VHF marine radio antenna 3db or 6db 1.5M or 2.4M

[Sneds]

After snapping our VHF aerial with too large a flag at the recent Harbour festival (!) I need to buy a new one, but which one?
I understand that the higher gain in db, say 6db will give a narrower more direct signal, like a focused torch beam but a 3db gain will produce a broader although less strong signal less affected by the rolling motion of the boat?
Is this correct?
I would have thought that the taller the aerial the better but looking at the narrow radiating pattern of the higher gain 2.4M aerial I'm not now so sure
Narrow beam or broad beam?
Any brand to avoid, is more expensive necessarily better, I was looking at a Glomex in Force 4

Any input very welcome

Thanks

 

[Richard.C]

I would go for the higher gain, assuming the antenna is kept vertical the narrower pattern will direct more toward horizon and less up in the air.

 

[Robg71]

With a 2.4m Galaxy I could get the south coast of England from Jersey, no problem.

 

[john_morris_uk]

The taller antenna better is intuitively correct but the answer is 'not necessarily.' (For reasons as to how you manufacture 'gain' in a vertical antenna.

On a Mobo that hopefully rolls or heels less than a sailing yacht you can have a higher gain antenna.

If you want an explanation of how gain is achieved with a vertical then I can give it a go, but I suspect your eyes will start to glaze over...

It's worth me pointing out that gain is always relative to something. dBi is 'relative to an imaginary single point source' (which is impossible to achieve). In practical experiments you plot dBd (relative to a dipole) which is very easy to achieve but already has 3dB gain over the imaginary dBi. The reason this is relevant is that manufacturers can be cunning and quote dBi and thereby get themselves an extra 3dB.

In practice you won't notice the difference except at extreme ranges etc.

 

[Oscar24]

Make sure that whatever antenna you get, it's designed and tuned to the VHF marine band.
The reason I say this is that I recently tested a chaps new VHF marine antenna (2.4mtr one brought from a well known marine chandler) with something called a Site-master.
The site-master can show what frequency the antenna radiates best on. I.e what frequency it's been 'tuned' for. (vswr)
The VHF marine band is centred around 158Mhz, so would have expected a good, or low vswr figure for 158Mhz.

What I found was that the chaps antenna was perfectly tuned to 148Mhz and was giving a poor vswr plot at 158Mhz. (vswr was 3:1)
148Mhz is the American VHF amateur radio band.
I don't know this, but suspect someone has been importing cheap American VHF amateur band antennas and rebranding them as 'VHF Marine'
In short his antenna still worked, just not as well as an antenna designed to radiate on the VHF marine band.
Unless you intend to be 30+ miles from land, i would go for the smaller antenna.

 

[Sneds]

Thanks and enlightening responses
It was an antenna claiming to be suitable for so many purposes that stopped me buying and posting on here, I now suspect this has come from the states in the manner you suggest, naughty and glad I didn't buy it as its a bit of a track to take it back.
I'll stick with a brand I know and check the 158 MHz suitably.
When you talk about vswr is this akin to the SWR meter we had in the 70s to check our CB radios?

 

[Richard.C]

Antenna and coax are things I would pay good money for high quality, at the end of the day it's normally a one time purchase for the time you are likely to own the boat and they are not big expenses compared to other things on boats.

 

[Piers]

My experience shows that Shakespeare, 6dB, 2.4m aerials are excellent with a good Ch 16 response (approx 157MHz). Better than Comrod which are more expensive.

 

[Oscar24]

Thanks and enlightening responses
It was an antenna claiming to be suitable for so many purposes that stopped me buying and posting on here, I now suspect this has come from the states in the manner you suggest, naughty and glad I didn't buy it as its a bit of a track to take it back.
I'll stick with a brand I know and check the 158 MHz suitably.
When you talk about vswr is this akin to the SWR meter we had in the 70s to check our CB radios?

Yep, a Site Master is like a very expensive SWR meter which can show the reflective power between two frequencies.
I use one to test commercial airband antenna systems.
Ahh, CB radio They were the days. Got one at 9 after watching the film 'Convoy' and nagging my dad for weeks!

 

[Richard.C]

My experience shows that Shakespeare, 6dB, 2.4m aerials are excellent with a good Ch 16 response (approx 157MHz). Better than Comrod which are more expensive.

Shakespeare would be at the top of my list, been making good antennas for donkeys years.

 

[Sneds]

The Shakespeare 6db2.4M is going to be the one

Thanks to all for your input

 

[Piers]

The Shakespeare 6db2.4M is going to be the one

Thanks to all for your input

Good choice. Just follow the installation instructions and it will serve you well.