[Elecraft] Experimental firmware for those who *do* want to listen to the AM band on the KX3...

[Wayne Burdick]

Hi all,

The KX3 is primarily an HF-6 meter radio. But it does have useful sensitivity in the 500-1700 kHz AM broadcast band (or "BCB" for short).

I'm looking for a few testers for a new firmware release (rev 1.84) that can improve AM-band sensitivity by as much as 10 to 20 dB for stations below 1300 kHz. This only applies if you have a KXAT3 ATU installed; the new firmware changes the way the ATU handles this range. The actual sensitivity improvement varies greatly depending on what kind of antenna is used.

Operating Details:

In the ATU MD menu entry, you can now tap '1' (PRE) to select one of two optimizations for the AM broadcast band:  BCB NOR or BCB=160. BCB NOR is the default setting: below 1300 kHz, the KXAT3's L-network is configured as a filter to reject harmonic images. BCB=160 uses the normal 160-meter L-network settings below 1300 kHz (i.e., the settings you obtained when you tuned up the ATU on 160 meters).

There's a tradeoff here. BCB NOR will provide much better rejection of BCB station harmonic images--as much as 50 dB at the low end of the band--but it also attenuates some signals significantly. BCB=160 doesn't reject harmonics as well, but it will bring signal strength in many cases.

Theory:

We made two design choices in the KX3 that affect AM broadcast band reception.

1. Like all Elecraft transceivers, the KX3 uses special low-resistance RF diodes called PIN diodes for transmit/receive switching. These diodes perform very well on 160 meters and up, but fall off in linearity toward low end of the AM broadcast band. For this reason, the KX3's T/R switch is designed to have an inherent high-pass response. By the time you get to 500 kHz, signals are attenuated by nominally 30 dB. This does a very good job of protecting the PIN diodes.

2. Being primarily a ham-band transceiver, the KX3 doesn't have low-pass ("half-octave") filters for frequencies below 160 meters. The KX3 is also a baseband SDR (software-defined radio) architecture, so tuning the VFO well below the low-pass filter cutoff can result in harmonic image responses if strong low-frequency signals are present. (These have no effect when listening above about 1.5 kHz.)

The KXAT3 ATU module can help with the latter issue by acting as a low-pass filter, assuming you have the BCB NOR option in effect. As you tune the VFO down through the AM broadcast band, the L-network in the ATU is automatically adjusted to values of L and C that provide this filtering. There are also extra capacitors in the L-network that are only used when the radio is tuned below 1300 kHz. However, this can also attenuate signals as mentioned above.

If BCB=160 is selected, the ATU will not be configured as a low-pass filter. It simply uses the 160-meter settings. 

73,
Wayne
N6KR