Testing VHF antenna connection?

[prv]

Fitted all new antenna and cables this winter, using good quality kit mostly from Salty John but the main downlead from a ham radio supplier (and yes, it is the right stuff). I can hear some other traffic, and a short-distance call I made the other weekend was received ok, but I'm a bit concerned that my AIS-equipped VHF is showing very few targets in a busy port like Southampton, and I don't hear the replies when VTS talks to ships that, given the calls being made, probably should be in range. In summary, I'm doubting the efficiency of the aerial system and would like to test it.

I'm aware there's a thing called an SWR meter. Is there a kind specific to marine VHF, or do I just get a ham one that covers 156ish MHz? Do they take any skill and judgement to use, or do you just plug it in and read the value? What sort of value should I expect? Are there any other checks I can do? How about testing the resistance between core and shield - I'm aware this depends on the antenna, anyone know the expected value for a Metz Manta?

Any suggestions of good stations to call for a radio check? I don't want to call the Coastguard, because their excellent aerials mean it's not a good test of the efficiency of mine. I really want to talk to someone at the edge of reasonable range from the Itchen - who wouldn't mind being called up for the purpose.

My installation uses quite thick cable for the long run down the mast (can't remember the designation, but it's about 10mm outside diameter) and then changes (via plugs and a barrel connector) to thinner cable for the short run across the deckhead and into the radio (the thick stuff would be too hard to work with). There isn't some RF-magic reason why this is a really bad idea, is there?

Any other suggestions welcome.

Cheers,

Pete

 

[Deleted member 36384]

I copied this thread (VHF aerial, SWR meter and lack of range conundrum) as I planed to replace my antenna and thought it may come in handy: -

http://www.ybw.com/forums/showthread.php?304288-VHF-aerial-SWR-meter-and-lack-of-range-conundrum

Hope it helps.

 

[prv]

Well, it confirms that an SWR meter is a useful thing to have . Also the idea of using an emergency aerial to narrow down the problem. But other than that I'm not sure what to take from it. At least unlike Simon my masthead is probably strong enough for lifting people, so I don't need to hunt down a suitable bridge

Pete

 

[Salty John]

The Metz will show a dead short to an ohmmeter so to check your cable connections you need to disconnect it. You should then see no circuit between braid and centre conductor for the cable run. There's not much else to it. By far the most common problem is badly fitted connectors, the main error being getting a single strand of centre core bent back when threading it through the centre pin of the PL259. Either tin the stranded core together first, or leave a long enough piece so that you can see that it all goes into the pin before the rest of the cable obscures your view.
The mix of large and small diameter cable is OK, although you'll lose a bit more signal than if you had the big cable all the way.
If you get a VSWR meter be sure it's attached to someone who knows what he's doing with it!

 

[bluedragon]

When I installed a V-tronix antenna a few weeks back the instructions suggested checking the resistance in Ohms between the cable centre and sheathing after installation - 10K was the figure given and that's exactly what I got - so I assume there are no major connection issues. I also called a marina office about a mile away on LOW power. They read me loud and clear. Are these reasonable indicators that all is well?

 

[prv]

The mix of large and small diameter cable is OK, although you'll lose a bit more signal than if you had the big cable all the way.

The "small" cable is your very own RG8X, so hopefully you'll agree that about four metres of it across the cabin is unlikely to introduce too much loss!

If you get a VSWR meter be sure it's attached to someone who knows what he's doing with it!

Given that I don't know any such person, I'm going to have to do my best to fulfil that role myself. Is there something important I need to know?

Pete

 

[Salty John]

Cable will be fine. If there is a serious problem, a vswr of more than 3:1 usually, most modern radio/AIS will show an antenna error signal - you should check that your radio has such a facility. A vswr reading of around 1.5:1 is ok. A radio check is really the best indicator that all is well.

 

[lenseman]

. . . . . Given that I don't know any such person, . . . .

I am in Haslar with the kit, Bird Thruline including a 50Ω 50 Watt load with SO239/PL259 connections if that is of any use?



.

 

[DogWatch]

I use one of these >>> http://www.radioworld.co.uk/catalog/av-40_avair_vswr_power_meter_144_470mhz-p-451.html

ex pro radio engineer, this is fine for boat use, easy to understand, good for fault finding as you have. Don't worry about the 12v input on it, it's only for the light.

 

[Alpha22]

Any HAM SWR meter is OK as long as it is designed for the frequency range. Most Hams will talk of 2 meters, which is about 144Mhz. Marine VHF is slightly higer than that, but not enough to matter.

