Poor AIS range with Furuno FA50

[keaton85]

Been experiencing poor range on a Furuno FA50, as in maybe 7-8nm and that average. Ive seen 2-3nm out and loss of information after the vessel passes.

There are two antennas on the rig one for AIS and one for VHF. So I tried switching to see if there was an antenna issue, still the same. Also, Navnet is set correctly for 24nm targets.

Any way to really check anything else? Ive been in the settings of the FA50 but nothing seems to catch my eye for range.

Thanks!

[DeepFrz]

Check your VSWR.

[keaton85]

VSWR checks out, and connections are all clean and perfectly dry.

Anything else to check?

[MarkJ]

Originally Posted by keaton85

VSWR checks out,

What does that mean? Whats your VSWR ratio?

If you are not close to 1:1 you are stuffed.


Mark

[ranger58sb]

I have an FA-50. No clue about range, but I'll have a look at returns later today.


I'm aware of no range settings, nor any other kind of settings, and the manual seems to confirm it's either on or off.


Range settings on our NN3D might apply to something like the radar... but our 64nm radar pre-dates NN3D, didn't integrate, so it's completely separate. In any case, I'd not expect any NavNet range settings to have anything to do with the AIS.


In addition to testing VSWR... some other questions come to mind.


Is it the antenna that originally comes with the FA-50?


How high is the AIS antenna mounted?


Has the problem only recently appeared?


Have you asked Furuno? Furuno USA Forum


-Chris

[keaton85]

I tested VSWR with a Shakespeare ART-2 and the VHF at the nav station. Both VHF and AIS antennas came up as under 2 on the scale.

The antenna height is about 105'

As for the AIS range, it was a solid 15-20nm before we noticed targets being lost at 7nm-8nm out.

[ka4wja]

Keaton,
1) Well, you haven't given us much to go on, specifically:
a) What is the LENGTH and "TYPE"/"MODEL" of your coaxial cable going to each of these antennas??? (I hope it is NOT RG-58 or RG-8x!!)
And, what connectors are you using, how old are they, and who installed them???

b) WHERE are your two antennas installed/mounted on your rig???

c) How FAR APART are these two antennas?? both horizontally and vertically???

d) Have you verified that your primary VHF radio's range (on voice communications) is the same on BOTH antennas...and please confirm that you DID observe the same limited AIS range on BOTH antennas, and not just measured the same "acceptable" SWR on both???



2) Not knowing the answers to those questions, among others, it's impossible to be precise....but here are some ideas for you...

a) The first thought is actually your antenna, and/or coaxial cables, and/or connections, are suspect / faulty....
Yes, I know everyone is caught up on "what's your SWR?"....But, when measuring VSWR at the radio end, there are HUGE errors...

In case you don't believe, just follow along with these simple calculations....
--- You measure a VSWR of 2:1 or less (that's a reflected power of ~ 10%), you with your primary vhf at 25 watts forward power, you'd have approx. 2.5 watts reflected power, or less....so far we all understand / agree, yes??

--- But, remember that you have LOSS in the coaxial cable....so, say you have a 100' run of RG-58 coax on VHF-Marine (a BAD idea, but it is done often!)....
So, now with your 25 watts out of the radio into 100' of RG-58, you have only 6 watts at the antenna end, and if you had a completely shorted-out cable (or completely "open" / no connection at all) at the antenna end, and had an infinite SWR (100% reflected power), this 6 watts becomes a bit less than 1.5 watts back down at the radio end, where your SWR meter is....and viola, you have a "measured vswr" of << 2:1, and you think all is fine....BUT...
But, in reality you may have a totally disconnected or shorted-out antenna / coaxial cable...
Understand, that I'm NOT saying this is absolutely what is wrong with your AIS range....but just saying, it could be!!!

If you hadn't mentioned swapping to your other antenna, and having the same results, I'd have made this an almost absolute cause....
(but to be clear, did you actually find the same AIS range issues, or was it just the same SWR measurements??)

Note that using 100' of RG-8x between the radio and antenna, would still show an SWR of 2:1 or less on 156mhz, when measured at the radio end, even if the antenna was completely disconnected or shorted out!!!


