How to Create a Bullet-Proof VHF/SSB Backup

In this short article I want to chat about ideas for making the radio bombproof in an emergency. I haven’t “invented” any of these ideas, others have discussed them and some are common for commercial installations. I am in the process of renovating my new boat and one of the significant jobs is a complete rewire. Two issues that have always concerned me are the vulnerability of the VHF to power failure and loss of performance on the SSB due to low voltage. The VHF is not super fussy and is usually fine if it gets at least 11V but if the battery goes dead obviously it won’t work. An SSB on the other hand is fussy about voltage. Icom states that their sets need 13.8V for full power with a minimum operating voltage of 12V. It is generally accepted that power drops off as the voltage goes down and Icom’s insistence that units need a 13.8V supply supports that. I cannot find any information online that confirms how much power is lost but, logically, it will be more than 10 percent, maybe quite a bit more. If anyone has a definitive answer to this, please comment.

So how likely are these problems to arise in the real world? For the VHF when are you likely to need to make a MAYDAY call? MOB, fire, flooding/sinking and medical emergencies are the main ones. MOB  and medical emergencies are no problem, a well-installed radio should be fine, but fire and sinking?

Fire. What is the most common cause of fire? Maybe an engine fault or a galley fire are up there but so is an electrical fire. What would be your most sensible response to an electrical fire, shut off the main battery switches would be mine. Does the radio still work? If it is directly wired as recommended in ABYC it might, that’s why they say to wire it directly from the battery but if you can turn the VHF off on the main panel it is now dead. If the fire is significant it may also be dead because the fire wrecked the batteries or burnt the wire.

Flooding. The second issue is flooding. Most designers place batteries low in the boat so that their weight does not affect stability and they are out of the way. If your batteries are in the bilges they are going to be one of the first things to flood. Even with sealed batteries, they will not work for long underwater because seawater is a conductor and effectively shorts the batteries, so again no radio!

SSB. SSB is a bit different. You have all the same issues but added to the list is the problem of voltage drop. A well-installed SSB will have short heavy wiring to minimize voltage drop in the cables but it is still limited by the actual battery voltage. In the real world, batteries will be somewhere between about 12.3 and 11.5V. For full power the SSB wants 13.4V, so unless you are running the engine you will be transmitting at reduced power. Having to run the engine potentially introduces more radio noise but also can just be noisy enough to make it more difficult to hear what is coming through so not ideal.

So those are the issues, what’s the solution?

Voltage solution. Like most of us these days my first thought was to go to ‘Uncle Google’ and see if someone makes a solution similar to an uninterruptible power supply that puts out 12V. I came up with nothing but if you have found something please leave a link in the comments.

So back to the drawing board, can I make something? Although I am a pretty fair electrician, I don’t build circuits so it had to be readily available components. What I found was very simple.

Boost buck converters are readily available as sealed units (IP67). They take a wide range of input voltages and convert them to 13.4V and are relatively cheap. For an SSB, you need one that provides at least 25 amps, if it is just for VHF, 10 amps is fine. The link below is the one I got. So that solves the voltage problem but still leaves the issue of backup power.

Backup power solution. Again, there is a simple solution. With a stable 13.4V supply, a small, sealed lead-acid battery in standby mode does not need a battery charger, simply a supply at the batteries, float voltage. Again, these are readily available and used for things like the backup batteries in burglar alarms or UPS systems. There are basically two options.

A high discharge sealed AGM (a 20 amp hour battery will run an SSB at full power for around 30 min). Or the more expensive option is to go for a lithium battery. You are going to need at least 30 ah to get the current needed for an SSB and that should give more than one hour of operating time. You will also need to go with a 14.6V converter, which is again more expensive. Total cost will be around $300 Canadian compared to around $160 for option 1.

CONCLUSION

These are my thoughts, I have not installed or tried the system out yet so take this as a design idea but it is all well-established tech so I don’t envision any problems but would welcome comments. For me, sailing in remote parts of the Northwest, looks worth it for the extra peace of mind. Whether it makes sense for you probably depends on your cruising area. I am going to go with the sealed lead acid option but go up to a 33 ah battery which should give around one hour transmission time and will not be stressing the battery close to its limit (20 ah is definitely the lower limit).