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