Steve, et al,
By the time I read this thread and found out it was about a VSWR issue, Steve is already at sea / on-passage...so, he's got what he's got...(I don't have enough time to read everything here! I wish it had "VSWR fault" or some such wording in the title...)
Quote:
Originally Posted by Emerald Sea
PS, we leave Namibia for St Helena later today so it will be 10-12 days before I receive CF emails. This support is great! Beers on me ... Should we meet up.
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But, at least I can offer a few tidbits of info that Steve, and all of you, will probably find helpful!
1) None of us being there with Steve, on Emerald Sea, further since we do not know what the "SWR" actually is, nor what it was, nor what coax was there, nor what is there now, we cannot be certain of anything.....so, we just do our best to offer the
advice that seems to make the most sense!!
So, just a tip for anyone else....take the time to give everyone here the most detailed description of the issues you can, with as much info/specifications as possible....
And, you're likely to get some great advice/assistance!!
2) As "continuouswave" and Paul have already explained in detail, a lossy
transmission line (old, lossy coax) can mask an antenna's high SWR....
But, be aware that even if replacing the coax with the same type/size, you
can see a marked decrease in loss, due to the old coax being old and more-lossy (if that a real word?), as well as because of new connections being better than the old ones!
So, a high SWR problem
can be noticed when replacing the coax, even if not replacing it with a low-loss variety...
3) But, big kudos to Paul for picking up on what I was going to write!
Higher-gain antennas, especially higher-gain collinear verticals, will exhibit a narrower operational bandwidth (both in gain and VSWR)....
Quote:
Originally Posted by Paul Elliott
This sounds like the higher-gain style of antenna I was mentioning. I've only done the measurements on a few of these, but at least on mine the antenna tuning is much sharper than it is for the short VHF whips. I measure a good SWR on channel 16, and a pretty bad one (the number escapes me) on the AIS channels.
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Paul hit the nail on the head here!!!
Combine the high-VSWR at 162mhz of the typical "6db" [sic] marine VHF antenna, with the lower loss of new coax (and/or lower-loss coax), and you have a damn good guess of what's going on with Steve's set-up!!!
While a shorter VHF
Marine Whip antenna (such as the typical 3' SS end-fed half-wave), should be able to have an
acceptable VSWR across the entire 6mhz wide "VHF Marine Band", even some of these do not....and certainly most of the longer, higher-gain 8' / 6db[sic] do not....and the problem is worse for the larger antennas...
{note that some "new" versions may use a stagger-tuning, to allow a good VSWR near 156.8mhz and 162mhz (with a slight sacrifice in gain)....but most do not..}
FYI, here is an VSWR plot of my own Shakespeare 3' SS whip, mounted at the masthead (fed with 70' of LMR-240uf and 25' of LMR-400uf)....
I also have another Shakespeare 3' SS Whip (designed/factory-tuned for the AIS freqs, at the top of the VHF Band) on a pole, at the stern...but cannot find the VSWR plot! ARGG!!!
But, take my word for it, it has a VSWR of about 1.5:1 at 162mhz....
I hope this info helps some, and most find it at least, interesting?
Fair winds..
John