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Thread: VHF Radio Testing

  1. #21
    Member 4merguide's Avatar
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    Quote Originally Posted by akdeweyj View Post
    I have a chart somewhere that I will try to post that shows separation req'd for different electronic units but I don't think it mentions anything about distance between antenna & radio.
    In a perfect world when transmitting all RF will be radiated out the antenna. But for most all accounts antenna systems are not perfect. When the antenna system is not properly matched to frequency then the coax will also radiate reflected RF. If the radio is real close to the antenna then, when transmitting, a lot of this stray RF will end up bouncing around inside the radio. Not good. The higher up in frequency you go, ie, VHF/UHF the more important antenna systems are. Poor quality (high loss) coax plays a huge factor at higher frequencies.
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  2. #22
    Member kobuk's Avatar
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    Quote Originally Posted by 4merguide View Post
    In a perfect world when transmitting all RF will be radiated out the antenna. But for most all accounts antenna systems are not perfect. When the antenna system is not properly matched to frequency then the coax will also radiate reflected RF. If the radio is real close to the antenna then, when transmitting, a lot of this stray RF will end up bouncing around inside the radio. Not good. The higher up in frequency you go, ie, VHF/UHF the more important antenna systems are. Poor quality (high loss) coax plays a huge factor at higher frequencies.
    My radio is mounted on the ceiling but i have a long coax. It's been a while but I remember it was quite long and I did coil it up under the dash and then back up to my radio. Would there be any issues if I mounted my VHF on my dash next to my GPS/Sounder combo? My new garmin uses an internal GPS antena. I would prefer the ceiling mount but a dash mount wouldn't be the end of the world. I supose using the morad antena without a stantion would be a little shorter and maybe loose a little range, but maybe being a better antena would be the offset? I'm probably overthing this too....

  3. #23
    Member akdeweyj's Avatar
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    Quote Originally Posted by cjjeeper View Post
    Will that screw right in to replace my garmin antenna? (if you happen to know...)

    Thx
    The threads should be the same. Here is a picture of my Morad > Adapter > SS Ratchet mount that is bolted to the side of my radar arch. I lay it back & lock it when travelling.

    20160906_173436_002.jpg
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  4. #24
    Member akdeweyj's Avatar
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    Quote Originally Posted by akdeweyj View Post
    Yes it mounts to the adapter that macncoke described & then into the SS ratchet that is mounted on the port side of arch. VHF is almost directly below antenna with about 3 vertical feet of separation. No stanchion. I have a chart somewhere that I will try to post that shows separation req'd for different electronic units but I don't think it mentions anything about distance between antenna & radio.
    Here is the antenna spacing chart that Nmea publishes.

    Nmea Antenna.jpg
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  5. #25
    Supporting Member iofthetaiga's Avatar
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    Quote Originally Posted by 4merguide View Post
    In a perfect world when transmitting all RF will be radiated out the antenna. But for most all accounts antenna systems are not perfect. When the antenna system is not properly matched to frequency then the coax will also radiate reflected RF. If the radio is real close to the antenna then, when transmitting, a lot of this stray RF will end up bouncing around inside the radio. Not good. The higher up in frequency you go, ie, VHF/UHF the more important antenna systems are. Poor quality (high loss) coax plays a huge factor at higher frequencies.
    This dovetails with another issue on small craft: RF exposure to personnel. FCC calls for 5 meters minimum distance between any part of the VHF antenna and personnel to prevent dangerous levels of RF exposure. Needless to say, on mastless boats it's impossible to get far enough away from the antenna to meet the requirement and mitigate exposure, and on a lot of boats we're standing only a couple feet if not only a few inches from a 6 or 9 DB gain antenna at ? watts... Might be somewhat less an issue with properly rigged aluminum boats, but definitely a concern with glass or composite construction. I am currently constructing the latter, and am scratching my head trying to figure out a suitable means of integrating functionally effective RF shielding into my pilothouse roof (upon which my antenna will be mounted)...
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  6. #26

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    Quote Originally Posted by akdeweyj View Post
    The threads should be the same. Here is a picture of my Morad > Adapter > SS Ratchet mount that is bolted to the side of my radar arch. I lay it back & lock it when travelling.

    20160906_173436_002.jpg

    yep cool.. thats what I bought. Thanks
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  7. #27
    Member kobuk's Avatar
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    Where is a good place to buy or order these antenas?

  8. #28
    Sponsor potbuilder's Avatar
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    Quote Originally Posted by kobuk View Post
    Where is a good place to buy or order these antenas?
    Call morad in seattle

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  9. #29
    Member kobuk's Avatar
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    Quote Originally Posted by potbuilder View Post
    Call morad in seattle
    Thanks and I'll give them a call.

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