Two AIS recievers sharing an aerial - OK?

[Amulet]

I have a NASA AIS "radar". (Not a nice instrument, but it once helped me out of Dover in fog in the dark.) It has a dedicated aerial on the pushpit. I am about to install an AIS receiver engine to talk to the computer. My belief is that, since they are both just receivers, they can share an aerial without a fancy splitter - I was planning to stuff in a television aerial T connector. I suppose they might not get a good enough signal, but I assume that, while my strategy might not work, it can't do any damage.

Advice from experts welcome.

For the curious, I want to keep the "radar" as I try to avoid total dependency on the computers.

 

[lw395]

You should use a splitter, but a simple passive one is OK.
Not sure what RF signal dividers are available retail though, it has to work at the 160 or so MHz of the VHF band.
Just t'ing the connection might give quite poor reception.
Better to use a low-grade aerial on the NASA than to compromise both?

 

[LadyInBed]

Assuming it is just coax feeding the NASA antenna, you should use an RG58 T connector.

 

[Amulet]

Thanks, about as I thought.

 

[Salty John]

You can use an A-B coax switch - where you switch from one receiver to the other by turning a switch - only one receiver is fed at a time so no sharing. XS-201 is a designation I remember, but there are several makes available.

 

[nigelmercier]

Assuming it is just coax feeding the NASA antenna, you should use an RG58 T connector.

No you cannot. It will change the impedance.

 

[rogerthebodger]

You can use an A-B coax switch - where you switch from one receiver to the other by turning a switch - only one receiver is fed at a time so no sharing. XS-201 is a designation I remember, but there are several makes available.

I proposed a change over version of that switch to switch Between 2 antenna's(mast head and pushpit mount) and radio /AIS but was shot down by another fora member as I was told it would it would not handle the frequency.

 

[ianj99]

Yes, but whether the resulting drop in signal strength at each receiver will be a problem is only something you can find out by trying it. For short ranges it may not be an issue.

 

[Amulet]

Despite the benefit of the forum's technical wisdom, I'm going to try a T-connector. If it fails I promise to create an "I told you so" opportunity for the naysayers. In that event I'm going to set up a primitive change-over switch implemented by plugging the aerial into the device I want to use.

 

[Salty John]

I proposed a change over version of that switch to switch Between 2 antenna's(mast head and pushpit mount) and radio /AIS but was shot down by another fora member as I was told it would it would not handle the frequency.

A switch like the Daiwa (or Jetstream) CO201, 50 ohm, 1.8 to 600 MHz should be fine.

 

[nigelmercier]

Despite the benefit of the forum's technical wisdom, I'm going to try a T-connector. If it fails I promise to create an "I told you so" opportunity for the naysayers. In that event I'm going to set up a primitive change-over switch implemented by plugging the aerial into the device I want to use.

A T-piece will probably work, but you will get vastly reduced range. The first reason is the signal is divided into two, much as a splitter would do. On top of this you have now double terminated the signal, so will lose further power (about 4.5dB IIRC). Furthermore, the impedance mismatch will cause reflections up and down the cable, you could be unlucky and end up with no signal at all.

I can't believe I saying this, but even a splitter would be better.

 

[LadyInBed]

Nigel, you speak with the authority of one who knows the subject, but we are only talking of a receiving antenna, so surely a bit of loss is an acceptable compromise in getting a signal to both bits of kit. If it doesn't perform at an acceptable range then the OP can look at the alternatives, feed just one item, splitter, second antenna.

 

[BlueSkyNick]

Nigel, you speak with the authority of one who knows the subject, but we are only talking of a receiving antenna, so surely a bit of loss is an acceptable compromise in getting a signal to both bits of kit. If it doesn't perform at an acceptable range then the OP can look at the alternatives, feed just one item, splitter, second antenna.

In simple terms, "a bit of loss" is half the power going to each unit.

 

[LadyInBed]

In simple terms, "a bit of loss" is half the power going to each unit.

A 3db loss on what signal strength? And as I said, if the range isn't acceptable then spend money on another option.

PS Nick, update your link - http://blueskymarine.com/BSM.shtml

 

[nigelmercier]

... if the range isn't acceptable then spend money on another option...

In my experience, it takes a lot more motivation to replace a bodge than to do a proper job in the first place.

 

[rogerthebodger]

A switch like the Daiwa (or Jetstream) CO201, 50 ohm, 1.8 to 600 MHz should be fine.

This was the type I was looking for to allow AIS and radio to be switch between mast head and pushpit mounted aerial so to allow higher reception for AIS but still allow max radio range when needed yen reduce the risk of damage due to no aerial connected when transmitting.

http://www.mfjenterprises.com/Product.php?productid=MFJ-1703

This shows only up to 30 MHz where this

http://www.mfjenterprises.com/Product.php?productid=MFJ-1702C

shows 300 - 450 MHz

 

[Salty John]

http://www.mfjenterprises.com/Product.php?productid=MFJ-1702C

shows 300 - 450 MHz

If you look at the download you'll see it covers a very wide range of frequencies but in the blurb uses 300 and 450MHz as reference points for certain quoted performance figures.

 

[lw395]

This was the type I was looking for to allow AIS and radio to be switch between mast head and pushpit mounted aerial so to allow higher reception for AIS but still allow max radio range when needed yen reduce the risk of damage due to no aerial connected when transmitting.

http://www.mfjenterprises.com/Product.php?productid=MFJ-1703

This shows only up to 30 MHz where this

http://www.mfjenterprises.com/Product.php?productid=MFJ-1702C

shows 300 - 450 MHz

A switch that shorts the unused input gives the best chance of blowing up your VHF transmitter.
For the OP, a switch is a poor idea, because from my experience the NASA AIS is best left continuously receiving.
A stubby helical on the pushpit will do the job, see the big metal objects at >10 miles, a handy fallback when your windoze won't boot.

 

[nigelmercier]

A stubby helical on the pushpit will do the job, see the big metal objects at >10 miles, a handy fallback when your windoze won't boot.

This would be my suggestion also, and it will double as an emergency VHF antenna if the mast goes down.

 

[rogerthebodger]

You are missing my point having the AIS aerial at the top of the mast will give you better range than on the pushpit. It also gives the best range for VHF. So the best would be to have 2 aerial's at the mast head but we know that the required separation would not be easy to get without interference.

The use of this switch http://www.mfjenterprises.com/Produc...uctid=MFJ-1703 would allow the AIS to have the best reception range abd the VHF to still be connected to an aerial on the pushpit. It would also allow quick connection of the VHF to the mast head aerial when max range of transmission is required say in case of a may day call. It also allows you still to use AIS and VHF alternately without fiddling with connections behind equipment in the case of a lost mast.

My setup is 3 separate aerials 1 mast head 2 on taga at the stern (at least 2.5m separation) for 2 VHF radios and one AIS but IMHO the changeover coax switch would allow more flexibility with the use of aerials.