Mounting antenna on backstay

[Albert Ross]

I'm thinking of mounting a whip antenna for a class B AIS transponder on my back stay (to get it about 3 or 4 metres above sea level).

The backstay is 5 mm diameter.

Any ideas how to attach it securely without damaging the stay?

Thanks for your comments.

 

[prv]

Not convinced that this is really a good place for an aerial but, that aside, I think I would start with two plastic blocks, cut a groove about 3mm diameter across both of them, then screw together with the wire lying in the groove. Attach the aerial to one of the blocks.

I fitted one of those barton shroud-cleats to my backstay the other day to take the ensign halyard, and that's how they work.

Pete

 

[rob2]

It will be quite difficult to make a mounting that holds the antenna in an acceptable orientation (too much leverage and windage for a point fixing). There were or maybe still are some mounts made which slide up the backstay, but they are a platform, extending either side with a bracing line down to the deck to stop them from swinging around. It would be much easier with twin backstays as you could bridge between them to get some stability.

Hope my ramblings don't put you off trying this - I'd be delighted to see a successful outcome!

Rob.

 

[PetiteFleur]

I would have thought that two blocks of wood, with a shallow groove for the backstay, bolted together, with the rear face vertical so that a standard aerial angle bracket so that the aerial is vertical. Alternatively a horizontal plate on top for the aerial. The cable then cable tied to the backstay and an Index deck gland for the cable.

 

[Albert Ross]

Thank you.

I have twin backstays but the gap between them at the height I want the antenna is about 3 metres so to make a bridge across might look a bit gross.

Another other ideas for fixing a whip antenna higher than the pushpit?

 

[duncan99210]

Use a piece of 25mm stainless tube as a mount. Fix bottom end to the transom and the mid point to the pushpit rail, mount the antenna on top, running the cable down the tube. I have one that extends about 2m above the pushpit and carries NAVTEX and gps antennas as well as a 'spare' VHF antenna.

 

[Pinnacle]

My set-up is here http://www.ybw.com/forums/showthread.php?383945-AIS-Antenna-Height&p=4557182&highlight=#post4557182

 

[William_H]

In fitting a VHF antenna here you have a compromise of wanting the antenna vertical to match vertical polarisation of VHF marine com versus the need to get the antenna as far away from the back stay as possible to avert interaction. So angled away from the backstay. a compromise of about 30 degrees aft from vertical away from the backstay perhaps a similar angle forward so 60 degree separation should be OK. All ok provided the backstay does not twist as when adjusting tension. good luck olewill

 

[Rigger Mortice]

Not convinced that this is really a good place for an aerial but, that aside, I think I would start with two plastic blocks, cut a groove about 3mm diameter across both of them, then screw together with the wire lying in the groove. Attach the aerial to one of the blocks.

I fitted one of those barton shroud-cleats to my backstay the other day to take the ensign halyard, and that's how they work.

Pete

That's how I fitted a DAB aerial on my backstay, only I had two such clamps so that the aerial was fixed top and bottom.

 

[KAL]

I wondered about this when I fitted mine but after trying it out in various positions, I fitted the stubby AIS antenna under the sprayhood, adjacent to the companionway. This gives me 10-15 miles of broadcast/reception on AIS, so I didn't bother worrying about fitting it any higher.

Suggest you fix it temporarily somewhere lower and give it a go, just to see?

 

[prv]

That's how I fitted a DAB aerial on my backstay, only I had two such clamps so that the aerial was fixed top and bottom.

So it was then lying along the wire? I'm no RF expert, but that doesn't seem like it would be good for an aerial.

None of this would work on my backstay, because the wire rotates as I alter the tension.

Pete

 

[Albert Ross]

KAL .. that's very interesting.
What make / length of "stubby antenna" are you using?
Does it still work well if you are heeled over?
I'm getting nothing like as good a range as you with a small antenna mounted on the pushpit, hence my questions about how to mount a standard whip antenna higher up.

 

[Salty John]

A few tips for getting the best out of your vhf antenna:

Height above sea level. The combination of your height and the co-respondents antenna height determines range. VHF antennas 'see' slightly beyond the horizon, so your horizon distance plus his horizon distance are important. If you have a very short horizon distance you'll be relying on his horizon distance so you'll only communicate with very high antennas such as on large ships or at coastguard stations.

