Gunnar, SG3P made a visit up to us during the eastern holidays. The plan was to build openASC power meters for his station (SK3W) and to build some spare cards which we can use if something breaks at either of our stations. We built seven power meters for him and three meters for me but we use different pickups. I use regular couplers using FT140-61 or FT240-61 cores while he wanted to use 2-3 GHz stripline directional couplers which he had gotten hold of a bunch. They had about 53dB attenuation on 28 MHz and around 77dB attenuation on 160m. The attenuation for 10m is pretty good for the design looking at the signal levels, but unfortunately the level is a bit low on 160m, so the frequency counter does not work properly on 80 or 160m. However, since he was going to use the power meters on a single band this was not a problem.
Another problem is that since SK3W is primarily a M/M station, transmission will occur on 10m at the same time as for example 160m. Since this will result in any reflected power getting into the antennas on 160m from 10m will be shown as 24dB higher, thus if for example 1 watt is inserted into the pickup on 160m from 10m this will result in the 160m meter showing 250w of reflected power, which is quite a big error in the measurement and will for sure trigger a high VSWR reading. So to avoid this I very quickly designed a few 2-pole bandpass filters in Elsie which were just optimized to give lots of attenuation higher in frequency and insertion loss was not an important factor. It took a couple of hours to make the filters and the result was superb when we calibrated the units. Even when transmitting with 100w in the reflected direction through the couplers they as most indicated 0.1w in the 160m pickup.
We did not build any BPF for 10m since I felt this was not needed. Only measurement error will be from harmonics which should be at such a low signal level that the measurement error will be very low.
After calibrating the units Gunnar seemed to be very happy and hopefully it will work well at SK3W. I think he will be very happy for the high VSWR protection feature in openASC, a step of making the stations safe for “The Tord”.
