Marine Electronics Forum - 4' vhf anntena pros and con ????

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View Full Version : 4' vhf anntena pros and con ????


cj
03-29-2012, 09:31 PM
:thumbsup:im in need of a new anntena as my comrad got wacked in half!! i am in the process of installing a radar arch on my skipjack 24 and would like to buy a 4' anntena if it will do the job!! ive looked at the digital 4' any feedback would sure be helpfull!! thanks


KismetLRC
03-29-2012, 09:39 PM
The higher, the better. Go with a quality 8' if you can. Digital or Shakespeare.

bradv
03-30-2012, 01:12 AM
Nearshore its fine but with antenna bigger is better just just for height but power.


bear685
03-30-2012, 08:03 AM
the 4' antennas have less power - even if you mount it high it will not have the same gain as an 8' antenna.

capecuddy
03-30-2012, 08:20 AM
8' is the way to go. Not legal to have more power than 25W with no license...so, the next 2 best things are 1) Gain, but no more than an 8 footer and 2) raise it as high above the water as possible to improve Line of Sight. Your arch will do the job nicely.

Birdman
03-30-2012, 09:26 AM
the 4' antennas have less power - even if you mount it high it will not have the same gain as an 8' antenna.

Nearshore its fine but with antenna bigger is better just just for height but power.

Antenna's do not have "power". ;) AND, a longer antenna will not have any affect on the "power" that is broadcast from the radio. The VHF's we use are 25 watt's, and 25 watts will leave both the 8' and 4' antenna's.

The only difference will be, the pattern the signal leaves the antenna in. The 8' antenna's pattern will be slightly tighter, allowing for SLIGHTLY better broadcast.

That said, I have NEVER found any difference in the TX (broadcast) capability of a Digital brand 4' vs 8' antenna in real life. I HAVE seen a slight difference in the RX (receive) side of an 8' vs a 4' Digital brand antenna. But the difference is not that significant. More important, is the height the antenna is mounted. As noted, higher is better.

If you will be going offshore, every inch of range could make the difference between life and death in a bad situation, so I'd suggest the 8'. But if it is not an offshore boat, a 4' will be just fine.


8' vs 4' pro's and cons:
Pro's:
- 8' better RX range (a higher gain).
- 8'ers are ussually mounted higher since it is longer, which helps range.

Cons:
- 8' is twice the lenght and weight of a 4', which means twice the stress on the antenna and mount, and whatever it is mounted to. Can be a consideration on soft tops, small boats...
- You might not have space for a 8' antenna to be mounted...
- 8' reqires more clearance for bridges, garages....

auntiepaula
03-30-2012, 09:31 AM
Antenna's do not have "power". ;) AND, a longer antenna will not have any affect on the "power" that is broadcast from the radio. The VHF's we use are 25 watt's, and 25 watts will leave both the 8' and 4' antenna's.

The only difference will be, the pattern the signal leaves the antenna in. The 8' antenna's pattern will be slightly tighter, allowing for SLIGHTLY better broadcast.

That said, I have NEVER found any difference in the TX (broadcast) capability of a Digital brand 4' and 8' antenna in real life. I HAVE seen a slight difference in the RX (receive) side of an 8' vs a 4' Digital brand antenna. But the difference is not that significant. More important, is the height the antenna is mounted. As noted, higher is better.

If you will be going offshore, every inch of range could make the difference between life and death in a bad situation, so I'd suggest the 8'. But if it is not an offshoreboat, a 4' will be just fine.


8' vs 4' pro's and cons:
Pro's:
- 8' better RX range (a higher gain).
- Is ussually mounted higher since it is longer, which also helps range.

Cons:
- 8' is twice the lenght and weight of a 4', which means twice the stress on the antenna and mount, and whatever it is mounted to. Can be a consideration on soft tops, small boats...
- You might not have space for a 8' antenna to be mounted...
- 8' reqires more clearance for bridges, garages....

Great information :)

colecaz
03-30-2012, 03:56 PM
Birdman:

+2 :thumbsup:

Flot
03-30-2012, 04:38 PM
I also changed radio brands, but I went from a brand new radio + 4' antenna on the previous boat, do a brand new radio + 8' antenna on the new boat, and I was astounded how much more chatter I picked up. Difficult to say how much MIGHT be because of the radio but was still a surprise to me, as I figured a 4' antenna on top of a t-top would be plenty.

capecuddy
03-30-2012, 07:53 PM
An 8' antenna is 3dB more directional than a 4'. That's not just a little bit...that's twice as much gain. and that's just one-way. It's 3 dB more on receive as well. That's another 2X.

