25 February 2014

QRPp and Power Measurements

It was cold and dark - And so I was operating the Ultimate3 in WSPR mode. All was guessing and hoping, that everything was right. At these low levels of power I could only measure the output with my scope. My other gear didn't work with power levels > 500 mW. (Also my trusted Elecraft T1 Auto-tuner didn't come alive and I had to pre-tune it with another rig and switch antennas in the hope that the expected 50 Ohm were still there.

But things have changed - somehow. I was looking for a power meter or an swr meter, which was able to measure power down to 1 mW or less and I found a solution. In an article written by   Loftur E. Jonasson:  Power and SWR Meter with dual bargraphs and sticky bar, using 2x AD8307 described the necessary circuit and even posted the software. But - it was no kit. The schematics for the PCB board are also published - well done - still I was no happy camper - no way to produce this piece of measurement device.

So I looked an asked and complained and asked and finally I was confronted with a solution which was ready made and calibrated. A good friend of mine had some time ago constructed the gear I was looking for.

It was a kit from Foxdelta PM3A (RS232) which has a nice display and can measure powers ranging from 0 - 5000 MHz. That was enough for me for I wanted to use this device only up to 30 MHz.  You can use this unit remotely with the RS 232 line and you can store your measurements in your local PC.

Of course you have to fetch the forwarded and reflected power from your antenna system. I was again lucky for this friend had already constructed a coupling system which could be connected with a measured attenuation of -26 dBm to the sensors. I only had to hook up the wires.

The critical measurements are made by two Analog Device AD8307 on the PM3 Sensor Board. I use a 8 pin DIP form which is good up to 30 MHz. The V/dB characteristics are impressing for my amateur workshop. This board can measure up to 1 kWatt if you provide necessary attenuators.

So, you see that I cheated a little bit,when I was saying that I was sending with 100 mW. In fact I was using a whole lot more of power: 126 mW.

I had to struggle a little bit to put this measurement equipment. The AD8307  has an effective linear rangefrom -80dBm to  +10dBm. On the board there is an input attenuation of  6,53dB. So you can measure power with this device without problems  with +16,5dBm down to ( -80dBm +6,53dB)  -73,5dBm.

But reality is somehow different. I had forgotten to subtract the attenuation from the directional coupler.
I had to add that attenuation of -26 dB to the system and ... see below.

My friend explained it to me:  

    Output power TRX    +30dBm (1W)
    Attenuation coupler       -26dB
    Input power  AD8307 +4,00dBm
    Attenuation of board    -6,5dB
    Real Power on  AD8307 -2,5dBm (from -80dBm to +10dBm)

After several days of using WSPR and my new Power Gadget, I can say that it does not only help me to be true and tell my real power, but I used it also finding faults in other LPFs. I tried it on other bands and compared it with the measurements on my scope. And they are right to the point. I also checked my step attenuator and compliments: The indicated attenuations are absolute correct at least for my amateur standard.

And this is my Ultimate3 after sending the WSPR-Message sitting on top of my K1. In the 10 minute frame I send my CW ID once. On the Display you see:

  1. Internal mode [6]
  2. Filter Bank No. 0
  3. Blinking heart from the GPS 1 pulse a second
  4. Frequency used
  5. Mode CW-ID
  6. (D)L1SDZ (The letter "D"  had already been transmitted.
Ain't that nice?

Stay Tuned!