AIS / VHF Antenna interference

[sebastiannr]

Hi there,

Apologies if this has already been covered in another thread - I couldn't find the answer.

My mast is down at the moment so I thought I would take the opportunity to install a separate antenna for my AIS receiver at the mast head. I have heard conflicting information about what is the best thing to do. My father reckons that if two antennae (i.e. VHF and AIS) are within 1m of each other they will interfere with each other, so he recommends using a single antenna with a splitter. Other sources have suggested that a splitter is not ideal as if it fails, transmitting on the VHF will fry the AIS unit.

What is the best solution, given that I have a good opportunity to install a second antenna while the mast is down?

Many thanks,

Sebastian.

 

[Playtime]

Hi there,

Apologies if this has already been covered in another thread - I couldn't find the answer.

My mast is down at the moment so I thought I would take the opportunity to install a separate antenna for my AIS receiver at the mast head. I have heard conflicting information about what is the best thing to do. My father reckons that if two antennae (i.e. VHF and AIS) are within 1m of each other they will interfere with each other, so he recommends using a single antenna with a splitter. Other sources have suggested that a splitter is not ideal as if it fails, transmitting on the VHF will fry the AIS unit.

What is the best solution, given that I have a good opportunity to install a second antenna while the mast is down?

Many thanks,

Sebastian.

The best solution, IMHO, is to mount the second aerial on the pushpit, ideally on a pole to give a bit more height and make it less vulnerable. This avoids the separation versus splitter problem altogether but still gives a range for AIS reception of at least 12 miles, which is quite sufficient for all practical purposes. It also provides a 'spare' VHF aerial in the event of a dismasting.

 

[Beadle]

If you separate them vertically you should have no problem

 

[dmayes]

Wavelengths

The marine VHF is around 150MHz and the speed of radio transmissions is around 300M Km/s This makes the wavelength of a full wave about 2 meters in length. VHF aerials are aroung 1m in length as this is a 1/2 wave and makes an efficient reception. Directional aerials have similar size unconnected rods about 1/4 wave distance from the actual aerial, this receives the signal and re-radiates it increasining the strength of the signal at the aerial and making the aerial more directional as it only increases the strength in the same line as it is placed in relation to the aerial. Look at UHF television aerials and you will see many more radiating rods so it looks like a ladder, this means you have to point the aerial at the television station. VHF needs to receive from all around so don't put a second aerial near the first, certainly not in a multiple distance of the 1/4 wave signal length.

 

[multihullsailor6]

Simrad's installation recommendation for my AIS stated that one should have at least 2m vertical separation between the VHF and the AIS aerials. Hence we placed the AIS aerial on the 2nd spreader which works just fine with excellent allround and long-range reception.

 

[pvb]

You don't need the AIS antenna very high to get adequate reception. Stick it on the pushpit, as already suggested.

 

[Elemental]

You don't need the AIS antenna very high to get adequate reception. Stick it on the pushpit, as already suggested.

+1 for sticking it on the pushpit.

I routinely see traffic 10NM away and have never seen anything suddenly "appear" much closer. Do you need to know about ships 20NM away?

 

[Sy-Revolution]

Got mine on the top of the radar arch (3.5m above S/L). I have 'seen' targets as far away as 250 miles.

No need to have it way up high, plus you've then got an emergency VHF ariel if you lose your mast.......

C.