Quoted: If you get the 857 you can turn down the power. I almost always keep my power down to 10 watts or lower. ...
But if you’re going backpacking to play radio your going to need to bring more battery anyway. So if your going backpacking to play radio the 857 is only 3 lbs heavier |
the primary difference between turning a QRO rig down to 10W and a true QRP rig is that the receive current is incredibly different. a true QRP radio, like the FT817 and Icom 703+, has by design extremely low receive current.
from Yaesu's numbers, the FT857D draws 1A on receive, and the FT817 draws 250mA on receive. so, what's a measly 750mA?
let's start with a 9lb 12AH battery and assume a 80% RX/20% TX duty cycle... we'll assume that the FT857 draws 2A on 5W transmit (i can't find any data on the web for this power level), and the FT817 draws 2A as well (from FT817 brochure).
FT857D:1A on RX, 2A on TX:
0.8 x 1A = 0.8A
0.2 x 2A = 0.4A
summed, 1.2A per operating hour.
12AH/1.2A =
10 hours operating time.FT817:0.25A on RX, 2A on TX:
0.8 x 0.25A = 0.2A
0.2 x 2A = 0.4A
summed, 0.6A per operating hour.
12AH/0.4A =
20 hours operating time. again, the purpose of a true QRP is to maximize the amount of operating time on a given battery, and by design one way to get there is to minimize the receive current.
moreover, an FT857 can not take anywhere near full advantage of a 12V battery -- it will cease to operate at a terminal voltage below about 11.8Vdc. i saw this firsthand at a recent camping trip. an FT817 will operate down to 8V, and a 703+ down to 9V. significant battery capacity is available below 11.8V, in fact the standard capacity calculation for SLA/AGM batteries ends at 10.5Vdc.
example datasheet, a 9 LB 12V 12AH AGM battery, e.g.,
www.bb-battery.com/productpages/BP/BP12-12.pdfhas the following capacities:
20 hour rate F.V.(1.75V/cell) (600mA to 10.50volts) 12.0 A.H.
10 hour rate F.V.(1.75V/cell) (1140mA to 10.50volts) 11.4 A.H.
5 hour rate F.V.(1.75V/cell) (2040mA to 10.50volts) 10.2 A.H.
1 hour rate F.V.(1.55V/cell) (7200mA to 9.30volts) 7.2 A.H.
in summary, on a given battery an FT817 can operate *at least* twice as long as the FT857D. and because of the aforementioned battery voltage issue, it is more likely that the FT817 will operate about four times longer.
btw, there are more specific, purpose built QRP rigs that have astonishingly low receive currents. google "elecraft".
ar-jedi