COAX

[dougg]

Salty johns web site says a 3dbi aerial is best, I pressume as the one I have is 2.4dbi this is very close.
What do you think the gain is on the one I already have, its part of the windex and I guess it's approx 350-400mm in length.

 

[starfire]

Salty johns web site says a 3dbi aerial is best, I pressume as the one I have is 2.4dbi this is very close.
What do you think the gain is on the one I already have, its part of the windex and I guess it's approx 350-400mm in length.

No idea, but, to put things in perspective, if you have a 10m run of coax, say RG58/u, in good condition, you have lost around 2.5dB just in the coax. (quick mental calculation only !)

If not in good condition, all bets are off.

Higher gain aerials are useful in some circumstances, on a masthead rolling around some 20 degrees is not one of them.

Think of the radiation pattern as a doughnut, with the aerial in the center of the hole, low gain, you have a nice thick doughnut, high gain, you have a very thin one.

 

[VicMallows]

And to put it another way .....

You only have a certain amount of power. You can choose in which direction you would like this power to go. On a boat you probably want it to point towards other boats and/or the shore. You also don't want to worry about which way your boat is pointing.

The standard boat VHF antenna achieves this to the best of its ability

Note that the 'GAIN' is only in relation to what would otherwise have been wasted sending power in a direction which would be a waste (such as straight up into space).

 

[mcframe]

Can any one tell me which coax to use for VHF / AIS. Thanks.

Geek Test #14: (but rather spoiled on this audience ;-)
"How many syllables in the word 'coax'?"

 

[Salty John]

Geek Test #14: (but rather spoiled on this audience ;-)
"How many syllables in the word 'coax'?"

Two in this case, but it gets more difficult when you need to coax the coax down the mast.

 

[dougg]

Gosh it all sounds very complicated, so as I understand it a high gain aireal is fine whilst upright but no use if you're Rolling about much.
As I look around th marina, a considerable amount of yachts have a 1 meterish aireal on the mast which is why I thought fitting one one give me a far greater range on the VHF.

 

[Salty John]

Gosh it all sounds very complicated, so as I understand it a high gain aireal is fine whilst upright but no use if you're Rolling about much.
As I look around th marina, a considerable amount of yachts have a 1 meterish aireal on the mast which is why I thought fitting one one give me a far greater range on the VHF.

Those "1m" masthead aerials are typically around 3dBi gain, and that's right for boating applications, particularly masthead locations. You can get a 1.5m, 1.8m, 2.4m and even longer antennas, with gains of up to 9dBi or more. Many are fishing rod style, others are steel whips, some are even steel whips with a 'protective' fibreglass sleeve over them - why the fibreglass should be protecting the stainless steel evades me!
Gain is achieved by focusing the signal, there is no actual gain in power. When the signal is tightly focused, as the boat rolls and pitches the signal dances around pointing now at the sea, now at the sky and not at the horizon where you need it. A less focused signal has at least some of the signal pointing at the horizon most of the time.
So, more gain doesn't necessarily mean better performance - however you define 'performance'.

 

[CreakyDecks]

..which is why I thought fitting one one give me a far greater range on the VHF.

VHF transmission is what they call "line of sight", so range is determined mainly by the height the antenna is mounted (because the horizon is further away if you are higher up). If the receiving antenna is also mounted at some height then that adds more distance, possibly considerably more if it is a tall land based mast.

 

[nigelmercier]

Geek Test #14: (but rather spoiled on this audience ;-)
"How many syllables in the word 'coax'?"

I think you are referring to the Americanism of not using a hyphen after the prefix "co"? I find this acceptable in nearly all cases, but I stumble every time I read the word coworker.