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Old 08-06-2017, 13:17   #1
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Masthead VHF Antenna - The devil is in the details...

Another thread regarding mounting multiple masthead antennas had a few nuggets of information that peaked my interest but I did not want to hijack that thread...

We have a VHF antenna mounted at the masthead via LMR-400-UF coax, and a dedicated AIS antenna on the first spreader connected to the AIS via LMR-400-UF coax. Knowing that height plays an important role in the performance of a VHF / AIS system, when we bought the boat in 2013 I decided to experiment with using the masthead antenna for both VHF and AIS by using a Vesper Marine AIS/VHF Splitter with built-in low noise amplifier. At the same time I upgraded the existing AIS to a Vesper Marine XB-8000 AIS, and I was introduced to the concept of VSWR through Vesper's Watchmate application software.

When connected directly to the AIS, VSWR indicated by the XB-8000 was 2.66:1 for the masthead unit and about 2:1 for the dedicated AIS antenna. However, when I used the splitter, the VSWR would increase on the masthead antenna to about 3:1 and 2.33:1 for the dedicated AIS antenna. I thought there might be a problem with the splitter, but after extensive troubleshooting with Vesper's tech support (which was excellent and responsive), we came to believe the splitter was functioning correctly. I went up the mast and inspected things and there was nothing obviously wrong so I lived with the higher VSWR and stuck my head in the sand. This issue continued to nag the back of my mind but I seemed to receive OK and do not use the VHF often, so I just assumed things were perhaps not optimal but still working OK.

Fast forward a couple years, this past winter we were approaching a port and I tried to contact the harbormaster. I could hear them, but they were really struggling to hear me and actually asked how far away I was because of the poor transmission quality... I had them in direct sight and was less than 1/2 mile from their antenna! This made me realize that I could not ignore this problem any further. I checked the VSWR and the Vesper application indicated a VSWR problem with a 4:1 VSWR. I swapped to the dedicated AIS antenna and found that VSWR had also increased to about 4:1, so the problem had gotten worse over the last three years. I eventually went back up the mast and took a closer look and again nothing obvious, but comparing the pics I took a few years earlier to present, there was noticeably more discoloration around the cylinder at the base of the VHF antenna (a Metz). Reluctantly, I undid the sealed connection from the coax to the antenna and found that water must have made its way into the loaded coil base and then down into the connection as there was corrosion evident inside the connector.

I have several questions that I am hoping folks can help with:

1) The instructions for my replacement antenna (a Digital Antenna 222-VW) specifically state that you should NOT mount the metal L-shaped mounting bracket so that the vertical part of the L is even with the loaded coil base of the antenna. That is, the mounting bracket should be mounted so the L is upside down with the antenna mounted to the top of the mounting bracket and the loaded coil base above the part of the L mounted to the mast. I believe that in an attempt to get the VHF antenna below the height of the lightning rod also mounted to our masthead, they mounted the L bracket upside down exactly as instructed not to in the instructions. Question: What is the effect of having the loaded coil base right next to the metal of the L bracket, and is it worth worrying about this and flipping the L mount around? I am reluctant to do so if not absolutely necessary as it means more time up, and more holes in, my mast.

2) In the other thread, there was considerable discussion about the effect of another antenna mounted in close proximity to the VHF antenna. While I don't have another antenna on the masthead, I do have the lightning rod within about 8" of my VHF antenna, and it is connected to a fairly large wire running to (I believe) the ground plane system of the boat. Question: Is this not essentially an antenna (or worse), and what are the ramifications of this setup to my VHF performance?

3) Replacing the masthead antenna did almost nothing to the high VSWR reading. Questions: Would this indicate that the corrosion extends into my coax cable and is affecting the impedance? Any other things I should look into to troubleshoot this?

4) If the answer to #3 is "yes", I have a few feet of extra coax slack at the base of the mast that I may be able to pull up and then cut some off the compromised end at the top of the mast. Questions: Is cutting the corroded end likely to be helpful, or is the cable beyond saving? Would I simply cut until I don't see any more corrosion? Any other guidance?

Thanks for your patience with the long-winded post, and any advice will be greatly appreciated...
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Old 08-06-2017, 19:34   #2
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Re: Masthead VHF Antenna - The devil is in the details...

A high swr could be many things.
get yourself a meter like
MFJ Enterprises Inc.
and extra coax so you don't have to climb up and down the mast a bunch of times.
then also a dummy load like this one
OPEK DL-60 - 50-Ohm VHF / UHF Dummy Load 60-Watt

and a pl-258 double male connector

connect up the pl-258 to the coax that's on the back of your radio, then connect the extra coax then the meter on the end of that then head up the mast with the dummy load.
ch 16 is 156.800 dial that in on the meter or as close as you can. what you are aiming for is a swr of 1:1 but 1:1.5 is acceptable.... What this means is 1 watt going thru the coax is giving the antenna the full 1 watt (there are cable losses) but 1:1.5 means 1 watt forward and .5 watt coming back down the line so you end up with .5 watt out.

so dummy load on the end of coax you should see 1:1 swr if not you have a problem with your coax, if you see 1:1 then it's a problem with the antenna.

