Pardon me for jumping in here. I don't have a K3 and don't have one on order. I'm trying to make that decision. I've read a lot of good things about the K3. Absence of a keying loop isn't one of them.
Although it's true that some modern QSK amps may appear to work fine without a keying loop, bear in mind that both Acom and Ten-Tec recommend using them. Elecraft should consider the recommendations of the amp manufacturers, not what they *think* will work correctly. Even if Elecraft is right, why force the owner of a $6,000+ amplifier to go against the manufacturer's stated recommendation? If something goes wrong with the amp's T/R relay, causing an expensive ship/repair/ship drama, will Elecraft indemnify the owner? Probably not. I sure wouldn't want to get in the middle of that finger-pointing exercise. Alpha 87A owners can relax: Alpha specifically discourages use of the 87A keying loop. That's not surprising, because the keying loop logic in that amp doesn't work correctly. It's not a problem because the PIN diodes switch almost instantaneously and there are no mechanical contacts to burn. You can drive that amp all day with the PTT output line. And that's what Alpha tells you to do. Let's look a little closer. The typical Jennings and Kilovac type vacuum relays used in QSK amps have rated switching times in the 6-8ms range. In some designs, the relay is triggered with a burst of high voltage which reduces the switching time to as little as 2ms. So, theoretically, there's no danger of hot switching if the transceiver has a delay between PTT and the start of the RF envelope of, say, 15 ms. But the problem is that you may not know what else the amp is doing besides closing its vacuum relay. For example, the Acom 2000a has a rather complex T/R switching sequence that involves several relays, and the timing may be longer if the amp has to retune. My point is that you can never be sure exactly what timing constraints a QSK amp may impose, so it's best to follow the manufacturer's recommendation and let the amp decide when it's safe to apply RF. In other words, use a keying loop if the manufacturer says to. In theory, relying on the transceiver to delay RF can reduce the maximum QSK speed (i.e., compromise the ability to hear between code elements.) But that's only true if the transceiver's delay can be reduced to less than the amp's switching time. If the delay isn't adjustable, then the excess delay will be present whether a keying loop is used or not. The best setup is a keying loop with a fully configurable PTT delay. At any rate, I can't attest to the effect of an extra 10ms or so of receiver muting at high speeds. The QSK experts will have to comment on that. A keying loop is also very desirable for preventing hot switching of antenna relays. A TX ENA or TX INH port can be used to suppress RF before and during any switching. It's possible to use PTT to prevent switching from taking place, but it's not as foolproof as suppressing RF -- there are timing windows where hot switching can occur. Also, if you use PC-based software to do your switching, it's a heck of a lot more difficult to detect when PTT has been closed than it is to raise TX INH. My point is that many contest stations, including mine, have switching systems based on the commonly-available keying loops found on popular rigs. Why force us to give up or modify those hard-won systems? This leads me to the key question: Why not implement a standard feature that the amateur community has come to rely on? After all, you wouldn't want to get a reputation of ignoring such things, like a certain other US-based manufacturer of amateur transceivers :-) I think it's unfortunate that Elecraft has made the decision to omit a keying loop. They may not have realized that the K3 is going to appeal to a whole different breed of users than the K2, including contesters who have a wide variety of equipment, station configurations and very demanding requirements. Lack of this feature is going to complicate my buying decision, for sure. But I'm always willing to resort to a mod if I have to (Warranty? What warranty?) Is there a point in the circuit where it would be possible to safely implement a TX INH or TX ENA function? If so, my soldering iron is heating up. ? If not, I hope K3 will reconsider a keying loop for the next major rev. 73, Dick WC1M _______________________________________________ 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
