Now that we have discussed the AN-762 amplifier, it's connection to the Elecraft K2 as an exciter, and the relay circuit, we still have a few elements to deal with to make the completed project a success.
The filters offered by Communications Concepts consist of 5 elements, 3 Toroids, and 2 capacitors (or 4 capacitors in parallel to get the desired capacitance). The readily available filters cover 160 to 10 meters, not including the WARC bands. The kits are straight forward and can be built in under an hour. The filters are the inserted into the RF chain after the amp and before the antenna, obviously.
FL-15 Harmonic filter for 15m band
I won't go into the calculations as to how they determine the appropriate inductance and capacitance for each band. Much of the more detailed discussion on this topic can be found in the ARRL Handbook.
Once the filter is completed, you will need to connect it to the RF OUT from the AN-762 and to the antenna. Very few builders will want to operate on one band only. If you spend $200 in parts and shipping, plus a lot of time, you will want to make your amplifier as versatile as possible. That means band switching. Band switching in my case is down by the use of 2 rotary switches. Why two? Fairly simple answer to that. One, because I only had SP7T rotary switches my junk box. Two, because those rotary switch terminals are really crowded, and to try to use a DP7T rotary switch just really would create a jumble of wiring.
Speaking of wiring, use COAX for all of your RF connections in the cabinet. I've got RG58 running between amp and the relays and the switches and the antenna out connector (SO-239 panel mount). This is critical, because you want to minimize losses and stray RF running around inside your amplifier cabinet. This is another reason why I used 2 rotary switches; because of the size of the coax wiring to the switches, etc.
So why two switches anyway? One switch is for "in from the K2," and the other is for "out to the antenna." If you use 2 switches like me, then you must be sure to switch BOTH before tuning and transmitting.
What about heat? I was concerned that the heat sink would not be enough once I enclosed the whole project inside a metal box. In fact, I did not use the full-size heat sink which I purchased with the amplifier kit. I cut it down to about 8 inches x 6 inches. Even so, the heat sink does seem adequate to keep things cool. But still, I'm a little paranoid about heat, especially since the MRF454 transistor can get pretty hot if you leave the key down. In particular, I noticed that one of the two transistors got warmer than the other. So, I added a 12 v fan that is blowing directly over the board from the back of the cabinet. It makes a little noise, and I am thinking about adding a thermal switch so that it will only run above a certain set temperature, but it give me a certain sense of comfort knowing it is there.
Everything else about finishing the cabinet is fairly routine. You run your wiring. Be careful not to take anything for granted and review all of your work for mistakes and bad connections. The front panel should have space for an On/Off switch, the band switch(es), an RX/TX switch, and a volt / ammeter. Hey, why do I need an RX / TX switch? I thought the relay circuit does that? Well, what if...what if you decide to hook up a transmitter (homebrew perhaps), that does not have a keying circuit? So you can then switch the amplifier into TX and then switch back to RX (just like the old days) when you want to listen to your receiver.
Analog Ammeter
The front panel also has, or should have room for some meters. The preferred meter is up to you. I currently am debating whether to install an analog ammeter like the one pictured above, or a combination ammeter / voltmeter like the one pictured below.
In either case, the meter will tell you volts and amps immediately when you are transmitting or not.
The back panel in my case has power jacks, an rca jack for the incoming switching signal from the Elecraft K2, an RF In and RF Out, and the fan.
Overall this project has been educational and fun. Granted there is some stress involved with applying power for the first time and also dealing with SMD capacitors and the general job of trying to build it "dead on," but it is a very worthwhile project. I now have 115w to 130w coming out of my amplifier, and I have the joy of using the K2 as an exciter.
The next add-on for the amplifier / K2 project is to deal with the K2's receive limitations. The K2 has adjustable bandwidth filters (4 filters) that are switchable from the front panel. Mine are 1.5khz, 1.0khz, 0.5khz, and 0.3khz. You can change these filter widths inside the K2.
This, to me, is not quite as much tailoring of the receive signal that I am looking for. So next up is the addition of a Sotabeams Variable Bandwidth Filter Module that can control both width and center frequency. More on this in an upcoming blog.
Until then 73 and all the best.
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