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Old April 19th 06, 02:13 PM posted to rec.boats.electronics
 
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Default Changing VHF antenna location...?

I have a small sail boat (26') used for coastal cruising. It
would probably never be beyond 25 miles from the shore
line (maximum) and normally no more than 1 to 3 miles.

At present... it has a 1/2 wave Shakespeare mounted about
5 feet above the water line on a stern rail. The previous owner
has indicated that this arrangement has worked for him... and
I am assuming the standing wave ratio and that sort of thing is
ok... or must be... if his radio checks/contacts have been
satisfactory.

My question is...

Would I be *much* better off... moving the antenna to the top
of the mast... which would be 30 feet higher than where it is now
situated... based on the aforementioned cruising criteria?

Cause... if the gain won't be that *significant*... I don't want to
go through the hassle of remounting everything. Kind of like...
"...if it ain't broke why am I trying to fix it?" (smile)

Tnx for your help...

73

Bill
Southern California


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Old April 19th 06, 05:23 PM posted to rec.boats.electronics
Ted
 
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Default Changing VHF antenna location...?


wrote in message
oups.com...
I have a small sail boat (26') used for coastal cruising. It
would probably never be beyond 25 miles from the shore
line (maximum) and normally no more than 1 to 3 miles.

At present... it has a 1/2 wave Shakespeare mounted about
5 feet above the water line on a stern rail.

My question is...

Would I be *much* better off... moving the antenna to the top
of the mast...


No.

There would be a small improvement but I don't think you would even notice.

I recommend you leave the antenna where it is and if you ever have an
emergency be sure to point your sailboat (if you can) so that the boat is
not between the antenna and the shore. If you are disabled and adrift off
shore then occasionally your boat will not be between your antenna and
shore. These are the best times to call for help but by all means, call for
help constantly until someone hears you. If you are out of sight from shore
then I recommend you get an epirb.

http://www.sarsat.noaa.gov/emerbcns.html

I also recommend you keep a cell phone aboard. I have found it to be much
more handy than marine vhf radios.




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Old April 19th 06, 07:09 PM posted to rec.boats.electronics
Bruce in Alaska
 
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Default Changing VHF antenna location...?

In article . net,
"Ted" wrote:

wrote in message
oups.com...
I have a small sail boat (26') used for coastal cruising. It
would probably never be beyond 25 miles from the shore
line (maximum) and normally no more than 1 to 3 miles.

At present... it has a 1/2 wave Shakespeare mounted about
5 feet above the water line on a stern rail.

My question is...

Would I be *much* better off... moving the antenna to the top
of the mast...


No.

There would be a small improvement but I don't think you would even notice.

I recommend you leave the antenna where it is and if you ever have an
emergency be sure to point your sailboat (if you can) so that the boat is
not between the antenna and the shore. If you are disabled and adrift off
shore then occasionally your boat will not be between your antenna and
shore. These are the best times to call for help but by all means, call for
help constantly until someone hears you. If you are out of sight from shore
then I recommend you get an epirb.

http://www.sarsat.noaa.gov/emerbcns.html

I also recommend you keep a cell phone aboard. I have found it to be much
more handy than marine vhf radios.





With both Marine VHF and Cellular Service, you are looking at the maximum
range of transmission being determined by LOS (line of sight) plus a
extending factor of signal bending, which is Frequency dependant, and
which is maybe 5 to 10% more, which makes up the Radio Horizon. USCG
Remote Radio Sites are typically located at high elevation points along
the coastline, so as to maximize their coverage out to sea, where
Cellular coverage is specifically designed to minimize adjacent Cell
overlap but give continious coverage along the coastline. These are
two, basically, Mutually Exclusive determinations.

Moving the Marine VHF Antenna to the top of the mast, WILL certainly
extend the VHF Range for the vessel over an antenna that is 5 ft
above the water. Remember it is just not the distance offshore that one
needs to consider, it is also how far the USCG Site is up or down the
coast from your position as well. Another thing to consider is the
Antenna Gain Specs of the Marine VHF Antenna, which will determine the
3db Vertical Antenna Beamwidth, which intern will affect the ability of
the antenna to communicate to shore stations when the vessel is healed
over while navigating crosswind.

For sail powered vessels, it has always been prudent to have the VHF
Antenna, as high as possible, with no more than 4.5 db of gain in
the antenna, so as to minimize the healing degradation of maximum
effective range. Cellular should NEVER be considered a substitute
for Marine VHF communications, but it can be helpfull as a backup
system.

Also remeber that Marine VHF will allow communications with other
vessels in your area, as well USCG Remote Radio Sites, where Cellular
MUST be able to communicate with the Shorebased Cellsite in order to be
of use in communications. There is no vessel to vessel communications
with cellular, if you are not within cellular coverage.

Bruce in alaska
--
add a 2 before @
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Old April 19th 06, 07:28 PM posted to rec.boats.electronics
brad
 
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Default Changing VHF antenna location...?

An interesting discussion. A couple of things to consider.

