Since my participation in Crossband Night several weeks ago I have been getting more and more interested in 630 meters. I have been listening to the WSPR beaconing stations most nights, and have found it is relatively easy to hear Europe and Hawaii when conditions are good. But, as always I am more attracted to the extreme. 2200 meters is the extreme!
I have attempted to hear stations on 2200 several times. It took two weeks to realize any success at all. I was barely able to detect signals from WD2XES and VO1NA on “below the noise” digital modes. It was obvious my system was deaf, but I blamed it on the antennas. After all, what could you expect from a “Beverage” that is less than 0.1 wavelength long? When I finally did hear stations, it was using an 80 meter inverted V. I did realize, however, that there were some issues with the Beverages not related to length. The transformers were not designed for this low frequency and probably had some loss if they worked at all. Later analysis with assistance from WA3TTS showed them to be about -3 dB. That’s not too bad. I also feed 26 volts DC down the coax to each Beverage feed box to power the direction switching relay. I use bidirectional Beverages made from WD-1A field phone wire. The chokes in the bias tees, it turns out, were 140 uH which barely offered 120 ohms of inductive reactance; clearly not enough. The capacitors in the bias tees were 0.1 uF or 12 ohms capacitive reactance; not too bad but it could stand some improvement.
After modifying the bias tees by adding 1 uF capacitors in parallel with the 0.1 uF and swapping out the chokes for some having almost 6 mH of inductance I found the system was still completely deaf. I built a W1VD preamp which measured 22 dB gain. That brought medium wave broadcast signals up to a level my receiver couldn’t handle. I had IMD products and garbage everywhere! Next I built a bandpass filter which rolls off more than 60 dB by the time it reaches 600 kHz and continues to roll of sharply above that. Now the IMD was gone with the preamp in line. For the first time I could hear some NDBs (nondirectional beacons used at airports for navigation) around 200 kHz, but still nothing in the 2200 meter band at 135.7 to 137.8 kHz.
At this point I began to sense my receiver itself (an FT-2000) might not be up to par on sensitivity that low in frequency. I had already discovered the built in 10 and 20 dB preamps acted like attenuators at this frequency! How to measure it? My Agilent E8285A test set is not specified to work below 400 kHz. While its signal generator will go down to 100 kHz I feared the level would be far from calibrated. I had a recently calibrated HP 3325B but its minimum output level is 0.001 volt peak-to-peak or -56 dBm; far too much to be of use in checking receiver sensitivity. Nevertheless I tried it and found the signal much weaker at 2200 meters than it was at 630 meters or 160 meters. At that point I grabbed every type N attenuator I had and started stringing them together. I realized very quickly that the receiver MDS (minimum discernible signal) was no better than about -75 dBm at 136 kHz! Wow, that is bad! Even my W1VD preamp would only bring it to -97 dBm, still deaf as a post!
Then an idea dawned in the dim recesses of my mind. I had a HP 1980B oscilloscope which I trusted to be in calibration at least for voltage measurements. I decided to use the scope to check the output of the Agilent service monitor at 160, 630, and 2200 meters. Lo and behold, I found it holds up pretty well, the output increasing only a couple of dB at 100 kHz. I then proceeded to measure receiver MDS at 1810, 475, and 136 kHz. On 1810 it was quite a bit better than -137 dBm, which is the lower limit for the Agilent. I didn’t bother using attenuators to find out just what it really is. Suffice to say it’s around -140 dBm. At 475 kHz it is -130 dBm or about 10 dB worse but still not too bad. My earlier test at 136 kHz was confirmed: -75 dBm. Wow. Just wow.
At this point I know the FT-2000 is useless for 2200 meter receiving. I will try something else. Stay tuned!