Thanks for all the information and suggestions. I finally finished the redo of my helm for my center console.
The top radio is for scanning, and the bottom radio is to be set on 16. I put a new Jensen stereo in, bottom right on the console, as well as a new compass. While I was at it, I upgraded my fish finder GPS Chart plotter to the Lowrance 5 DSI. No idea how the unit works as I am not tech savvy to figure it out 1 2 3.
I love the two 8ft antennas along with the 4ft AM/FM antenna. Only issue is when its rough, the two antennas hit each other. Is there a product that would attach to both to keep them separated? I had to mount them close due to the canvas. I rarely use the bimini top so Ill just lower the antennas when in use (usually when anchored out)
The antennas look awfully close. FWIW, NMEA says 4 ft VHF antenna to VHF antenna spacing though you are certainly not the 1st person to "live on the (spacing) edge ". Have you tried transmitting at full power through each VHF yet to see how the other 2 radios handle it? Even standing that close while transmitting reportedly ...could... be deleterious.
Here's an interesting article on VHF antennas , with the gain section particularly interesting to me . http://www.boat-project.com/tutorials/vhfant.htm
It also mentions ,"Safety. There is a safety concern - at least from this boat owner's point of view - about electromagnetic radiation from the antenna. Most radio manufacturers will recommend one to three meters separation between occupants and the antenna. This may be difficult to achieve on a small boat., While you might minimize this problem by using a lower gain antenna to reduce the ERP, this is no assurance that you are safe from excessive electromagnetic radiation. The only safe practice is to read and heed the VHF radio manufacturer's safety instructions. "
I have an 8' vhf antenna on a 2' extension above the mount that I felt could use a stabilizer. I cut a piece of nice white (~3/4"dia.?)thin-wall PVC conduit at the exact level (& almost fore and aft) distance between the antenna and the side forward part of my marlin tower rail tube, taking into account shaping the ends to match the curve of antenna casing and rail tube. Then ran light white braided cord through the center to secure at both ends. Works great in very rough conditions and has probably avoided several antenna replacements.
Perhaps something like that fairly low would prevent inter-contact? Or maybe even a length of light shock cord to keep them from contrary-whipping enough to contact?
They are fairly close I'd prefer more distance between them but what's the worst that can happen? The transmitting radio can de-sensitize the other radio's receiver. Otherwise when both are receiving there won't be any issues perhaps there's a remote chance with the antennas that close they could be directional to a small degree. One might act like a reflector or a director similar to a 2 element Yagi, but I doubt it.
I think it looks fairly good Mark. I thought you were going to mount the antennas towards the stern at opposite sides that would have given more separation but perhaps the cables wouldn't be able to have been run this way?
As far as safety there are 2 standards for MPE (maximum permissible exposure) controlled and uncontrolled environments. You would be in a controlled environment so the higher level of MPE would be acceptible. With that being said the percentage of use or the number of minutes transmitting in a 24 hour period would be very low. You really have no health issues even with the units transmitting at high power. The levels were really developed and changed because of the cellular industry placing antenna arrays on Condo and apartment buildings (uncontrolled) and to protect tower workers (controlled). But cellular transmitters operate 24/7/365 unlike your marine VHF radios. In my opinion you have no MPE worries at the power levels you will be using and for the percentage of use in a 24 hour period.
Bill
"The transmitting radio can de-sensitize the other radio's receiver. Otherwise when both are receiving there won't be any issues perhaps there's a remote chance with the antennas that close they could be directional to a small degree. One might act like a reflector or a director similar to a 2 element Yagi, but I doubt it."
+1 to Bill - in practice the distance between the two really have very little performance difference unless they are both in use transmitting. This comes from using a boat for 8 seasons with a closer installation. Relax and use the boat and have a great time as you likely will not see anything or a problem.
The two 8ft VHF antennas are roughly 39" apart. The AM/FM 4ft is inches away from one 8ft VHF antenna.
I did not want to transom mount the antennas for a few reasons. 1. I am not a fan of putting holes into the transom. 2. I want to thru bolt the base, if it were the transom it would only be screws. 3. running the wires would have been a pain in the rear. 4, I would have some extra holes on the center console..
To stop them from slapping in rough seas try tying fishing line between them at the very top. This might help stop them from swaying outward and springing back into each other. Someone else can answer as to if this might be a problem electronically if the line gets wet.
To stop them from slapping in rough seas try tying fishing line between them at the very top. This might help stop them from swaying outward and springing back into each other. Someone else can answer as to if this might be a problem electronically if the line gets wet.
A wet fishing line tied to them probably wouldn't contribute to a high VSWR. It could work. If that's a problem perhaps a small drilled piece of PVC pipe slid over them would help. I just don't see antenna movement being a problem.
Bill
Billy, its not really a problem. They slap together and make an annoying sound. Nothing I cant deal with. Just dont want one antenna or both to crack over time from the slapping...
If you tip one antenna slightly forward and one slightly backwards it should reduce the slapping, but at rest the visual may be more annoying than the slapping.
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