Digital Antenna 992, wideband VHF/AIS antenna with easy cabling
Both Ben Ellison and I have recently installed Vesper’s Cortex VHF Radio, AIS, and boat monitoring system. Cortex uses a single VHF antenna for VHF voice and AIS, which calls out for a wideband antenna that covers both frequency ranges well. Ben E. selected a Shakespeare 6500-WB antenna, and I’ve been testing a Digital Antenna 992 wideband antenna. We don’t have the ability to perform in-depth performance comparisons, but here’s a look at what you should expect from the antennas, and also the installation differences.
The eight-foot Digital Antenna 992 has 6 dB of gain compared to the four-foot Shakepseare 6500-WB‘s 3 dB. The 992 also claims a much wider band than the 6500-WB, covering from 134 to 176 MHz, but only matches the Shakespeare’s claimed 1:5:1 VSWR over VHF voice and AIS frequencies. As Ben E. noted, that’s as good a spec as can be found in marine VHF antennas.
Typically, a boat with a VHF radio separate from its AIS transceiver uses one antenna tuned for VHF voice and one for AIS. Even though VHF/AIS antenna splitters have long been available, separate antennas were generally considered the highest performance option. Before Cortex, even VHF radios with AIS transceivers built-in had two antenna connections.
But the software-defined radio used in the Cortex hub handles VHF voice and AIS messages in a highly integrated fashion, and one result is its single antenna connection. Also the conventional wisdom about dual antennas is changing, especially given newer broadband models that perform well from the emergency channels low in marine VHF range through the AIS channels at the high end.
For instance, Digital Antenna’s conventional VHF voice antennas are all centered on channel 16’s frequency of 156.8 MHz with 8MHz coverage. That means they’re tuned to perform best from 152.8 MHz to 160.8 MHz, with AIS frequencies just outside that range. Shakespeare’s Phase III VHF voice antennas are also centered on 156.8 MHz but with 3 MHz of bandwidth, so the AIS frequencies are further outside the best performance range.
But the Digital Antenna 992 and Shakespeare 6500-WB earn their wideband designation by extending performance across both VHF voice transmissions (156 to 157.6 MHz) and AIS transmissions (161.975 and 162.025 MHz). However, that entails some compromise. As John Jones, Digital Antenna’s vice-president of engineering explained, “Optimum antenna performance comes when an antenna is tuned for a single frequency. You can tune the aperture of the antenna on the receive side so you get much less of anything else other than the frequency you’re hunting.” So, by widening the frequency range these antennas are receiving, you’re also increasing the amount of noise that makes its way to the receiver.
So, two antennas are always better, right? Not necessarily. There are myriad other considerations for getting optimal performance out of your radios. One such consideration that’s given me trouble on Have Another Day is antenna separation. Two antennas too close to each other will affect performance, and the closer together, the worse the results. It’s not just antennas you need to worry about; any metal structure, like a mast or support, can reduce the antenna’s performance. So, in many cases, one well placed, properly selected antenna can outperform two compromised antennas.
A VHF antenna’s ability to focus RF is measured in gain. Gain is expressed in decibels (dB) on a logarithmic scale. 0 dB represents a 1:1 ratio of power into the antenna and out of the antenna. 3 dB represents a doubling of apparent radiation energy through the antenna, 6 dB is four times the energy, and 9 dB is eight times the energy. But, an antenna is a passive device, so it’s not creating energy out of thin air. Instead, it’s focusing the radio frequency energy flowing through it. The diagram above shows happening. At 0 dB (no gain) the antenna collects energy in a sphere centered on the antenna. As gain increases, the radio signals are focused into a narrower range that increases apparent signal strength but also decreases the coverage pattern. Fisheries Supply has a good primer on marine VHF antennas that discusses marine VHF basics.
Images courtesy Shakespeare (https://shakespeare-ce.com/marine/antenna-selector/)
As soon as gain is introduced, the coverage sphere begins to get squished, the higher the gain, the more squished. Eventually, the pattern will be a wafer-thin pizza shape, instead of a giant ball. As beamwidth gets narrower, the chances a transmission is missed go up, especially as a boat rolls in a sea-way. Shakespeare’s antenna selection tool illustrates the effects of both gain and antenna height in a way I find easy to understand.
Digital Antenna uses a mini-UHF connector on the end of the cable the 20-foot cable that comes attached to the antenna. The connector is barely larger in diameter than the cable itself so it should be possible to fish it through nearly anywhere the cable fits. A mini-UHF to PL-259 adapter is provided with the antenna, so the entire install can be performed without any soldering but with the confidence of factory-quality terminations. The cable is permanently attached to the antenna, so if you need to replace the antenna you will also have to fish the wire again.
It’s hard for me to assess the antenna’s performance accurately. Firstly, I’ve done my testing at Legacy Harbour on the Caloosahatchee River in Fort Myers, FL. I’m about 10 miles up a river in the shadow of four large high-rise buildings that have, I think, a negative effect on my VHF performance. Plus, I’ve always been suspicious of the VHF performance on Have Another Day. It has three VHF antennas across the aft edge of the hardtop that are too close together, and it’s seemed anecdotally that other boats have longer AIS receive range. For my testing I’ve laid down all the other antennas near this one.
I have noticed that AIS performance is as good or better than the VHF voice tuned Shakespeare 6225-R Phase III antenna it replaces. Watching targets move up and down the river, I can see them a little further — out to about four nautical miles either way — than with the VHF voice antenna. Plus, with the VHF voice antenna, Cortex consistently threw an antenna VSWR error, but only on AIS transmissions. I suspect that’s because the antenna’s VSWR climbed rapidly outside of its design frequency range. With the 992, I haven’t seen any indications of VSWR trouble.
The Digital Antenna 992 is a good match to Vesper’s single-antenna VHF/AIS radio. Covering the entire frequency range in a single antenna requires some compromises and purists may prefer two antennas each tuned for a smaller frequency range. But, I believe there’s a good argument for using a single antenna and focusing on optimal placement, good cabling, and clean connections. If you subscribe to that argument, I think the 992 is an excellent choice.