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.


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.