If you have water in the coax you should replace it. Soldering on the pl-259 isn't all that hard but look up some videos if you aren't sure how to do it, most non hams do it wrong.

yes a metal object less than about 15' will have an effect on the performance.

hope this helps
KB8VRU
long time ham....
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Old 09-06-2017, 06:25   #3
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Re: Masthead VHF Antenna - The devil is in the details...

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Originally Posted by Pyxis156 View Post
3) Replacing the masthead antenna did almost nothing to the high VSWR reading. Questions: Would this indicate that the corrosion extends into my coax cable and is affecting the impedance? Any other things I should look into to troubleshoot this?
Very likely. Once water gets into the coax it wicks along the length of the coax pretty fast. Trying to salvage it by cutting back the ends is almost always an exercise in frustration. You can run through a lot of connectors trying.

My recommendation would be to replace the coax and use good connectors at both ends. Avoid the crimp-on connectors sold in chandleries. You can generally pull new coax with the mast in place unless there is a big drip loop or someone wire tied cable inside the mast.
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Old 09-06-2017, 06:55   #4
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Re: Masthead VHF Antenna - The devil is in the details...

LMR400 has open-cell foam insulation and will be toast, if any water at all gets into it. It is also quite soft and the insulation can be compromised if the cable is pulled against anything, like edges of the mast where it comes out. Just a little indentation may bring the shield and core close enough to cause weirdness in your signal.

I don't like this cable for boat use despite its excellent attenuation properties, and I use RG213 and 214, which has solid insulation, so is much less vulnerable to mechanical compromise or water, for mission-critical VHF and (separate!) AIS antennae. Theoretically there is more attenuation with 213, but at VHF frequencies it's no big deal. In any case, 25 watts is vastly more power than you need for normal marine VHF usage, and will be plenty to deal with a fair amount of attenuation in the transmission line. Much more important is flawlessly installed and correctly protected connectors.

I do use LMR400 for non-critical satellite and UHF antennas, where the attenuation is a much bigger deal, but with great care to protect from the above-mentioned problems. I wouldn't use it for marine VHF radio or for AIS.

As to connectors -- I use Type N wherever possible rather than UHF (SO258/PL259) connectors, the best money can buy, and I crimp them (with some types you still solder the core). The kind which are all soldered are not as durable, and it's real byatch to solder the shield without burning the cable, especially at the masthead in a bosun's chair Downside is the proper crimpers are bloody expensive.

UHF connectors are awful on boats, but it's hard to find antennae and other gear with N connectors. When forced to use UHF, I protect with multiple layers of heat shrink tubing and a "telecomm wrap" -- self-amalgamating tape protected by regular electrical tape. But even a well protected UHF connector is a p*ss-poor substitute for a proper outdoor connector like a Type N.

I do have Type N for my masthead VHF antenna, but was forced to use UHF for my AIS antenna. All coax cables on my boat run without a break or connector all the way to the nav table. Adding a disconnect at the mast base as per usual practice does make it just slightly easier to pull the mast (how often do you do that!?) but adds a major weak point in the cable, and adds insertion loss of the connectors. I figure I will be replacing all the coax anytime I pull the mast anyway, so the unbroken cable run will be no problem.

But even in the worst case, where you don't want to replace it for some reason, you can just pull it back out and tape it to the mast base, or in the even worse case, just cut it. It won't be too late to insert the connector then.

For an antenna, I carefully chose one with an internal dipole which doesn't require grounding on the masthead truck, where getting a good ground is fraught with problems. I have the Shakespeare Galaxy XT.

Almost five years later, this installation is still going strong. I once got a "weak but readable" signal report from Solent Coast Guard from 60 miles away across the Channel in Cherbourg -- USING 1 WATT TRANSMISSION POWER. I normally use 1 watt transmission power and LO receive attenuation settings on my Icom M604. As any radiohead will tell you -- the antenna and transmission line account for at least 80% of the performance of your radio.
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Old 09-06-2017, 07:34   #5
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Re: Masthead VHF Antenna - The devil is in the details...

What are you using to measure SWR? The xb8000 youmentionis not an SWR meter.
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Old 09-06-2017, 08:35   #6
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Re: Masthead VHF Antenna - The devil is in the details...

2:1 SWR is not ok for a working brand-new install on VHF. You should be seeing 1.1:1 to 1.5:1 normally. At 3:1 you're losing 25% of your power, and a modern radio will reduce its power output to protect its internal amplifier.

I'm doubtful that the location of the antenna nor the way the bracket is mounted is causing your SWR problems. You've likely got a bad coax like you suspect, or a bad connector.
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Old 09-06-2017, 10:02   #7
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Re: Masthead VHF Antenna - The devil is in the details...

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Originally Posted by S/V Illusion View Post
What are you using to measure SWR? The xb8000 youmentionis not an SWR meter.
The Vesper Watchmate application [for mobile devices] has a software VSWR meter function.

