VHF issue investigation without an swr meter

[laika]

Hmm...in defence of those people who *do* do radio checks with someone other than the marina they're starting from (and yes yes, NCI...)...

My VHF seems to be performing very poorly since the last time I actually used it to call anyone other than Brighton marina from Brighton marina (over 18 months ago). The squelch is up quite high. Marinas can hear me when I'm close but a couple of miles away I'm apparently distorted. Also I can only hear occasional coastguard messages and those are often distorted. A problem with my antenna, which has obviously suffered some stress over winter (the close hauled "wings of my hawk vanished over winter)? Fortunately I have a spare mast-head antenna and cable. Switched over. No better. splitter in the AIS? Fed the cable directly into the VHF. No better. The cable which connects the connector at the bottom of the mast to the VHF? I've checked resistance between the conductors (infinite). I got my multimeter out and using a long piece of wire checked the resistance between both ends of each conductor: In both cases rather less twice the resistance of the piece of wire I was using to provide my multimeter with a circuit. I'm not sure what more I can do without buying a long piece of RG85 a couple of connectors and a couple to replace it all, but seeing as coax isn't cheap I'm wondering if it's worth it given the resistance seems low. On the other hand I have minimal RF knowledge.

Ideas?

I'm thinking of heading into weymouth to see if I can find an emergency VHF antenna but ideas on further debugging I could do appreciated (and also ideas on the soundness of my idea that low impedance in wire means there's not a corroded connection issue).

 

[Skylark]

Wind indicator reference arms being damaged over the winter period is not uncommon and is usually attributed to birds. I’ve lost two arms in the last 6 seasons ?

Fault finding your radio is probably easiest using substitutions. Fitting an emergency antenna, bought or fairly easily made, begged, stolen or borrowed, should help. Connecting a portable radio to your main antenna will also help diagnosis.

I don’t think that a multimeter is much use in these circumstances. SWR meters are really good and inexpensive. I prefer twin needle, analogue. That said they have been made less useful (can’t yet say redundant) by the incredibly cheap availability of VNAs (vector network analysers). These are available for a few pounds if you have the eyesight of a child or a few pounds more for one with a larger screen. Happy to find a link if needed. These will show a trace of SWR across the entire frequency.

Form other, similar threads, I believe that there are plenty of posters that own such equipment and would, probably, be happy to help. Another option is to find your nearest amateur radio club and ask for help. Where are you based?

 

[kwb78]

There are a couple of possibilities that can cause difficulties with VHF performance. The obvious one is the condition and quality of the antenna and feed - coax which is poor quality or degraded from moisture or corrosion can have a significant effect on both reception and transmission. Moisture ingress into the upper end of the coax can wick along inside the cable which can affect the RF characteristics of the coax as well as cause corrosion damage. That is something which won't necessarily show up by simple resistance check, but would show up with an SWR meter or VNA. Moisture ingress into the antenna itself could also cause problems. If you are currently using RG58, it's probably worth replacing it with something more suitable anyway (there was a recent thread discussing this) as losses over the length required for a masthead antenna on a yacht can be significant.

It's possible that interference can be caused by noise on the DC supply, perhaps from an alternator, inverter or some other device like a poorly filtered switched mode PSU for a laptop/phone etc. Perhaps do some tests to see if the problem persists with all other electronics disabled while running off battery alone. If you could borrow a radio from someone (or use a handheld with suitable antenna adaptor) and try it on your antenna it would eliminate any problems with the radio itself - alternately try your radio on someone else's boat if it's easily removable.

The antenna and coax would be the first place I'd look however, since that's the part which is constantly exposed to the elements and can degrade significantly over time. AIS performance can give a indication of how well your antenna system is working - are you receiving ships right out to at least the radio horizon when at sea? (should be 30ish miles depending on mast height) If it's less than that from a masthead antenna something isn't right. How reliably is your boat received by other vessels at range? Have a look on something like marinetraffic to see if you are received consistently and out to the expected range - again you should be visible 20-30 miles away from a good receiver even with a class B transponder. You can switch on a layer which shows the locations of receivers being used, and if you click to bring up its details, you should see 'source', which if you click on it takes you to the page giving details about that particular receiver.

 

[Martin_J]

The other thing that many SWR meters can do is to measure the actual power output from the radio. That's why a lot are known as SWR/Power meters.

The things like the NanoVNA are great but won't (unless I'm mistaken) measure actual power.

For all we know, the output circuit in the VHF could be on its way out, hence the need for a power meter.

Regarding asking others to see if they can hear you. With the incredibly good discrimination circuitry in modern radios, you could be just transmitting a few hundred milliwatts and they'll say they can hear your clearly. You really need the ear of an experienced radio amateur to possibly tell the difference without recourse to the right test equipment.

 

[Martin_J]

If you do look to buy an SWR/Power meter, the first thing to check it that it covers around the 156MHz frequency that marine vhf uses.

I use something like the SX400.

www.nevadaradio.co.uk diamond-sx-400

 

[st599]

Having 2 antennas at the masthead isn't a good idea. The spare will be within the near-field of the operating antenna and significantly affect the radiation pattern.

 

[Slowboat35]

How can the average yottiescientifically measure the performance of an antenna system without a SWR meter?
Is it even possible - leave alone practical?

 

[laika]

It's not that I'm reluctant to buy stuff, it's just that I'm trying to make running repairs while cruising so don't currently have the luxury of being easily able to receive stuff. Moreover with marinas seemingly unbookable and variable weather it's hard to say where we'll be in 3 days' time.

