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What size VHF antenna do i need for offshore fishing ?

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What size VHF antenna do i need for offshore fishing ?

Old 06-22-2016, 04:24 AM
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Default What size VHF antenna do i need for offshore fishing ?

I have a 98 pro sports 18' 3" cc . What size VHF antenna do i need ? I cant go anymore than 20 miles offshore max . I do not have a t-top so i will be mounting it to my CC.
Old 06-22-2016, 05:50 AM
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Get a standard 8' antenna.
Old 06-22-2016, 05:54 AM
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I agree 8' antenna will work fine.
Old 06-22-2016, 06:17 AM
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I just dont have any room to mount an 8' antenna . I was also worried about the antenna hitting someone with it being mounted so low. i dont have a t-top
Old 06-22-2016, 06:51 AM
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If you don't want to mount an 8' antenna on a ratchet mount on the side of the console, then either a 3' SS or 4' fiberglass is your other option.
Old 06-22-2016, 10:56 AM
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Can you please post a picture of the boat and console?
Old 06-22-2016, 12:51 PM
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Height is more important than antenna gain. So if no T-Top and you do not want the 8 foot get a four foot antenna, not really many options for you.


Jim
Old 06-22-2016, 01:03 PM
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Is this the flats boat model you are taking 30nm out?

With no T-top, if you have room for a 4' antenna you have room for an 8'er. Anything longer would have too high a gain for your relatively small boat. Presumably you are thinking about communications with other boats near you out that far, not to shore. A sat. phone would suffice for that.
Old 06-22-2016, 06:14 PM
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If you don't want to install an antenna use a handheld VHF. Just remember antenna height is directly proportional to distance transmitted. You can get some skip but don't rely on it.
Old 06-22-2016, 11:51 PM
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The comments so far are not exactly correct.

Antenna gain, amplifier power, and height above the ground all affect the distance of transmission.

A lower gain antenna (lets say 3dBi) will transmit energy more spherically than a higher gain antenna (say 11dBi) which compresses the signal on the Y-axis and transmits further on the X plane. With boating (unlike 4WDing, etc), because the terrain is flat a higher gain antenna will always be better.

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The power of the amplifier defines how far out this transmission pattern goes, but does not change the pattern itself.

And a higher antenna position will allow it better "line of sight" with things further away.

Also, if you want maximum distance, make sure you have a VHF radio and not a UHF (here in AUS people use both VHF and UHF) radio as the lower frequencies of the VHF (at the same power) will better curve around the Earth.

So, get the highest gain antenna, mount it as high as possible, with the most powerful radio (5W typically) and make sure it's a VHF.
Old 06-23-2016, 02:59 AM
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Originally Posted by alexcim View Post
The comments so far are not exactly correct.

Antenna gain, amplifier power, and height above the ground all affect the distance of transmission.

A lower gain antenna (lets say 3dBi) will transmit energy more spherically than a higher gain antenna (say 11dBi) which compresses the signal on the Y-axis and transmits further on the X plane. With boating (unlike 4WDing, etc), because the terrain is flat a higher gain antenna will always be better.

Attachment 680613

The power of the amplifier defines how far out this transmission pattern goes, but does not change the pattern itself.

And a higher antenna position will allow it better "line of sight" with things further away.

Also, if you want maximum distance, make sure you have a VHF radio and not a UHF (here in AUS people use both VHF and UHF) radio as the lower frequencies of the VHF (at the same power) will better curve around the Earth.

So, get the highest gain antenna, mount it as high as possible, with the most powerful radio (5W typically) and make sure it's a VHF.
You are forgetting that boats, especially small boats, rock and pitch in rough water. That creates the exact same situation shown in your diagram where a hi gain antenna actually has less effective range than a low gain antenna. Thats why sail boats often have low gain antennas mounted hi on the mast.
Old 06-23-2016, 03:34 AM
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Originally Posted by raudi View Post
I just dont have any room to mount an 8' antenna . I was also worried about the antenna hitting someone with it being mounted so low. i dont have a t-top
If antenna length is an issue, you can go for a short, approx 3 foot quarter wave monopole.
That means its 'ground dependent' and becomes a half wave 8 foot by use of its own image in the water.
The ground plane for this type of antenna can be obtained by using copper foil, glued to the inside of the hull. At RF frequencies the ground can penetrate the hull and not need a physical connection to the water.
Old 06-23-2016, 04:01 AM
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Originally Posted by Yrral3215 View Post
You are forgetting that boats, especially small boats, rock and pitch in rough water. That creates the exact same situation shown in your diagram where a hi gain antenna actually has less effective range than a low gain antenna. Thats why sail boats often have low gain antennas mounted hi on the mast.
Yes that's a good point.

I basically just wanted to say that mounting an antenna high won't automatically make it better if its the wrong gain for what youre trying to achieve.
Old 06-23-2016, 04:34 AM
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Originally Posted by alexcim View Post
Yes that's a good point.

I basically just wanted to say that mounting an antenna high won't automatically make it better if its the wrong gain for what youre trying to achieve.

Height is always more important, it is the center of radiation that determines the range to the horizon.


Power is not in the equation here, all fixed mount VHF's put out 25 watts of power.


Antenna gain makes very little difference going from a 4 foot to an 8 foot. Antennas do not have active gain, they compress the radiated signal into a small vertical beam width , and as noted may not be a good choice especially if mounted high on a sailboats mast.


For an 18 foot vessel with no T-Top a 4 foot antenna is most practical.


Jim
Old 06-23-2016, 04:52 AM
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Originally Posted by Yrral3215 View Post
You are forgetting that boats, especially small boats, rock and pitch in rough water. That creates the exact same situation shown in your diagram where a hi gain antenna actually has less effective range than a low gain antenna. Thats why sail boats often have low gain antennas mounted hi on the mast.
Yes that's correct but also signal strength varies when at the extremes of beam pattern coverage, so too distance between transmitter to receiver.
Radio's have automatic gain control or AGC which saves users the trouble of continually adjusting the volume control. This is different to the squelch control which detects a valid signal over the noise.
So the quality of the radio also determines usable range regardless of all the antenna physics.
The receiver sensitivity is specified in micro volts which is a specification of the signal over the noise at 12 dB which is determined as 'intelligible speech' and known as SINAD. It's basically the ratio of the signal strength to the noise and distortion.
The point is, if you have a good quality radio, buying a cheap quality antenna doesn't do the radio justice as intelligible reception at maximum distance is what the purpose is. That extends also to the installation. A cheap radio properly installed and tested will perform better than a top of the range radio installed by a Muppet. As for power if the SWVR is poor the power is reflected back and is not transmitted. If the modulation is poor then you have high power garbled audio.
Old 06-23-2016, 05:57 AM
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I had 3' metal Whip style antenna on my boat and replaced it with a 8' Digital brand. Huge difference in transmission distance.

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