Two VHF Antenna on mast top. Can they be beside each other.

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Kinsale373

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I'm planning to fit the economical Onwa KS 200 AIS transponder. The advise from Onwa is to fit a 2nd VHF Antenna. I think that I'll go ahead and do it rather than a splitter. Just wondering if there is any risk of interference between the existing VHF antenna at the top of the mast and the new one I'm planning close to it for the AIS ? Do they need a minimum distance between them or is there no issue?

Would the VHF Antenna for the AIS be high enough on a Pole on the transom of the boat, would it give 15 to20Kn Coverage , I'm planning to mount the GPS Antenna on the Transum Rail anyway ?

Regards, Kinsale 373
 
You shouldn't mount them next to each other...there should be (opinion varies) but about 2m distance horizontally, and 1m distance vertically between the VHF and the AIS antenna.

I don't know if you really mean sufficient coverage for 15 to 20kn speed (!?), seems unlikely with a mast, or if you're mixing units up and mean 15 to 20 NM coverage.

Of course the higher you go, the larger the coverage you'll get...there are many calculators for VHF range by height of antenna online you can consult. Can't say if you'll get 15-20NM coverage as it depends how high your mast is.

All I can say is that many boats have the AIS antenna on the stern pushpit and have more than adequate distance coverage for sailing speeds.
 
AIS antenna on a pole is a fairly standard arrangement. It has the advantage that if dismasted you can connect the VHF to it and still transmit to get assistance. As for range use this calculator Distance to the Horizon Calculator but remember that any other vessel will have its antenna raised above sea level so the range achieved will be the sum of both vessels range to the horizon. Big fast ships of the sort you need to see will have their antennas easily high enough to give the range you need even if your antenna is on a six foot pole, keeping the transmitted signal above your head height. Two antennas at the mast head need to be separated vertically to avoid possible damage to the receivers.
 
With the antenna on a pole, say 10' above the water, your radio horizon will be about 5 nm. To this you add the radio horizon of the other vessel. A yacht transmitting from a masthead antenna at 50' above sea level would have a radio horizon of about 10 nm, so you can communicate at a range of 15 nm. A ship with antennas at 100' above sea level has a radio horizon of about 15nm, so your range to him would be 20 nm. This is perfectly adequate for AIS, I would say.
Two antennas at the masthead would need to be 1m apart. Theoretically you can argue they can be 0.45m apart and I've seen several installations like this and the owners report no issues. An antenna at the rail gives you back-up VHF availability in case of dis-masting or other masthead antenna failure.
 
Thanks for the feedback and very useful infomation.
I think that I will go the easier route and mount the antenna on a pole at the back. I can always head up the mast if i.m not happy with performance and it will be a lot easier to set up on a pole at the back. I didn't know about the recommended distance between VHF antenna -good to know,

Thanks again Guys , Kinsale 373
 
Hi had my AIS aerial mounted on a pole 4' above my pushpit rail (10'o/a above waterline) and it worked no problem and ranges were in access of 20 m
Yes it is well recommended not to have any aerials etc less than 1m apart,however the VHF aerial was mounted ontop of the main mast.

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What you can get away with will depend on the spec of the radio and the consequences of one transmitter interfering with the other as a receiver.
VHF and gps can probably be right on top of each other without issues, as they are in an EPIRB, because the frequencies are so different.
Radar has a high peak power and can do damage, most other things will at worst jam another receiver.
1ft vertically above the radome is probably better than 10ft away, right on boresight.
You probably won't notice your AIS transmit burst blocking your VHF telephony for a fraction of a second. If your voice transmission kills a few AIS receive slots, it's unlikely to be a big issue.
 
lw395 said:
What you can get away with will depend on the spec of the radio and the consequences of one transmitter interfering with the other as a receiver.
VHF and gps can probably be right on top of each other without issues, as they are in an EPIRB, because the frequencies are so different.
Radar has a high peak power and can do damage, most other things will at worst jam another receiver.
1ft vertically above the radome is probably better than 10ft away, right on boresight.
You probably won't notice your AIS transmit burst blocking your VHF telephony for a fraction of a second. If your voice transmission kills a few AIS receive slots, it's unlikely to be a big issue.
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That's only one of the issues that the antenna separation is trying to prevent.

Having one antenna close to the other will change the radiation pattern of both. You will no longer have equal antenna gain in all directions around the boat. The closer, the more significant the effect.
 
st599 said:
That's only one of the issues that the antenna separation is trying to prevent.

Having one antenna close to the other will change the radiation pattern of both. You will no longer have equal antenna gain in all directions around the boat. The closer, the more significant the effect.
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Your ideal radiation patterns are out of the window as soon as you put say a VHF aerial anywhere other than on top of the mast with nothing parallel to it.
Ideally perhaps we'd put all the antennas one on top of the next at the masthead.
Two VHF antennas would have to be quite close together to change the pattern very much, whereas putting one on the pushpit near the backstay with the mast in the way of a forward view is technically horrible, but rarely matters in practice.
 
Yes go for antenna on a pole on the pushpit. My main VHF is on a pole about 50cm from pushpit to bottom of antenna. Works very satisfactorily for me. I prefer the antenna at the back because i drop the mast often and so have no wiring in the mast. ol'will
 
William_H said:
Yes go for antenna on a pole on the pushpit. My main VHF is on a pole about 50cm from pushpit to bottom of antenna. Works very satisfactorily for me. I prefer the antenna at the back because i drop the mast often and so have no wiring in the mast. ol'will
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I'm not sure you've understood the OP's question.
 
I'm not sure you've understood the OP's question.

I do understand OP question however rushed I did not complete the argument. Certainly it would be bad to have 2 VHF antenna next to one another on mast top. It would be difficult to get decent spacing on mast top. A vhf antenna on the push pit is the best alternative used for AIS but also available for VHF com if needed by mast top antenna failure. My statement therefor is to assure OP that a push pit will give good results for AIS and useable results for VHF com in an emergency. ol'will
 
I have my AIS antenna mounted directly on the pushpit (no pole) and routinely see vessels at 12NM+ Assuming that stuff I'm interested in will have a max speed of around 20kt then that still gives me around 30 mins notice - enough to stay safe. Too much range could even be counterproductive as you risk cluttering your own and other vessels' displays.
 
Kinsale373 said:
Thanks for the feedback and very useful infomation.
I think that I will go the easier route and mount the antenna on a pole at the back. I can always head up the mast if i.m not happy with performance and it will be a lot easier to set up on a pole at the back. I didn't know about the recommended distance between VHF antenna -good to know,

Thanks again Guys , Kinsale 373
Click to expand...
I had exactly this choice to make two years ago and went with a 1.5m pole on the pulpit for the AIS aerial. It gives me 20-30nm range on big ships fairly reliably (thanks to their high aerials).
My experience is that I only use ranges of 12 miles and under for monitoring AIS except when I'm bored on a night watch and want to check that there actually are other vessels out there!
 
GPS and VHF together not a problem but 2 x vhf would be an issue if two close, receive antenna likely to be AIS input when other transmits , other way round power lower and short burst so radio likely not to care much
 
differentroads said:
Its a single VHF band aerial, not two. the point being made about aerial seperation. applies to aerials in similar frequency bands. The GPS aerial part of this device is a different frequency band and aerial type. There will be little interaction between them.
Click to expand...

True, I think JBJag's point was that NMEA standards does particularly state a separation distance for GPS and VHF antennas.
 
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