Radio Communications Systems During Crisis Situations (Power)

This is part 4 in an ongoing series on radio communications during a crisis event.

How much RF power do I need?

• As much as possible, but only use the least amount of power required to complete the communication.  There are reasons for both high power and low power.
• Low power conserves battery consumption and keeps the range of communications as short as possible when trying to maintain a low profile.
• High power is used to reach out longer distances when conditions form weak propagation but propagation is still possible. No amount of power will reach out long distances if the conditions for propagation are not favorable.
• During the best propagating conditions one can talk (voice) around the world on 5 watts or less using a gain antenna. Using a low power mode such as CW or PSK31 even milliwatts (less than a watt) is all that is required to communicate long distances.
• I know I previously said as much power as possible and sure that would be nice.  However it’s not practical to have all that power as the battery or power source requirements would not be feasible in SHTF conditions.  100 watts is the most feasible transmitter power running from a 12 volt system. That’s from a fully charged battery putting out 13.8 volts.  To get 200 watts of transmitter power the voltage requirements jump up to 85VAC for the ICOM IC 7700 ($7000 retail, almost 9 times the cost of the FT857D).  This radio now requires a generator to operate and dino fuel is scarce and expensive WTSHTF.  So how much better will 200 watts sound at the receiving end compared to 100 watts? Only about 25% better because of the inverse square law.  However depending on how noisy the conditions are at the time 25% might mean the difference between copying the communication before the transmitter batteries die and not hearing the transmission at all.  To use the full legal limit power of 1500 watts you’ll need a full house generator to supply the required power and the expensive power supplies to provide the amplifier the required power. Expensive equipment and expensive fuel consumption!  Instead of using transmitter or amplifier power to boost the transmitted signal which costs exponentially more money and consumes exponentially more resource power there is a better way to amplify the RF signal using an antenna. We’ll talk about antennas more later.

 What type of power sources should I use?

• Unless you have unlimited fuel and will be running generator power throughout the crisis you will need some sort of battery bank to run the radio equipment. If you stick to the radio types recommended here the most power you will require is from a 12 volt deep cycle battery bank for the mobile/base radios to AA or AAA batteries for the portable hand held radios.  The 12 volt batteries should be charged via solar or wind power through a charge controller so as not to over-charge and damage the batteries. The AA or AAA batteries should be rechargeable via direct solar charging from the solar panels or 12 volt battery bank and not through wall warts that use converted DC to AC/AC to DC voltage.  There is substantial waste converting the DC to AC and then AC back to DC to charge the batteries. 
• Save wasted power by not using 12 VDC rectified power supplies that run off of 120VAC. Again much needless energy waste happens in the conversion process.  Stick to direct battery power where the only losses are in charging the battery and cable resistance loss.  Speaking of power cables, they should be heavy enough for the 22A current draw when transmitting at 100 watts of power and I’d even go up a gauge in size to help reduce the power cable loss even more.  The min gauge is set for safety reasons but the cable will still be wasting energy which you can feel as the cable warms up.  Use the largest feasible wire (cost/ease of workability/available connectors) possible so as to conserve precious battery power.

How long can I expect to operate off one battery charge?

• A typical 12 volt lead acid battery is capable of taking a charge from a 14 volt (14.4 typical) bulk charge source before it starts to damage the battery.  At 13.4 volts (float charge) the rated output power of a typical mobile/base radio will be close to 100 watts. So while the charging source is applied to the battery your radio will be transmitting at or near 100%. Once the charging source is removed a fully charged battery will output about 12.7 volts. At this level your radio will produce about 92 watts. At 12.4 volts your battery is about 75% capacity and at this point it should be recharged.  If not then sulfur will build up on the plates and begin to shorten the life of the battery. At this level your radio will only be outputting about 90 watts. As the battery drains down to 12.2 volts the radio will only put out about 88 watts. At this point the battery is getting to 50% capacity.
   
• A 100Ah battery will run the radio in receive mode (squelch open mode 1A) for about 50 hours before it reaches 50% capacity.  However, for longer battery life (longevity) it is recommended not to run the batteries below 75%. So the most you can run off a 100Ah battery in receive mode is about 25 hours or one day.  If the radio was to transmit at full power (22A) the time to 75% capacity is now at just over 1 hour.  Reducing transmit power to 25 watts will get you about 4 hours of transmit time. 10 watts will get you 10 hours of transmit time. 5 watts of output power consumes about as much power as the radio in receive (squelch open) mode or about one day of operation or 25 hours.  I’ve used rounded numbers here for easy understandable math.  These are just rounded ball park figures to give you an idea on battery drain compared to output power. Remember 5 watts of power during the best propagation conditions will travel all around the world with the right antenna.

In the next series we'll discuss what antennas to use.