Your output power is restricted by law, so make sure you don't waste it. If you use undersized cable, corroded cable, badly made connections and a cheaply constructed antenna you'll rapidly lose your signal power. 20m of RG58 cable with perfectly made joints will lose half the signal strength, add in a bad joint and a bit of corrosion and you'll have practically no transmission power. Use RG8X for longer than 6m runs and RG8U/RG213 or better for runs over 25m.

Keep the antenna vertical, you want the signal to point at the horizon, not sea or sky. Use a whip antenna with a fat radiation pattern so you always have some of the signal pointing at the horizon. Using a high gain fishing rod antenna leaning backwards might look sexy but it's useless for communicating. A 1m whip antenna on a 1.5m pole is much better than a 2.5m fishing rod antenna.

Keep the antenna away from other antennae (about 0.5m minimum) and away from vertical bits of metal.

Antenna quality counts - the gauge of the loading coil and how well it's supported effect transmission.

Mounting on the backstay is OK if the antenna is vertical and the wire is big enough and rigid enough to prevent the antenna wafting around. You'll need to be able to remove the antenna bracket to tension the stay.

Hope this helps.

 

[Albert Ross]

Thank you.

John, does a 1 metre whip antenna always result in a fatter radiation donut pattern than a "stubby" antenna?
If a whip antenna and a stubby antenna both have the same gain (3 db) do they therefore have the same radiation pattern or is the stubby one's pattern more like a thin disc and the longer one more like a fat donut?

 

[Modulation]

Fitted stub aerial for mine on the pushpit rail. Gets stuff from 20 miles away so perfectly happy. Height about 1.8m above water level

 

[prv]

On Ariam I have a Metz Manta at the masthead for both AIS and VHF (combined unit) and I pick up ships into next week . However, on Kindred Spirit the AIS used a rubber-duck at deck level (actually sikaflexed to the cabin top just aft of the mast) and I had no complaints as to range.

Pete

 

[Salty John]

Thank you.

John, does a 1 metre whip antenna always result in a fatter radiation donut pattern than a "stubby" antenna?
If a whip antenna and a stubby antenna both have the same gain (3 db) do they therefore have the same radiation pattern or is the stubby one's pattern more like a thin disc and the longer one more like a fat donut?

A stubby antenna, typically 0.3m to 0.5m has no gain (0dBi), a 1m whip has a gain of 3dBi, a 2.5m fishing rod has a gain of 6dBi and a 6m fishing rod has a gain of 9dBi. Depending on which marketing man writes the copy, any of these antennas can have a higher or lower gain - miss out the little i, and you go up 3dB, for instance - hence your 3dB stubby. There's also a dBd, but we won't go there. The only meaningless measure is, in fact, dB gain. The figures I've given above are the most widely accepted by the reputable manufacturers - such as Shakespeare and Metz.

So, no a stubby has a slightly different pattern to a whip, but it's close, but weaker. As the antenna height increases the pattern flattens out and it becomes more disc-like, more focused, less appropriate for an unstable platform such as a boat. That's why a whip antenna on a pole is more effective than a longer antenna. The trade off is always radiation pattern versus height above sea level. Best of all worlds for the size of boats we all use is the whip antenna. On a stable shore base you'd go for large gain and height, on a boat you have to compromise.

When you are communicating with shore base or big ship antennas they have large gain, height and power to their advantage and can reach your rather puny horizon, but other smaller boats will need to be much closer to engage with your radio horizon.

 

[KAL]

KAL .. that's very interesting.
What make / length of "stubby antenna" are you using?
Does it still work well if you are heeled over?
I'm getting nothing like as good a range as you with a small antenna mounted on the pushpit, hence my questions about how to mount a standard whip antenna higher up.

It's a Digital Yacht transceiver - the previous model not the current. Works like a charm and is connected to a SH CP180i, which overlays the targets onto the chart. Hope you find the right solution.

 

[Albert Ross]

John, KAL ... thank you.

I'm using a digital yacht GV30 combined GPS antenna and AIS antenna. It's very neat and tidy and fits conveniently on the pushpit but unfortunately the AIS is not reliably showing big ships even 2-3 miles away.
I'm trying to work out if there is something wrong with the set up, or whether I need to put the antenna higher up (on a pole or the backstay) or whether I need to buy one of your pucker (Salty) american VHF-AIS antennae. And if so, exactly where to put it!