[Elecraft] Dramatic improvement in K3 RX sensitivity below 500 kHz -- looking for testers

[Wayne Burdick]

Hi all,

In honor of the FCC's proposed "2200 meter" allocation (137 kHz), we decided to see if we could improve the K3's receive sensitivity in the 100-500 kHz range. This extended coverage is made possible by the new synth (KSYN3A). 

After a lot of digging, we discovered the cause of the wide-band low-frequency noise. We also found a way to shift the passband of the KBPF3 lower. 

With a couple of simple modifications, the MDS (minimum discernible signal) can be improved by about 25 dB at the low end (100 kHz). Here are some test results (preamp off, 400-Hz bandwidth): 

  137 kHz  -114 dBm   FCC's proposed 2200 meter band
  160 kHz  -117 dBm   low end of the "Low-Fer" band
  250 kHz  -126 dBm   test frequency in the long-wave beacon band
  472 kHz  -133 dBm   630 meter experimental band

This represents an improvement throughout the range, but especially at the low end. I believe -114 dBm MDS should suffice at 137 kHz, given the high band noise in this range. 

Another indication of how well the mods work: With no antenna connoted, the K3's S-meter now just barely flickers (S1) at 100 kHz. Without the mods, the no-antenna S-meter reading could be as high as S7 due to the noise source we've now identified (a voltage regulator). (By the way, this regulator noise had no impact on frequencies about about 500 kHz.)

Here's a summary of the changes required:

  RF board:              Add one leaded capacitor
  KRX3 (if applicable):  Add one leaded capacitor
  KBPF3:                 Remove two inductors; parallel three new capacitors

If you're interested in this low frequency range, and you'd like to try the mods, please email me directly. 

Note: To tune this range with the K3, you'll need the new synth (KSYN3A), a KBPF3 module (on the main and/or sub receiver), and a KXV3 module. A KXV3 is required because the antenna for the 100-500 kHz range should be connected to RX ANT IN or XVTR IN. The ANT1/ANT2 path goes through the T/R switch, which has a high-pass filter to protect the K3's fast PIN-diode T/R circuitry.

73,
Wayne
N6KR