VHF antenna problems?

[Quandary]

This year we have noticed that reception on our VHF seemed short range, we very rarely transmit but monitor the inshore weather forecast avidly. When we were sheltering on the pontoon in Gairloch, the skipper of the yacht beside us, a ham enthusiast, kindly brought his meter aboard and tested it for us.
He was able to ascertain that the problem was at the top of the mast: all the connections inside the boat were tested and one was resoldered. The antenna is the V'tronix one with the windex on it. It is only three seasons old and the cable is continuous from the connection in the duct in the table leg at the foot of the mast to the antenna with no joins at the top. I had a similar antenna on a previous boat which also seemed to give a weak signal.
Before I attempt the unappealing task of purchasing another antenna and going to the top of the 16m. mast to fit it and pull the old cable through and feed the new cable down from there, can anyone recommend a good quality, reliable and durable masthead vhf antenna; I am prepared to change the brackets and fit a separate windex if this extra trouble is necessary to get a reliable product. Or is V'tronix stuff Ok and have I just been unlucky twice? Can it perhaps be repaired in situ. or is the skip the best place for it.

 

[lw395]

The V tronix should work well, if the connection to the cable is good.
Water in the cable is a very common failure mode, corrosion soon takes its toll.

You may find a new cable is all that's needed.
Maybe you can bring the aerial down and test it with a short lead?

 

[Quandary]

There are no connections at the top. The original V'tronix factory fitted cable comes out from the base of the antenna to the inspected and tested connections inside the boat?
I would not fancy trying to shorten the cable and resolder anything up there, unless the internal connections are mechanical?

 

[lw395]

There are no connections at the top. The original V'tronix factory fitted cable comes out from the base of the antenna to the inspected and tested connections inside the boat?
I would not fancy trying to shorten the cable and resolder anything up there, unless the internal connections are mechanical?

I thought my aerial was a V-tronix, it had a cable fitted by means of a clamp nut and rubber seal, a bit like the cable side of a clamp-up BNC connector. The connection to the inner was a brass spike captive in the antenna.

May have been a similar make, or they may have changed the design.

Another possibility is that water has got in part way down the cable due to chafe or something. Or the inner could fatigue and break.

 

[pvb]

I thought my aerial was a V-tronix, it had a cable fitted by means of a clamp nut and rubber seal, a bit like the cable side of a clamp-up BNC connector. The connection to the inner was a brass spike captive in the antenna.

May have been a similar make, or they may have changed the design.

My V-Tronix aerial is exactly the same. The cable is terminated in a screw-in fitting which has an O-ring seal, and just screws into the base of the aerial.

 

[Quandary]

Sounds like its for the skip then as I don't want to wait until the mast is down.
Does anyone know of a replacement antenna which is more reliable than the V'tronix?

 

[mireland]

see here

http://www.saltyjohn.co.uk/boatproducts.htm

 

[earlybird]

see here

http://www.saltyjohn.co.uk/boatproducts.htm

I thought about a Metz before deciding on a Vtronix last winter
I was a bit doubtful about the mast-head connecting plug which seemed to rely for weather-proofing on being wrapped in tape. I decided the Vtronix 0-ring system might be better. I did anoint them well with silicone grease.
Oh well, time will tell !!
The old antenna had a non-detachable cable and had lasted for years, perhaps that's the way to go.

 

[tt65]

VHF aerial stuff.

Mine is of the Dog's Dick type (Helical), similar to a hand portable VHF aerial. Less easily damaged but you will need a different windex system. The range for both reception and transmission is the same as a full length aerial (15m mast). The VSWR was recently measured and is the same as for the more common dipole. (VSWR + Voltage standing wave ratio = the only true measure the the effectiveness of aerial and cabling) Is now 27 years old. The join between aerial and cable is moulded over so cannot get into it anyway.
I believe that V Tronix has been taken over by Shakespeare antenna's which I think is American.
To measure VSWR correctly testing kit should be set up for measuring 156 mHz.

