[Elecraft] K2 rev B low sensitivity

Wed Feb 12 17:43:00 2003

If you define "sensitivity" as the ability to hear very weak signals, it
is very easy to tell if your K2 is capable of hearing as weak as signal
as ANY K2 or ANY receiver is capable of receiving at your QTH.

Select your narrowest filter. 

Turn up the gain so you hear "band noise" - the background QRN that is
always there.

Disconnect the antenna. 

If the noise level drops noticeably, you K2 is hearing as well as any
receiver with a similar bandwidth. The only receiver that could hear
better on that antenna is a receiver with a narrower bandwidth, since
narrower bandwidths reduce the amount of noise coming in through the
antenna. And that's why it's important to do the check at the narrowest
filter setting you use. 

If the noise level does not drop when you disconnect the antenna, then
the noise you are hearing is that being generated internally inside your
K2. And, since you couldn't tell when you disconnected the antenna, the
internally-generated noise is "masking" the weakest signals. This is
more likely to happen with very small (low gain) antennas and at the
higher frequencies - above 15 MHz typically. That's why Elecraft
included the pre-amp, of course. Elecraft included it to improve the
overall noise figure in those situations. The pre-amp makes less noise
than the mixer stage that the signals normally hit first, so it boosts
the signals when needed so they will over-ride the internal noise. The
quieter the QTH and the lower the receive antenna "gain" the more likely
the pre-amp will be needed. 

Because the wider the bandwidth the more antenna noise you'll hear, even
a cheap receiver usually shows plenty of sensitivity up through 15 MHz,
and most of them will show all the sensitivity they can use up through
30 MHz. But, when you add a narrow i-f filter to the receiver like the
K2 has, the receiver must be inherently "quieter" in order to avoid
masking weak signals, especially on the higher bands, at an
exceptionally quiet (low noise) QTH, or with a low-gain antenna. 

So that takes care of "sensitivity" - or the ability to hear weak
signals. You don't get an objective comparison with every other K2 that
way, since every antenna and QTH is different with respect to gain and
noise. But it tells you that there isn't a receiver in the world at any
price that can hear better, given the same i-f bandwidth, connected to
your antenna at your QTH. 

The other issue is, of course, overall gain. How loud are the weakest
signals? That's a function of the audio gain system and you can usually
get more overall system gain by turning up the Audio Gain. 

SOME K2's seem to have relatively low audio system gain. Ops complain
that they have to run the Audio Gain control way up - almost full "open"
to get adequate gain and then it is not enough in noisy environment like
some automobiles when going mobile. To correct that, some ops have
gotten in the habit of using the Pre-amp on their K2 a lot. Normally
that is as BAD practice. Using a pre-amp when you don't need one means
that you are reducing the dynamic range of the receiver. Strong signals
can cause overload and generate noise that sounds like QRN as well as
produce other strange "spurious" signals on the bands. Fortunately, the
K2 has a VERY robust "front end" and in most cases ops can get away with
running the pre-amp simply to make things louder without noticing any
problems, but it's a bad practice that can cause problems in other
receivers or even in a K2 when it's operated close to other
transmitters. 

Personally, I find that I use the K2 pre-amp only on occasion on 10
meters. My K2 is quiet enough that it is not needed on any lower bands.
Or, as the case may be, the bands are too "noisy" to need a
pre-amplifier on the lower bands.

It's possible that a K2 may, for a variety of reasons, might have a
different AGC characteristic and that would affect how the "S-meter"
behaves, since the "S-meter" is just measuring the AGC response to noise
and signals. But that is not a good indicator of whether your K2 can
"hear" weak signals as good as another receiver.  

Some ops wonder if it shouldn't take something a lot more sophisticated
than a "pull the antenna" test to establish whether they have all the
sensitivity that they can use, but that's all it takes. It's like trying
to decide if it's sunny out today. I could log onto the internet and
check with the weather service, but it's more accurate and easier to
look out the window. 

Ron AC7AC
K2 # 1289

-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Wallace, Andy

....What I am trying to determine is not whether my S meter is
calibrated compared to someone else, but whether my rig is as sensitive
as someone else's. 

... I'm just trying to get a handle on whether it's up to snuff. With
the preamp on, it sure sounds like it is. 

Andy

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