Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 01-12-2009, 10:58   #1
Registered User
 
svHyLyte's Avatar

Join Date: Mar 2006
Location: Tampa Bay area, USA
Boat: Beneteau First 42
Posts: 3,434
Images: 25
Question Masthead Antenna Orientation

This AM I was stopped by a friend at our YC who needed some help with a replacement Masthead Bracket for his yacht. Last year his masthead antenna was hit by lightening, which fried all of his electronics. He is concerned about a recurrance and has decided to remount the antenna with the whip pointing down, along the side of the mast, rather than up and is convinced that with such orientation he's less likely to be hit; and, if hit, the dischagre is more likely going to pass through the mast to the keel than through the VHF Coax to the electrics. Will one of the electronic wizzards hereabouts kindly comment on the efficacy of the foregoing including the effects on transmission capability, if any?

Thanx...
__________________

__________________
"It is not so much for its beauty that the Sea makes a claim upon men's hearts, as for that subtle something, that quality of air, that emanation from the waves, that so wonderfully renews a weary spirit."
svHyLyte is offline   Reply With Quote
Old 01-12-2009, 11:15   #2
Senior Cruiser
 
senormechanico's Avatar

Cruisers Forum Supporter

Join Date: Aug 2003
Boat: Dragonfly 1000 trimaran
Posts: 5,832
I doubt it would make any difference in lightning damage avoidance.
__________________

__________________
Memento,homo, quia pulvis es, et in pulverem reverteris.
senormechanico is offline   Reply With Quote
Old 01-12-2009, 12:28   #3
Senior Cruiser

Cruisers Forum Supporter

Join Date: Jul 2006
Location: Chesapeake Region and Maine
Boat: 42' Bob Perry sloop
Posts: 4,038
Images: 4
....but it WOULD make a difference in VHF reception and, especially, transmission.

Dumb idea.

Lightning's gonna hit where it's gonna hit. Period. Nothing you can do with a VHF antenna's going to make a difference.

Bill
__________________
btrayfors is offline   Reply With Quote
Old 01-12-2009, 12:39   #4
Moderator Emeritus
 
GordMay's Avatar

Join Date: Mar 2003
Location: Thunder Bay, Ontario - 48-29N x 89-20W
Boat: (Cruiser Living On Dirt)
Posts: 31,586
Images: 240
Quote:
Originally Posted by btrayfors View Post
....but it WOULD make a difference in VHF reception and, especially, transmission.
Dumb idea.
Lightning's gonna hit where it's gonna hit. Period. Nothing you can do with a VHF antenna's going to make a difference.
Bill
Methinks, that in the interest of politeness, Bill understated the objections (with which I concur).
__________________
Gord May
"If you didn't have the time or money to do it right in the first place, when will you get the time/$ to fix it?"



GordMay is offline   Reply With Quote
Old 01-12-2009, 13:44   #5
Senior Cruiser
 
sandy daugherty's Avatar

Cruisers Forum Supporter

Join Date: May 2008
Location: near Annapolis
Boat: PDQ 36 & Atlantic 42
Posts: 1,178
Reassure your friend that lightning can strike twice in the same place. Suggest he raft up next to a much taller mast.

Science has long overlooked the fact that lightning is merely one of Mother Nature's ways of expressing herself, and she has a lot to say about many things. Perhaps the most egregious provocation is the mistaken assumption that something we invent will shut her up. You need to be married to understand this, but its far better not to do that.
__________________
sandy daugherty is offline   Reply With Quote
Old 01-12-2009, 14:14   #6
Registered User
 
svHyLyte's Avatar

Join Date: Mar 2006
Location: Tampa Bay area, USA
Boat: Beneteau First 42
Posts: 3,434
Images: 25
Hummm...

well. Those answers are rather what I expected but with some people, one is wise to keep one's own counsel...

Quote:
...Suggest he raft up next to a much taller mast.
Ah.. No... That might be me!

Thanx All...
__________________
"It is not so much for its beauty that the Sea makes a claim upon men's hearts, as for that subtle something, that quality of air, that emanation from the waves, that so wonderfully renews a weary spirit."
svHyLyte is offline   Reply With Quote
Old 01-12-2009, 15:38   #7
Registered User
 
bill good's Avatar

Join Date: Oct 2006
Location: Brisbane Australia
Boat: sold
Posts: 721
Why not mount it in the centre of the spreader? It would not matter whether it is up or down but more which way is clearest from other metal parts of the boat.Some shielding will occur but I doubt you will notice any major loss of range. Leave a sacrificial vertical whip grounded on top of the mast for lightning protection. If you look at the damage done by the strike you will find the whip prob. "vaporized" without any other visible damage to the top of the mast. (which proves it works).The coax cable to the radio should include a gas discharge lightning protector bonded to mast & earth plate. Other wise go with the "look for a taller mast" advise!!
Bill
__________________
bill good is offline   Reply With Quote
Old 01-12-2009, 21:45   #8
Senior Cruiser
 
FSMike's Avatar

Cruisers Forum Supporter

Join Date: Aug 2009
Location: Bahamas/Florida
Boat: Solaris Sunstar 36' catamaran
Posts: 2,654
Images: 5
[QUOTE=bill good;367913]Why not mount it in the centre of the spreader?
Bill[/QUOTE

The lower the antenna the shorter the transmission range. That's the reason for the masthead installation in the first place.
__________________
FSMike is offline   Reply With Quote
Old 02-12-2009, 12:39   #9
Registered User
 
denverd0n's Avatar

Join Date: Jun 2008
Location: Tampa, FL
Posts: 3,953
Images: 6
Quote:
Originally Posted by bill good
Some shielding will occur but I doubt you will notice any major loss of range.
I think that if you spent some time working with any of the better antenna modeling software that is available, you would change your mind. Even being next to the shroud wire is enough to create significant lobes in the antenna's radiation pattern. Being next to the mast? You will no longer have an "omnidirectional" antenna by any stretch of the definition of that word. You would probably get an improvement of range in one or two directions, but a drastic loss of range in other directions. This is not how you want your VHF to function.

The VHF antenna needs to be at the masthead, mounted in the clear, up above the mast. If you have any other antennas up there, they need to be adequately separated, depending on the frequencies for which they are used.
__________________
denverd0n is offline   Reply With Quote
Old 02-12-2009, 15:21   #10
Registered User
 
bill good's Avatar

Join Date: Oct 2006
Location: Brisbane Australia
Boat: sold
Posts: 721
FS MIKE & DEVERDON. Yes there will be some degraded performance but the increased protection is what I am looking at!! The line of sight cal. is only one part of the equation & this drop of height is practice is about 1mile in 20. If the antenna is located on the top spreader & actual field strength test conducted you may find the results a more than acceptable. How do I know well I have done it. How many boats have lightning protection put into the coax of coax outer bonded to the mast & earthed to a proper plate not many!! About the same number of boats recording lightning strikes I suspect. It might not prevent damage but it is better than putting up a come & get me rod on top of a mast!!
__________________

__________________
bill good is offline   Reply With Quote
Reply

Tags
antenna

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

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


Similar Threads
Thread Thread Starter Forum Replies Last Post
Multiple Antennas at Masthead? denverd0n Marine Electronics 7 09-10-2009 09:39
cable question for VHF masthead antenna ribbony Marine Electronics 12 29-04-2009 16:26
Masthead Antenna Cable Waterborn Monohull Sailboats 16 01-04-2009 20:18
What co-ax for masthead VHF? Weyalan Marine Electronics 16 26-06-2008 07:53
fractional or masthead which is best? philip van praag Multihull Sailboats 24 20-06-2007 09:28



Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 23:50.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.