These FACTS above shows both the fallacy of measuring swr at the radio end (on VHF and/or UHF freqs), and why if you DO measure a bad SWR at the radio end, then the most probable cause is a bad coaxial cable connection right there at the radio end (or in the bilge, etc.), NOT at the antenna end!!




b) Now, if these antennas are in close proximity to each other (less than 5' - 6' away), then there is a good chance that you've damaged the FA50's receive front-end, reducing its reception capabilities...
Although, this might not be an issue in your situation....if your antennas are too close, this is a high possibility!!




3) Also, be sure to tell us WHERE you are located, and what other tests you've done...

Originally Posted by keaton85

Been experiencing poor range on a Furuno FA50, as in maybe 7-8nm and that average. Ive seen 2-3nm out and loss of information after the vessel passes.

There are two antennas on the rig one for AIS and one for VHF.
Where are these antennas....and how far apart are they??

So I tried switching to see if there was an antenna issue, still the same. Also, Navnet is set correctly for 24nm targets.

Any way to really check anything else? Ive been in the settings of the FA50 but nothing seems to catch my eye for range.

Thanks!

Originally Posted by keaton85

I tested VSWR with a Shakespeare ART-2 and the VHF at the nav station. Both VHF and AIS antennas came up as under 2 on the scale.
See info above...

The antenna height is about 105'
That's some tall mast!!
And, with that size boat, you are only using a Class B AIS???

PLEASE tell me what coaxial cable you are using, and what connectors....and HOW LONG is this run of coax??

As for the AIS range, it was a solid 15-20nm before we noticed targets being lost at 7nm-8nm out.

Originally Posted by keaton85

VSWR checks out,
Probably not....
Please see info above!!

and connections are all clean and perfectly dry.
Please tell us what connectors you are using, how old are they, who installed them, etc.???
Anything else to check?

I hope this helps...
Please answer all of the above questions, and we'll get you going..

Fair winds.

John

[ka4wja]

BTW, even though the Furuno specs are a bit on the cautious side, they are a VERY good rule of thumb to follow!!!

Anyone wishing to learn how to properly install an AIS transponder and antenna (or even just an AIS receiver), please read the whole Furuno FA-50 (or FA-150) manual...

FYI, here is an excerpt:

The location of the AIS VHF-antenna should be carefully considered. Digital communication is more sensitive than analog/voice communication to interference created by reflections in obstructions like masts and booms. It may be necessary to relocate the VHF radiotelephone antenna to minimize interference effects.

To minimize interference effects, the following guidelines apply:

• The AIS VHF antenna should be placed in an elevated position that is as free as possible with a minimum of 0.5 meters in the horizontal direction from constructions made of conductive materials. The antenna should not be installed close to any large vertical obstruction. The objective for the AIS VHF antenna is to see the horizon freely through 360 degrees.

• There should not be more than one antenna on the same plane. The AIS VHF antenna should be mounted directly above or below the ship's primary VHF radiotelephone antenna, with no horizontal separation and with a minimum of 2.8 meters vertical separation. If it is located on the same plane as other antennas, the distance apart should be at least 10 meters.

• Install the VHF whip antenna (option) referring to the outline drawing at the back of this manual. Separate this antenna from other VHF radiotelephone antennas as shown below to prevent interference to the FA-50.

Also, keeping the coaxial cable loss to a minimum is best for AIS transponders, as well!!
Typically the "3db loss, or less" spec of VHF-FM voice communications will still be "okay", but with only a 2-watt transmitter and typically poorly installed antennas (and/or antennas only a few feet off the water), it is best to try for less loss, especially if the AIS VHF antenna is mounted low...

Note that with RG-213 and LMR-240 cable having 2.8db and 3.0 db loss (respectively) per hundred feet, these are usually the choices to recommend, unless an atypically-long length is needed...







Oh, and don't obsess over the connectors....Amphenol PL-259's have been working well for decades at VHF....but, just have a look here at what 23 different connectors/adapters do...not much...











