Tuning VHF antenna for use with AIS

[DavefromNZ]

I don't think AIS is just line of sight and there is more to this than VHF 'skip'. I have been 40nm off the coast of two big harbors in NZ that are in volcanic cones. The big ships I saw on AIS were directly behind hills. Do the ground station retransmit signals maybe?

PS you can figure out the scale but here is me and a big ship showing about 80nm away and another smaller ship in Manukau Harbour about 100nm away;



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[transmitterdan]

There is something known as Fresnel effect that explains why VHF waves are able to go over hills. The hill "bends" (technically refracts) the wave so it can go over a hill. There is a lot of attenuation due to the refraction process but enough signal gets through to make it work.

[Ziggy]

Originally Posted by Nicholson58

Cable should be low impedance. Old & high impedance cable will likely influence results.

In order to avoid reflections (i.e, minimize VSWR), cable characteristic impedance should be matched to the antenna, connectors, and the transmitter. For normal marine VHF equipment, this means 50 ohms. No more, no less.

[hellosailor]

"I don't think AIS is just line of sight and there is more to this than VHF 'skip'."
AIS is just VHF and VHF is "just" line of sight, unless you want to become a radio technician. Yes, there is more to it than that. As Dan said, you can get geographic influences. Einstein might have called that gravitational bending and distortion not just diffraction.(G) Yes there is skip and ducting from atmospheric conditions. Yes, there is range reduction due to local noise levels, and yes, the height of BOTH antennas must be counted for the "line of sight" range.


Bottom line is that it doesn't really matter what else affects the VHF line of sight range, that's the one simple number, and the only number, that you can RELY ON 100% of the time. The rest is all icing on the cake.


Good cable, good antenna installation, good power supply, and that's all you need to worry about in terms of the range you can rely on.

[Cadence]

Originally Posted by DavefromNZ

I don't think AIS is just line of sight and there is more to this than VHF 'skip'. I have been 40nm off the coast of two big harbors in NZ that are in volcanic cones. The big ships I saw on AIS were directly behind hills. Do the ground station retransmit signals maybe?

PS you can figure out the scale but here is me and a big ship showing about 80nm away and another smaller ship in Manukau Harbour about 100nm away;
Attachment 110881


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You may want to consider the height of the ships antenna? Depending on the ship. The Bridge deck might be 90 ft. +/- off the water and the antenna considerably more?

[ka4wja]

Steve, et al,

Oh goodness....such a great deal of misunderstandings here...
I'll do my best to sort some of this out for you all...


But, let me give you a simple/quick answer first:
No, you usually will not need to "tune" a VHF antenna for AIS use...
For the reasons and exceptions, as well as what you should do, please read further..



1) First off, if you look at a few of my earlier postings you will see LOTS of details on VHF (and AIS) range, and radiowave propagation, etc...

http://www.cruisersforum.com/forums/...ge-149499.html


As well as detailed info on VHF/AIS "splitters"/"relays"...also, info on antenna VSWR (both real world VSWR measurements and the inaccuracies involved in measuring antenna VSWR from the transmitter end of lossy transmission lines!)

http://www.cruisersforum.com/forums/...ld-130803.html



Here is the SWR plot of a standard Shakespeare 3' SS whip antenna (non "AIS-version"), mounted at the 64' masthead, fed with 70' of LMR-240uf and 25' of LMR-400uf...
[you'll note the VSWR of 1.2:1 at 156.8mhz...and 1.9:1 at 161.98mhz...both within the normal operating parameters of any marine VHF transceiver and/or AIS transponder...]






http://www.cruisersforum.com/forums/...ml#post1934240


http://www.cruisersforum.com/forums/...ml#post1802846

http://www.cruisersforum.com/forums/...ml#post1883519

http://www.cruisersforum.com/forums/...ml#post1876295




2) As for specifics here...aside from the additional losses incurred thru the coax with an elevated VSWR (which is transferred as heat), there is NO power lost in the system when it has an elevated VSWR...
Although most transmitters will reduce power or shut-down completely if the VSWR is above a certain level....typical power-fold-back starts at VSWR's of 2:1, and transmitter shut-down (if so designed) occurs at VSWR's above 3-4:1...

BUT...

But poor VSWR is also telling of poor installation, poor connections, possibly old lossy coax, old/corroded connectors, etc...
And, these things can be an issue!!!!
SO..
So, without being on-board those boats, without knowing the type/model/age of coax and connectors, and especially without knowing the coax lengths, antenna types, and where they are mounted/installed....and without testing/measuring, troubleshooting, examining coax/connectors, it is impossible to know anything to any certainty at all....
But, it IS possible to say that the "bad swr" is a sure sign that everything else should be checked thoroughly!!



Further...whether it Voice comms (Marine VHF-FM) or Digital comms (AIS), except for the lower power of the AIS, the RF is the same....the coax loss / connector issues are the same....line-of-sight is the same....radiowave propagation is the same...

What is different here, in the real world:
--- The 2-watt AIS Class B transmitter

--- The installation....(where the AIS antenna is mounted/installed....what are the additional losses such as thru a "splitter"/"relay"....how old is the antenna/coax, etc.....and how the Marine VHF Voice communications system has such a large margin (10's of db's) above threshold, compared to the Class B AIS system...)

