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  1. #11
    Join Date
    Jul 2003
    Location
    West Australia
    Posts
    11,671

    Default Re: VHF/AIS cable, antenna and connections

    Should you go to the trouble of using low loss cable or not. That is the question. The purists would say definitely yes. Indeed the larger coax may be more robust against corrosion etc simply being bigger. However the loss of performance may never be noticed. Essentially radio signal strength works on a logarithmic scale so half power is barely noticed.
    Re splitters yes definitely fit an AIS antenna on the stern rail rather than a splitter. Performance is very adequate for AIS and gives you a standby antenna in case of top antenna failure or loss of mast. ol'will

  2. #12
    Join Date
    Dec 2007
    Location
    Boat: Falmouth. Work: Cambridge
    Posts
    1,386

    Default Re: VHF/AIS cable, antenna and connections

    A couple dB is probably not that critical to range as it's usually limited by the curvature of the earth. Here's a graph of path loss versus range which I prepared earlier (full article on the CA website):

    vhf_range_1.png

    As can be seen, a 2dB change in attenuation makes rather little difference to the range, hence I think the thicker and thus less flexible cable isn't actually that important.

  3. #13
    Join Date
    Jun 2017
    Posts
    483

    Default Re: VHF/AIS cable, antenna and connections

    Quote Originally Posted by jdc View Post
    A couple dB is probably not that critical to range as it's usually limited by the curvature of the earth. Here's a graph of path loss versus range which I prepared earlier (full article on the CA website):

    vhf_range_1.png

    As can be seen, a 2dB change in attenuation makes rather little difference to the range, hence I think the thicker and thus less flexible cable isn't actually that important.
    And @William: thank you.

    As I understand, for normal boat to boat communications (combined radio horizons <20NM) it should not make a difference. Question is, what is the case when communicating with coastal stations installed on e.g. 100m tall cliffs or SAR planes?

    Other question is the ideal antenna gain (=length?), but it seems to me that the usual 3dBi should be fine.

  4. #14
    Join Date
    Jul 2018
    Location
    Bewdley, Worcs
    Posts
    513

    Default Re: VHF/AIS cable, antenna and connections

    Quote Originally Posted by GTom View Post
    Which type of cable did you have before? High quality RG8X or just an RG58?
    I had RG58 before, ~ 5mm diameter.

  5. #15
    Join Date
    Jul 2018
    Location
    Bewdley, Worcs
    Posts
    513

    Default Re: VHF/AIS cable, antenna and connections

    Quote Originally Posted by jdc View Post
    A couple dB is probably not that critical to range as it's usually limited by the curvature of the earth. Here's a graph of path loss versus range which I prepared earlier (full article on the CA website):

    vhf_range_1.png

    As can be seen, a 2dB change in attenuation makes rather little difference to the range, hence I think the thicker and thus less flexible cable isn't actually that important.
    But, if you are replacing the cable, and the cost difference is minimal, why would you not use it? You are correct, in good conditions, it may not make much difference, however in marginal conditions, it could be the difference between being intelligble and not.

    Remember, our radios are FM, not AM/SSB ... there is a very sharp cut-off point at which the FM discriminator is unable to resolve the signal and it goes from being slightly hissy to a bag of crackles. 2dB here could make a lot of difference. On an SSB signal, it just gets a proportionally weaker with respect to the noise, on FM there is a sudden and quite severe change.

    I appreciate that in *most* circumstances it will make little difference, but in marginal conditions it could be the difference between your distress call being intelligible and not, and for the sake of a couple of extra quid on the rable run, I'll take it.
    Last edited by rszemeti; 20-09-19 at 11:01. Reason: smelling pistakes

  6. #16
    Join Date
    Jun 2005
    Posts
    10

    Default Re: VHF/AIS cable, antenna and connections

    Quote Originally Posted by rszemeti View Post
    But, if you are replacing the cable, and the cost difference is minimal, why would you not use it? You are correct, in good conditions, it may not make much difference, however in marginal conditions, it could be the difference between being intelligble and not.

    Remember, our radios are FM, not AM/SSB ... there is a very sharp cut-off point at which the FM discriminator is unable to resolve the signal and it goes from being slightly hissy to a bag of crackles. 2dB here could make a lot of difference. On an SSB signal, it just gets a proportionally weaker with respect to the noise, on FM there is a sudden and quite severe change.

    I appreciate that in *most* circumstances it will make little difference, but in marginal conditions it could be the difference between your distress call being intelligible and not, and for the sake of a couple of extra quid on the rable run, I'll take it.
    Well exactly - you want to minimise losses as best you can. If you have the opportunity to minimise losses, unless there is a good reason not to (ie you can't physically fit it), then it would seem to me that it is prudent to do so. If you take the attitude that "it's only a few dB", and apply it to every stage in the chain - cheap antenna with slightly worse performance, cheap coax with slightly more attenuation, cheap connectors with slightly higher insertion loss etc, you can quite quickly end up with a quite large difference in performance.