 

[Heckler]

The Vtronix fitting to the antenna is actually a brass bullet, screwed on to the cable with a plastic nut to join it up. My mate Juggler had probs with his, turns out that water had migrated in to the joint and was shorting it out. So he can take it appart at the top to check.
Stu

 

[Quandary]

Mine is of the Dog's Dick type (Helical), similar to a hand portable VHF aerial. Less easily damaged but you will need a different windex system. The range for both reception and transmission is the same as a full length aerial (15m mast). The VSWR was recently measured and is the same as for the more common dipole. (VSWR + Voltage standing wave ratio = the only true measure the the effectiveness of aerial and cabling) Is now 27 years old. The join between aerial and cable is moulded over so cannot get into it anyway.
I believe that V Tronix has been taken over by Shakespeare antenna's which I think is American.
To measure VSWR correctly testing kit should be set up for measuring 156 mHz.

Tim
Can you give me a brief idiots guide to testing?
Thanks

 

[john_morris_uk]

The VSWR was recently measured and is the same as for the more common dipole. (VSWR + Voltage standing wave ratio = the only true measure the the effectiveness of aerial and cabling)

I am sorry to contradict your assertion, but the VSWR measured at the tranceiver end only tells you a little about the impedance of the transmission line "as seen" and where in the line it is measured.

There are several reasons why you you might have a good VSWR and a completely useless antenna. Even if you have an excellent transmission line, matching a working antenna, the good VSWR doesn't tell you anything about how "effective" the antenna is.

You need some fairly sophisticated instruments to measure the effectiveness of the antenna.

 

[Beadle]

Here we go again

I'm keeping out of this one.

I've often thought expertise isn't a question of how much you know, but rather a question of understanding how much you don't know.

I suspect you may well be about to see a few examples to support my theory.

 

[john_morris_uk]

Here we go again

I'm keeping out of this one.

I've often thought expertise isn't a question of how much you know, but rather a question of understanding how much you don't know.

I suspect you may well be about to see a few examples to support my theory.

I agree - I take a deep breath every time I see posts on this subject - but I have strong feelings about people making claims that just aren't true in a subject area that I used to work in. A little knowledge is a dangerous thing -especially when it comes to commonly available VSWR meters.

 

[lw395]

I agree - I take a deep breath every time I see posts on this subject - but I have strong feelings about people making claims that just aren't true in a subject area that I used to work in. A little knowledge is a dangerous thing -especially when it comes to commonly available VSWR meters.

Likewise, except I'm still in the industry.

Would we agree that VSWR is a good test for antenna match, provided that the cable is known to be low loss?

This proviso is of course void once the cable has been on a yacht for a week or more.

For the non RF people, VSWR meters indicate how much power is being reflected back from the antenna. If everything is low loss, what isn't being reflected back must be being radiated by the antenna. Both open circuits and short circuits reflect back the power, so it is a good test for these faults.

However if the cable is corroded, or wet, it becomes lossy and not only is the power absorbed on the way to the antenna, but also the reflection is absorbed on the way back, so a 'good' VSWR can be indicated despite very poor radiation.

That's the simplest useful explanation I can offer. People still write books full of hard sums about the subtleties. Also I don't use 'amateur' type VSWR meters, but I understand there are pitfalls with them, in particular many are narrow band and not intended for 'our' VHF band.

 

[Pasarell]

V-Tronix

I bought a V-Tronix aerial the same as yours last year to go with my new Icom VHF. The cable and antenna come separately and need to be connected at the masthead by screwing up a compression fitting that pushes the cable core against a spike in the antenna base.
Installed it but got virtually no reception at all. I called V-Tronix in the UK to ask if I had done anything wrong to be told there was nobody in the UK who could give me any advice. I then sent an e-mail to Shakespeare in the USA expressing my surprise at lack of customer service. Got a phone call back by return explaining how to make the connection effectively and saying they would send me a complete new aerial in case of problems. Following morning a new aerial arrived complete with a VSWR meter to check the system. Following day the UK technical guy called me to offer any help!
I think the original aerial may have been faulty - but it may have been the way I connected antenna and cable at the masthead. Whichever it was I couldn't fault the final response.
No connections with company etc

 

[john_morris_uk]

Likewise, except I'm still in the industry.

