When to use the preamps on the KX2/KX3 vs. K3/K3S

[wayne burdick]

Occasionally we have a KX2 or KX3 user claim their rig has low receive sensitivity. Often this is simply because the preamp is turned off.

These users may be used to superhet receivers like the one in the K3/K3S, where preamp-off sensitivity (MDS) is typically in the -130 dBm range (or better). The preamp is rarely needed below 15 meters on these rigs.

In contrast, the KX2 and KX3 are quadrature downconversion designs that have the A-to-D converter farther up the signal chain. Even very high-performance ADCs have quite high noise figures, resulting in preamp-off MDS of around -120 dBm. So the preamp is needed more often. Typically, the preamp on the KX2 or KX3 should be used on 20 m and up unless extremely strong interference is present, or if you have a very high ambient noise level.

Related notes:

All Elecraft transceivers keep tabs on how strong incoming signals are. If the signals exceed a certain threshold, firmware will automatically turn off the preamp. (This is preceded by a “HI SIG” warning on the display.)

If signals are so high they might cause damage to the front end--say, a watt or two as measured at the antenna jack--firmware takes the further step of temporarily turning off all of the low-pass filters. This drops the signal level at the receiver’s front end by some 60-80 dB. The actual level of this “carrier operated relay” function is configurable in some cases (COR LVL menu entry).

A typical case of this would be at field day, where antennas for two different stations are inadvertently placed extremely close together and one or both are used at high transmit power. The cure is to move them farther apart. The radio will then reenable the normal signal path.

Another case is what we call “HF Pack Meeting Mode,” where two or more pedestrian mobile operators get their whips crossed :)

73,
Wayne
N6KR

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[na5n]

Wayne Burdick writes:

> If the signals exceed a certain threshold, firmware will automatically  
> turn off the preamp.

I generally operate in a fairly low noise environment and little strong  
signal interference.  Still, it is nice to learn the KX2 has such  
safeguards built in, especially since I plan on using mine at a 2 or 3  
station FD operation shortly and been wondering about what effect the SSB  
QRO station next to me will have.  I feel better now.

> Another case is what we call “HF Pack Meeting Mode,” where two or more  
> pedestrian mobile operators get their whips crossed :)

What?  No Crossed whip anti-overload vectorized buffer disrupter sensor?
:-)

72, Paul NA5N
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