Thursday, May 22, 2014

How Long Can One Operate on Emergency Power

I have seen many a set-up in YouTube and other SHTF Preparedness web sites of amateur radio gear, with go boxes, batteries, solar power backup, but I have seen very little of what I think is a well thought out set-up which addresses maximum utilization of meager energy resources should such a situation arise.  For example, I have seen go boxes which employ high current draw rigs – even on receive.  For an objective look at the comparison between various rigs, I have collected data over the years on how HF and VHF radios consume  power on receive and transmit. Based  on this data, one can put together a set of expectations on how long such a set-up will be operable on back-up or emergency power.

Several variable are at play
Receive current draw – the amount of current a system consumes in amperes or milliamperes while just listening
Transmit current draw – the amount of current a system consumes in amperes or milliamperes while transmitting – key down carrier
Duty cycle – the percentage of time the mode is in full key-down (100% duty)

Let us consider some common modes of transmission
SSB – 25% Duty Cycle (Source QST)
CW – 33% Duty Cycle (Source QST)
PSK31 – 100% Duty Cycle
RTTY – 100% Duty Cycle

What one needs is an instrument that measures current draw during receive only and during key-down (100% duty cycle).  I have put together operating time expectations using a 14 ampere-hour depletion to 50% capacity – the maximum depletion one would want to push a deep cycle battery to without severely affecting longevity of the battery.

As one can plainly see, operation at 20 watts greatly reduces the operating time.  Not all rig / amplifier combinations are ideal.  Some are quite power hogs.  The most miserly of the transceivers are also amongst the most limited in terms of operating mode - but the rigs can practically operate "forever" with solar power.  For example, the Elecraft KX1 only consumes 55 milliamps on receive and less than 1 amp on TX.   It is a CW only rig and the version I had was a 4 bander 10,20,30 and 40m.  There are now several of this type of radio on the market.   This will just about double the operating time at 30% duty cycle to 11 hours.  The most versatile ones are the Elecraft K2, KX3, and Yaesu FT817.  The power output on the latter is limited to 5 watts.  Operation of the Elecraft rigs greater than 10W will require an external amplifier.  External amplifiers for QRP radios are manufactured by several sources and can be purchased as kits,

Elecraft KXPA100 and KPA100 (K2 only)
Juma PA100 
Ten Tec Model 418

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