Notes from a Small Station
As a change from his usual piece, Joe Chester M1MWD looks this month at the KXPA100 amplifier from Elecraft.
As a change from his usual piece, Joe Chester M1MWD looks this month at the KXPA100 amplifier from Elecraft.
There is design, and then there is design. We all recognise good design with hindsight; oh, that’s a really neat bit of kit, sort of thing. But it requires amazing insights to work out in advance what good design means. I came across a supreme example recently. So, I’m going to amuse you with the story. But let’s get the rules straight before I start. What do you understand by the phrase good design?
First up, it works. Not only that, but it works well. And then there is the mystery ingredient. Call it style, or magic even. The thing that puts a smile on your face. The wow factor, if you like. Now that we understand the framework, we can move on to examples, if you like. I’m quite certain you all have your favourites; the PCB that worked first time when powered up, the first QSO with the new rig, or the even better one with that new antenna. But....
So, a box slightly larger than a shoe box arrives at my door, to be instantly opened, revealing a new linear amplifier. It’s about the same size and weight as a bag of sugar, with a fancy set of LEDs on the front, and connectors on the back. There were a couple of bags with cables too. RTFM, screamed my brain. But....
Really?.......just plug the wires into the sockets, it’s transparently obvious because there is nowhere else for them to go. This is crazy, but in less than ten minutes it’s all wired up and ready to test. Cold feet intervened. Cup of tea, and a look at the diagram in the manual. Well, it’s an expensive piece of kit. And then it’s switch-on time.
Maybe I had better say what I’m talking about here. You will know that I’ve had an Elecraft KX3 for some time, as my go-to for portable operations. I’ve had a lot of fun with QRP and that transceiver, but I am going to ask more of it in the next few years. Being a fan of portable operations means that I like to travel. Which means the KX3 is being mounted in the car, so that I have it with me on my excursions. And occasionally I think I might need a bit more power. I haven’t worked this out exactly, but it sounds right. And if this is correct, then the question is which linear? I was impressed with the review of the XPA125B in the March PW, but Richard G3UGF mentioned a few “minor shortcomings, if you are not already a Xiegu transceiver user”, which I am not. I also looked at the Hardrock kit, but......I don’t think I’d want this to be my very first buildathon.
The matching 100W linear from Elecraft is a bit more expensive than either of these, even in its kit form. But, as I said, I’m not into kit building. And as local suppliers had none in stock, I ordered it built from the factory. I added the on-board ATU as well. Given current reports in the newspapers about import problems, I sat back, expecting a lengthy delay. I was surprised when it arrived a week after I ordered it.
Back to Testing
So back to the testing. I had mentioned its arrival on an 80m net that morning, and was offered an immediate opportunity to test it out on air, after the net. One more check of the wiring, push the ATU button (on the KX3 − more on this in a minute), relays sang for a few seconds, lights flash on panels, 1:1 match. I cautiously turn up the power to 100W, press the PTT and call. Its midday, so 80m propagation is the pits (it wasn’t good earlier either). But back comes “you’re very weak”. I sign off and begin to breathe again.
The following day, in a much more sober frame of mind, I spend the morning reading the manual. Its immediately obvious that I didn’t do anything wrong; that weak signal report was just conditions. Let me now explain in more detail.
First Impressions
I’m sorry, but I just have to say it – wow! It’s a beautifully crafted piece of kit, with a full sized heatsink on the top. There is a set of LEDs on the front panel. One set, the top line of LEDs, shows power out in watts, then an ATT LED, a TX ON one, and one marked ON for power. The ON/OFF switch is to the right of this line of lights. The first nine of these LEDs have secondary functions, to highlight faults. For example, the first LED permanent ON indicates an SWR problem caused no doubt by a really bad mismatch. The bottom set of LEDs are basically for the on-board ATU, the first five being the SWR reading.
The next two show which of the two antenna ports is being used, and the last three show in which state the unit is operating – bypass, manual, or auto. There are two buttons to the right. One is marked TUNE, which engages the ATU, and the other is called MODE, which selects one of the three modes I just mentioned. I should also mention that that ATU can tune a clothesline, anything up to a 10:1 mismatch.
The rear panel has a pair of SO239 antenna sockets, and another for RF IN. Above these there is an RJ45 socket for rig control, a KEY line jack for use with if the KXPA100 is used to power another linear amplifier, and an RS232 port for a PC connection. There are two Andersen Powerpole inputs provided, one for external power, and the second to provide DC output, such as for the KX3. It seems a lot to take in at first, but it isn’t. Please read on.
