T2 and low power on 10 meters was K2 Binocular Core

[Don Brown-4]

Hi

I have also found that T2 is the most critical part affecting the power out on 10 meters. Here are a few things I do to get it right. It is important that the red winding covers most of the core. After tightly winding the red wire, spread out the winding evenly around the core so the ends match up with the holes in the PC board. Then wind the green winding but allow some extra length on the start lead. Count over about 3 or 4 turns of the red winding and wind the green so each turn interleaves the red turns and do not cross over the red windings. Is important that you end up with alternating red and green turns. Count the red turns that are on either side of the green winding, they should be equal or within one turn of each other. If not remove one turn from one end and add one turn to the other so they are as close to equal as possible. This is why you need to allow some extra start wire length so you can adjust the windings if needed. Now the red winding should be covering 80-90% of the core and you should have the green leads coming out at the sides covering about 50% of the core. Bend the green leads forward close to the core away from the red leads The wires will come off of the sides of the core. Strip the insulation from the wires. I use a small 1/8 inch thick square of double stick foam tape on the PC board under the toroid to space T2 off of the board. This spacing is also important in getting the required power out on 10 meters. Install the toroid making sure the leads are just tight enough to remove any excess length. Do not allow the green winding to get scrambled up over the red by pulling the green leads to tight. The green leads should still exit the core on the sides and make a gentle curve to the PC board holes.

I have not really seen T4 winding methods affect the power out much. Just make the windings tight and the correct number of turns.

One other thing that is sometimes missed is the voltage drop in the power lead. The K2 will pull up to 3 or so amps at full power. The power draw is greater on the higher bands so this may also show up as low power out on 10 meters because the drop in the power lead is starving the final amp of voltage on transmit causing the current draw to increase. The power supply must be able to provide 13.8-14.0 volts at 3-4 amps for the QRP K2 or 20 amps for the K2/100 or you may not be able to reach full power.

Do not use smaller than 20 gage wire or longer than 3 or 4 feet for the power wire. I use 18 gage when I build K2 power cables. That is about the largest that will fit into the round connector. Radio shack sells some red/black 18 gage speaker wire that I use for this that looks and works well. For the K2/100 Elecraft supplies 12 gage red/black wire for the KPA100 so use only that wire or larger or you may have problems with the K2/100 reaching 100 watts. If the power cord is getting warm then you have a problem and you need to use a bigger wire.

If you must mount your power supply remotely then use something like a RigRunner with a large gage cable from the power supply to the RigRunner (14 to 12 Ga for a QRP K2 or 10 Ga for a K2/100). Use a short cable from the RigRunner to the K2 with the 18 gage wire for the QRP K2 or the 12 gage for the K2/100 The RigRunner is fused for each power tap so you do not need to build in a in line fuse for the K2/100 power cable if you install a 20 amp fuse into the power tap for the K2/100.

Don Brown

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