Reply
 
Thread Tools Search this Thread
 
Old 08 July 2016, 16:09   #1
Member
 
Country: UK - Scotland
Make: HumberOceanOffshore
Length: 8m +
Engine: Volvo KAD300/DPX
Join Date: Oct 2002
Posts: 4,962
Ooo...dirty trick!

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.
Attached Thumbnails
Click image for larger version

Name:	Antenna 1.jpg
Views:	140
Size:	283.3 KB
ID:	114327   Click image for larger version

Name:	Antenna 2.jpg
Views:	133
Size:	305.2 KB
ID:	114328  
__________________

__________________
JW.
jwalker is offline   Reply With Quote
Old 08 July 2016, 17:01   #2
Member
 
Country: USA
Town: S. Carolina
Boat name: D560
Make: Avon
Length: 5m +
Engine: 2016 Merc 115hp CT
Join Date: Aug 2013
Posts: 1,120
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
__________________

__________________
Richard
Gluing geek since 2007
office888 is offline   Reply With Quote
Old 08 July 2016, 17:49   #3
Member
 
Country: UK - Scotland
Make: HumberOceanOffshore
Length: 8m +
Engine: Volvo KAD300/DPX
Join Date: Oct 2002
Posts: 4,962
Quote:
Originally Posted by office888 View Post
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.
__________________
JW.
jwalker is offline   Reply With Quote
Old 08 July 2016, 17:53   #4
RIBnet supporter
 
Pikey Dave's Avatar
 
Country: UK - England
Town: South Yorks
Boat name: Black Pig
Make: Ribcraft
Length: 5m +
Engine: DF140a
MMSI: 235111389
Join Date: Feb 2008
Posts: 7,595
RIBase
Quote:
Originally Posted by jwalker View Post
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
__________________
Rule#2: Never argue with an idiot. He'll drag you down to his level & then beat you with experience.
Rule#3: Tha' can't educate pork.
Rule#4: See rule#5
Pikey Dave is online now   Reply With Quote
Old 08 July 2016, 18:26   #5
Member
 
Country: UK - Scotland
Make: HumberOceanOffshore
Length: 8m +
Engine: Volvo KAD300/DPX
Join Date: Oct 2002
Posts: 4,962
Quote:
Originally Posted by Pikey Dave View Post
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.
__________________
JW.
jwalker is offline   Reply With Quote
Old 09 July 2016, 01:37   #6
Member
 
Country: UK - England
Length: 3m +
Join Date: Sep 2008
Posts: 2,251
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.
__________________
ShinyShoe is offline   Reply With Quote
Old 09 July 2016, 04:13   #7
RIBnet admin team
 
Poly's Avatar
 
Country: UK - Scotland
Boat name: imposter
Make: FunYak
Length: 3m +
Engine: 2 stroke YAM 20 HP
MMSI: 235089819
Join Date: Sep 2005
Posts: 10,114
Quote:
Originally Posted by ShinyShoe View Post
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!

Quote:
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?
__________________
Poly is offline   Reply With Quote
Old 09 July 2016, 06:38   #8
RIBnet admin team
 
willk's Avatar
 
Country: Ireland
Make: Redbay Boats
Length: 9m +
Engine: 370hp
Join Date: Feb 2008
Posts: 12,926
RIBase
Quote:
Originally Posted by Poly View Post
... 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....
__________________
"Who knows what evil lurks in the hearts of men?"
.
willk is offline   Reply With Quote
Old 09 July 2016, 08:26   #9
Member
 
Country: UK - Scotland
Make: HumberOceanOffshore
Length: 8m +
Engine: Volvo KAD300/DPX
Join Date: Oct 2002
Posts: 4,962
Quote:
Originally Posted by Poly View Post
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!!

Quote:
Originally Posted by Poly View Post
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?
__________________

__________________
JW.
jwalker is offline   Reply With Quote
Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off





Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -5. The time now is 13:09.


Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.