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Old 24th Jan 2023, 8:18 pm   #1
Iangebbie
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Default Poor VSWR on Fibre glass construction boats

Poor VSWR also RX ground planes

Many years ago, I installed several VHF Marine band radios on lifeboats for offshore, these were not the normal lifeboats that you may be thinking on but what is referred to as TEMP (Totally Enclosed Motor Propelled)

https://www.vanguardelifeboat.com/pr...osed-lifeboat/

The construction is fibre glass and the antenna is mounted on the roof at the rear, a VHF marine base transmitter capable of 25 W was also fitted, 12V batteries were used with a vent and hosed to the outside to get rid of any gas from the cells (Note: even if a battery says sealed for life, it still gives off gas)

Issues due to construction, these TEMP boats were fully certified and could not be altered (IE: drilled or adding or modifying in any form)

The top roof area (3/4) had steel pipe work, set up as a sprinkler system to keep the vessel cool if it went through any fires due to oil in the sea, so the antenna had to be located to the rear.

The antenna was installed by the manufacture at build

VSWR was so poor on the original set up (Typical 1:9) measured on a trusty Bird Through line meter,

Many methods could be used to resolve this issue / Metal plate to act as a ground plane fitted to the inside below the antenna.
Or connect four radial wires (Again to the inside) and attached to the base of the antenna at about wavelength of the centre frequency used.

But these would invoke the Certification of the TEMP itself, I ended up using a RF shielding spray
https://www.mgchemicals.com/products...spray-coating/

Under where the antenna base was, I sprayed about 1m square, end result was near 1:1 VSWR
Over the many years of maintenance this spray continued to hold on the fibreglass and the routine checks (VSWR) remained the same.

Regards Ian Gebbie
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Old 25th Jan 2023, 12:16 pm   #2
Iangebbie
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Default Re: Poor VSWR on Fibre glass construction boats

And now its big brother / holds about 100 people, same issues as before with VSWR, same solution applied, this is called a free fall life boat, unlike its smaller brother that has to be lowered, this just takes off into the sea.
When inside you sit backwards and attach a head strap to your head, to stop any movement when it hits the water.
Original trials asked for volunteers stating you would pay a fortune on any entertainment park to do a similar thing!
One other thing the boats (New) have to go through re certification every 5 years, one of the tests is to prove that life can survive, Photos attached of the start of the test with a bag inserted, the final outcome of the test is revealed at the end (This was the start of the Internet and these photos spread like wildfire, the company concerned was slated by 1000s of companies and individuals)
Enjoy regards Ian Gebbie

Note Will attempt to reduce the size of the Free fall photo
Attached Files
File Type: pdf Start of test.pdf (160.2 KB, 47 views)
File Type: pdf Test underway.pdf (180.1 KB, 43 views)
File Type: pdf Out of the Bag.pdf (197.6 KB, 44 views)
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Old 25th Jan 2023, 12:18 pm   #3
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Default Re: Poor VSWR on Fibre glass construction boats

Free fall life boat link = https://www.verhoef.eu/freefall-lifeboats/

Regards Ian G
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Old 25th Jan 2023, 2:17 pm   #4
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Default Re: Poor VSWR on Fibre glass construction boats

There were 1/2-wave marine-band antennas available [essentially a half-wave end-fed dipole] which minimised the need for a ground-plane.

I also successfully used "Antenna Specialists" VHF half-wave antennas on vehicles with GRP ~High-top~ roofs.
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Old 25th Jan 2023, 2:52 pm   #5
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Default Re: Poor VSWR on Fibre glass construction boats

Many thanks for the reply and the information Tanuki,

The main issue was the boat supplier / could build boats but had no in-depth knowledge of radios, hence the basic VHF antenna supplied, unfortunately this arrangement was fully certified and to alter the design would require a re certified boat, information was passed onto several builders but was never implemented hence my only possible solution
Regards Ian Gebbie
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Old 25th Jan 2023, 2:53 pm   #6
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Default Re: Poor VSWR on Fibre glass construction boats

So what is the procedure for dealing with faults - if you are not allowed to change anything??

I would declare a VSWR problem as a fault.

But a key question is this: did you measure it when floating in the sea? It may well not work well when out of the water.
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Old 25th Jan 2023, 5:16 pm   #7
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Default Re: Poor VSWR on Fibre glass construction boats

Many thanks for the questions GMB, the original build was performed and certified by others outside the oil and gas telecoms or instrument group, the maintenance spec stated 1:1 when testing, you have brought up a good point as to what was it like in the sea, unfortunately
all were tested in place on the platform itself / to do any tests within the sea would have taken a lot of permits / and the fact that you would have to remove the capability (Remove from service) a TEMP would just not happen, from my point of view the original set up was not fit for purpose, this was fed back to senior managers onshore, again who did the acceptance testing when new? doubt if telecoms were involved, I was involved in detailed design and specifications within the industry (2015 onwards) I clearly stated in the. specification that any TEMP must have an antenna with a ground plane built into the boat that must achieve 1.1 VSWR,
Statoil used both Marine and UHF radios in their life boats (UHF required as part of any muster in the Norwegian sector) ABB Norway telecoms group were copied in the specification that I devised.
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Old 25th Jan 2023, 8:29 pm   #8
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Default Re: Poor VSWR on Fibre glass construction boats

I am a great believer in end-to-end testing. If you have a boat and it has a radio then you test it when floating. The sea is a much bigger ground plane than anything on the boat so it only needs the radio to couple to it somehow, and I would have expected that was how it would be designed.

There is a danger that if you made the VSWR perfect when 50 foot in the air then it might be very bad when in the water.

It's probably OK, but you don't know until you actually try it for real.
And of course you could have been (un)lucky - the VSWR in a bad sitution tends to depend on cable lengths so it might have got lucky and been perfect, but again would change when in the water I suspect.

But no need to worry, I have already made a note to always take my own radio when out at sea.
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Old 25th Jan 2023, 8:36 pm   #9
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Default Re: Poor VSWR on Fibre glass construction boats

Many thanks again GMB, again a issue with the double radios (Marine+UHF) the UHF radio would be used before deployment into the water as part of the final muster and the Marine once in the water, will contact some of my past work mates in Norway to see if the design document was followed to the letter
Regards Ian Gebbie
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Old 25th Jan 2023, 8:48 pm   #10
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Default Re: Poor VSWR on Fibre glass construction boats

Surely there would have been FAT or Factory acceptance/witness tests carried out? as well as in this case sea trials before a design was accepted and signed off?

This being the case then I would have thought a full functionality test procedure of all aspects would have been written into the test documentation prior to acceptance?
Once the design is proven and then frozen for production all foreseen 'difficulties' should have been well and truly ironed out at that stage.
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Old 26th Jan 2023, 11:06 am   #11
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Default Re: Poor VSWR on Fibre glass construction boats

This is going far off topic for the forum. Time to close

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