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Old 09-06-2017, 10:34   #8
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Re: Masthead VHF Antenna - The devil is in the details...

Pyxis,
1) First off, this discussion will probably be long and controversial...
But, as long as you don't mind the long discussion and give some benefit of the doubt, we should be fine..
Also, hope you don't mind if I digress a bit and clarify some points that should be understood by all, before moving forward??



EDIT:
Just saw the other posts here...I spent over 3 hours (on and off) typing all of this...

And, see that Dockhead mentioned connectors...
We disagree here...
Nothing wrong with a properly installed PL-259 (UHF connector), and N-connectors will add no improvement in performance at all (although are easier to weatherproof!)


And, Dave mentioned not using chandlery crimp-ons!! Yes, please heed this advice!!
But, FYI professional crimp-on connectors are great!! (you will just spend more for the crimp tool and dies than you will for a couple runs of cable and connectors on your boat!)






2) Second, the use of LMR cabling is one of the more frustrating things in our lives these days...

[I understand that the initial moisture intrusion in your installation may have come from the antenna / connector itself, and the main issue you started with was probably a bad connection (actually likely no antenna connected at all!)....I think you'll see that if there wasn't LMR cabling being used, this issue would probably have been less severe...and possibly not occurred at all..]

In some RF systems gaining a 1/2db to 1db is important, and I'd never recommend someone go out and willy-nilly lose a db on purpose (except, of course, on HF receiving in noisy environs)...
But the fact is, using "low-loss" cabling for short runs, for VHF-FM, for mostly line-of-sight comms, with 40+db margins on average...well, it's just marketing driven, not actually necessary...
And, since there are issues with connections on LMR cables and weatherproofing these connections, you do end up with moisture intrusion...and a lot more problems than most would ever imagine!


Now why is moisture intrusion an issue with LMR cabling, versus "normal cabling" (like RG-213)??
a) Dissimilar metals!! (as well as the moisture itself, causing issues)
With aluminum foil and copper braid (the "tinning" of the braid is VERY thin, and corrodes very quickly) inside the cable jacket, all it takes is a little bit of moisture to get in there (and this can happen even when the cable is NEW and still on the spool in the shop!) and you end up with dissimilar metal corrosion!! And, usually end up with lossy and intermittent connections at best, or simply no connection (no antenna actually connected) at worst!

b) Low-density-foam dielectric!!
Obtaining low-loss is all about conductor size (surface area) and low-loss dielectric (a vacuum is best, dry air is great, low-density foam is good, high-density foam is okay, solid dielectric is worse...)
But, while a low-density foam allows low-loss, it also allows moisture to wick in very quickly!!

c) Unlike some other low-loss cables (like Davis RF Bury-flex) LMR cables, like most specialty cabling, require specialty connectors, NOT the ones that every other cable of similar size uses...
And, these are usually not available off the shelf, so most usually use improper connectors...
And, while this use of improper connectors can further exasperate moisture issues, this can also simply make the cable/connector junction poor or unusable!! (meaning that if the wrong connector is used, you're apt to not make a good connection, if making any connection at all!!)


Now, if you place these cables in a moist environ, especially with salt water and salt air nearby...well, all it takes is just a little bit of an issue to turn into a big problem in very short order!!

Please understand that I've used low-loss cabling for 35 - 40 years in various applications....and I do use LMR cabling on my boat, but...
But, I understand where, when, and how to use it....
I understand that this probably sounds like "do as I say, not as I do"...and I do not want it to come of like that!!
So, please just know that using LMR cabling is NOT recommended by anyone that knows their stuff in the marine electronics business, unless for some specific purpose and then only if full-disclosures are given!!

{Ever wonder why everyone who has been doing this stuff for decades, recommend RG-213???
It's because it works, and it lasts....
Well, you do still have to weatherproof the connections, but it will last!}

Also, do you have any splices / disconnects in the bilge or at the mast base (most of us do) to allow disconnecting of the mast wiring when unstepping the mast??
As, these are many times the cause of the problems described here!!




3) Whether my preferred Shakespeare 3' SS Whip or your Digital Antenna 3' SS Whip, what you refer to as the "loading coil" is not a loading coil....
These are end-fed 1/2-wave antennas and are not "loaded" antennas...
What is in the base mount is the feedline matching, which is a parallel tuned circuit that allows feeding a very high-imp (1500-2000 ohms) end-fed 1/2-wave antenna with 50-ohm coax...

As to why / how your antenna's base got water into it...I don't know, I'm not there, I can't see it...
But, if the instructions talk specifically about how not to mount the antenna and it was installed/mounted that way (the improper way), that is a big tip-off to me...(perhaps they know that their antennas need a particular mounting inorder to not break/crack??)
Also, the fact that Digital has some sort-of "twist-off" mechanism for their antenna, that may in fact impart more stress / force on this base section?? And/or may allow some pathway for moisture to get in???

Again, I'm not there...can't see it, and don't know how/why the moisture got in there....

But, I'd recommend getting a new antenna (Shakespeare is what I recommend), and of course ripping out the LMR cabling and using RG-213!!



