory fit VHF aerial. Is the coax attached directly to the aerial at the top of the mast or is the a PL259 I can undo when I get there? I ran out of time to check when I was there yesterday.
  • On changing the cable, is there a cable gland at the mast head that needs to be undone? Or can I simply tie a mousing cable to the old coax and pull it through without dismantling anything at the mast head?
  • Is the deck gland factory fit also? I note the one on the transom was leaking also, so I took that apart and revealed it. Can you get a replacement rubber inner for it? How have others made these glands water tight?
  • Does anyone have any photos of their head box?? Or their VHF aerial setup aloft please?

SorinCT

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Re: VHF aerial and high VSWR
« Reply #5 on: June 27 2020, 21:12 »
I had the same issue last week and ended up being a bad solder/connector installation. Just make sure everything is perfect down below before climbing up.

Yngmar

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Re: VHF aerial and high VSWR
« Reply #6 on: June 27 2020, 21:35 »
Is the deck gland factory fit also? I note the one on the transom was leaking also, so I took that apart and revealed it. Can you get a replacement rubber inner for it? How have others made these glands water tight?

The deck gland is factory fit, we have the same ones. These are designed to seal around a single cable and that's how it is from original. If used like that, they are watertight.

Someone must've replaced that single cable with a bundle of separate wires, probably due to replacements/upgrades. This doesn't work, because the wires are round and thus there are gaps between them, much like there are in a stack of cylinders.

Simplest fix for your situation is to squeeze some sealant between the individual wires in the bundle, but very carefully as it's easy to make a mess of it. Or install a different type of deck gland or waterproof plug/socket.
Sailing Songbird (Blog & YouTube)  ⛵️ Bavaria 40 Ocean (2001)

Lyra

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Re: VHF aerial and high VSWR
« Reply #7 on: June 29 2020, 09:47 »
From your pictures it seems that the coax cable is a non  tinned type - so could be corrosion of the copper strands (inner and sheath) - you may be surprised as to how deep into the  insulation this corrosion creeps so cutting several cm at the ends do not help much.
I know that on my boat I took great care in making the connections and it did not help much with the VSWR - my plan is to replace the cable from mast top into the cabin.
S/Y Lyra
B36 / 2004

SorinCT

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Re: VHF aerial and high VSWR
« Reply #8 on: June 29 2020, 15:05 »
One radio enthusiast told me there are tools and different ways how to troubleshoot and pinpoint the cause of a high VSWR, although best practice is to check the wiring and connectors yourself as you get more familiar with the setup and start to find problems you did not know you had  :))

MagicalArmchair

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Re: VHF aerial and high VSWR
« Reply #9 on: June 30 2020, 09:59 »
Now I am resolved to go up the mast at the weekend, I have my new VHF aerial plus coax. It's one of these (or one very like it):

http://www.glomex.it/leisure/eng/antenna-vhf-RA106SLSSB25.php

It has a RA106 solderless connector at the base of the antenna, one of these: http://www.glomex.it/leisure/front/upload/usermanual_RA106solderless.pdf

I don't fancy making the connection whilst aloft, so the order of the day will be the below, I think, unless anyone can spot anything daft about it.

  • Make up the RA106 connection in the garage, so we will be feeding the coax DOWN the mast, as opposed to pulling it UP the mast.
  • Prepare by using binoculars to look up the mast, and hope the bracket already there will fit the new aerial. Cut the plug off the bottom end, pull it through the cable gland, and get it the bottom of the mast, on deck, ready to be pulled down (I'd rather do it this way as that gland will offer up a lot of friction when pulling the cable through).
  • Climb up the mast, don't fall off the mast.
  • Remove old VHF aerial
  • (optional) If the old VHF bracket does not work with the new VHF, drill out rivets and rivet on the new Glomex one with monel rivets, being sure to use TEF-GEL to stop any galvanic corrosion.   
  • Attach the new aerial to the bracket
  • Cut RG58 cable off the old VH aerial, attach the end of the new coax to the end of the old coax using whipping twine. Wrap the connection in electrical tape making sure the width of the cable stays broadly the same.
  • Hope the lovely assistant can spare 30 seconds away from stopping the three kids down below from jumping overboard, ask said assistant to pull the cable through. I'll give the cable a light coat of silicone lube to help it along its way. Feed the cable down the mast from the top as Liz pulls it from below.
  • Climb down the mast, don't fall off the mast.
  • Post cable end through deck.
  • Attach plug.
  • Test.
  • Seal up deck gland.

Have I missed anything? There is a distinct possibility I'll need to come down to pull the cable through as the first mate might be busy with the crew. Anything daft anyone can spot?

I'll go up on the spare main Halyard, with the spinny halyard as my backup line with my fall preventer on it.

Yngmar

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Re: VHF aerial and high VSWR
« Reply #10 on: June 30 2020, 10:55 »
That looks a lot like the original(?) one we got our mast, so you might be in luck with the bracket.

Don't use Tef-Gel, it's a scandalous rip-off and Duralac is far better suited for the job (although not exactly cheap either - buy smallest tube as it does go off after a couple years and you only use a bit usually).

Use liquid soap instead of silicone grease as temporary lubricant. It'll wash away in the future, which is desirable. Silicone grease will stay forever and the cable ends up coated in dust/dirt and/or forever greases anything else it may touch in there.

Squeeze any connectors full of silicone grease before plugging them in, then wipe off excess squeezeout - it'll keep any moisture out and thus prevents corrosion. This tip makes a huge difference on boats I've found.

