VHF antenna questions

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Plevier

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I'm trying to resolve mediocre VHF performance.
This is the antenna https://plus.google.com/photos/1135...ms/6005398180777883409?authkey=CNKs78Ps0prOeQ
Does anyone recognise what make it is please and if so is it decent or poor reputation? I can't identify it to any make I can think of. It's 2008 vintage I believe.
I hope it's not relying just on that boot for weatherproofing :-(

Someone in our club was talking about improving performance by tuning the cable. Is there anything in this? I didn't think it made a difference with long low impedance co-ax.
 
The antenna looks like an CELMAR0-1, fairly decent marine whip antenna. If it hasn't been damaged by a lightning strike (it is not internally grounded) it should be OK.

The installation looks a bit dodgy with the L bracket so far down the mast that the mast could be limiting performance.

Check the cable joints at any connectors, make sure the cable has not corroded. I can't see what type of cable you have but it looks reasonably heavy - RG8X or RG8U/RG213, so OK if it has no corrosion. If it's RG58 it shouldn't be used for runs much over 6m, RG8X is OK to about 25m and then you're in Superyacht territory and the 9.5mm cables, RG8U and up.

Cable length only matters in that you lose power as the cable gets longer so need bigger cable for longer runs - as per above.

I assume you've checked that any recently added LED nav lights aren't interfering.

My first check would be cable joint integrity and then get the antenna clear of the masthead. If that doesn't help you'll need someone with a VSWR meter which will probably confirm you have a problem but won't tell you where it is. You need to define 'mediocre' - VHF is line of sight and has a range limited by your horizon distance and your co-respondents horizon distance. Check on my website and blog for a few articles on the subject.

Hope this helps, good luck.
 
Hi Arno

That could well be it, especially if it's a common one in the Netherlands (that's Monnickendam in the background!).
The shape looks the same, and the bracket. I don't know if there is a name on the band round the middle of the outer sleeve. I've a nasty feeling I'm going to have to go up there and check it all out.
Ah this looks convincing http://vmsrv02.starwebb.se/~truehead/public/img/user/VHF_antenna.jpg
Thanks

Mike
 
Salty John said:
The antenna looks like an CELMAR0-1, fairly decent marine whip antenna. If it hasn't been damaged by a lightning strike (it is not internally grounded) it should be OK.

The installation looks a bit dodgy with the L bracket so far down the mast that the mast could be limiting performance.

Check the cable joints at any connectors, make sure the cable has not corroded. I can't see what type of cable you have but it looks reasonably heavy - RG8X or RG8U/RG213, so OK if it has no corrosion. If it's RG58 it shouldn't be used for runs much over 6m, RG8X is OK to about 25m and then you're in Superyacht territory and the 9.5mm cables, RG8U and up.

Cable length only matters in that you lose power as the cable gets longer so need bigger cable for longer runs - as per above.

I assume you've checked that any recently added LED nav lights aren't interfering.

My first check would be cable joint integrity and then get the antenna clear of the masthead. If that doesn't help you'll need someone with a VSWR meter which will probably confirm you have a problem but won't tell you where it is. You need to define 'mediocre' - VHF is line of sight and has a range limited by your horizon distance and your co-respondents horizon distance. Check on my website and blog for a few articles on the subject.

Hope this helps, good luck.
Click to expand...

Thanks John.
Yes I wondered about the mast affecting it, but it can only be about 100-150mm overlap with the whip, is that significant?
I'm afraid I think it's RG58 cable, it definitely is from the radio to the mast base which is about 3m, then PL259s and barrel connector (dry, inside) and then the mast is 18m. I would certainly like to fit RG8X but don't plan to take the mast off! I don't know if it will pull through (Selden mast, hopefully it's in a duct but it may be clipped.) The grey stuff looks the same dia as the RG58 but I haven't found markings on it, not much exposed.
The poor performance is with the Lopolight off, haven't tried with it on. Must do so!
I can't exactly quantify it but range is much less than with my previous 2 boats that both had VTronix antennas that people seem to sneer at, with RG58 cable. The radio on this is a Raymarine 240E that has just been checked by Raymarine and they say is working to spec.
 
I think the location of the antenna could have an effect. The centre of the radiation pattern will be half way up the whip but there will be some masking of some of the signal from certain directions.

I'd check that joint at the antenna first and then put the antenna out on a stand-off bracket. Glomex do one that will get you 200mm higher and 100mm off the mast face.

The RG58 is costing you 3.6 dB over that distance, RG8X will lose 2.5 dB. A loss of 3dB is a halving of the power. Worth considering when you get round to replacing the cable.
 
Salty John said:
I think the location of the antenna could have an effect. The centre of the radiation pattern will be half way up the whip but there will be some masking of some of the signal from certain directions.

I'd check that joint at the antenna first and then put the antenna out on a stand-off bracket. Glomex do one that will get you 200mm higher and 100mm off the mast face.

The RG58 is costing you 3.6 dB over that distance, RG8X will lose 2.5 dB. A loss of 3dB is a halving of the power. Worth considering when you get round to replacing the cable.
Click to expand...
All cable loses some power, the difference between 3.6 and 2.5 is not much.
The loss of old cable is often much higher as even small amounts of oxidation make a big difference.
I tend to suspect the cable rather than the aerial in most instances.
The bracket is a compromise with masking the navlights, I doubt there will be much difference, but I'd at least raise the antenna so the bottom of the thin bit is level with the nav lights?
 
Done some checking today. The plot thickens.
First, the grey cable that goes up the mast. It's 5.5mm OD, good tinned braid coverage over foil which is bonded to foam dielectric, and a thick multi strand tinned core, definitely heavier than the normal RG58 it joins to. So is this some RG58 variant or something else, anyone recognise it? I can only see a foot or so of it and there are no markings visible.
I then remade the connection on the grey stuff and took an RG58 from there direct to the AIS. Woo hoo! Picked up a vessel 45NM away.
Went back to the normal system i.e. from the grey connector with RG58 to the EasyAIS splitter (nobody tell Nigel!) and thence the EasyAIS receiver and the Raymarine 240 VHF. All looked fine, seeing ships at 30NM+ and hearing things on VHF.
Tried a radio check with Solent CG. Nothing.
Looked at plotter; AIS now showing nothing over 2NM!
Replaced the direct link to AIS; still the same.
Connected AIS to 1 metre of wire on the end of a bit of RG58 and hung from deck hatch; now see ships at 12NM.
Tried multimeter on antenna lead; under 1 ohm.
Removed connector from grey cable in case of internal short; still under 1 ohm reading directly on the wire.
So I seem to have an intermittent short in either the cable or antenna, which occasionally recovers but then breaks down, possibly with stress of transmitting power.
I guess the next move is a trip upstairs to remove the PL259 from the antenna and see what it's like and see if the short is still apparent in either the cable or antenna.
Any better ideas anyone?

Later - oh dear Google tells me that some aerials do have effectively zero DC resistance, that confuses things. I thought they usually showed a few '00s or '000s.
The cable seems to match the spec of H155. That's a new one on me but looks good performance.
 
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I would suggest that the OP buy and fit an emergency VHF antenna on the stern rail. This will provide AIS quite adequately without use of a splitter and will provide useable VHF com until the top antenna is fixed. It will always remian as a backup for VHF com if mast is lost.
When Op gets up the mast take a screw driver so that he can temporarily short the centre to outer while a helper down below measures resistance with a multimeter so checking for cable continuity and for shorts. However problem is most likely to be in connector on cable at the top. good luck olewill
 
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