4) You notice I didn't mention VSWR yet??
That's because I wanted to get some of the above basics out-of-the-way first....AND...


And, because while I think Vesper makes excellent AIS units (and are really nice guys!), I'm not a fan of "software VSWR measurements"....mainly because if they're shown out a decimal point or two, or three, everyone thinks that they are accurate, but they are NOT....in actual practice, even "lab grade" power/VSWR readings are +/- 10%, and at VHF many consumer grade devices have twice the variances, or more....
Not saying that you cannot believe the readings you took, to the contrary I'm going to assume they are pretty good readings, but just understand that when measuring small amounts of power at VHF frequencies, absolute accuracy is tough, so let's just say that these readings are +/- 10%...okay??


{Also, please note that when measuring VSWR at the radio end, rather than at the antenna itself, means the "loss" in the cable actually improves the VSWR you see....meaning that the VSWR is worse at the antenna than you are seeing on the meter (unless there is a bad connection at the radio end, in the bilge, etc..), see below for details... }



Further, you've made no mention of the type / brand / quality / length, etc. of the coaxial jumper cables that you used to connect these devices together...(such as those connecting your radio - Vesper SP-160 - Vesper XB-8000 - antenna)
The reason this is important is that I've seen brand-new, out-of-the-box, pre-made coax jumpers being intermittent and/or produce bad VSWR!!


And, even further, we have no idea if these connections were tight and secure??
Which can also cause intermittents and VSWR issues...


And, do you have any splices / disconnects in the bilge or at the mast base (most of us do) to allow disconnecting of the mast wiring when unstepping the mast???




a) Looking at the Digital 222-VW SWR plot in their spec sheet shows it to be a bad choice for AIS (or VHF/AIS) use...it shows 2:1 at 153.8mhz and 159.8mhz, dipping to 1:1 (?) at 156.8mhz...
This is not what you'd want for AIS (at 162mhz)...as I suspect that it is as high as 3:1 at 162mhz (but this is masked by the losses in the cable)...



Here's a shot of my own masthead VHF antenna's VSWR plot...this isn't as good as my stern-mounted AIS antenna (the 5215-AIS), but is perfectly fine...



b) Looking at the numbers that you write, it seems likely that originally you had connection to the antenna, but since I don't know if your system actually worked or not, I cannot be sure! (understand that a dummy load has a perfect VSWR, but does not work as an antenna...and an antenna that is up in the clear with a bad VSWR will still work okay....)
VSWR is ONLY a indication of whether the antenna is matched to the feedline, NOT if the radio system is actually working....for that you have to make contacts and/or chart receiving signal strengths vs. distances, etc...
But, for sake of furthering the discussion to try to find solutions (rather than speculating endlessly), let's assume you did originally have a connection from the radio (radio and AIS) to your masthead antenna (Digital 222-DW)??
And, at some point water got into the antenna base (and connector), and either corroded things beyond ability to maintain contact and/or the water froze in the winter and just broke the connection altogether??



You mention that you don't use the VHF often??
You wrote that it "seemed to receive OK", and that you "do not use the VHF often", so you just assumed things were not optimal but OK??

Please note that our VHF Marine Radio systems have incredible margins built-into the system design...and as most of our comms are within 15-20 miles (which is line-of-sight distance between 2 sailboats' masthead antennas), this margin is typically 40+ db!!
So, even mediocre installations can still work "OK"...
So, without any real indication from you whether your VHF radio system ever worked well at all, it's difficult to troubleshoot from 1000+ miles away!!
But, I am trying...



Since your masthead antenna and "dedicated AIS antenna" are fed with different cables, I'm assuming you (and Vesper) discounted the cabling as proximate cause of these issues???
But, I'm still sticking with the cable and connectors AS the proximate cause, or at least the original cause!! (this is assuming the antenna was "new" and in good shape when installed)

And, especially the mastbase / bilge connections (if you have any)....and/or the short coaxial jumper cables you use to connect these devices...



Take note, as mentioned above, that when measuring the VSWR from the radio end, even without any antenna connected (infinite VSWR at the antenna end), you will show some VSWR that might make the user think things are not optimal but "OK", when in fact things are pretty bad...



Using some round numbers:

100' of LMR-400UF (1.8db loss/100' @ 150mhz)

25 watts - 1.8db loss = ~ 16.5 watts at masthead

16.5 watts reflected (infinite VSWR) - 1.8db loss = ~10.9 watts reflected power at bottom end of coax
10.9 watts - approx. 1.2db loss thru SP-160 = ~ 8.3 watts measured reflected power...
8.3 watts is 33% of 25 watts = 1/3 reflected power measured = VSWR of 3.7:1 at radio...
BUT...
But, this assumes the cable is at factory spec'd loss (on additional moisture-intrusion loss) and no connection/connector induced loss...
So, if there are additional losses in the system, the measured VSWR at the radio will be lower....and if there are less losses (such as in a shorter run of cable) the VSWR measured at the radio will be higher!!
And, even more importantly is the additional loss in the cable when operated at high VSWR!!
This can be as much as an additional 4 - 5 db loss in an otherwise 1.5db loss cable!!!


