In the fall of last year I decided it was time to take on the emergency power aspect of an amateur radio go kit. Presently I do not have a home backup power source, making the addition of battery power for radio communication beneficial during a power outage, or in the field during an emergency.
I knew it is common to include an HF rig in a go kit, but having no experience with HF I did not understand the benefit an HF radio can provide in an emergency. Additionally, I had not upgraded my license therefore I had no plans to include an HF radio in my kit - or at my home station for that matter.
When I did add HF capability in January, I discovered there is significant interference on the HF bands when using my AC to DC power supply with the new radio. I was not excited about the cost required to buy a new power supply, and as I had recently built an emergency power system the problem was solved - to a point.
The new radio easily connects to my battery power system, but I bought a battery based on calculations of power needed to operate just my VHF and UHF dual band radio over a given amount of time. If I want to use both of these radios on battery backup when the power is out at home, or when deployed, as Tim the Toolman would say, I “need more power”!
In case some of you have not gone through the process of determining how much battery you would need to operate your radio equipment, for the remainder of the training I want to touch on this process as it is a key consideration when putting together an amateur go kit.
The method I used takes into account the amp draw during transmit, plus the amp draw when receiving. Factor in the amount of time you expect to transmit vs time listening, then times the result by the number of hours you expect to be in operation. For the purpose of this discussion we will assume the radio equipment to be powered includes at least one 12 volt mobile radio as a base station.
First, review the user manual for each radio that will be powered by your battery. Most mobile radios have low, medium and high transmit output settings. Considering the possibility it will be necessary to operate on a simplex frequency to communicate with distant stations, I suggest using the specs for the maximum transmit power when you make this calculation. For my Kenwood TMD700 the spec sheet indicates a draw of 11.5 amps while transmitting at 50 watts in the 2 meter band, with a 1 amp draw when receiving in either band.
Next, determine the duty cycle you could expect if you were operating in an emergency situation. This relates to the time spent transmitting, versus time spent listening. If you are acting as net control, for example, would you expect to transmit at least 50% of the time, maybe more? If you are not net control would you transmit 20% of each hour, or more or less?
Using the specs from my Kenwood TMD700 with an estimated duty cycle of 20% transmit and 80% receive, I would multiply the 11.5 amps draw during transmit by 0.2, with a sum of 2.3 amps used over 20% of the hour spent transmitting. Then I multiply the 1 amp receive draw by 0.8, with a sum of 0.8 amps used during the 80% of the hour when I am listening.
Add the two sums together and the total draw is 3.1 amps over the course of 1 hour for a 20/80 duty cycle. If I planned to volunteer at an event with an expected operational period of 8 hours, I would times the sum of 3.1 amps needed per hour by 8 to determine the total amp requirements for 8 hours. In this case I would need a battery with at least 24.8 amp hours - as long as the battery can deliver the required amps for the entire 8 hour event.
Remember to consider the power consumption of any auxiliary devices you may use when you choose a battery for your system, such as a laptop or mobile device to use for digital communication with Winlink. Also, if you think an HF radio might be in your future I encourage you to “build up” the capability in advance. In the case of my HF rig, the max draw during transmit is 21 amps and during receive the draw is 1.25 amps - with the volume at its highest.
When commercial power is available it’s a slight inconvenience to recharge my battery periodically to keep my HF radio on the air while avoiding the interference from the power supply. But, to be truly effective as an emergency power source, a higher amp hour battery would extend operating time when recharging with commercial power is not possible.
Another option would be the addition of an off-grid recharging system for my smaller amp hour battery, effectively extending the time I could be on the air - but that will be a topic for another net.