Hi Paul: The test on one band isn't valid on other bands because the band QRN changes a great deal with frequency. The fact that you hear little or not change in background QRN when you remove the antenna on 10 meters with the preamplifier off is because the K2 is optimized for maximum sensitivity and greatest dynamic range across the HF spectrum.
The sensitivity of a receiver - its ability to hear weak signals - is a function of the bandwidth of the receiver, the overall gain of the receiver and the amount of internal noise generated in the receiver. Most receivers, including the K2, have more than enough overall gain, so the things to look at are the bandwidth and internal noise generated in the receiver. Narrower bandwidths make a band sound "quieter" because they filter out noise as well as signals on adjacent frequencies. The most rigorous test is to check the receiver at the narrowest bandwidth you'll use. (That's why a weak CW signal is easier to copy than an equally weak SSB signal. The SSB signal requires a wider bandwidth at the receiver, so more noise comes through that reduces the signal-to-noise ratio.) The internal noise in a receiver is a concern only when it competes with a weak signal being received. Of course, in order to hear the weak signal on any receiver the signal must be strong enough to be heard over the band noise - the background QRN - being received along with the signal. If, when you connect the antenna, you hear the background noise increase you can be sure your ability to hear signals is limited by the unavoidable background QRN and not the receiver. On the higher-frequency bands, typically 14 MHz and up, the background QRN may be low enough that the internal noise in the receiver might mask weaker signals. You sense when that's happening when you disconnect the antenna while listening to the background noise and the noise level does not change! The K2's preamplifier is a low-noise amplifier that boosts incoming signals so they easily over-ride the internal noise. In order to do this, the preamplifier must be placed at the input to the receiver, ahead of the I.F. filters, so it amplifies not only the desired signal but signals over a wide range of frequencies around it as well. On the lower frequencies, too much "front end" gain (ahead of the I.F. filters) such as provided by the preamplifier can lead to overloading of the receiver by extremely strong signals outside of the passband. So the K2's preamplifier can be turned off when it's not needed. That results in a receiver that has the low noise figure needed for the higher frequencies and optimum strong signal handling capability on the lower frequencies. While your test is a good quick check to see if the receiver is limiting your ability to hear weak signals, a more meaningful test for comparing receivers is to measure their signal-to-noise plus noise and minimum discernable signal (MDS) levels. Normally such measurements require some rather costly test equipment. However, Elecraft created a simple test oscillator that you can use to measure the signal-to-noise plus noise ratio at 1 uV and, based on that, you can estimate the MDS with good accuracy. It works with any receiver covering 80, 40 or 20 meters. The oscillator produces accurate crystal-controlled outputs on the three bands. It can also be used to calibrate your K2's S-meter and do other tests. Check out the XG2 Three Band Receiver Test Oscillator/S-Meter Calibrator on the Elecraft web site at: http://www.elecraft.com/mini_module_kits/mini_modules.htm It's the second-to-last item on the page. Ron AC7AC -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Paul Del Negro Sent: Monday, January 15, 2007 7:11 AM To: elecraft@mailman.qth.net Subject: [Elecraft] K2 receive sensitivity Hi to all on the reflector. I'd like to put something out here and see if others experience the same thing or if I possibly have a problem with my K2 sn5834. Someone (I don't remember who) mentioned that a great way to test receive sensitivity is to simply check for an increase in noise level when switching from a dummy load to a resonant antenna on a dead band (like 10m is lately), and I am a great believer in that method. I have used it for years with various commercial and homebrew rigs. So here's the deal. K2 set for 10 meters, preamp off, antenna is resonant wire (and also used a beam). When switching between dummy load and antenna, there is no (or barely perceptible) change in receiver noise output. With the preamp on, the change is noticeable, as the noise increases both with dummy load and antenna connected. What I'm concerned about is the preamp off situation, as all other receivers I have tested do show a marked change in receiver noise when doing this test. I have already set the agc threshold so that there is no reduction in band noise when turning agc on and off. Band pass filter alignment was done with a minimum signal from a signal generator and IF and filter alignments were done with Spectrogram and everything seemed to check out OK. BTW, the I used two antennas for this test, a 3 el tribander at 40 ft and a OCF dipole cut for 160 and tuned with ATU. Do others have the same or similar results with this test? I'd really like to know if my K2 is behaving normally or I have a problem. BTW, the results improve as you lower the frequency, but the first band that shows a really marked increase in noise level with this test is 17m. Any and all comments are appreciated. 73, Paul, N2PD _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com _______________________________________________ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