{as example, if you had only a 75' run of LMR-400UF, you'd have only 1.35db loss (26%) in the coax...25- 1.35db loss = 18.5 - 1.35db loss = 13.7 - 1.2db loss = 10.4 watts

10.4 watts is 42% of 25 watts = VSWR of 4.5:1}


Now do see why I don't mention VSWR very much here!!!
It's 'cuz many of the measurements made are inaccurate and most times actually using the VHF radio can be a better indication that things are actually working!!


And, further in the realm of VSWR and what might be the issue....I come back to one of my original thoughts:
Are there splices / connections at the mastbase??
And, what condition are they in??
If you have none there, or they are perfect (making perfect contact), then I'm back to my number 2 cause, the connections up top, at the antennas!!


Of course we do know you need to replace the antennas, but I suspect the cables will also need to be replaced as well...(unless you have enough slack to cut-back a few feet and inspect the cable interior, and if it's good, you could just install new connectors, but I'd NOT recommend this....just replace the whole darn run of cable!!)







5) Although, I'm not there, and cannot be certain, here are some specifics....that are my pretty good!
Quote:
Originally Posted by Pyxis156 View Post
When connected directly to the AIS, VSWR indicated by the XB-8000 was 2.66:1 for the masthead unit and about 2:1 for the dedicated AIS antenna. However, when I used the splitter, the VSWR would increase on the masthead antenna to about 3:1 and 2.33:1 for the dedicated AIS antenna.
Assuming the mastbase connections (if any) are good, I suspect that you didn't have a good connection at the masthead, and/or either never had the antenna ever really connected, or had a bad antenna to start with!!

I thought there might be a problem with the splitter, but after extensive troubleshooting with Vesper's tech support (which was excellent and responsive), we came to believe the splitter was functioning correctly.
Seems odd...but, possible...
But, what about the coax jumpers???
I went up the mast and inspected things and there was nothing obviously wrong so I lived with the higher VSWR and stuck my head in the sand. This issue continued to nag the back of my mind but I seemed to receive OK and do not use the VHF often, so I just assumed things were perhaps not optimal but still working OK.
I'm not sure how high the mast, nor where you are sailing...but..
But, with a 54' boat, and a normal-height mast, you should easily / daily be able to maintain perfect contact with other like-equipped boats at 15 - 20 miles!! And, even at 25 - 30 miles, should still be solid copy...
And with USCG much farther than that....and to other shore stations (harbor master, marina, yacht club, etc.) at least 10 - 15 miles over open water, and even in harbors at least 5 - 10 miles...

Have you tried any of these??
Or how about SeaTow's Automated Radio Check system...


Fast forward a couple years, this past winter we were approaching a port and I tried to contact the harbormaster. I could hear them, but they were really struggling to hear me and actually asked how far away I was because of the poor transmission quality... I had them in direct sight and was less than 1/2 mile from their antenna!
At this point, I assume you didn't have any antenna connected at all!
And, possibly didn't even have any signal getting up the mast?? (mastbase connections???) (coaxial jumper connections???)
This made me realize that I could not ignore this problem any further. I checked the VSWR and the Vesper application indicated a VSWR problem with a 4:1 VSWR. I swapped to the dedicated AIS antenna and found that VSWR had also increased to about 4:1, so the problem had gotten worse over the last three years.

I eventually went back up the mast and took a closer look and again nothing obvious, but comparing the pics I took a few years earlier to present, there was noticeably more discoloration around the cylinder at the base of the VHF antenna (a Metz). Reluctantly, I undid the sealed connection from the coax to the antenna and found that water must have made its way into the loaded coil (not a loading coil, but fairly irrelevant to this discussion) base and then down into the connection as there was corrosion evident inside the connector.
This is actually quite common when using LMR cabling!!
(see my details above)
But, it is also certainly possible that the moisture came in thru the antenna itself, and then through the connector, and then wicked into the coax!!
You will most likely need to replace the coax!! (and antenna, of course!)


I have several questions that I am hoping folks can help with:

1) The instructions for my replacement antenna (a Digital Antenna 222-VW) specifically state that you should NOT mount the metal L-shaped mounting bracket so that the vertical part of the L is even with the loaded coil base of the antenna. That is, the mounting bracket should be mounted so the L is upside down with the antenna mounted to the top of the mounting bracket and the loaded coil base above the part of the L mounted to the mast. I believe that in an attempt to get the VHF antenna below the height of the lightning rod also mounted to our masthead, they mounted the L bracket upside down exactly as instructed not to in the instructions. Question: What is the effect of having the loaded coil base right next to the metal of the L bracket, and is it worth worrying about this and flipping the L mount around? I am reluctant to do so if not absolutely necessary as it means more time up, and more holes in, my mast.
First off, if the base of the antenna is metallic / the base matching section is inside a metal enclosure (as the Shakespeare antennas are), then the position of the L-bracket is highly unlikely to effect the antennas performance....as long as the radiating element (the whip) is not obstructed (by the mast, etc.), having the base next to the L-Bracket (and mast) would have very little effect!!
But, if the base is plastic / fiberglass / etc., then its close proximity to the metal of the L-bracket could effect its tuning! (although it seems unlikely that someone designed and sells an end-fed 1/2-wave marine VHF antenna with a plastic base....I don't have the Digital 222-VW to look at!!)

