Vhf antenna length/make?

Fishery Pokery · 11 January 2010, 21:29

Hi guys,
I've trawled through past posts but can'tfind one that addresses my question.
I am after an antenna for my 4.6m boat, but cannot decide which length/ make to go for.
I will be using the boat within 10 miles of the shore and already have a handheld radio, so am installing this for better range. However I don't want too long a length for transporting.
The pacific ones on ebay look good (Cheap

), what do you guys think?
Cheers,
Geoff

Downhilldai · 11 January 2010, 21:56

Are you mounting it directly onto the transom?

Channel Ribs · 11 January 2010, 22:07

Quote:

Originally Posted by Geoff_Jubb

The pacific ones on ebay look good (Cheap

), what do you guys think?

Pacific antennae are anything but cheap, third best on the market perhaps. Very highly recommended from me at least.

Size really comes into its own for receiving, as a minimum option the 0.3m helical antenna that Pacific do is very good, the Seamaster Pro version even better.

Mounting height is perhaps more important than length.

Fishery Pokery · 11 January 2010, 22:16

I am thinking of mounting it on the transom, or some kind of instrument mount/console thingy I have yet to concieve. Either way, it will be about transom hieght.
With the pacific antennas, the ones I am looking at seem to start at £25. Is this the same make that you talk of?
Thanks for your help,
Geoff

Channel Ribs · 11 January 2010, 22:22

Quote:

Originally Posted by Geoff_Jubb

With the pacific antennas, the ones I am looking at seem to start at £25. Is this the same make that you talk of?

Yes, should be. Mounting low can be a problem, not least of which you have quite a bit of power radiating out when you transmit.

Fishery Pokery · 11 January 2010, 22:30

Is thr radiated power dangerous?
Does anyone have any ideas for mounting an antenna without an A frame, as mine is a SIB for fishing and I would prefer not to go down that route if possible?
Cheers again for your advice,
Geoff

Channel Ribs · 11 January 2010, 22:38

Quote:

Originally Posted by Geoff_Jubb

Is thr radiated power dangerous?

Given the small amount you will probably use it to transmit, no. But it is something to think about.

Using a small pole with the antenna on top would be good.

Downhilldai · 12 January 2010, 00:48

As it's going to be sitting at transom height, I'd suggest you go for a 1.5m antenna on a fold down base and fix it on the inside face of the transom.
A pole would be a good solution. You could make the pole removable, if it helps with storage.

9D280 · 12 January 2010, 14:36

I had a removable pole on my old boat. Easy to either hinge at the bottom so it folds onto the floor or pull out with an inline waterproof connector.

The main issue I could see with transom height is you will make a very good absorber for the outgoing signal, esp. if you are on 1W ......

Height (above water level!) is good!

Channel Ribs · 12 January 2010, 14:44

Pacific do a mount for their Pro range that sits inside 1" tube, the antenna itself is then removable to you do not need an inline connector so long as you are happy with whatever length of tube you go for.

This is the 1m version of the Seamaster Pro...

http://www.pacificaerials.co.nz/Mari...ssAntenna.aspx

The antenna, cable, pole and tube fitting should come to less than 30 quid.

kawasaki · 13 January 2010, 20:08

Just a quickie
For info only
VHF works by line of sight

So if boat 1 has a 10ft high spike
And boat 2 has the same

Theoreticlly each will 'see' each other for 4 miles
Therefore they can communicate if they are 8 miles apart.

I,m sure you all wished to know that

OK
I,ll feck off and go and watch the footy Then

Poly · 13 January 2010, 20:22

Quote:

Originally Posted by kawasaki

So if boat 1 has a 10ft high spike

is that measured from the base of the antanae or the top? makes a big difference on a small rib/sib especially with no a-frame

Fishery Pokery · 13 January 2010, 21:39

Quote:

Originally Posted by 9D280

The main issue I could see with transom height is you will make a very good absorber for the outgoing signal, esp. if you are on 1W ......

And I haven't even got a profile picture... ;D

Fishery Pokery · 13 January 2010, 21:44

I think I will go with what you describe Malthouse, but there is a small chance I will build some kind of DIY A frame.
Does your shop stock the things you mention?