Would we agree that VSWR is a good test for antenna match, provided that the cable is known to be low loss?

This proviso is of course void once the cable has been on a yacht for a week or more.

For the non RF people, VSWR meters indicate how much power is being reflected back from the antenna. If everything is low loss, what isn't being reflected back must be being radiated by the antenna. Both open circuits and short circuits reflect back the power, so it is a good test for these faults.

However if the cable is corroded, or wet, it becomes lossy and not only is the power absorbed on the way to the antenna, but also the reflection is absorbed on the way back, so a 'good' VSWR can be indicated despite very poor radiation.

That's the simplest useful explanation I can offer. People still write books full of hard sums about the subtleties. Also I don't use 'amateur' type VSWR meters, but I understand there are pitfalls with them, in particular many are narrow band and not intended for 'our' VHF band.

No problem - and we will agree to differ on the concept of 'reflected power' because it makes no difference to end result. Besides which the maths of transmission lines are too convoluted and complex to go into here...

Given a low loss line, then the VSWR at the transmission end is usually a good indication of the power transfer (and hopefully emission) at t'other end.

(I recall that one of the ways to get some aircraft antennas to exhibit 50 ohms impedance over a large bandwidth is to add a suitable non inductive resistor at one end of the antenna! It did nothing for the efficiency of the antenna, but gave it a very wide bandwidth... The inefficiencies didn't matter as there was so much spare gain in the system.)

 

[lw395]

No problem - and we will agree to differ on the concept of 'reflected power' because it makes no difference to end result. Besides which the maths of transmission lines are too convoluted and complex to go into here...

Given a low loss line, then the VSWR at the transmission end is usually a good indication of the power transfer (and hopefully emission) at t'other end.

Not sure that we differ on understanding of reflected power?

The assumption of a low loss line is the key, even 'perfect' antenna cable can have very significant loss on a yacht mast, IIRC the IOR/RORC/ORC regs say it should be no more than 3dB/50%. That's enough to make VSWR meters a fairly blunt tool.
Compared to the network analyser on my bench anyway!

Of course the transmitter will not be a brilliant match either, so you have to consider whether it is better to match to the ideal 50ohms or to the transmitter. The catch being that your radio may (will) have a different match in receive, so do you want best Tx, Rx or a compromise?
In practice, VHF does not have to be near perfect to work extremely well, a perfect setup will just extend the range a little more over the horizon. Mostly we are looking for cables that are nearly completely shorted or broken, bearing in mind that a VHF can still operate over short range with 99% loss in the cable!
There is a lot to be said for testing over the air (unfortunately).

 

[john_morris_uk]

We are in agreement - its the idea of power being reflected that is in common currency, but doesn't accurately describe for what is really going on - but what I was trying to say is that the descriptive term becomes irrelevant as the end result is what counts.

I also get frustrated by people trying to chase the VSWR meter reading down much below 2:1 The difference between 1.8:1 and 1.1:1 is not going to make the slightest bit of difference in the grand scheme of things.

Its nice if the resonance of the antenna is neatly in the middle of the marine bit of spectrum though, but as 156 Mhz doesn't refract very far beyond the horizon under normal propagation conditions it isn't that critical anyway.

I do also agree that over 90% of problems in small craft installations are almost certainly due to poor co-ax that has corroded and/or got water in it, or is just plain broken or shorted out.

 

[lenseman]

. . . . The range for both reception and transmission is the same as a full length aerial (15m mast). The VSWR was recently measured and is the same as for the more common dipole. (VSWR + Voltage standing wave ratio = the only true measure the the effectiveness of aerial and cabling) . . . . . To measure VSWR correctly testing kit should be set up for measuring 156 mHz.

Oooooh, fun.

David pulls up a deckchair and is watching the battle unfold!

xxxxxxx