Class B AIS on OpenCPN not working

[Bill4]

I have been given a Comar USB dual-channel AIS receiver by someone who has upgraded to a transponder. It is in "full working order" apparently.

I've configured OpenCPN to see the AIS and Class A targets are displayed but Class B targets aren't showing up at all. Several Class B's were within line-of-sight and between 400 metres and a couple of kilometres away for 20-30+ minutes. They were transmitting according to Marine Traffic Android app on a phone (different receiving station, I know, but...)

Any ideas why OpenCPN isn't picking up the Class B's? Is there a setting somewhere in OpenCPN I've missed?

Using Version 4.0.0 Build 2015-01-08.

[Neptune's Gear]

I'd suspect a poor antenna connection/function. Class A has much more transmit power than class B. There is no filter in OCPN that might cause this (except the don't display anchored targets perhaps if the Class B's were really slow?)

[Bill4]

Originally Posted by Neptune's Gear

I'd suspect a poor antenna connection/function. Class A has much more transmit power than class B. There is no filter in OCPN that might cause this (except the don't display anchored targets perhaps if the Class B's were really slow?)

One Class B was anchored (according to Marine Traffic) the rest were sailing/motoring at > 4 knots. One in particular was motorsailing at approx 6 knots and passed directly in front of me approximately half kilometre away.

Interesting comment about the antenna - I had assumed because Class A's were being received OK that Class Bs should be received too. I'll take a look (can't check until the weekend though).