VSWR Readings - Poll

[Playtime]

I have just purchased a VSWR meter (£29) and taken some readings for the 2 antennae on Playtime.

The masthead antenna gives a reading of 3.5 which is not brilliant. However, the antenna and cable are 11 years old so maybe it's to be expected. I plan to drop the mast (for the first time) next winter and will probably replace it then.

The pushpit anntenna is brand new, only just fitted (some idiot wiped out the previous one without owning up) and gives a reading of 1.2, which sounds a bit too good to be true. It's a VTronix with about 6m of cable. It also gives a reading of 10K ohm across the cable (as per the spec) so it looks as though it is connected correctly.

Having got these results I thought it would be interesting to see what readings others have found and compare notes.

I don't expect a large response to this poll as VSWR meters are not commonplace but just a few results should be interesting.

 

[john_morris_uk]

The high VSWR for the masthead antenna could be from no antenna at all on the top of the mast (or no connection to it) and a lossy and old bit of co-ax.

The 1.2:1 sounds perfectly reasonable for the new antenna on the stern. New co-ax - new antenna - might easily be that sort of figure.

I would be looking at the masthead antenna before next winter!

 

[Playtime]

I would be looking at the masthead antenna before next winter!

Umm. I was hoping to avoid this.

I've just looked up that a VSWR of 3.5 indicates that approx 30% of the transmitter power is lost, which is not good.

However, for the optimist, 70% is still radiated.

Is there a simple formula for calculating range based on radiated power (as opposed to height of antenna)?

 

[ProDave]

I voted, but should add my <1.5 VSWR is after I re made all the coax connections.

Before I did that, my VSWR was close to infinity, due to one connector not soldered at all, one having a short due to the cable being pulled partly out, and one having a short due to simply being put together wrong (through deck N type socket)

If checking out your cabling, the other very useful thing is a 50 ohm dummy load. So if you have a length of cable say going to a through deck fitting like mine (where the cable from the mast plugs in) you can plug the dummy load into that socket and you should read an almost perfect VSWR. If not, then that length of cable or it's connectors is faulty. Doing that on my boat initially gave an almost infinite VSWR with the dummy load until I re made the connections.

 

[SHUG]

I have no idea.
Oh God!... something else to worry about.
Is this a measurement which should be made annually.

 

[Playtime]

If checking out your cabling, the other very useful thing is a 50 ohm dummy load. So if you have a length of cable say going to a through deck fitting like mine (where the cable from the mast plugs in) you can plug the dummy load into that socket and you should read an almost perfect VSWR. If not, then that length of cable or it's connectors is faulty. Doing that on my boat initially gave an almost infinite VSWR with the dummy load until I re made the connections.

Good idea re the dummy load. I think there is a connection under the headlining after the cable passes through the deck. Clearly this is a good place to start.

The poll results are starting to indicate that my 3-4 reading is out on a limb so I guess I better start investigating!

 

[Playtime]

Is this a measurement which should be made annually.

Could be.

I thought my VHF was working well but from the poll it looks like I am out of line with everyone else.

What I don't know is how badly the radio range is affected by a poor VSWR. I am still awaiting an expert response on that one!

 

[john_morris_uk]

Umm. I was hoping to avoid this.

I've just looked up that a VSWR of 3.5 indicates that approx 30% of the transmitter power is lost, which is not good.

However, for the optimist, 70% is still radiated.

Is there a simple formula for calculating range based on radiated power (as opposed to height of antenna)?

No there isn't - well not in simple terms. I used to do microwave analysis of signal paths and you can calculate all the losses and come up with expected signal levels - but this was from satellites - or radar returns in line of sight. VHF is 'sort of' line of sight - and actually the 30% loss is diddly squat compared with the path losses you incur, but the calculations will be based on some assumptions and its not worth doing.

You ought to realise that its just not doing the PA stages of your VHF any good either. Actually, I've just remembered that lots of VHF sets have the PA stages 'protected' and the set might be additionally reducing the power output to cope with the poor match into the antenna.

 

[Beadle]

It certainly isn't good.

The key question is whether it actually works or not.

If it works reasonably in that you can talk to people on it and they can hear you then the probability is that your co-ax has gone a bit rotten ie wet.

If people can't hear your signals or even have difficulty in hearing you - lots of requests for repeats, the as John says you may well have no aerial on the end.

Case1 you could leave until you drop the mast, case 2 best you look at doing something sort of now-ish.

 

[Blue5]

So if I want to participate in this poll which is the best buy VSWR meter? or is this thread a prelude to some future PBO test.

 

[Playtime]

So if I want to participate in this poll which is the best buy VSWR meter? or is this thread a prelude to some future PBO test.

Not necessarily - you could borrow one or get someone with one to give your rig a test. If you berth in Gosport (or Northney before 1 April) I would be happy to have a go.

I bought the AV-201 from eBay - £29 plus postage. It's easy to use and seems to be giving useful readings.

... and no - this is nothing to do with PBO, although it might make an interesting article or technical Q&A response.

 

[Boathook]

I borrowed a friends meter after I found I could receive but not transmit at a 20 mile range. Found that part of the cable had corroded in a 'waterproof' decking fitting. Luckily had enough cable to be able to cut out the poor bit and remake the connection below decks. Still another bit of cable to pull through but it all works to a suitable level. I do want to replace the aerial and wire but the wire is stuck somewhere in the mast conduit so I have to try and drop a new cable down the mast it self. All fun and games but it all takes time .....

 

[Playtime]

There's no doubt about it now - I need a new antenna and cable.

I checked the antenna cable coupling in the saloon headlining this afternoon and dirty brown water started wicking out of it. Note, this is a continuous cable from the below deck coupling to the antenna on top of the mast. Cutting back the cable 6" revealed that the braid is also badly corroded, probably all the way to the antenna as there are no visible cracks or nicks in the outer sheath that I could see.

Clearly the VSWR has been a useful indicator of the problem!

Thanks to everyone who has responded on this one - it has helped me to reach the right (and safe) decision quicker than I might otherwise have done.

 

[Bosun Higgs]

Is there a simple formula for calculating range based on radiated power (as opposed to height of antenna)?

No. You can send a signal across the atlantic with milliwatts, and ham operators regularly do so. For that matter the power output of a gps satellite is about the same as your fixed radio - 26 watts I believe - and you receive the signal here on earth some 6000km away.

So I dont think you need to worry about the 70% though it would be better if it were 100%. But you do need to worry about the dirty brown water because that will lead to failure altogether

 

[Playtime]

But you do need to worry about the dirty brown water because that will lead to failure altogether

Agreed - hence the decision for a complete replacement.

Now, where's that Bosun's Chair.

 

[Strathglass]

I have built a VSWR meter into the boat beside the chart table. I can connect it either to the VHF or HF rig.

I find it quite useful - mainly to check if the auto ATU on the HF ham rig is doing what it should.

I will soon be changing the VHF whip to a stub as I find the signal varies too much in high wind and seas.