Good luck, especially with the not falling off part  :))
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Symphony

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Re: VHF aerial and high VSWR
« Reply #11 on: June 30 2020, 11:50 »
Rather you than me! Pity your children are so young as once they get to teenage they become the modern equivalent of chimney boys as they seem to love going up the mast. Better having two people doing the pulling through bit as the top one will need to feed the new cable through and keep a bit of tension on to keep it straight. Probably better the lower one is on deck as well rather than inside as pretty sure there will need to be communication between the two.

MagicalArmchair

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Re: VHF aerial and high VSWR
« Reply #12 on: June 30 2020, 12:33 »
Secretly, I quite enjoy going up the mast. I have some good, solid kit which helps with the general anxiety related to the downsides of gravity (no pun intended).

The boy setting up my kit on my old boat :).



Typical, Saturday is now going to be a howling gale with 40mph gusts and rain - not ideal for going aloft. Sunday is looking a better day for it.

Re the soap, I'm away's shy just in case there is something eventually corrosive in it (such as a scent additive for example), I want this cable to still be fine in 20 years time. What about something made for the job like https://www.screwfix.com/p/ideal-yellow-77-wire-cable-pulling-lubricant-950ml/44462 - then I have it in stock for future use too?

Good idea re the silicon grease!

MagicalArmchair

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Re: VHF aerial and high VSWR
« Reply #13 on: July 06 2020, 16:52 »
At the weekend I went up the mast, all the way to the top, twice (my quads and shoulders are telling me all about it today!).



The old aerial bracket will fit the new aerial, so that is the good news, the bad news is that the splice on the spare halyard means I can't get quite to the top without standing with locked knees in the stirrups, making it quite hard to work, and any work on the aerial is done at full extension.

To try and get the nut off the bottom of the aerial, I gripped the top section with my mulgrips. As the plastic split (I gripped a bit too high up the aerial to start with) water poured out of the body, showing water had managed to enter the aerial body itself - it had probably then travelled all the way down the coax, destroying it as it did. I, rather stupidly, didn't tie the mulgrips on to bosuns chair, they slipped and fell straight into the water (thankfully not the deck! (or onto any hapless passer by...)), however, with no mulgrips, with the spanner on the nut, the aerial just span no matter how hard I tried to grip the top section with my pliers. I did consider drilling out the bracket and replacing that too, however I had forgotten my Duralac also, so I didn't want to start that process. Drilling fully extended didn't feel desperately sensible at any rate...

I am left ordering new mulgrips, and going back to have another try in a few weekends time when my quads have recovered. What is my best option? I'm surprised that nut has galled so badly onto the body of the aerial/the bottom of the bracket (Alley onto Stainless I suppose?) 

  • Use some Plusgas, spanner and mulgrips should get it undone do we think? Or am I wasting time there?
  • A nut splitter just won't work as too much force will be required. What about a Dremel with a cutting disc? Sounds more dicey than just drilling off the bracket?
  • Do I get on with it and just drill out and replace the bracket and stop messing around? Installing monel rivets at height using my long, levered, riveters, is not for the faint hearted. 

Either way, I'm going to try and pull a bit of that splice into the mast (without getting it wedged), to try and get a bit higher up... come to think of it, I could always cut the splice off and replace it with a Bowline?? The primary use for this halyard will be going aloft anyway?







Yngmar

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Re: VHF aerial and high VSWR
« Reply #14 on: July 06 2020, 17:48 »
Dremel would do the job, or simply a small hacksaw down the slot of the bracket.

When doing jobs up the mast I find it helpful to do the entire job in my head first. Not just plan it, but do every move. Usually that helps me find three more things I need to take up there with me :)
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MagicalArmchair

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Re: VHF aerial and high VSWR
« Reply #15 on: Yesterday at 09:41 »
The heat generated from the Dremel grinding disc might help break the galling of the nut onto the aerial also if it doesn't free off easily this time.

I'm going to chop the splice off the end of the rope. I am uncomfortable (on reflection) going up on someone else's splice anyway (it looked pretty good mind you), so I'll chop the end off the spare 12mm Halyard and join it on to my 10mm climbing rope with a Flemish bend and two stopping knots, and that should let me get a good foot higher, making the whole job a lot easier and safer. The knot I joined my climbing rope to the halyard with was a double bowline finished with two half hitches (which I've always used when climbing masts), however, reading around it, eventually that knot might kill me ;D.

New Mulgrips get delivered today! This time I'll remember to tie them on to myself.... probably....

Symphony

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Re: VHF aerial and high VSWR
« Reply #16 on: Yesterday at 11:08 »
Suggest you replace the splice with a Selden halyard knot to a shackle. This knot is less than 50mm long, much less than a splice and will butt right up to the sheave. I have no splices on any of my running rigging.

MagicalArmchair

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Re: VHF aerial and high VSWR
« Reply #17 on: Yesterday at 15:49 »
Suggest you replace the splice with a Selden halyard knot to a shackle. This knot is less than 50mm long, much less than a splice and will butt right up to the sheave. I have no splices on any of my running rigging.

Interesting. I could put one of these on the end of my halyard I use to pull my "climbing" rope up with (with the TopClimber gear, you must ascend on a 10mm braid on braid, so you hoist that first, before climbing that rope), and one on the climbing rope itself, shackling the two together and then seizing the shackles? This would make it quicker to set the kit up in future too I suppose.