Some folks are criticizing the ARRL for not modifying the Field Day rules in response to the Wuhan virus epidemic. Most of them are looking for a way to operate Field Day from home but still have a club score of some kind. I posted my thoughts here: Don’t Mess With The Field Day Rules.
The Field Day (FD) rules allow for a home station with commercial power to participate in FD as a Class D station. However, Class D stations cannot work other Class D stations for points. If the home station has emergency power (batteries, gasoline generator, etc.), then it is a Class E station that can work all FD stations for point credit.
Emergency Power: Too Difficult?
I’ve heard some hams argue that it is too difficult to set up emergency power for their home station. In many cases, the argument is actually that it is too expensive to do this. I can see this point if you run out and buy a name brand gasoline generator…a Honda EU1000i costs about $950.
This raises the question of what is the lowest-cost way to equip a home station for emergency power? Let’s consider the case of a typical 100W HF transceiver such as an IC-7300 or FT-991A. These radios require a 12 V power supply at 22 A maximum on transmit. Receive current is much lower, typically 1 to 2 A. Under FD rules, we don’t need to power our computer or other accessories from emergency power, just the radio. [Update: if the computer performs the keying of the transmitter, as in WSJT modes, it must use emergency power.] If we assume a 50% duty cycle, this class of radio consumes about (22+2)/2 = 12 A average current. (Yes, you could choose to operate QRP and really stretch the battery but let’s stay with the 100 W scenario.)
Get A Battery
So what is the cheapest way to get this done? Let’s take a look at using a deep-cycle battery. Walmart has an RV/Marine battery for $75, rated at 101 AH. Assuming 12 A of current, this battery would support about 8 hours of radio operating. This is going to be way short of the 24 hour operating period of FD but it might be enough to support a less intense operation. We could also do some things to stretch out the battery life, such as reducing our transmit power. Dropping to 50 W would roughly double the operating time to 16 hours, which should be enough for a single-operator station.
Of course, another option is to double the battery capacity by using two batteries. These amp-hour ratings on batteries are always a bit idealistic and our transmit duty cycle might be more than 50%. Let’s assume we buy two batteries to give extra margin and allow us to run 100W. We will also need a simple charger, which costs about $25. So there you have it, 2 x $75 plus $25 = $175 for a decent emergency power source. (If we decide to use only one battery, the cost drops to $100.)
Now $175 is a significant investment and only you can judge how well your ham radio budget can support this. For many people, this is affordable and the real question becomes is this how you want to spend my hard-earned cash.
This is my best shot at a low-cost emergency power source. Do you have a better idea?
73 Bob K0NR
Update 25 May 2020: Various people have pointed out that your vehicle is a good emergency power source. You can either run your transmitter directly from the vehicle battery or use the vehicle to charge a separate battery.
I started with a marine battery. It was well within my budget, and served me for many years of public service outings and the occasional net. About a year go, I started participating in Parks on the Air. There is a strong push among members of the Facebook group to ditch the heavy lead acid batteries in favor of LiFePO. These are delightfully lighter but considerably more expensive, but the claim is they last longer and so prove themselves cost effective in the end. I am in the midst of proving (or disproving) that assumption.
Also, I have a power monitor inline between the radio and battery, and on both my Kenwood TS-590SG and Yaesu FT-891, peak current while operating SSB at the 100W setting is well below the published 22A. More like 15-16A. That was true with the marine battery as well as the lithium one.
Good point about the peak SSB current. This reminds me that the average SSB transmit current is quite a bit lower than the max spec. It will rise and fall tracking the voice modulation. This is in addition to the 50% duty cycle assumption and will increase the operating time for a given battery capacity. This might be enough for a 100 W SSB station to get by with one battery instead of two.
If your computer is used for keying the radio, then it must also be powered by something other than the commercial mains. So, if you’re operating any of the digital modes or using something like N1MM to operate your keyer, better add whatever your computer requires when sizing the battery.
Good point. My notebook PC draws about 2 A at 18 V, so not a huge amount of power but still needs to be accounted for.
The computer runs on 117VAC, therefore it needs an inverter to be able to be powered from a battery. The cost and inefficiency of the inverter must also be added to the overall calculation if you want to be correct about it.
The computer may operate off AC line voltage. It could also be a laptop or Raspberry Pi that runs off DC.
Your calculation is optimistic in some and pessimistic in other ways. The radio doesnt draw as much using ssb, that has been said before. But what hasnt been said: Dropping the output power of the radio by half wont nearly cut the input current in half, it uses way more than half the input power, so qrp isnt really helping as much as you might think. Dont take my word but do your measurents, its interesting. The second thing is: using 100Ah out of a 100Ah lead acid battery means its totally flat. Do that a couple of times and the battery is dead. For standard cheap lead acid starter batteries you shouldnt discharge more than 50%, better 30% to get a long life from the battery. Lithium batteries are another thing, those can deliver close to 100% of their charge without being harmed (but be sure not to undercharge them further, one time might be enough to kill them). I use old laptop battery cells for my solar setup (a few kWh of storage) and go packs for my radios. Those are very light compared to lead acid and very cheap (old laptop packs) but there is a lot of work involved and you definitely need to know what you are doing if you dont want to burn them down. Always be extra careful near any kind of batteries. For me, making the battery packs and the solar setup is part of the hobby. Enjoy guys!
Alex, thanks for the helpful comments. My Ah / current calculations are rough estimates, not a precise estimate.
On the discharging of lead-acid batteries, note that I specified a deep cycle battery, not a starter battery.
On the QRP power level, I came across this video of Yaesu FT-891 current measurements vs RF output power.
http://oh8stn.org/blog/2018/03/12/yaesu-ft-891-current-consumption-test/
These results show that to get half the current consumption, the power would need to be dropped from 100 W to about 10 or 20W, supporting your point. The measurements also show the max current as 15A, not the specified 23 A.