[Elecraft] K3 power out - key down

[Jim Brown]

On 7/20/2014 10:32 PM, Wes (N7WS) wrote:
> Not if exchanging call signs takes 10 minutes instead of ten seconds. 

There are excellent reasons for QRP.

As chance would have it, I spent an hour or two today at the NCDXC club 
picnic chatting with AF6RT, who spent Friday and Saturday lighting up a 
very rare grid on 6M (CM79, I think) that involves lots of hiking over 
rough terrain carrying your gear, then making additional round trips to 
get more gear. The hike is on the order of 2-3 hours one way. He's not a 
CW op, and he said he ran down his batteries calling CQ.

The objective is to pass out this very rare grid (99.99% is water) to 
those pursuing the Fred Fish Award to work all grids in the US.

One of my suggestions was to encourage him to use JT9 (24dB of noise 
rejection referenced to a 2.8 kHz bandwidth) and a strategy where he 
announces (on the Ping Jockey JT65 chat) a JT9 window where the 
deserving will call him, and he will respond only to those who he hears. 
W7GJ/KH8 is successfully using this strategy on a moon-bounce expedition 
to American Samoa. I also suggested that he try to use the KX3 barefooot 
when the indicated S/N is better than about -10 dB, and use only enough 
amp to make the Q. The objective, of course, is to minimize current 
drain, and thus maximize operating time.

The WSJT-X software has a multii-decoder, and gives a readout of the S/N 
for each signal it copies. I also encouraged him to replace his K3 with 
a KX3 and KXPA100, because of the KX3's greatly reduced current drain on 
RX (250 mA as opposed to about 1A for a K3).

I'm looking at Li-Ion battery packs that we might pitch in and buy for 
him, and that raises a few questions for Wayne. I can find nice Li-Ion 
packs with 3 cells (11.1V nominal) or 4 cells (14.8V nominal).

1) Can the KXPA100 and KX3 tolerate the higher voltages of a fully 
charged 14.8V battery pack? (In the range of 16V).

2) Do the KX3 or KX3 pull proportionally less current for the same power 
output from a 4-cell pack as compared to a 3-cell pack?

3) I'm seeing some 20Ah packs rated for a max of 10A and 14A load 
current, with active protection to limit the current to those levels. 
How would the KXPA100 react to that, if he wanted to run 40-50W?

4) Where can we access a graph of KXPA100 load current vs. 6M power out, 
not considering tolerances from one piece to another?

5) Am I correct that a KX3 does not benefit from an outboard preamp in 
this application?  That is, a very quiet QTH, he's current starved, the 
guys who want to work him have power and big antennas (but probably more 
RX noise), and he's got to fight for every dB.

Weight for a K3 as compared to a KX3 + KXPA100 is about equal, 
especially when interconnect cables are taken into account. The 
advantage is current drain, and is a big deal if 90% of your time is RX.

He's already carrying a very nice M-Squared antenna and mast to hold it, 
and LMR-400 to feed it.

Those reading the mail should appreciate this as a very practical 
engineering problem. :)

We don't need fast response on this -- thanks to work commitments, his 
next window of opportunity is Memorial Day 2015. :)

73, Jim K9YC