Ooo...dirty trick!

[jwalker]

So, I've snapped my vhf antenna, it's a 2.4mtr antenna, 2.4mtr should give a gain of 6dB, it was advertised as a 6dB antenna - what's inside it? a 1.5 mtr antenna!! That'll have again of 3dB then - half power!

If you happen to have a Silva, 2.4mtr Silver Star 6dB antenna, you probably haven't.

[office888]

I'd switch to a 5/8 wave antenna (basically, a Metz). The signal radiation is a little better for RIBs in rough seas. It only drops it from 6 dBi to 3 dBi

[jwalker]

Originally Posted by office888

I'd switch to a 5/8 wave antenna (basically, a Metz). The signal radiation is a little better for RIBs in rough seas. It only drops it from 6 dBi to 3 dBi

Too late, I've fitted a new, claimed 9dB, antenna.

[Pikey Dave]

Originally Posted by jwalker

Too late, I've fitted a new, claimed 9dB, antenna.

That will have a very flat compressed radiation pattern, possibly not the best choice for a RIB (IMHO)


Lions led by donkeys

[jwalker]

Originally Posted by Pikey Dave

That will have a very flat compressed radiation pattern, possibly not the best choice for a RIB (IMHO)


Lions led by donkeys

Nah, it'll be fine, I don't often need to speak to aeroplanes.

[ShinyShoe]

Neither do i, but when I'm trying to talk to the helo that's coming to save my bacon I may change my mind!!

In seriousness - it's the roll of the waves that will matter. So a nice vertical 9dB ant is emitting relatively parallel to the 'ground'. Roll the boat over a wave (as you might when broken) and the up hill side is now pointing to the moon and the down hill to the sea.

[Poly]

Originally Posted by ShinyShoe

Neither do i, but when I'm trying to talk to the helo that's coming to save my bacon I may change my mind!!

In seriousness - it's the roll of the waves that will matter. So a nice vertical 9dB ant is emitting relatively parallel to the 'ground'. Roll the boat over a wave (as you might when broken) and the up hill side is now pointing to the moon and the down hill to the sea.

JW has a big cabin humber. Its going to be a pretty stable platform in all but the very worst conditions - I suspect it doesn't spend much time airborne off big waves either. He's also pretty well informed on all things VHF so I'm sure he understands the implications of the design. I guess he also has a H/Held for comms with the helo overhead!

Too late, I've fitted a new, claimed 9dB, antenna.

do you have a "stud"/"pipe" detector thingy... does it reveal how long the working part is?

[willk]

Originally Posted by Poly

... does it reveal how long the working part is?

Probably the first 1.5m and the rest is pipe?

I wouldn't worry about the 9db....

[jwalker]

Originally Posted by Poly

JW has a big cabin humber. Its going to be a pretty stable platform in all but the very worst conditions - I suspect it doesn't spend much time airborne off big waves either. He's also pretty well informed on all things VHF so I'm sure he understands the implications of the design. I guess he also has a H/Held for comms with the helo overhead!....

Oh, you know me so well! Spooky!!

Originally Posted by Poly

do you have a "stud"/"pipe" detector thingy... does it reveal how long the working part is?

No, I don't but I do have divining rods - no laughing, they work and they are as spooky as you, too.

The new antenna is two piece and it screws together, I can see the element soldered to the threaded joiner so it's at least half the length of the outer. Hehe.

More seriously, I'd be very surprised if the new antenna is actually 9dB gain since it's 2.4mtrs long and also antenna manufacturers are notorious for their exaggerated claims on antenna gain. With respect to beam angle, when looking at a general representation of a polar diagram of radiation they are often drawn with a single lobe and one might have the impression that there is no radiation outwith that main lobe. In reality there will likely be many lobes of varying signal strength emitted from the antenna and, even if the beam is well focussed, there will be signal of a lesser strength above and below the main beam. In theory, the least radiation will be vertically above the end of the antenna but I bet in practise there will still be a usable signal for that helicopter just a few hundred feet away.

Where there may be a limitation is in a situation such as a narrow Scottish loch where getting the signal out over the hill might be difficult but even then reflections can be useful. I've certainly been in that situation and the coastguard could read me but his incoming was unreadable.

With regard to the angle of the boat, I suspect that the actual angle of lean in rough water is very much less than one might imagine and I can't think of even one occasion where I've been attempting to make a vhf call whilst jumping off a wave. Anyway, isn't a rib a nice stable platform?