Thanks everyone for your help,
Geoff

Ian Sharlot · 13 January 2010, 22:03

This probably won't be relevant to you Geoff but one thing that we find is that a large proportion of ariels are supplied with a five meter cable and we would advise avoiding any joins or extension cables where possible so may be worth checking that the aerial that you are looking at will have a long enough cable with it

kawasaki · 14 January 2010, 07:11

Quote:

Originally Posted by Polwart

is that measured from the base of the antanae or the top? makes a big difference on a small rib/sib especially with no a-frame

The theory is, 10ft above sea level.

So in realty it is just a fairly accurate guidline.

Yachts for instance.
Will enjoy better ship to ship communication with their 'spikes' mounted on top of a tall mast.

30ft (above sea level)for those chaps would give them about 8 miles each.
Therefore a 16 mile 'range'

Then again weather conditions, the sea bobbing us up and down etc will affect reception etc.

Basically, the higher the better really though.

Obviously HM Coastgaurd has antennas dotted about the place.
Which is why sometimes you may hear the CG transmitting when you are 20 miles from Him
But you wont hear the vessel responding.

For instance
I like to stick me radio on Ch 84 Holyheads CG's Ch for 'routine traffic' normally and stick the 'dual watch' facility on.

So 16, cos everything starts there, or should!
Plus on 84 I can hear the CG and sometimes the vessel responding.

Just kinda makes me feel safe, to know who is about and whats going on.
Plus I,m a nosey git

Poly · 14 January 2010, 08:29

Quote:

Originally Posted by kawasaki

The theory is, 10ft above sea level.

So in realty it is just a fairly accurate guidline.

Yachts for instance.
Will enjoy better ship to ship communication with their 'spikes' mounted on top of a tall mast.

30ft (above sea level)for those chaps would give them about 8 miles each.
Therefore a 16 mile 'range'

Then again weather conditions, the sea bobbing us up and down etc will affect reception etc.

Basically, the higher the better really though.

Obviously HM Coastgaurd has antennas dotted about the place.
Which is why sometimes you may hear the CG transmitting when you are 20 miles from Him
But you wont hear the vessel responding.

For instance
I like to stick me radio on Ch 84 Holyheads CG's Ch for 'routine traffic' normally and stick the 'dual watch' facility on.

So 16, cos everything starts there, or should!
Plus on 84 I can hear the CG and sometimes the vessel responding.

Just kinda makes me feel safe, to know who is about and whats going on.
Plus I,m a nosey git

I know that - I am familiar with the numbers which have all been posted here in the past. But you didn't actually answer the question - can I expect to be able to talk to an identically rigged boat from at 8 miles if I have a a 10ft long aerial mounted at the top of the transom or only if it is in fact 10 ft above the gunwale?

kawasaki · 14 January 2010, 09:07

Quote:

Originally Posted by kawasaki

The theory is, 10ft above sea level.

So in realty it is just a fairly accurate guidline.

I don't think it would make a great deal of diffence if it was mounted on the transom or the gunwhale.

Higher the better obviously.

Just trying to point out the type of range we can expect rib to rib.

VHF as we know is 'line of sight' .

But as others have posted.
There are other things to consider.

Tother thing
Like land in the way
Sometimes I have locked into Port Dinorwic marina.
The Dockmaster has an ariel on His office roof.
Dunno
Could be 40ft up.
Until I clear a headland approaching from the North its about a mile away until He hears me
Tother way I can raise Him from about 3 miles away
The spike on me little mobo is about 10ft above sea level.
Figures I gave are probaly 'in the best possible scenario'

Just as a guidline.

Poly · 14 January 2010, 09:12

Quote:

Originally Posted by kawasaki

I don't think it would make a great deal of diffence if it was mounted on the transom or the gunwhale.

Higher the better obviously.

I think your misunderstanding me. I'll post a piccy later.

Channel Ribs · 14 January 2010, 09:35

Quote:

Originally Posted by Polwart

is that measured from the base of the antanae or the top? makes a big difference on a small rib/sib especially with no a-frame

I tend to measure effective heights of antennae from two thirds up the active length.

There is a proper rule, I will see if I can dig it out.