SWR meters come in a variety of styles. The common one needle version has a FWD and REV switch and a Calibrate knob. Set to FWD, transmit and set the cal knob so the meter shows full deflection. Stop transmitting and switch the switch to REV. Transmit again and the meter will indicate the SWR.
1:1 is ideal (unusual)
1.5:1 is typical with good kit, in an open area.
2:1 is acceptable, but wasting transmitter power in the system somewhere.
Anything higher can certainly be improved upon and with an all new system should be investigated. You are wasting most of your 25 watts and will suffer low range problems.

Cross needle versions are OK, but sometimes difficult to get an accurate reading if the Tx power is less than full scale deflection.

 

[VicMallows]

When I installed a V-tronix antenna a few weeks back the instructions suggested checking the resistance in Ohms between the cable centre and sheathing after installation - 10K was the figure given and that's exactly what I got

Yes, that's a very useful check with current V-tronix antennae. Older V-tronix though present a near dead short like the Metz so others reading should not automatically jump to conclusions if they have never tested their antenna's resistance before.

 

[earlybird]

A basic VHF SWR is fairly simple and very cheap to make. I made one years ago to a 2m design passed to me by a Ham friend. It's mainly bits of brass and a few diodes, resistors and capacitors. A cheap micro-ammeter is needed for read-out, (ebay!).
There seem to be several designs on the web these days.
I find mine useful for occasional checks, eg after raising the mast, to ensure that there is no deterioration in transmission, although a good reading is not an absolute guarantee that all is well.

 

[forelle541]

I had exactly the same problem this year when I had connected up a new VHF with AIS. It turned out to be the plug connection on the end of the aerial lead, new plug and a dab of solder and all working great now.

 

[rob2]

A VSWR meter is not difficult to use, just try it first on low power Tx to check for gross errors. Doesn't a lot of VTS traffic in Southampton use a duplex channel? That might explain why you're not hearing both sides of the conversation. If you're going to call the CG on any other matter, ask for a radio check as well. Their response will give you the readability and strength of your signal - as they have good aerials, you don't have to wonder whether the fauklt is at your end or theirs!

Rob.

 

[nigelmercier]

... I don't hear the replies when VTS talks to ships

While you should check your SWR, it will not affect your receive capability if it is wrong.

Could the ships perhaps be using duplex channels?

D'OH! Just seen Rob's reply.

 

[prv]

Doesn't a lot of VTS traffic in Southampton use a duplex channel?

Nope, all on Channel 12, simplex. Plus I think 09 for pilots and a high one (71?) for tugs. I can hear vessels around the docks, just not much further south than Calshot, which I would have expected to.

Pete

 

[Skylark]

It's handy to have a connection at the mast because it will enable a bit more flexible testing with a VSW meter. This is where various patch leads and plug/socket adaptors become worth their weight in gold. N, BNC and SO/PL259 are all fairly common to find. Maplins and e-bay provide good access for these.

Don't worry too much about frequency. Even a Cretins Band (27 MHz) will do the job for you. The SWR gauge with 2 meters is easier to use than a single meter with changeover switch, in my expereince. You'll also need a dummy load; anything in the 50 to 75 ohm region is just perfect. 25 watt power rating.

At the mast junction. Connect the dummy load to the SWR meter "output". Connect your coax plug or socket to the input. Set your VHF to low power. Ket the mic, any frequency will do for this exercise. Set the forward power knob to read full scale. Read reflected power. It should be very close to 1 to 1. This shows that the radio and coax plus plug/socket is correctly wired. Now, remove the dummy load and connect the SWR output to the aerial, Repeat the test. As said already by others, a value around 1.5 to 1 is ok. This will test the whole system. If you find a high SWR, you now know which "half" to investigate.

If you are really struggling to get hold of an SWR meter, dummy load, connectors et al, please send me a pm.

 

[prv]

Just to close this one off - turned out to be user error / poor UI design in relation to the AIS mode of the radio. Turns out I unknowingly had a 1-mile range filter enabled . Oops.

I began to suspect my aerial might be better than I had feared when we were picking up CROSS Jobourg in the entrance to Lymington river, and Brixham Coastguard from just outside Hurst Castle

Pete

 

[VicMallows]

Turns out I unknowingly had a 1-mile range filter enabled .....

...... picking up CROSS Jobourg in the entrance to Lymington river, and Brixham Coastguard from just outside Hurst Castle.

Good man for admitting it . And yes, with the high pressure even Swansea was coming in strongly last w/e. Must make life horrible for the Coastguard watch at such times.

 

[lw395]

For radio checks, I sometimes use a marina at a suitable distance, just after they've finished some traffic and not at a hectic time.
But I only do radio checks if I have reason to suspect all is not well.