Need help diagnosing VHF/AIS antenna problem

[jt11791]

My boat has two antennas at the masthead, one for the VHF and one for the AIS. All was working well until recently. I had the mast down for rigging work and as it was down anyway, I had the yard replace the coax antenna cables. They also replaced one of the antennas as they said it looked corroded. Apparently, they fouled something up.



As I left port, I found that my VHF was not working and I am only seeing AIS targets out to about 3nm. The first check I made was the connections at the base of the mast. They appeared okay to me. I tried swapping the two cables, so if it was antenna A -> VHF and antenna B-> AIS it is now A->AIS and B->VHF. This got the VHF working again, at least I have a range of several nm, but no change on the AIS.


This seems strange to me. If the B antenna is good I'd expect the AIS to work better when connected to B, but it doesn't, or at least not obviously.


I put my multimeter on the antenna cables at the base of the mast and they both read 0.1-0.2 ohms. Are there any additional tests I can make from deck level? I have an SWR meter, but I'm not sure how to use/interpret it.


Thanks for any ideas, I'm not thinking too clearly as I rage at the yard

[donradcliffe]

When you say your VHF was not working, was it not receiving or not transmitting?

Do you still have the "corroded" antenna? Duct tape it to your lifelines or bimini and run a shorter piece of coax to the AIS and VHF radios. Compare its performance with each masthead antenna.

I never considered two masthead antennas because they are too close together unless separated vertically. Not only will they disrupt the radiation patterns, their proximity will overload and may damage the front end of the listening radio when you transmit on the other.

Pro AIS should give you the SWR on the AIS antenna. If you put your SWR meter into either antenna wire it should give you SWR, but make sure that the meter is rated for 150 mhz frequencies.

[S/V Illusion]

Too many variables, e.g., bad coax, poor coax connections, any broken connections where coax exits mast base, etc… which makes this a guessing game absent more info.

If you have or can make a short section of coax, connecting the SWR meter might be informative but not always.

[jt11791]

Originally Posted by donradcliffe

When you say your VHF was not working, was it not receiving or not transmitting?

Do you still have the "corroded" antenna? Duct tape it to your lifelines or bimini and run a shorter piece of coax to the AIS and VHF radios. Compare its performance with each masthead antenna.

I never considered two masthead antennas because they are too close together unless separated vertically. Not only will they disrupt the radiation patterns, their proximity will overload and may damage the front end of the listening radio when you transmit on the other.

Pro AIS should give you the SWR on the AIS antenna. If you put your SWR meter into either antenna wire it should give you SWR, but make sure that the meter is rated for 150 mhz frequencies.

I don't have the corroded antenna, but I do have another I can try at deck level. It's difficult for me to test range effectively as there aren't a lot of boats in my area and I don't want to fill the airwaves with dozens of radio check requests. This also makes evaluating the AIS performance difficult.



I went with two antennas for redundancy. Maybe that was a bad idea but they have performed very well for years. I could install a splitter and just use one, I guess.



I tried the SWR meter. It reads the expected power, 1W and 25W, but it reads the same with the antenna attached or not.

[S/V Illusion]

Originally Posted by jt11791

I went with two antennas for redundancy. Maybe that was a bad idea but they have performed very well for years. I could install a splitter and just use one, I guess.

Splitters come with their own set of problems, worst among them they can and do fail and do so without apparent notice. Some also have a high insertion loss. The interaction between two antennas in the same plane can serve to detune both but typically that effect isn’t significant.

I tried the SWR meter. It reads the expected power, 1W and 25W, but it reads the same with the antenna attached or not.

Isn’t possible except under dubious circumstances - the SWR meter is dysfunctional, connected backward or the coax and/or connectors are bad.

[Jim Cate]

I would guess that one or more of the coax connectors that the yard put on the new cables was poorly installed. This is a job t hat is often botched, even by folks who think they know how to do it. The ones at the mast head are hard to inspect, too!

Jim

[SailFastTri]

Cut off the coax connectors at both ends and install new. You’ll save time vs troubleshooting. Your problem is in one of them, in both cables.
Having two antennas next to each other destroys the effectiveness of both radiating patterns. I’d move the AIS antenna to a rail. The VHF is more critical, and the lower antenna will still be effective for AIS at 10-15 miles or more, depending on ship’s antenna heights.

[Jammer]

Hello JT


Is the coax one continuous run? Or is there a disconnect of some kind at the base of the mast?


What kind of coax? RG8, RG8X, RG213, RG58, etc. It should say on the jacket.


Can you post photos of the connectors? Some are inherently better than others.

Originally Posted by jt11791

I tried the SWR meter. It reads the expected power, 1W and 25W, but it reads the same with the antenna attached or not.