Very. Big. Difference.

sickcat
03-30-2012, 08:40 PM
What are you doing with a Skippy in TN anyway??? :D

Crabpot Man
03-31-2012, 02:51 AM
An 8' antenna is 3dB more directional than a 4'. That's not just a little bit...that's twice as much gain. and that's just one-way. It's 3 dB more on receive as well. That's another 2X.

Very. Big. Difference.

Which could actually hurt more than help unless sitting in flat calm waters. :thumbsup:

capecuddy
03-31-2012, 05:36 AM
Which could actually hurt more than help unless sitting in flat calm waters. :thumbsup:

Please explain that one....

The pattern that causes needless signal fluctuation due to pitch/yaw/roll is the 9 dB gain of a 16 footer. Its pattern is more directional but narrower vertically.

The 8 footer (6 dB gain) is close to optimum as it has a pattern that is fairly constant vertically through pitch, yaw and roll. A 4 footer is also good in pitch. yaw and roll but you are missing out on useful gain of the 8 footer.

jhebert
03-31-2012, 06:53 AM
All antenna gain results from a concentration of the radio-frequency energy into a smaller beam. Antennas with higher gain have narrower main lobes.

Antenna gain tends to increase with antenna array length, but it is not a guarantee. A well-made 4-foot antenna may work better than a poorly made 8-foot antenna.

The height of the antenna base is very important. Since you have a RADAR arch the base of the VHF Marine Band antenna will be nicely elevated.

I describe the installation and performance of the 4-foot antenna I have been using in

http://continuouswave.com/whaler/reference/VHFAntenna.html

You may find that article to have useful information for you.

I also see many commercial, law enforcement, or coast guard boats have 4-foot antennas. The 4-foot antennas tend to be more durable. Perhaps that is why they are used in commercial, law enforcement, and coast guard boats in preference to longer antennas.

Crabpot Man
03-31-2012, 01:37 PM
Please explain that one....

The pattern that causes needless signal fluctuation due to pitch/yaw/roll is the 9 dB gain of a 16 footer. Its pattern is more directional but narrower vertically.

The 8 footer (6 dB gain) is close to optimum as it has a pattern that is fairly constant vertically through pitch, yaw and roll. A 4 footer is also good in pitch. yaw and roll but you are missing out on useful gain of the 8 footer.

All antenna gain results from a concentration of the radio-frequency energy into a smaller beam. Antennas with higher gain have narrower main lobes.

http://www.boat-project.com/tutorials/vhf23.jpg

Image courtesy of http://www.boat-project.com/tutorials/vhfant.htm which on first look appears very informative.

cyclops2
03-31-2012, 05:00 PM
Dam. I hate modern sales pitchmen. Most lie worse than their competitor. Where does that lead to ??

Height will ALWAYS win out in the marine bands.Even a fishing rod & wire on a 50' hill top will ALWAYS outreach the best tuned boat antenna.
So find a deck spot just in front of the windshield on the glovebox side.where the covers will not be a problem.
I use a 8' with the lots of teeth in the clamping gears. Tilt down for low clearences. Then back up. 10 seconds each way.

Big boat with a arch ? not to strong of a arch ? Use the tilting to keep it low for wind resistance. Need it ? stand it up.

Go for the brand name with a large signal gain. Getting the right length FACTORY MADE antenna cable is important. Some old units were tunable for the cable & antenna. Not sure about new stuff. Do NOT let a boat mechanic shorten the cable.
GREAT way to cause a loss of range, if done wrong.

cj
03-31-2012, 05:06 PM
many thanks guys ,,very helpfull

Mist-Rest
03-31-2012, 05:12 PM
I installed 4 foot Digital antennas because The boat looked better with 4's and I don't boat 35 plus miles from shore.

LI32
04-01-2012, 04:31 PM
I have a 4' Shakespeare XT on my small boat, juts because there is a 10' high bridge I take it under on manny occassions, and I don't have to lower it. I would say in the inshore areas the boat gets used (up to 5 nm out), it works just fine. Never done a comparison wiht the 8' Shakespeare XP's on my bigger boat.



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