Antenna vs. Bimini?

ElBichoMalo · #1 05-16-2008, 08:51 AM

I have a 20SF. I plan to add a bimini top, and I'm not sure where to mount the VHF antenna. If I mount to the gunwhale, it would be in the way when fishing, and still probably in the way of the top. On the console, seems like I wouldn't be able to have the top and the antenna up at the same time. I'm in sunny florida...the shade is a must, and I do not want a t-top. With the popularity of center consoles, surely there is an easy answer.

ScottM · #2 05-16-2008, 04:40 PM

If you don't venture offshore, try a 3-foot SS whip or 4-foot glass antenna on the console which should fit under the bimini.

3rdday · #3 05-17-2008, 12:33 AM

I have a new Big bimini that I will sell you cheap.

CaptLloyd · #4 05-20-2008, 08:32 AM

I have a 8' high gain antenna attached to the grab rail for the windshield with a swivel mount. When I put the bimini up, I angle the antenna back so it sticks out from under the bimini. It's a little ackward, but going offshore, I feel a big antenna is a must. And I agree with you, gunwale mounted would get in the way.

Sorry I can't show you a pic, I'm on a trip and won't see my boat for at least a month.

Lloyd

BigLew · #5 05-21-2008, 12:09 AM

Regarding antenna height and sensitivity, one has relatively little to do with the other. Let me explain.

VHF is essentially a "line of sight signal", which means that if the transmitting antenna has a "theoretical" line of sight to the recieving antenna, then there should be success in reaching one to the other. An 8 foot antenna with its tip 11" above the water surface (gunnel or other mounting surface height) transmitting to an 8' antenna similarly mounted will have a farther range than a 3 ft. antenna transmitting to another 3 foot antenna mounted in the same fashion. The reason for this is that at some point of separation, the earth's curvature will block the "line of sight." Another example is the typical 3' antenna mounted on top of a sailboat's mast will have a farther range than the same antenna mounted on a SC's gunnel.

Having said all that, there is also the other and more important consideration of what radiating pattern the antenna has. Most of the antennas in use today on our type of boats are 6 dB in design. Antennas also come in 3 dB and occasionally 9dB. The 9 dB antenna will actually radiate reliably transmit a usable signal a greater distance. The 3 dB a shorter distance and the 6 dB some point in between. Why doesn't everyone then use a 9 dB antenna you ask; because of wave motion.

Essentially all of the antennas transmit in 360 degrees. If you look at the transmitted signal as a donut with the antenna tip in the middle, the radiated patterns will look like the following: 3 dB- smaller overall diameter, but very high altitude (think a small but very fat donut) 6 dB is like taking the 3 dB and flattening some, it spreads out wider but it is not as thick. the 9 dB gets really flattened out(longer from outer edge to outer edge but very flat.

Here's the thing. As our boats roll side to side and up and down waves the angle of radiance from the antenna does not change- it remains at 90 degrees from the antenna's axis. If we roll to starboard, the radio signal looses range to starbord because the radio signal hits into the water sooner than if the boat was sitting level in flat water. Like wise on the port side the signal goes out at an upward angle. If the receiving antenna is not within the donut, then it doesn't receive the signal! This is also why a sailboat will typically use 3 dB antennas; the fatter donut allows for the roll and heeling of the boat. A sailboat under way is seldom truly level in the water, but the height of the mast offsets the range issue due to the grater overall height of the antenna tip. Also think of the big sport fish with 8 and 12' mounting posts that the antenna is mounted on top of.

The 9 dB is generally not used because of the high likehood of the recieving antenna not being within such a "flattened" transmission pattern.

More than you probably wanted to know. If you want to see a illustration of all this check out a West Marine catalog. They usually have a page of explanation near the section listing VHF antennas. Look at page 24 in the 2007 W/M catalog.

With regard to choosing an antenna for you center console, I would probaly go with a 8" 6 dB antenna mounted just frward of the bimini and plan to lay it down on the gunnel while I fished. If I wanted some radio capability while fishing, I would have a hand held or a 3" 6 dB antenna mounted below the bimini of out on the gunnel where I could easily get a line up and ove the 3 ' antenna if need be.

Remeber, it can be your life line if trouble occurs and don't be out of communication because you saved $20-30 on a cheap antenna. What you don't see on from the outside is the quality of electrical components on the inside. Also, squeeze a cheap antenna and then squeeze a more expensive one. You should notice that the costlier one has a thicker outer shell and will take more abuse and so will the solder connections and the heavier gauge metal inner compnents. Typically the antennas I would recommend for most coastal boats like ours typically cst between $70 - $85 a year or two ago.

Just my 2cents.

ElBichoMalo · #6 05-21-2008, 04:03 PM

BigLew....already read the BoatUS article. I am only considering 8', just not sure where to mount. I do not plan to go offshore far or often (but these are old boats, we just might need the radio, and why not have a little overkill anyway). Not everyone on this site has a t-top. Anyone have photos of where your antenna is mounted?

BigLew · #7 05-21-2008, 05:09 PM

ElBichoMalo,

I understand. I just wanted to help if there were points that you didn't know of or other issues you might consider.

For what it is worth, I would mount an 8' antenna on the gunnel just froward of the most forward part of the bimini. That way, you could run whenever you want and still have both the bimini up AND VHF available. If/when you drop the bimini for fishing, then you could also fold down the antenna along the gunnel and fish "over" it. If you needed to use the VHF then you could simpley raise the antenna and use the radio.

Just my 2 cents. Good luck.

PS- Consider getting a hand held VHF sometime. They make great gifts!

gordonhg · #8 06-02-2008, 03:01 AM

I have a 20 CC with a bimini. I mounted my antena on the starbord rear corner of the boat, about 6" forward of where the transom boxes come accross, about 1 1/2 foot in from of the transon. Mine is mounted on the inside if the hull so I can walk along the gunnle with out stepping on it, it is quite out of the way when it is laying down flat and parallel to the floor. I use a rubber clip at the end and middle to keep in in place. It is mounted on a cam/latching swivel mount, similar to West Marine part #1986868. When I want to put it up, it sticks up from the stern of the boat out of the way of the bimini. Its mounted about 6" down from the top of the gunnel so it is not in the way of the floor or scuppers..

I suppose if you really wanted to you could mount one like that, install an antena switch and mount a small 3ft antena on the console under the bimini.. Then you could use one or the other..

Aloha

It is 8ft feet or so.