Mike wrote: Late yesterday, about 2330 EDT I heard a weak station at around 3720. Zeroed him in with the K6XX tuning indicator, and replied to his call. He pause for a bit, then resumed calling CQ. Okay, I don't always get a reply -- my antenna is definitely not optimal for 80m, a 40m horizontal loop at about 20 ft, tuned with an externa tuner.
I then tuned up the band a few hundred hertz, and to my surprise found the same station calling at 599!! I answered him on this frequency, and we had a brief QSO. My report was not that great, 479/QRN and it turned out he was quite local, only about 30 miles away. A textbook example of NVIS I suppose, given my antenna. So what did I hear the first time? Could that have been some kind of image on the other sideband leaking through? Groundwave? (whatever that means...) This is sort of like low band crop circles.... ;-) Anyway, the KXB3080 seems to be working fine, and I will check out the other bands over the next few days. The receiver in the KX1 seems better since I made the mods, but that could just be a result of better alignment than before. -------------------------------------- Yes, you heard an "image". All superheterodyne receivers (like the KX1) that use a BFO for CW/SSB reception have two of them because they have two mixers: one at the input to convert the signal frequency to the intermediate frequency (I.F.) and another at the detector to convert the I.F. to audio. It works like this. When you inject a signal and a second local signal (local oscillator or BFO) into a mixer, the mixer reproduces the signal at its output on frequencies that are the sum and difference of the input frequencies. The KX1 uses an I.F of about 4913.6 kHz. All signals coming in at the antenna are converted to this frequency, amplified, filtered and passed to the detector which is a mixer. A 'beat frequency oscillator' (BFO) produces a signal at 4913.0 kHz. That means the detector puts out two signals, the sum of these two frequencies and the difference between these two frequencies. The sum is a very high RF frequency - over 9 MHz. That's easily filtered out. The difference frequency is what we're interested in: 600 Hz. That's the audio frequency you hear in the phones. Now suppose you tune slightly so the same signal produces an I.F. of 4912.4 kHz. That mixes with the 4913.0 BFO to produce 600 Hz again! You'll hear the same signal at two places on the "dial" separated by exactly twice the beat frequency. In this case, they're 1200 Hz apart. All superhets do this. In earlier receivers before modern crystal filters, it was a very well understood phenomena. As the I.F. filters got better, they would reject one of the signals. In this case, a high-performance crystal filter (such as in the K2) would reduce the signal at the I.F. of 4912.4 so much you'd not likely hear it at all. Manufacturers called this "single signal" reception: each signal now only appeared once as we tuned across the bands. Amazing! <G>. We've come to expect "single signal" reception in all modern gear these days, but it depends entirely on how strong the signal is and how good the I.F. filter is. The tiny filter used in the KX1 is not as good as the those used in larger and more expensive rigs like the K2. Its "stop band" attenuation isn't as good. That, coupled with your hearing a very strong signal on the band, allowed you to hear that "other" signal that's normally suppressed. Even with the simple filter used in the KX1 you should only be able to hear the second signal when listening to a very strong station! Ron AC7AC _______________________________________________ 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