Wiring Up
I’ve already covered wiring up. Because all the sockets on the KX3, and its companion panadapter the PX3, are different sizes, it’s just not possible to connect the supplied cable incorrectly. Well, I suppose you might deliberately try to do this, but you wouldn’t. The control cable between the PA and the transceiver as supplied is only 15cm long, but you can replace it with any length up to 7m of shielded CAT6 cable, very useful for situations requiring a longer run. Now, before I launch into using it with my KX3, a word about using it with other rigs. The KXPA100 can be used, in basic mode, with any other transceiver. In fact, Elecraft can supply a set of cables for the FT-817 as well, allowing much the same level of integration as you will read about in the next section. And there are basic settings to allow it to be used as a manual standalone amplifier with any low power transceiver.
Setting up with the KX3
There is just one setting to be made using the menu of the KX3 – set the PA setting in the MENU to ON. And that’s it, you are good to go! The KX3 controls all the KXPA100’s features from the front panel of the transceiver. It’s just an amazing level of integration, which of course I knew existed, but had not seen it myself. I don’t even have to turn the amplifier on, the KX3 does this automatically! The output power control (PWR) for the KX3 on its own runs from 0 to 15W. With the linear it now reads 0-110W! There is an automatic 3dB attenuator in the linear should I exceed its power handling capacity (with another transceiver). Press the ATU switch on the KX3 briefly, and the KXPA100’s ATU finds a match, bypassing the ATU in the transceiver, the SWR value showing on the LEDS, and on the screen of the KX3. A longer press of this same control button changes which of the two antenna sockets are being used. It’s so easy, I wonder why I never considered the KXPA100 as an option previously.
Two Antenna Sockets
Which brings me to my ongoing discussion, about which you have been reading. Developing my small station has been my major preoccupation recently and, in particular, the question of handling a low noise receive antenna. I have looked at various options for this, using my IC-7300 as the main rig, many of these solutions being quite complex. It all revolves around antenna switching options, and the need to make sure I’m not transmitting into one or other of my transceivers.
The ATU in the KXPA100 solves this problem. I can now put up something like an EWE as a receive antenna, using one socket on the ATU, with the other as my transmission antenna. The linear does the switching between antennas safely, at the push of a button. I’m sorry – but wow again!
On Air
Well, what did I expect? On day two, I set up early for my usual 80m net, using the KX3/ PX3/KXPA100 combination. I tuned up the IC-7300 with my manual tuner as I would do normally to act as a standby if anything went wrong. But nothing did, and my KX3 participated in an 80m net for the very first time. My audio was said to be slightly different, but the KX3 has a TX graphic equaliser on board to adjust this. I haven’t done it already because I normally use it /P with minimal transmission, basically ‘59 73’ type QSOs. Now I need to give this some attention.
Apart from this, no one seemed to notice very much difference, and why would they? At one point I nudged the PWR up to 110W for an over just to see if it may any difference, or caused any problem, which it didn’t. But it was all so simple, and easy.
I should also mention that reception with the KX3 is excellent, and clearer than with my IC-7300. You will recall that the KX3’s receiver sits several places above that of the IC-7300 on Rob Sherwood’s famous table. In practice that morning I heard stations on the KX3 that I could not hear on the IC7300. This will be tested more fully on another occasion, to see if this also happens on other bands, and at other times.
Conclusion
What more can I say, really. The KXPA100 is just a wonderful piece of engineering, well designed, and works flawlessly with the KX3. The level of integration is superb, even down to leaving the ON/OFF switch in the OFF position! As a package, I think this is almost the ideal transportable station, and this will be my next step. My new system did very well in the 80m test, and the previous week, as I described last month, my KX3 did very well ‘barefoot’ (running QRP) in the ARRL SSB contest.
Yes, you can buy your own components and build yourself a system to achieve the performance I now get from my small station, and you can also spend big to buy this level of performance in a one box transceiver. But, in my view, the KX3/PX3/KXPA100 system reaches the level of performance of something like a K3, in a more convenient, more modular, but fully integrated form. And you can still take the KX3 out on its own for SOTA or other /P activity if you wish, and plug it back into the system when you get home. Catch you on the air!