AIS reception is terrible: 4nm max it seems so probably not the radio. I managed to buy the last emergency antenna in the portland and weymouth area (everywhere is out of pl239s so my dipole construction skills were not called upon). hoist to about 5m I couldn't get a response to a radio check request from st. alban's head NCI (we're in portland) but
- transmission and reception to/from the handheld about 500m away were significantly clearer than with the masthead
- direct connection was not significantly better than through the wiring from the mast base, so the latter is ruled out.

The radio does suffer from interference from the fridge but everything was off during my test. Lack of response from a station in line of sight 15nm away using the emergency antenna was not encouraging but I'm leaning towards it being an issue with the antennae.

Having 2 antennas at the masthead isn't a good idea. The spare will be within the near-field of the operating antenna and significantly affect the radiation pattern.

The separation is about 1m. I was aware that that was too close for 2 active antennae but is it an issue if one is not connected? Performance used to be acceptable until recently and I kind of liked the idea of having a spare. Not that it's helped me on this occasion....

 

[wooslehunter]

Having a spare antenna is always a good idea. If your only antenna is at the top of the mast, & you lose the mast, the antenna probably be under water or no longer connected.

The alternative is a secondary pushpit mounted antenna or a handheld radio. If you have the former, simply swap the cables over at the radio & you're back in comms with full DSC (assuming you had that in the first place!!!!!).

Second advantage is that you can use the pushpit antenna for fault finding. It won't be as good as the mast head as it's lower down, But, it should be able to identify a mast head antenna fault.

 

[kwb78]

AIS reception is terrible: 4nm max it seems so probably not the radio.

Almost certainly that's an antenna problem of some sort. Digital modes like AIS are generally much more resilient to noise and interference than analogue or voice, and a class A transponder is easily powerful enough to be picked up at the radio horizon or significantly further in the right atmospheric conditions. That you are only seeing 4nm or so indicates there is significant signal degradation.

You might be able to find an SWR reading from your AIS transponder via the configuration software if you are able to connect it to a laptop via USB. Some models have this, but others don't - Digital Yacht ones have it, as do most Raymarine for example. Some will just report an error rather than give you a specific number, but it might worth a try.

 

[laika]

Good thinking. Unfortunately the raymarine version of proais doesn't seem to be too diagnosticy. It's telling me transceiver status is "OK" giving rssi values of 60-ish (but is this just the strength of all the vessels in the marina around me?) , forward power 151, reverse 25.

perhaps there's more I can get from just looking at the raw nmea...

 

[laika]

perhaps there's more I can get from just looking at the raw nmea...

From my reading of ALR sentences, none of the alarm thresholds (including VSWR and noise thresholds) are being exceeded. I'm still guessing I should be seeing traffic further out than 4nm from portland marina

 

[Martin_J]

You didn't say which, if any active antenna splitter you were using.. I assumed you are using one because you mentioned only using one masthead antenna.

I'm not on the boat at the moment but I found a previous thread where I mentioned my values from ProAis

forums.ybw.com ais-splitter.393015


It looks like my Transponder was showing about 1/12th power reflected whilst yours is about 1/6th reflected. Although they are just numbers some improvement could be made.

Out of interest. What style emergency antenna is it? One of the short rubber duck style? I don't wish to berate them all but they've usual got rather a thin lossy cable on them and the short stubby antenna isn't the best either.

My emergency antenna is a pole mount approx 1m whip and I've got a couple spare as well, all using well terminated aircell 7 cable.

Bear in mind... other yachts might only be transmitting from deck level and have badly made up connections. Maybe there aren't many ships near enough. Shipping lanes might be to far away from you and hidden by the Bill.

 

[Martin_J]

My problem is that having achieved a reasonable antenna (15m above sea level), AIS targets start to overload the plotter.

Screenshot below whilst tied up in Southsea Marina, top left.

The cursor/pointer mid shipping lane is at 61 miles (distance calculated at bottom left) and I'm sure there is one ship showing from a few miles further away than the cursor..



Having an SWR/power meter on board and knowing how to use it is so useful.

 

[kwb78]

It's telling me transceiver status is "OK" giving rssi values of 60-ish (but is this just the strength of all the vessels in the marina around me?) , forward power 151, reverse 25.

The RSSI figure is just a measurement of the overall signal strength being received on that channel, not necessarily of the ais targets being received. It’s mainly useful to see that the receiver isn’t being overwhelmed with interference so isn’t really significant for this.

The reverse power is about 16% of forward, which implies an SWR of a bit under 2.5:1. That’s somewhat higher than it should be (too high for my liking) but SWR is much more important for transmission than reception so given your poor AIS range there is also likely significant attenuation.

That attenuation will be masking the true SWR, since the transmitted power is attenuated as it travels down the coax to the antenna, then the reflected power is similarly attenuated on the way back. The measured reflected power at the receiver is going to be only a percentage of the true amount, so it would appear to be lower than it actually is. That makes the true SWR significantly higher than 2.5:1 which means the antenna wont be working well for transmission or reception.

Unfortunately unless you find something obviously wrong that you can repair like a corroded or broken connection, you probably won’t be able to fix it without replacing the coax, antenna or maybe both.

 

[grumpy_o_g]

If you can easily remove the radio itself is it an option to try it on another boat that you know works OK? Or, if such a thing exists, you could try a handheld with an external antenna option on your antenna cable perhaps?

 

[st599]

The separation is about 1m. I was aware that that was too close for 2 active antennae but is it an issue if one is not connected? Performance used to be acceptable until recently and I kind of liked the idea of having a spare. Not that it's helped me on this occasion....

Yes, the presence of metal changes the beam pattern. (That's how a TV antenna works - only one of the bars is connected, the rest are just concentrating the signal in one direction)