Secondly, there may be some specific mechanical reason that Digital recommends this???

Third, I suppose they may also want to encourage installers to mount the antenna in the clear (a good thing!) and by recommending the whole antenna stick up above the bracket, there is a better chance of this???

Fourth, unless you just cannot source a Shakespeare antenna from your present location (??), just return the Digital and buy a Shakespeare 5215 (or 4200)
5215 Classic AIS Squatty Bodyģ Antenna | Shakespeare Marine Antennas


2) In the other thread, there was considerable discussion about the effect of another antenna mounted in close proximity to the VHF antenna. While I don't have another antenna on the masthead, I do have the lightning rod
If this is a narrow pointy thing (usually with a point at the top), then it's called an "Air Terminal"...if it looks like a toilet brush, it is called a "Static Dissapator"...within about 8" of my VHF antenna, and it is connected to a fairly large wire running to (I believe) the ground plane system of the boat. Question: Is this not essentially an antenna (or worse), and what are the ramifications of this setup to my VHF performance?
Well, being so close to your antenna it will have some effect on directivity...but as I wrote in the other thread, it should not be detrimental...
If this air terminal is longer / taller than your VHF antenna (I suspect it is), then you will have some decrease in signal in that direction and a slightly increase in the opposite direction....(if this is shorter than your VHF antenna, the effects will be less and fairly unknowable from here....but if it is significantly shorter, such as 1/2 or 2/3 the length of your VHF antenna, the effect will be very slight if at all...)

But, FYI....
Of course, this is NOT the cause of the water in your antenna base, coax, etc...and of course replacing your VHF antennas and all cables / connectors (I suspect your AIS antenna and cables, too) is what you will want to do...
But, please check your mastbase for connections, too!!
So, this is "air terminal" might just be a red herring of discussion...



3) Replacing the masthead antenna did almost nothing to the high VSWR reading. Questions: Would this indicate that the corrosion extends into my coax cable
Yes, absolutely!!
You've got to replace all of the cabling and connectors!!
and is affecting the impedance? Any other things I should look into to troubleshoot this?
Replace all the cable and connectors!!
{BTW, in my decades of electronics experience, especially here in Florida and on boats, I find 80% - 90% of all "electronics" problems are caused (or originally caused) by the wiring / connections, NOT by the "electronics" themselves!!
As I've been saying for years:
It is the wiring / connections / programming / commissioning that makes or breaks the installation, not the brand of equipment!!}



4) If the answer to #3 is "yes", I have a few feet of extra coax slack at the base of the mast that I may be able to pull up and then cut some off the compromised end at the top of the mast. Questions: Is cutting the corroded end likely to be helpful,
Highly doubtful!!
or is the cable beyond saving?
Replace all the cable!!!
Use quality RG-213 (Belden, Davis, Commscope, Times, Andrew, etc.) and either hire a pro to install all the connectors properly, or buy pre-made cable assemblies (Texas Towers, DX Engineering, Davis RF, etc....)

Would I simply cut until I don't see any more corrosion?
Not sure what your budget is, but you can replace the cables with professionally installed, pre-made jumpers/assemblies of RG-213, for a little more than $1/ft....
A 100' jumper complete, is $109!

If this is beyond your budget then, yes....you can cut back the cable until you see clean / shinny cable, and then cut back another foot or so...
BUT...
But, what connectors are you using for the LMR cable....'cuz you cannot use regular connectors, you know??

Any other guidance?
Yes, replace all the cables and connectors!!


Thanks for your patience with the long-winded post, and any advice will be greatly appreciated...
Well, in addition to what I wrote above in direct answers....
Here's a piece of advice to end with:
Do not believe the BS that oh so many will try to tell you...

Now, the big problem is...how do know I'm not BS'ing you??
This is tough, because if I write the truth here (which I'm doing), it come off like I'm boasting...so...
So, as I wrote right up front...if you give the benefit of doubt, you'll be fine..

So, for one thing, I'm trying to help you make your VHF system work well and be reliable!! (not trying to tell you what is "best", nor "what I do", etc.. or what I read here....nor what someone told me, etc. etc.)
Secondly, I've been working with electronics / RF communications since I was a kid in the 60's and in addition to majoring in physics, have run my own electronics firm for > 30 years, and have close to 40 years experience in VHF / UHF antennas, antenna systems, repeaters, etc...(I actually own the highest-rated 2m repeater in the SE US...KA4WJA/R on 146.970...)
Third, I'm an ocean sailor, made multiple Atlantic crossings on my own boats, thru tropical storms and gales, etc...but, more importantly have had to find/fix electronics problems at sea and especially on others boats...




And, finally....
Please have a look at these threads here:
There is a LOT more there than the titles make you think..


