The Antenna Splitter Decision

Man Overboard the right way

When installing any AIS transponder on board, owners are inevitably faced with ‘the splitter decision’. AIS communicates with other vessels over VHF and therefore requires a VHF antenna; however, in some cases using a splitter with an existing antenna is a great alternative to installing another dedicated antenna. There are pros and cons to each option, but what if you could have your proverbial cake and eat it too?

What exactly is a splitter? In its most basic form, a VHF/AIS splitter is a device that splits an RF signal allowing it to be shared across multiple devices, in this case allowing a VHF and AIS to share a single antenna. Some splitters introduce problems, which has led many people to have a negative opinion of them. Many inexpensive splitters simply split the signal and offer no protection to either device. This can cause damage between the devices when the AIS or the VHF transmit into the other device, rather than out the antenna. On receiving a signal, splitters will typically split the signal equally between the AIS and the VHF outputs, meaning neither gets the full strength it otherwise would. The best you can hope for is a 3dB drop. A no-loss splitter like our SP160, or the capability built into Cortex, will negate the drop by amplifying the signal output after its been split. To put the drop in context, a 1dB drop in a 25W system equates to ~5W of loss, which is pretty significant. Transmission is slightly different, and we’ll touch on what makes Cortex so special a bit later in this article. A notable concern that should also not be ignored, is that adding additional connectors into an antenna cable can create points of failure or degradation of the signal. Any technician will tell you the more connections in a system the higher the resistance or impedance, which all equal less performance both in range and reception.

Choosing an antenna versus a splitter

One of, if not the most common questions we get asked when talking to a new AIS customer is 'Do I need another antenna, or can I use my existing antenna with a splitter>' There are several benefits to using a splitter including saving installation time and cost, fewer holes on the boat, and not having to find a location for another antenna. The main downside to using a splitter is the potential to block the AIS from sending out a position report if the boater is actively transmitting on the VHF. An external splitter will likely also require its own power supply, adding to installation complexity. In our experience, most sailboats elect to use a splitter to take full advantage of a single antenna being located on the top of the mast which is the single best position on board given its height. Powerboaters on the other hand typically have more flexibility with additional antenna mounting locations. It is common however for powerboat owners to have multiple VHF Radios and a splitter offers a great solution for accommodating this with single antenna placement.

How does Cortex make all of this clean and simple?

With Cortex being the VHF as well as the AIS there is no need to consider an external splitter or having multiple antennas for a normal installation. Firstly, Cortex contains an internal no-loss splitter, providing the same functionality as our existing SP160. But this is used to provide a pass-through signal output from Cortex to a second VHF, and shouldn’t be confused with the purpose of using a traditional splitter to handle the VHF / AIS split. The special part is how Cortex handles the VHF / AIS signals. Because it’s all internal in the same device we don’t actually need a splitter at all!

Handling VHF and AIS transmission without a splitter

Since Cortex is both the VHF (Voice and DSC) and AIS, we know exactly when the next AIS update needs to be sent. We also have full control of the VHF transmission for Voice or DSC. At the exact point, the AIS message is transmitted, Cortex will break the VHF transmission and then resume it immediately after. You may think this would create a poor VHF transmission that may sound glitchy if we kept interrupting it. The time needed to broadcast the AIS message is about 24 milliseconds and is barely detectable by someone listening. This means that without needing a splitter, Cortex provides continuity of both VHF and AIS simultaneously using a single antenna.

Receiving without a split signal

Cortex is not like traditional VHF Radios which predominantly rely on hardware to split and manage their signal. Cortex is software-driven, which means it has a slightly different rule book. When Cortex receives signals through the antenna, it is not solely concerned with just one frequency band like a dedicated AIS or traditional VHF. Cortex instead captures the entire frequency range without splitting it. Cortex then uses software to distribute the frequency bands used for different broadcast types for VHF Voice, VHF DSC, and AIS. By using software processors to manage the signal, there is actually no need to physically split the signal from the antenna in the Cortex Hub.

No compromise

With Cortex there is no compromise sending AIS messages while using the VHF to communicate. The need for costly and added complexity of multiple antennas or splitters is eliminated. The potential for a poor or degraded signal through multiple devices or poor quality splitters or contacts is all but eliminated. With Cortex's internal integration of VHF and AIS functionality, and being the world’s first software-based VHF, it redefines the rules on how VHF signals are split and managed on board your boat.

Learn more about Cortex's VHF features

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