Hi, Matthew! The less gain you have before signals reach the I.F. filters, the greater range of signals your rig can handle without overloading. Very strong signals outside the I.F. bandpass can cause many problems if they are strong enough to drive one of the stages into non-linear operation. The result may be what sounds like signals that are not signals at all (so called "birdies"), excessive background QRN and other noises that may sound like someone's signal is splattering across the band and, in really extreme situations, desensing of the receiver (all signals are reduced in strength).
However, too little gain ahead of the I.F. filters may mean that the weakest signals aren't strong enough to be heard above the internal background noise created in the receiver, so a balance between some gain and not too much gain in the "front end" (part of the receiver before the I.F. filters) is required. Normally, the preamp would be used only on the higher frequency bands (above 10 or 15 MHz) where the band QRN is very low and you need the gain to be sure weak signals are stronger than the internally-generated noise. That depends on the background QRN at your location and the antenna you're using. A simple way to see if you have enough "front end gain" is to set your K2 to the narrowest I.F. filter setting you'll use, tune to a spot in the band where you cannot hear any signals, just the background QRN, and disconnect your antenna. If the noise level drops significantly, your ability to hear weak signals is limited by band QRN and not the receiver gain. If the noise level drops when you disconnect the antenna with the pre-amp off or the attenuator on, those settings will provide maximum protection from overload without compromising your ability to hear weak signals. Indeed, by avoiding overload and the apparent band noise that can cause, you may hear weaker signals with the pre-amp off and the attenuator turned on than you would otherwise. The K2 has excellent dynamic range characteristics, so you can use your preamp much of the time without serious side effects, if you wish, but it's not the best operating practice. Ron AC7AC -----Original Message----- Hi All, My operating experience is fairly limited, so I have a question about operating the K2. What circumstances would cause one to turn off the pre-amp? And related, what circumstances would cause one to turn on the attenuator? Even my old FT-101 had an attenuator to switch in, but I never came across a situation where I thought I'd want to. 73, Matthew AE6UP _______________________________________________ Elecraft mailing list Post to: [email protected] 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
