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
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