Testing the miniHFPA2 with the Elecraft KX1

I received a query from a reader yesterday regarding tuning up an antenna using the internal ATU in his Elecraft KX2 if it were connected up to the miniHFPA2 amplifier. The reason he wanted to know about this was because he was considering ordering one for use with his Elecraft KX2.

He wanted to know if it is it possible to run an antenna such as a doublet, by tuning the antenna using the KX2’s internal tuner with the amp in bypass mode, then switching the toggle switch on the amp to put it inline. He said that it appeared to him that this procedure would not work because the amp, once in line, would probably change the SWR of the antenna.

I decided to conduct a test to find out if this would in fact be the case. This is what I found:

I carried out the test in two parts:
Firstly, with the KX1 (powered by a 12V supply voltage) connected to a PWR/SWR meter and 50 Ohm dummy load.

On keying down, the PWR/SWR meter read the following: output 3.5W with SWR of 1.0:1

A straight through test with the signal going to the dummy load.Secondly, connected up the miniHFPA2 directly to the KX1, and the Pwr/SWR meter and dummy load.
With the Amp in bypass mode, the reading was 3W output and 1.0:1 SWR.

The only difference now is that the amp has been connected up but left in bypass mode.

With the Amp switched on and placed inline, the reading was 18W output with SWR 1.0:1.

With the amp in action, the SWR remained 1.0:1

Note: the lower than expected power output with the amp inline was due to the fact that the amp is set up for an input of 5W. My KX1 can only supply it with 3W, so the final output of the amp will be lower too. With 5W in, as is the case when connected up to my Yaesu FT-817, it outputs 30W.


Elecraft KX1 mod

I have now replaced R11 (now 5.6 Ohms) and R30 (now 30.8 Ohms), and squeezed the turns of L2 together more. The result has been most satisfying.

L2 in position before L1 was installed.

L2 in position before L1 was installed.

The power output is now as follows:
9V = 1.87W
12V = 3.14W
13.8V = 3.92W

9V = 1.66W
12V = 3.07W
13.8V = 3.98W

Measuring power levels.

Measuring power levels.

The rig is now performing as it should according to the specifications.

Completing my Elecraft KX1 and first on air QSOs

The final stage of the build process involved final assemble and testing. The last few resistors, a zener diode and a trimmer pot were soldered in. Then it was time to wind and install toroidal inductors. The only tricky one was the transformer, which has two windings.
Next came inserting the metal standoffs onto the PCB.

With the standoffs in place, it is beginning to look like a radio.

With the standoffs in place, it is beginning to look like a radio.

Once that was done, it was time to scrape off the paint on the enclosure so that there is a good earth between it and the standoffs. I did this with a rotary tool.

Having the right tools makes the job easier. Sand paper will also do.

Having the right tools makes the job easier. Sand paper will also do.

I then installed the red filter over the LED display opening, soldered in the final transistor and carried out some voltage checks to see if all was okay. I then attended to the installation and wiring of the internal battery holders. This was a little tricky but didn’t present any problems, except that I applied too much solder to one of the crimp terminals and so ruined it. A quick email to Elecraft resulted in two new ones being mailed out to me that same day. Awesome service.

The PCB in place mounted to the upper part of the enclosure.

The PCB in place mounted to the upper part of the enclosure.

Time now for final alignment and testing.

Testing the transmitter requires taking power readings on both bands with an accurate power meter; one that can read power at QRP levels. I have two; a QRPometer, and a Hendricks dummy load/power meter. I used both.

QRPometer on the left, Hendricks dummy load/power meter on the right.

QRPometer on the left, Hendricks dummy load/power meter on the right.

This is what I read:
20m @ 13.8V: 3.8W
40m @ 13.8V: 2.66W

Hendricks dummy load/power meter
20m @ 13.8V: 3.5145W
40m @ 13.8V: 2.66W

The formula I used to calculate power with the Hendricks meter is as follows:

The instruction manual says that power on both bands should be between 3 and 4 Watts, so clearly my 40m readings are a bit low. I emailed Elecraft and was told the readings look typical, but I want a little more power, especially on 40m as Down Under it was come in handy. I was told to change the values of R11 and R30 to between 4.7 and 5.6 Ohms, and 27 to 33 Ohms respectively. That is the next thing I will attend to, when I can source such low value resistors.

The rig complete and ready for the first on air test.

The rig complete and ready for the first on air test.

Time for the first real test. I plugged in my Touchkeyer P3K and tuned around on 40m on 1 March 2013 and heard ZL1IG Robin calling. I answered and completed my first QSO with this rig. He gave me a 339, which I was pleased about, especially since the distance between us was 1,549 miles. He is in Invercargill and I am in Brisbane.

Two days later, I decided to try 20m and answered FK8CE Dominique, who was calling from New Caledonia, a distance of 891 miles away. He replied and we had a satisfying QSO. He gave me a 599. After that, I tuned around and heard HB9BQR Roland calling from Solothurn in Switzerland. I answered and he gave me a 579. That was fantastic because the distance between us was 10,133 miles. We had a long QSO, so it wasn’t just a case of luck.

