Strange AIS antenna result.

[greeny]

Just got round to installing the kit supplied by marine traffic to receive and forward data to their AIS page. As with all things like this, I jury rigged it on my balcony and checked everything worked before going to the more difficult installation phase. I was amazed that when directly interrogating my receiver, I was receiving ship and boat transmissions from up 40 miles away and occasionally up to 60 miles. The antenna LOS on the balcony is obstructed by nearby buildings and a nearby lamp post.
I then fitted the antenna to my roof top that is 12 metres higher with no obstruction to the seaward view. The performance dropped dramatically. Now only receiving up to 10 miles away max. All terminations look good and no obvious problem with installation. Except for reduced range of reception. Puzzling. Tomorrow first job is to re position the antenna back to the balcony to see if I can replicate the much higher range. But in the meantime, have I missed anything obvious. Normally vhf antenna go as high as you can in clear air. Open to suggestions.

 

[kwb78]

VHF is largely line of sight, so your primary limit will be the radio horizon. Since you are aiming to receive transmissions from stations at sea level, that could be as close as 10 miles (or even less) depending on the terrain near you. I would suggest going to HeyWhatsThat and setting up a panorama for your antenna location - Click on 'new panorama' at the top, then find your location on the map. Set the antenna height and give it a name, then submit. After a minute or two you will get a map showing the radio horizon at your location. The red shaded areas are those which are directly visible without any terrain obstruction. This will give you a good idea of a baseline for performance. If you are getting reception roughly from the indicated area then that's not too bad.

Radio is voodoo though, and you will find there are occasions where your performance drastically differs from the prediction. You can get atmospheric ducting which can greatly increase the range of reception and you could see targets from hundreds of miles away. It could be that this explains the difference.

Another thing that could be happening is that you may have introduced some noise somehow - I don't know what the receiver Marine Traffic supply is like, but some receivers are quite susceptible to out of band interference. Strong signals can overload the front end of the radio and effectively blind it. Putting an antenna up high in clear air might have exposed it to a strong transmitter that was previously obstructed. RTL-SDR dongles are particularly susceptible to this problem, but I don't know if their receiver is based on one of those or not.

If you have another radio or sdr capable of tuning at VHF frequencies you could use it to check what the antenna is seeing - it would show up if there's any problem with poor signal or excessive noise.

 

[greeny]

VHF is largely line of sight, so your primary limit will be the radio horizon. Since you are aiming to receive transmissions from stations at sea level, that could be as close as 10 miles (or even less) depending on the terrain near you. I would suggest going to HeyWhatsThat and setting up a panorama for your antenna location - Click on 'new panorama' at the top, then find your location on the map. Set the antenna height and give it a name, then submit. After a minute or two you will get a map showing the radio horizon at your location. The red shaded areas are those which are directly visible without any terrain obstruction. This will give you a good idea of a baseline for performance. If you are getting reception roughly from the indicated area then that's not too bad.

Radio is voodoo though, and you will find there are occasions where your performance drastically differs from the prediction. You can get atmospheric ducting which can greatly increase the range of reception and you could see targets from hundreds of miles away. It could be that this explains the difference.

Another thing that could be happening is that you may have introduced some noise somehow - I don't know what the receiver Marine Traffic supply is like, but some receivers are quite susceptible to out of band interference. Strong signals can overload the front end of the radio and effectively blind it. Putting an antenna up high in clear air might have exposed it to a strong transmitter that was previously obstructed. RTL-SDR dongles are particularly susceptible to this problem, but I don't know if their receiver is based on one of those or not.

If you have another radio or sdr capable of tuning at VHF frequencies you could use it to check what the antenna is seeing - it would show up if there's any problem with poor signal or excessive noise.

Thanks KWB. All interesting stuff and thanks for the link. Its raining here this morning so will probably delay the playing around outside until later. As you say, LOS and I obviously thought the higher and clearer view the better. Actually now its getting light the sky looks promising so maybe I'll get up there later this morning. In the meantime, coffee and play with that link you sent. Thanks.

 

[Iliade]

The building acting as a reflector, intensifying the signal?
The balcony acting as a ground plane?

 

[greeny]

Thanks KWB. All interesting stuff and thanks for the link. Its raining here this morning so will probably delay the playing around outside until later. As you say, LOS and I obviously thought the higher and clearer view the better. Actually now its getting light the sky looks promising so maybe I'll get up there later this morning. In the meantime, coffee and play with that link you sent. Thanks.

Just run the link and input my location, elevations and frequency and wow surprise. The models show reception distance at 10 miles with antenna on the roof and up to 50 miles with antenna on the lower balcony. Lower balcony has other buildings obstructing the line of sight but still shows much better reception distances. Mysterious forces at play here. lol. Problem is the wife doesn't want the antenna on the lounge balcony. Negotiations underway.

 

[greeny]

Now dropped the antenna by 4 metres onto bedroom balcony, range increased back to 50 miles as opposed to 10 max on the roof. Must be an invisible force field up on my roof! Seriously though, unbroken line of sight up on the roof versus obstructed view with buildings lower down.

 

[andsarkit]

I assume you are using the same coax for both positions.
It could be caused by reflections from buildings or land cancelling out part of the signal. Try looking up Fresnel zones for radio waves but I don't know if this is a likely cause.

 

[mjcoon]

I assume you are using the same coax for both positions.
It could be caused by reflections from buildings or land cancelling out part of the signal. Try looking up Fresnel zones for radio waves but I don't know if this is a likely cause.

Ooh, perhaps these balcony balustrades are acting as Yagi directors/reflectors! ;-)

 

[greeny]

Antenna back on roof now but in a different location. Back to 50 miles. I'm now looking at the receiver information and its pretty good. I logged onto marine traffic to compare their current map to the one I'm seeing real time from my set up. Its amazingly different. Always knew the internet based system was a toy and not to be used but my word, boats are miles from where the internet version shows.

 

[greeny]

I assume you are using the same coax for both positions.
It could be caused by reflections from buildings or land cancelling out part of the signal. Try looking up Fresnel zones for radio waves but I don't know if this is a likely cause.

Just googled fresnel zones. Looks like good bedtime reading.

 

[Marsali_1]

Antenna back on roof now but in a different location. Back to 50 miles. I'm now looking at the receiver information and its pretty good. I logged onto marine traffic to compare their current map to the one I'm seeing real time from my set up. Its amazingly different. Always knew the internet based system was a toy and not to be used but my word, boats are miles from where the internet version shows.

Just out of curiousity, at what range do you have your potential traffic conflict alarms set and is your street considered a "narrow channel or fairway" under Colregs ?

 

[kwb78]

Antenna back on roof now but in a different location. Back to 50 miles. I'm now looking at the receiver information and its pretty good. I logged onto marine traffic to compare their current map to the one I'm seeing real time from my set up. Its amazingly different. Always knew the internet based system was a toy and not to be used but my word, boats are miles from where the internet version shows.

Yes, small changes can sometimes make a big difference. It's hard to say what's going on there as there could be a lot of explanations. I find marine traffic to be quite delayed and it won't update automatically unless you have one of the premium accounts. I think you should get access to that though for feeding data.