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#1 2021-07-11 15:36:07

Duncan_Hill
Committee Member
From: Dun Laoghaire, Ireland
Registered: 2017-03-14
Posts: 153
Website

Ever checked your SWR for your radio + antenna?

When idly looking at the NMEA sentences on my boat network, I noticed some AIALR sentences coming from the B600s that indicated it might be very unhappy about the SWR to the antenna. I asked around at the club, and one of the members had a SWR meter that he could bring down for checking.

In the meantime, I contacted the manufacturer of the AIS unit, and got some clarification about the sentence that made me think there was a problem - turns out that V,A meant it was inactive, and A,V meant it was active. I was seeing V,A, so it was not actually a problem (and the warning light on the AIS unit was not coming on) - but I wasn't transmitting when I was seeing the sentence (dun dun dun foreshadowing).

Today, we hooked up the SWR meter anyway, to take a baseline reading so any future antenna work could be checked against the baseline. Started with the AIS unit out of the path, got a SWR of 1.1. Imagine my surprise when we put the AIS back in the path and the SWR promptly hit 2.5+ and the warning light came on. Pinned the problem on some 90 degree SO-239 to PL-259 connectors that I was using. Removed them from the path and the SWR came back down to 1.1.

High (2+:1) SWR can damage the transmitter. Worth checking just to know what your transmission path is like. Note that you need a SWR meter that's designed to handle the ~156 MHz of VHF transmission.


Victoria 34 Cutter - 'Blue Opal' (the non-bowsprit edition)

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#2 2021-07-11 15:43:08

Duncan_Hill
Committee Member
From: Dun Laoghaire, Ireland
Registered: 2017-03-14
Posts: 153
Website

Re: Ever checked your SWR for your radio + antenna?

https://www.diamondantenna.net/sx200.html is what we used to check my installation - about 80 quid online in the UK at hamradio.co.uk.


Victoria 34 Cutter - 'Blue Opal' (the non-bowsprit edition)

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#3 2021-07-13 20:14:25

Charles_Grossie
Committee Member
Registered: 2017-08-10
Posts: 162

Re: Ever checked your SWR for your radio + antenna?

Hi Duncan,

Interestingly, I installed a new 'Digital Yacht AIT5000' Class B transponder a few weeks ago. I had pre-configured it 'on the bench' at home at Easter time and everything functioned as it should.

After installing the unit on Anitra a few week's back, all looked to be working, however, 'Marine Traffic' wasn't showing her position information nor were other local boat/yacht users able to pick her up on their AIS receivers/chart plotters, which was weird.

All the basics looked to be OK and the power connection was already there as there had been an iCOM AIS receiver installed before.

Applying 'Kepner Tregoe' Problem solving techniques, it was a case of establishing what had changed from my initial successful configuration and testing of the unit at home versus the setup on the boat. The external GPS antenna had been installed and the cable routed through the boat and the boat's power was now being used. The yacht's VHF aerial was also being used versus me using my 'Emergency VHF' antenna too. So, there were three possibilities without considering the unit to be at fault.

The 'Digital Yacht' transponder has the ability to connect a laptop to it for the purposes of configuring it, but also to look at the vital signs and status.

When replacing the standing rigging two years ago, I replaced all of the mast's wiring and also installed a new high spec VHF aerial. I knew that everything up the mast was OK and the VHF radio was receiving and transmitting (apparently) well.

The SWR ratio was measured by the 'Digital Yacht' diagnostics, which showed a value of 1.1, which is very good. The unit has its own internal splitter too.

1626202154_ais_status1_low_res.jpg

The 'Digital Yacht' dashboard (above) also indicated that everything had passed self-test and that everything appeared to be working, but it wasn't.

To cut a long story short, there was a bad negative return to the battery, which only presented itself when the AIS unit momentarily (20 milliseconds) transmitted. I believe that the outputted data was being corrupted due to the momentary voltage drop. As it turned out, the 'split charging' Voltage Sensing Relay had also failed meaning that both of the house batteries were lower than they should have been.

I ran new heavy gauge cables to one of the house batteries (fused of course) and also replaced the VSR. I tested the unit again. Everything worked.