Read the instructions for the meter closely. You may be reading power into a dummy load built into the meter. What you really want is the SWR ratio, which should be close to 1:1. There may be several steps required to determine this, depending on the meter.


The connectors are a likely source of trouble but there are other possibilities too, damage to the coax if it has been crushed by something or bent too far for example. Replacing any readily accessible connectors could be useful if you have the materials, tools, and skills to do so in a reliable manner, if there is any doubt about the ones the marina installed.



If you have a handheld VHF you can use that to evaluate range. Use the low power setting on your boat radio and see how far away you can hear it. Some handheld VHFs have DSC capability that can trigger an automated transmission from the boat which can help with determining range.

[smac999]

Originally Posted by jt11791

I tried the SWR meter. It reads the expected power, 1W and 25W, but it reads the same with the antenna attached or not.

You never transmit a radio with no antenna.

You need to read the manual for the meter and take an actual swr measurement. Power out is only 1 step.

Or just plug the ais into your laptop and it’ll tell you…

[jt11791]

Thanks for all the ideas, I'll make more tests today and report back.

[jt11791]

OK, some new info.

The new antenna cables are RG8X and the connectors at the base of the mast are Shakespeare no-solder crimp ons. I assume those at the top of the mast are the same.

I put my multi-meter on the connectors and found both cables shorted, center pin to shield. I cut one connector off and tested the cable directly, center conductor to shield, and still shorted. So the short is in the connector at the masthead, or in the cable in between (not so likely). FML.

I had better luck with the second cable. I cut the connector off and now the cable tests open. I installed a new connector and viola! A working antenna!

I also read the fine manual for my SWR meter. The shorted cables gave a reading >3, my emergency antenna 1.2, and the repaired cable/antenna gave 1.0.
So progress, but I'm still steaming that I paid $$$ for all this f*ckery. It's such a mistake to let any "professionals" touch your boat. I want to go back to the yard and make this guy go up the mast and fix his crappy work, but I'm now 40nm away and it will be a huge waste of my time to go back, and what confidence can I have that his second try will be any better?

[Jammer]

Originally Posted by jt11791

The new antenna cables are RG8X and the connecftors at the base of the mast are Shakespeare no-solder crimp ons. I assume those at the top of the mast are the same.

Most Shakespeare no-solder connectors are screw-on compression connectors that are not particularly reliable. True crimp-on connectors, installed with proper tooling, are best practice.


RG8X is a foam dielectric cable. While its theoretical electrical performance is very good and it is lightweight, it is far more susceptible to mechanical damage from tight bends and crushing than RG58 or RG213/RG8. High temperatures make it more fragile. I use it for long runs in my house (which doesn't move much unlike a boat) and for temporary installations where low cost and light weight matter most.

I put my multi-meter on the connectors and found both cables shorted, center pin to shield. I cut one connector off and tested the cable directly, center conductor to shield, and still shorted. So the short is in the connector at the masthead, or in the cable in between (not so likely). FML.

I had better luck with the second cable. I cut the connector off and now the cable tests open. I installed a new connector and viola! A working antenna!

Well, if you can find feedline problems with an ohmmeter, they're bad problems.

I also read the fine manual for my SWR meter. The shorted cables gave a reading >3, my emergency antenna 1.2, and the repaired cable/antenna gave 1.0.

Good for you. Those are plausible results, and they likely indicate how well your antennas will actually perform.

So progress, but I'm still steaming that I paid $$$ for all this f*ckery. It's such a mistake to let any "professionals" touch your boat. I want to go back to the yard and make this guy go up the mast and fix his crappy work, but I'm now 40nm away and it will be a huge waste of my time to go back, and what confidence can I have that his second try will be any better?

The key thing that gets lost in all these VHF antenna discussions is feedline reliability and durability. Someone who is a complete hack can install a feedline made of doubtful material, inadequately secured, and poorly terminated, and 95% of the time it will work fine for the first month, often longer. But a good installation with proper tooling, correct materials, and attention to detail should work reliably for 20 years.

[smac999]

Lots of antennas show 0 ohms. Unless you disconnect the cable up top you can’t trust the bottom reading

The Shakespeare antennas Actully come with a tag saying if the antenna shows shorted or not. Probably because they had so many calls / returns of a “shorted ant”.

The Sha crimp ones are crap thiugh. .

[Orion Jim]

Originally Posted by jt11791

OK, some new info.

The new antenna cables are RG8X and the connectors at the base of the mast are Shakespeare no-solder crimp ons. I assume those at the top of the mast are the same.

I used the same Shakespeare connectors. I had issues, intermittent of course, with my AIS and VHF which are on the masthead and incorporate a splitter. I finally paid a pro to install proper soldered connectors and it cured all my issues.