Elecraft KX1

kx1Spectrum Analysis

The following plots were taken with a recently-calibrated Hewlett-Packard HP-8563A spectrum analyzer. I am uncertain how much variation there may be from one KX-1 to the next given the fact that different builders may have different techniques with such tasks as winding toroids and making clean solder joints. These plots represent my own KX-1 and may or may not be typical of others. The built-in auto-tuner was, of course, disabled for these tests.

Values below represent decibels below carrier. Click on the thumbnail for a larger version of each band’s plot.

Plot
Fundamental
2nd
3rd
4th
5th
40mkx1t
7.0 MHz
-40
-53
-60
-66
30mkx1t
10.1 MHz
-35
-63
-71
-68
20mkx1t
14.0 MHz
-58
-60
-73
-70

Two Ways to Power Your KX1 with 12V from an Internal Battery

The portable KX1 operator faces a choice: the simplicity and compactness of using 6 internal AA cells with the resulting reduced output power caused by this 9V supply or an external 12V battery pack that will power the KX1 to its full potential.

I always made the latter choice when operating afield with the rationale being that I would likely be using a compromise antenna, shorter and lower than I’d prefer or have available at home – therefore, I wanted every milliwatt I could get. But I never liked having to attach an external battery pack to the rig as this seemed to “undo” one of the features I like best about the KX1 – its integrated portability.

1. As my external battery-pack-of-choice evolved from alkalines to NiMh and finally to Lithium-Polymer batteries KX1LiPo(and as my LiPo collection grew), I found that a particular 3-cell (12V) 900 mAh would fit in each side of my KX1’s bottom case – exactly where the AA holders are normally mounted. So I ordered an additional bottom case and the appropriate power connector from Elecraft and one more 900 mAh LiPo battery and wired them in parallel for 1800 mAh of capacity.

The results are:

  • Preserved integration of the KX1
  • Full power out of the KX1 for the duration of the battery’s charge (over 5 hours)

Yes, there are caveats associated with lithium-based rechargeable batteries but I have them in my laptop and my cell phone and see no reason not to put their advantages to use in a QRP rig for the same reasons they are used elsewhere. I don’t leave this bottom case on the KX1 when I’m not actually in the field with it and I also don’t charge the batteries while this case is attached to the rest of the rig.

The chart below summarizes the difference between batteries:

batteries

This chart illustrates the difference between the “best case scenario” with internal AA cells by plotting high-powered lithium batteries against the 2-pack LiPo batteries shown in the photo. Alkaline AA cells would fare even worse and NiMH would be worse still.

Lifetime of these two types of batteries is similar – the main difference is that the LiPos will provide full power out of the KX1 for the duration of their charge whereas the lower voltage provided by the 6 AA cells will not allow the rig to operate to its full potential. Also note that the starting voltage of the AA cells is less than what I consider to be the end-of-charge voltage for the LiPo batteries.

Keep in mind that this plot is based on a constant current draw of 300 mA. This is done for comparison purposes; both batteries would typically last longer in actual QRP use.

2. A simpler way to have an internal 12V battery in the KX1 utilizes both of the rig’s existing 3xAA holders. At least one company, BatterySpace.com, sells rechargeable Li-Ion batteries in an AA-size form factor. These batteries have 3.7V each rather than the typical 1.2 to 1.5 volts for NiMH or alkaline batteries.

If the KX1’s two battery holders were re-wired so that they were in parallel with each other rather than in series, six of these batteries would provide 12V with 1500 mAh of capacity.