Back to your point about the importance of the SWR ratio. It is very important to ensure that the SWR ratio is a low as possible. At least in my case, the AIS transponder has this built in, which is very useful.

Never a dull moment with yachts!  smile


Victoria 34 Cutter - 'Anitra'

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#4 2021-07-13 21:27:31

Duncan_Hill
Committee Member
From: Dun Laoghaire, Ireland
Registered: 2017-03-14
Posts: 153
Website

Re: Ever checked your SWR for your radio + antenna?

The SWR ratio was measured by the 'Digital Yacht' diagnostics, which showed a value of 1.1, which is very good. The unit has its own internal splitter too.

1626202154_ais_status1_low_res.jpg

The 'Digital Yacht' dashboard (above) also indicated that everything had passed self-test and that everything appeared to be working, but it wasn't.

Supply voltage looks a bit dodgy too!

Never a dull moment with yachts!  smile

Or houses for that matter... I think it's best to hold the attitude of "boats force you to learn new things"; whether you want to learn them or not!

The AMEC B600s can detect SWR issues, and will turn on a warning light (and I presume if I had a buzzer hooked up it would have buzzed too), but it's in the wiring locker because I reasoned that I don't need to see it 99.99% of the time. I don't know if the sentence it emits will actually tell me the SWR. I suppose I could put it back into a faulting mode tomorrow morning, hook up the laptop, and capture the alert sentences to see if it produces anything useful.

Of course, I had tested everything without the 90 degree elbows last year, and transmission was fine. Just never thought after installing the elbows to test it again with the locker door open. Ah well, it's fixed now.


Victoria 34 Cutter - 'Blue Opal' (the non-bowsprit edition)

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#5 2021-07-13 21:56:36

Duncan_Hill
Committee Member
From: Dun Laoghaire, Ireland
Registered: 2017-03-14
Posts: 153
Website

Re: Ever checked your SWR for your radio + antenna?

Charles_Grossie wrote:

To cut a long story short, there was a bad negative return to the battery, which only presented itself when the AIS unit momentarily (20 milliseconds) transmitted. I believe that the outputted data was being corrupted due to the momentary voltage drop. As it turned out, the 'split charging' Voltage Sensing Relay had also failed meaning that both of the house batteries were lower than they should have been.

I ran new heavy gauge cables to one of the house batteries (fused of course) and also replaced the VSR. I tested the unit again. Everything worked.

Yeah, I put a lot of work into Blue Opal's wiring over the last few years - the wiring loom from the yard is a bit anaemic for a modern boat setup, but replacing it completely is more than I wanted to deal with so far; just upgraded everything else around it! Found I really liked the OceanFlex cabling, far easier to work with than the Ancor stuff.


Victoria 34 Cutter - 'Blue Opal' (the non-bowsprit edition)

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#6 2022-10-18 12:38:29

Colin_Reid
Member
From: Gloucestershire
Registered: 2003-11-02
Posts: 37

Re: Ever checked your SWR for your radio + antenna?

There's an article in the current Cruising Association magazine about checking the SWR in the VHF antenna, instead of calling up the coastguard for a radio check. It mentioned that many AIS sets have a built in SWR meter. I had a look at mine (Vesper) and it did, so I plugged in the VHF antenna and checked it. All fine. Likewise the AIS antenna. A useful feature.

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#7 2022-10-31 12:45:24

Duncan_Hill
Committee Member
From: Dun Laoghaire, Ireland
Registered: 2017-03-14
Posts: 153
Website

Re: Ever checked your SWR for your radio + antenna?

Colin_Reid wrote:

There's an article in the current Cruising Association magazine about checking the SWR in the VHF antenna, instead of calling up the coastguard for a radio check. It mentioned that many AIS sets have a built in SWR meter. I had a look at mine (Vesper) and it did, so I plugged in the VHF antenna and checked it. All fine. Likewise the AIS antenna. A useful feature.

Alas, my B600s doesn't have a meter built in, not in a way that you can get to easily. It will flash a red light, but no more than that. It's possible somewhere in the NMEA data stream that there's a value showing the actual SWR, but there's a lot of data flowing on my NMEA network.


Victoria 34 Cutter - 'Blue Opal' (the non-bowsprit edition)

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