    It's really a cost/benefit trade-off. If you only ever sail up the coast and back and use VHF mainly for talking to marinas you probably won't notice any difference. If you travel further afield and want to have the best possible reliable coverage, especially for AIS, then it's worth taking the time and spending a little extra to get the best out of your equipment.

  7. #17
    Join Date
    Dec 2007
    Location
    Boat: Falmouth. Work: Cambridge
    Posts
    1,386

    Default Re: VHF/AIS cable, antenna and connections

    Quote Originally Posted by kwb78 View Post
    Well exactly - you want to minimise losses as best you can. If you have the opportunity to minimise losses, unless there is a good reason not to (ie you can't physically fit it), then it would seem to me that it is prudent to do so. If you take the attitude that "it's only a few dB", and apply it to every stage in the chain - cheap antenna with slightly worse performance, cheap coax with slightly more attenuation, cheap connectors with slightly higher insertion loss etc, you can quite quickly end up with a quite large difference in performance.

    It's really a cost/benefit trade-off. If you only ever sail up the coast and back and use VHF mainly for talking to marinas you probably won't notice any difference. If you travel further afield and want to have the best possible reliable coverage, especially for AIS, then it's worth taking the time and spending a little extra to get the best out of your equipment.
    Hmm, I'm not 100% convinced (and I have spent my life doing RF engineering, where we say that any RF engineer would sell his grandmother for a dB). My reason is that I've looked at quite a few VHF installations and have really very often found 20 or more dB of excess loss due to decaying antenna matching components, poorly made up connectors and/or chafe (especially bad around the mast base and where the cable passes through a deck-gland and so the connector has been taken off and on a few times for mast removal), and bad grounding and supply to the sets themselves,.

    Inline connectors usually BNC, are nearly always accompanied by dry-joins if the BNC is the solder type, or shoddy connections and discontinuous grounds where the BNC is a crimp type since nobody (even yards) ever have the proper crimping tool.

    So as a 'triage' I'd put use of RG-58' rather low down the list of reasons for poor performance. Of course it's better to use better cable, and I use RG-8X on my boat (not RG-8U as it's so hard to make connections if doing it up the mast while in a bosun's chair except when using one of these http://shakespeare-ce.com/marine/pro...p-g-connector/ into which RG-8X fits but not RG-8U).
    Last edited by jdc; 21-09-19 at 16:28.

  8. #18
    Join Date
    May 2007
    Posts
    38,313

    Default Re: VHF/AIS cable, antenna and connections

    It's surprisingly common for radio systems to work better with a little bit of loss between antenna and transceiver.
    As jdc says, you're missing the point if you use 'good' cable then ruin it with poor connections.
    The biggest crime is allowing any trace of damp to get into the coax, you can get 10dB of loss before you know it, or worse!

  9. #19
    Join Date
    Feb 2003
    Posts
    15,662

    Default Re: VHF/AIS cable, antenna and connections

    Quote Originally Posted by GTom View Post
    Planning to shop for an AIS and as the 13m tall mast is down at the moment, thinking on replacing my corroding venerable RG58 cable. Couple of questions:

    - Is it wiser to get an antenna that has a cable already joined? Regarding cable, is the 2dB difference between RG213 (10mm diameter) and RG58 relevant on about 18m (60') distance at sea? RG213 is much more difficult to handle being twice thicker.

    - Where shall I put the splitter? I thought about right next to the deck gland where the cable enters? Yearly mast unstepping is much easier if all cables have a connector at the mast base.
    At the end of the day, in the real world, I used RG58 cable, has it impacted on my connecting to whoever I wanted to? Nah!

  10. #20
    Join Date
    Jun 2017
    Posts
    483

    Default Re: VHF/AIS cable, antenna and connections

    Quote Originally Posted by lw395 View Post
    It's surprisingly common for radio systems to work better with a little bit of loss between antenna and transceiver.
    As jdc says, you're missing the point if you use 'good' cable then ruin it with poor connections.
    The biggest crime is allowing any trace of damp to get into the coax, you can get 10dB of loss before you know it, or worse!
    Agreed, decided to take a few precautions. As for connections, I'll keep the cable in one piece at least until the splitter. Likely going with RG8X, just received word that the antenna kit I was interested in has 12dB/100m loss on the RG8X cable. Obviously with a 30m mast serving a significantly larger horizon, I'd go with something much better.

    Still undecided regarding splitter: I like simplicity and found an AIS with integrated splitter, which sounds appealing. Obviously some losses can be expected here as well compared to having an individual antenna.

    Quote Originally Posted by kwb78 View Post
    If you travel further afield and want to have the best possible reliable coverage, especially for AIS, then it's worth taking the time and spending a little extra to get the best out of your equipment.
    If I go further, I get HF gear and a satphone... I don't nurture high hopes about beyond-horizon communications on VHF.
    Last edited by GTom; 22-09-19 at 09:57.

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