What this all means??
Is that marine electronics guys need to be more conscientious when dealing with AIS, especially Class B AIS!! (and no matter how many years they may have doing this, some just aren't too conscientious!)

Originally Posted by stevecody

In recent times I have installed three AIS units that run through a splitter to share the antenna with the VHF radio.

When something like AIS is installed which gives real numbers/performance to quantify the antenna performance, it becomes obvious that something is not up to scratch.

3) Steve, there is a lot that you can do...

Originally Posted by stevecody

I'm looking to this forum to ask "what next?". Should I just blindly change the antenna - which involves a trip up the mast - and hope that it comes good? Should I go prowling for connector loss and cable quality issues? Are there any more tests I can do to be a little more deterministic about the cause of the signal loss? What experiences do others have in this area?

a) First off, learn about radiowave propagation...
http://www.cruisersforum.com/forums/...ge-149499.html

b) Secondly, learn about coaxial cable losses...(and the fallacy that PL-259's are "lossy")...
And, learn to work in db and dbm, rather than watts...it'll make life easier!
Read the posts I referenced / linked to, above...

c) Third, understand that antenna placement, especially height, is by far the most important determining factor when considering VHF communication range with limited power...

d) Fourth, understand that adding a minor amount of loss from use of a high-quality VHF/AIS "splitter"/"relay" (such as the Vesper SP-160), is almost always far exceeded by the massive increase in range from using an antenna mounted higher and in the clear...
(and, even if this "extra loss", of about 1db, just doesn't sit well with you....understand that you can use lower loss coaxial cable, to offset the "splitter"/"relay" loss....and in some circumstances actually end up with less overall loss!)


Once you've learned these basics, then you're ready to start investigating / troubleshooting what might be wrong with some installations...
And in reality, once you know the above basics, in many cases you'll not need to do much measuring at all, sometimes simply looking at things and/or asking the owner a few questions, will be 90% of all the "troubleshooting" needed!!
{Trust me...
I've owned/operated my own communications firm for > 30 years....and have been working / experimenting / studying (and teaching) communications for > 40 years...and while I've got a plethora of test equipment costing as much as many cruising boats, sometimes all you need to do is ask a few questions and look with your eyes....BUT...
But, you must know/understand how all of this works, first!!}







Oh, and for those that harp on "connector losses", for VHF systems and for REAL PL-259's/SO-239's/PL-258's, it is a NON-ISSUE!!
Note that I'm talking about name-brand (Amphenol, Times, Andrew, Commscope, or Kings) connectors....NOT those "Shakespeare toys" or other "made-in-China" crap!


I know some will disagree, but unless you're in a lab with 10's of thousands of dollars worth of equipment (and a few trained techs/engineers to set up the tests), you will NOT be able to measure the infinitesimal differences!!!
If you want to SEE what I mean, have a look at this photo....the transmitter is outputting exactly 100 watts (measured with that same meter/slug), and after 23 connectors/adapter, etc. N's, UHF's, BNC's, including "T's" and "elbows", a short RG-58 jumper, and even including some "F" connectors and "phono" plugs/adapters (neither of which were ever designed to handle RF), the results are only marginally measurable....showing 95 watts out (a 5% power loss), which is a loss of 0.2db...

So, next time someone wants to tell 'ya that using an "N" connector will have less loss than a PL-259 {"UHF" connector), you can see for yourself that this will have no effect at all (at least until the freq gets past 400mhz)....take note that in my professional comm work, connector losses ARE figured into system design, link budgets, etc. but these losses are very slight, typically 0.01 to 0.04db, depending on connector and freq...
Enjoy...












I do hope this helps....

BTW, I only read Steve's original post here, and the first half-dozen replies, before I started this posting....so, if I'm rehashing something that others have posted, while I was away from the screen, sorry about that!!!


Fair winds..

John

[ka4wja]

Now that I've read the rest of the posts here, I hope some don't think that my thoughts are contrary to Bill's?

--- As Bill highlights a scenario where the AIS transponder's transmitter didn't like the VSWR (and likely was folding-back its transmit power?)....and this is, of course, where you should try to prune the antenna slightly (trim a half-inch off the whip, from the bottom of the whip), and retest...

And, Bill's scenario is also going to be less likely with masthead-mounted antennas, due to the increased transmission line losses, versus a rail-mounted antenna with its shorter run of coax...{FYI, my stern-rail mounted antenna IS tuned for the AIS freqs...but not my masthead antenna...}

Originally Posted by ka4wja

But, let me give you a simple/quick answer first:
No, you usually will not need to "tune" a VHF antenna for AIS use...
For the reasons and exceptions, as well as what you should do, please read further..

--- As far as "splitters"/"relays"...
Well, Bill and I just have a polite difference of opinion...

But, I only recommend the Vesper SP-160....and only if the application (more reliable AIS coverage when in heavy seas, etc.) calls for greater antenna height than can be attained from the stern-rail..




I hope this helps..

Fair winds..

John