Vesper AIS SP-160 "relay/splitter" test results, lab/real world


VHF and AIS Radiowave Propagation and VHF and AIS Radio Range







I hope this helps..

Fair winds...

John
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Old 09-06-2017, 11:08   #9
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Re: Masthead VHF Antenna - The devil is in the details...

Quote:
Originally Posted by Auspicious View Post
My recommendation would be to replace the coax and use good connectors at both ends. Avoid the crimp-on connectors sold in chandleries. You can generally pull new coax with the mast in place unless there is a big drip loop or someone wire tied cable inside the mast.
Thanks for the input, between your recommendation and those below, the consensus is clear...
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Old 09-06-2017, 11:49   #10
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Re: Masthead VHF Antenna - The devil is in the details...

Dockhead, thanks for taking the time to provide the detailed description of your installation. It is hugely helpful as I will be pulling the mast probably next year and will likely mimic your installation. A few more questions below if I could trouble you further:

Quote:
Originally Posted by Dockhead View Post
I don't like this cable for boat use despite its excellent attenuation properties, and I use RG213 and 214, which has solid insulation, so is much less vulnerable to mechanical compromise or water, for mission-critical VHF and (separate!) AIS antennae.
By saying "...and (separate!)..." I believe you are advocating that the masthead antenna not be shared by the AIS, am I interpreting this correctly?


Quote:
Originally Posted by Dockhead View Post
As to connectors -- I use Type N wherever possible rather than UHF (SO258/PL259) connectors, the best money can buy, and I crimp them (with some types you still solder the core). The kind which are all soldered are not as durable, and it's real byatch to solder the shield without burning the cable, especially at the masthead in a bosun's chair Downside is the proper crimpers are bloody expensive.
We are planning on casting off in the not too distant future for an open-ended cruise to Latin America and the South Pacific, and I am trying to learn everything I can to become self-sufficient and that includes accumulating the tools as well. Fortunately (or unfortunately) the boat was originally spec'd with two extra runs of LMR-400 to the first spreaders and I was able to make use of one of the spare runs to install a cellular booster. To complete the installation, I paid the big bucks for a quality crimper so I could attach the Type N fittings for the cellular antenna.

Quote:
Originally Posted by Dockhead View Post
UHF connectors are awful on boats, but it's hard to find antennae and other gear with N connectors. When forced to use UHF, I protect with multiple layers of heat shrink tubing and a "telecomm wrap" -- self-amalgamating tape protected by regular electrical tape. But even a well protected UHF connector is a p*ss-poor substitute for a proper outdoor connector like a Type N.
Thanks for describing the "telecom wrap", I will be sure to use this on my setup.

Quote:
Originally Posted by Dockhead View Post
I do have Type N for my masthead VHF antenna, but was forced to use UHF for my AIS antenna. All coax cables on my boat run without a break or connector all the way to the nav table. Adding a disconnect at the mast base as per usual practice does make it just slightly easier to pull the mast (how often do you do that!?) but adds a major weak point in the cable, and adds insertion loss of the connectors. I figure I will be replacing all the coax anytime I pull the mast anyway, so the unbroken cable run will be no problem.

But even in the worst case, where you don't want to replace it for some reason, you can just pull it back out and tape it to the mast base, or in the even worse case, just cut it. It won't be too late to insert the connector then.
This is a good point! I do currently have a break in the cables and barrel connectors at the base of the mast. But considering the frequency of pulling the mast (i.e., 10 years or so?), replacing the coax with every mast pull seems wise and as you suggest, I can always cut the cable at the mast base and insert connectors if I need to for some reason before replacing it.

Quote:
Originally Posted by Dockhead View Post
For an antenna, I carefully chose one with an internal dipole which doesn't require grounding on the masthead truck, where getting a good ground is fraught with problems. I have the Shakespeare Galaxy XT.
I selected the Digital Antenna 222 due to good reviews and an emphasis on sealing the unit to prevent water intrusion. However, given your experiences with the Shakespeare and John's recommendations below as well, I may switch to the Shakespeare when I pull the mast and redo everything. Until then, I will give the Digital Antenna a try as I am pretty sure it isn't my current problem.

Quote:
Originally Posted by Dockhead View Post
Almost five years later, this installation is still going strong. I once got a "weak but readable" signal report from Solent Coast Guard from 60 miles away across the Channel in Cherbourg -- USING 1 WATT TRANSMISSION POWER. I normally use 1 watt transmission power and LO receive attenuation settings on my Icom M604. As any radiohead will tell you -- the antenna and transmission line account for at least 80% of the performance of your radio.
Well, that is just slightly better than my "How far away are you?" query from the harbormaster when I was literally just off their docks! And an excellent data point for a system design that works, thanks again for taking the time for the detailed response... much appreciated!
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Old 09-06-2017, 11:54   #11
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Re: Masthead VHF Antenna - The devil is in the details...

If you have any slack in your coax replace the connectors. It is unlikely the coax has failed unless it has been beat to death inside the mast. The antennas are tuned.