The antenna I was using was a simple dipole up at around 15 feet.
Now to wait for the crimp connectors to arrive and then to source some resistors, and the rig will be complete. Perhaps I don’t really need that additional Watt of power!

Stand for my Nissei SWR & Power Meter

I decided it would be nice to have a wooden stand for my Nissei SWR and Power Meter. I wanted to have the meter standing at an angle so that it would be easier to read while operating at the bench in my shack.

So I found some ply wood and some sheets of foam material and got to work. Once the basic design was proved, I screwed it all together and applies two coats of mahogany varnish. Then I cut the foam to fit the stand.

I am happy with the end result. And it works too.

The stand angles the meter so that it is easier to read while operating. Looks good too.


Calibrating an MFJ-860 SWR Wattmeter

Now that I know the characteristics of my Cushcraft Ringo 6m vertical antenna, I wanted to see if my MFJ-860 SWR Wattmeter was any good. You see, I thought I could now run 6m without going through my MFJ-941E antenna tuner. My antenna is tuned for the SSB segment of the band, so I should be able to operate with the antenna connected directly to my Yaesu FT-450 rig.

I was keen to do this as it would allow me to free up a connection on my coax switch that I use to connect the rig to two antenna, a G5RV for general HF work and the 6m vertical. I can then use that connector to connect up a QRP rig as well as the FT-450 to the G5RV instead.

All this would only happen if I could rely on the MFJ-860 being accurate, as I do like the idea of being able to monitor power and SWR levels while operating.

I connected up the meter and keyed the rig up at 100W on 6m. Way off …

There was nothing for it but to recalibrate the unit.

This is remarkably easy to do. I opened up the unit by screwing off the lid of the enclosure  and was faced with internals that featured four variable pots. Two of these are for forward power  (two scale settings – 300W and 30W) and two for reflected power readings (60W and 6W).

These are the four pots.

Then it was a simple matter of connecting the rig to the transmitter connector and a dummy load to the antenna connector on the meter, choosing a power setting on the rig, keying down to produce a signal and seeing what the reading was. Then, with a small screwdriver, I adjusted the setting of the one pot (trial and error to see which one moved the needle on the meter) until it lined up with the 100W mark on the meter.

Calibrating the reflected scale was carried out in the same way, but with the antenna and transmitter connections reversed and remembering to reduce power (I chose 50W).

Then it was simply a matter of screwing back the enclosure and testing the meter. Final tests proved it to be more than acceptable. Now I have another useful piece of gear on the bench in the shack.

Measuring the performance of my 2m/70cm vertical antenna

For VHF/UHF communications I use my trusty Yaesu FT-7800 rig, which I have mounted to the bench in the shack. Up on the roof I have a DX-60 vertical 2m/70cm antenna that is fed with a length of RG-213 coax. And it seems to work well.

I have just bought myself a nice little SWR/Power meter, a Nissei RS-40 with which I can see what is going on regarding my signals on UHF/VHF. Until now I have been operating ‘blind’.

The Nissei RS-40 is robust and makes a nice addition to my bench.

I decided to measure the SWR on a few frequencies on both bands to get a better idea of how my antenna is doing. This is what I found:

146.400 1.5:1

146.300 1.5:1


145.650 1.5:1

439.000 1.3:1

433.200 6.0:1

438.425 6.0:1

439.300 6.0:1

Imagine then my surprise. Seems the dual bander is really a mono bander! From now on I will restrict my operations to 2m using this antenna and will put up a rotator and 2 element beam for 70cm, thanks to Wallace, VK4CBW who gave me the goods.

Watch this space.

Building a Hendricks Dummy Load and Power Meter

Having just completed building a Hendricks PFR-3 rig, I noticed that whenever I transmitted, the power output as read on my MFJ-941E power meter showed around 10W. This was strange as the rig is rated at around 5W.

I posed the question as to why on the PFR-3 forum and the general consensus was that these meters are not really accurate at QRP levels. What I needed, I was advised, was a piece of gear with which I could read RF voltages such as the Hendricks Dummy Load and Power Meter.

I placed my order and two weeks later the kit arrived. Time to melt solder once more.

The contents of the kit.

I started by soldering all the resisters, and there are a lot of them.

It´s handy using a vise to hold the PCB steady for soldering the leads of the resisters.

The dummy load is very neat and tidy and attaches to the BNC connector of the rig.

A piece of spare resister wire is used to connect the resisters to the BNC connector.

One the dummy load was complete (it took about an hour to build), I connected it up to the PFR-3 rig and applied 12V DC. Then it was a simple matter of taking a DC voltage reading with a digital multi meter across the two little terminals on the top of the PCB, and using the formula P=(E+0.3×0.707)squared/50, I would have the power put out by the rig. The formula takes into account forward voltage drop across the 1N5711 diode in the dummy load. That is the 0.3v added to the voltage E as read on the digital multi meter.

The dummy load in position ready for measurements.

So now I know that my rig puts out 5.9W on 20m, 5.15W on 30m and 5.29W on 40m.