I hope this helps others, as well as Keaton...

Fair winds..

John

[MarkJ]

Originally Posted by keaton85

Both VHF and AIS antennas came up as under 2 on the scale

1:2 is no good at all.

You need to be close to 1:1

Mine is 1:1.1 Thats good.

As the scale is logrithmic the differecne between my 1:1.1 and your 1:2.0 is HUGE.

You need new cable and/or new connections.


Mark

[ranger58sb]

Originally Posted by keaton85

The antenna height is about 105'

As for the AIS range, it was a solid 15-20nm before we noticed targets being lost at 7nm-8nm out.

Woof! That's serious antenna height!

I just checked returns displayed in our system. Furthest are 19.8nm to the south, and 13.6nm to the north. That's just who happens to be out there on the water right now (with an AIS transmitter).

Our antenna is approx. 18' off the water.

Given that you're experience some kind of change from previous, did you do any work on or near an antenna? Nearby lightning? Bird strike? Or...?

-Chris

[keaton85]

To give a quick update as I don't have all the info

- All coaxial is RG-213
- Connections at the base of the mast and at the unit is clean, but I have not had the inspiration to try and get to the antenna connections due to the location.
- Total cable run is probably around 130-150ft
- Both VHF and AIS antennas are on the same plane about 3ft apart at the tip of the mast, vertical, although the primary VHF antenna is on the mizzen. Thus the main mast VHF is hardly ever used.
- All antennas are whip style
- The only thing that has changed recently is the Navnet computer being replaced.


I have not tested VHF range on any of the antennas. Looks like I have some work to do before going any further.

Thanks guys for all the help! great reading....

[MYTraveler]

For what it may be worth, my Furuno FA150 routinely returns targets at ranges in excess of 99 nm.

[ka4wja]

Keaton,
Now we're getting somewhere!

1) At 156.8mhz, 150' of RG-213 (with 2:1 SWR) has about the same loss (within 0.2db) of 100' of RG-8x....about 4.5db - 4.6db...(but would be even higher loss, if the SWR was higher)

Which would mean approx. 1/2 watts reflected power measured at the radio end, if there was an SWR of 10:1 at the antenna end, which would show as an VSWR of approx. 1.35:1 on your meter at the radio end!!!
SO...

So, a VSWR reading of < 2:1 at the radio end of your 150' of RG-213, will not show you much about the antenna / antenna-end of the coax...



2) Without going up the mast, you can very easily check how these antennas compare, by using a distant transmitter (such as a NOAA weather radio station > 50 miles away, if you are here in the US???), or use another vessel at ~ 40 - 50 miles away, and compare the two / three different antennas.....the should all be similar (although the mizzen antenna may have a slight disadvantage on the longer distance signals due to its lower height)....




3) Not sure I understand completely....

Originally Posted by keaton85

- Both VHF and AIS antennas are on the same plane about 3ft apart at the tip of the mast, vertical, although the primary VHF antenna is on the mizzen. Thus the main mast VHF is hardly ever used.
You have an VHF-AIS antenna, AND a VHF-FM antenna on the main mast 3' apart, but do not use this VHF-FM antenna much...
And, your primary VHF-FM antenna is on the mizzen mast...

Do, I understand correctly???
If so, it is still possible that your main-mast VHF-FM antenna's radio caused some damage to your FA-50 receive input....


- The only thing that has changed recently is the Navnet computer being replaced.
Not sure how this could've caused any problem with the AIS reception....but perhaps the new Navnet computer has some issue showing AIS targets???
Or possibly, a VHF antenna cable has been damaged / kinked, etc.???



I have not tested VHF range on any of the antennas. Looks like I have some work to do before going any further.
This is the VERY first thing to do....
Still not sure WHERE you are at, so I cannot give you an exact answer / exact procedure....but if you are in the US, you can use NOAA weather radio stations, or a couple of SeaTow's automated radio check stations (use ones 40+ miles away)....

Please let us know WHERE you are at...

4) In addition to telling us where you are at, please tell us what VHF radios you are using...




I hope this helps...

Fair winds..

John