It might save you some grief cheap.
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Old 09-06-2017, 12:02   #12
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Re: Masthead VHF Antenna - The devil is in the details...

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Originally Posted by ka4wja View Post
Pyxis,
1) First off, this discussion will probably be long and controversial...
But, as long as you don't mind the long discussion and give some benefit of the doubt, we should be fine..
Also, hope you don't mind if I digress a bit and clarify some points that should be understood by all, before moving forward??

...

I hope this helps..

Fair winds...

John

John,

Wow...

A huge amount to digest (which is fantastic!) and I immensely appreciate you taking the time to share your expertise. I have responded to a few other posts this morning and didn't want you to think I am blowing you off by not responding right away, but there is so much here and I have to leave for an appointment so I will have to provide a detailed response this evening.

Thanks again, I sincerely appreciate it and more to follow shortly!
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Old 09-06-2017, 12:57   #13
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Re: Masthead VHF Antenna - The devil is in the details...

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Originally Posted by ka4wja View Post
1) First off, this discussion will probably be long and controversial...
This is a fan letter.

John is one of about five people (six if you count me) I can think of who have a firm grip on the theory AND a lot of practical experience applying that theory and understanding the boundary conditions that drive actual performance.

When John writes one of his dissertations (he's the only person I can think of who writes longer posts than me) I read it. It may take a couple of days to wade through (I'm busy) but I do.

I suggest that when John and I disagree you should pay special attention. First we respect each other so you can count on the discussion being civil. Second, it is likely that we are exploring the edges of things. This is a learning opportunity for us and therefore for others. Questions help - "I don't understand why you say ... because I see ..." gives us an opportunity to think yet harder and share what we know.

best, dave
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Old 09-06-2017, 21:33   #14
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Re: Masthead VHF Antenna - The devil is in the details...

Always brings myths, beliefs, and facts, this VSWR/SWR topic.

Try reading this before taking too much of the first two into your memory:

https://www.arrl.org/files/file/Tech...f/q1106037.pdf

To whet your appetite, here is an extract (credit K5DVW) :

Does Higher SWR Lead to Lower
Power Being Transmitted?
Not always so dramatically. Believe it or
not, 100 percent of the power is actually trans-
mitted in both of the previous examples. In the
first case, with a 50

antenna, itís easy to see
how all the power is transferred to the antenna
to be radiated since there are no reflections.
In the second case, the 33 percent voltage
reflection travels back down to the transmitter
where it doesnít stop but is re-reflected from
the transmitter back toward the antenna along
with the forward wave. The energy bounces
back and forth inside the cable until itís all
radiated by the antenna for a lossless trans-
mission line. An important point to realize
is that with extremely low loss transmission
line, no matter what the SWR, most of the
power can get delivered to the antenna.

Cheers, Terry (VK4JC)
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Old 10-06-2017, 03:04   #15
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Re: Masthead VHF Antenna - The devil is in the details...

Quote:
Originally Posted by ka4wja View Post
. . . Just saw the other posts here...I spent over 3 hours (on and off) typing all of this...
And, see that Dockhead mentioned connectors...
We disagree here...
Nothing wrong with a properly installed PL-259 (UHF connector), and N-connectors will add no improvement in performance at all (although are easier to weatherproof!)
. . .
Well, thousands of hams and sailors use UHF connectors successfully -- certainly nothing fatally wrong with them.

BUT:

They have significant disadvantages.

1. They are totally unweatherproofed and hard to seal. They were never intended for any kind of outdoor use.

2. Contrary to what you say, UHF connectors do produce a significant impedence bump and have a non-trivial insertion loss at marine VHF frequencies:

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The UHF type connectors under network analysis



By itself, this is not enough to ruin your VHF comms, but together with the other drawbacks of a typical boat installation, it adds up. Particularly if there are several sets of connectors as you often find, with disconnects at the mast base, etc.

Type N connectors have almost unmeasurable insertion loss at VHF frequencies, but what is more important, they are far more solid and robust, they are weatherproof, and designed for outdoor use. The professional crimp-on types are almost foolproof.

I guess the mechanical properties are key for our use. But electronically, the difference is also non-trivial in my opinion. According to this guy I respect:

[F]rom our results we can see that utilisation of the UHF connector at 146 MHz for FM type transceivers is not such a problem. A cheap rugged connector is probably an advantage as many FM units are used for mobile applications. However, for 144 MHz SSB type work where low loss and good signal to noise ratio is very desirable, again I would not recommend the use of UHF type connectors. The UHF connector still has a place in many applications where a robust economical RF connector is required but for serious applications its use should be limited to below 100 Mhz. As we have shown the N type is far superior in performance. . . "

Emphasis added.

The UHF type connectors under network analysis

As our fixed marine VHF sets are items of critical life safety gear, with VHF DSC emergency signalling being probably the primary means of distress signalling for most boats, I think we can say that this is a "serious application", and we are, naturally, above 100mhz. Therefore, following the quoted advice, I avoid UHF connectors where possible, on marine VHF sets, in favor of Type N, which are superior in every way, both electronically and mechanically.

YMMV.
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