Another reader mentioned that VHF signals are essentially line of sight,
which means that the effective range of your radio is a function of,
among other things, the height of your antenna and the height of the
receiving antenna. Since you dont necessarily know the height of the
receiving antenna, it's hard to predict the range of the radio with any
precision. However, you can get a relative indication of the range of
an antenna mounted at 5' versus, say, 25'. At 5', the horizon is about
2 1/2 miles away. At 25' it's about 5 1/2 miles away. So, all other
things being equal, moving your antenna to the top of the mast should
roughly double your range. However, all other things are not equal, and
this should only be used as the roughest of estimates.

There are a couple other considerations. At 155mhz, there's a
reasonable amount of signal loss in the cable itself. For instance,
let's say that your marine radio is on high power, 25 watts. Let's say
also that you're using relatively low budget Radio Shack RG58, and that
the total cable length from the radio to the base of the antenna is 35'.
That's almost 2dB of signal loss, or roughly 16 watts of power at
the antenna. Obviously the shorter the feed line, the less loss.

The other obvious consideration is what happens if you lose your rig and
need to call for help? An antenna at the top of the mast performs
better, assuming the mast is still standing!

On my last boat, I solved this conundrum by having 2 separate VHF
radios, one with an antenna on the stern rail, and the other with the
antenna at the top of the mast. That way I was protected from a failure
of either of the antennas or the electronics themselves.

On the current boat, I have the luxury of having 2 masts. Each mast has
2 small high gain antennas at the top, offset by about 2'. The four
antennas are all interchangeable, so if one fails I can simply switch to
another. The four antennas are connected to a VHF marine radio, a dual
band VHF/UHF ham radio, an AIS receiver, and to the stereo system.



wrote:
I have a small sail boat (26') used for coastal cruising. It
would probably never be beyond 25 miles from the shore
line (maximum) and normally no more than 1 to 3 miles.

At present... it has a 1/2 wave Shakespeare mounted about
5 feet above the water line on a stern rail. The previous owner
has indicated that this arrangement has worked for him... and
I am assuming the standing wave ratio and that sort of thing is
ok... or must be... if his radio checks/contacts have been
satisfactory.

My question is...

Would I be *much* better off... moving the antenna to the top
of the mast... which would be 30 feet higher than where it is now
situated... based on the aforementioned cruising criteria?

Cause... if the gain won't be that *significant*... I don't want to
go through the hassle of remounting everything. Kind of like...
"...if it ain't broke why am I trying to fix it?" (smile)

Tnx for your help...

73

Bill
Southern California

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Old April 20th 06, 02:24 PM posted to rec.boats.electronics
GregS
 
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Default Changing VHF antenna location...?

In article , Bruce in Alaska wrote:
In article . net,
"Ted" wrote:

wrote in message
oups.com...
I have a small sail boat (26') used for coastal cruising. It
would probably never be beyond 25 miles from the shore
line (maximum) and normally no more than 1 to 3 miles.

At present... it has a 1/2 wave Shakespeare mounted about
5 feet above the water line on a stern rail.

My question is...

Would I be *much* better off... moving the antenna to the top
of the mast...


No.

There would be a small improvement but I don't think you would even notice.

I recommend you leave the antenna where it is and if you ever have an
emergency be sure to point your sailboat (if you can) so that the boat is
not between the antenna and the shore. If you are disabled and adrift off
shore then occasionally your boat will not be between your antenna and
shore. These are the best times to call for help but by all means, call for
help constantly until someone hears you. If you are out of sight from shore
then I recommend you get an epirb.

http://www.sarsat.noaa.gov/emerbcns.html

I also recommend you keep a cell phone aboard. I have found it to be much
more handy than marine vhf radios.





With both Marine VHF and Cellular Service, you are looking at the maximum
range of transmission being determined by LOS (line of sight) plus a
extending factor of signal bending, which is Frequency dependant, and
which is maybe 5 to 10% more, which makes up the Radio Horizon. USCG


I would say this is definately for reliable transmission.
I always get cellular service around 60 mi up at lake Erie. Its hard not to
keep out of the Canadian stations on the US side, in fact its hard to get in the US
sites on the US side from the coast. I always think of an article or rig named Puddle Jumper.
This was a 2 meter superregen rig, mabe by Heathkit, not
sure. They talked about talking out over one of the Great Lakes I always think of this.
Of course, heat and cold air over the lake do wonderfull things.

greg


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Old May 23rd 06, 07:00 AM posted to rec.boats.electronics
Jester
 
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Default Changing VHF antenna location...? line of sight calculator web page

So,

antenna height feet, horizon miles

Boat Coast Horizon
6 100 17
30 100 22


The range with the coastal stations doesnt change so much and the DSC calls
will be received from much further away. For emergency calls to the coast
the antenna difference is not so much, boat to boat is a different story.

J


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Old May 25th 06, 03:53 AM posted to rec.boats.electronics
Larry
 
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Default Changing VHF antenna location...? line of sight calculator web page

"Jester" wrote in :

For emergency calls to the coast
the antenna difference is not so much, boat to boat is a different story.


And you have to factor in the damned marinas and their 70' antenna towers
running 25 watts to talk to the motoryacht 200 ft away coming towards their
docks. FCC should restrict marinas to antennas 20' off the water running
1/2 watt.....or walkie talkies.



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