Thursday, 5 March 2015

Notes from the Shack - Where there's a will there's a way

Some of you know that I've been having antenna troubles. A permanent solution awaits in the Summer, but until then I've been making do with my flaky 30 year old Butternut HF6V. For a vertical, it's a great antenna - when it works, which is some of the time on some of the bands. But even when it's not working, contacts are still possible.

Now that the year of W1AW is in the history books, I've been trying to fill in some of the missing bits of my log. The sunspot cycle is heading towards a trough, so I've been focused on 12m during the day while the conditions are still somewhat favourable. At night, I've mostly been working 80m as it is my weakest band in the 10 - 80 range. One night, the noise level on 80m was unusually high at S9+20 and almost as high on every other band. My noise blanker works on some types of noise but not others, and this night I was happy to see it drop to about S6 after hitting the switch.

Because the antenna was working so poorly, I decided to use JT65/JT9 as it works well with low power. With the noise blanker on the display was quite unusual.

The signals appeared as adjacent bands in the spectrogram (a.k.a. waterfall) which was clearly an artifact of the noise blanker. This was confirmed by the text display.

Just like the waterfall, each signal had shadows above and below the actual frequency. This is very similar to when Ron VE3CGR is working JT65 on the same band as me at the same time. In that case I just turn my preamp off and everything is fine. The other strategy is to make sure I'm transmitting on the same time slot as he is and it solves the problem for both of us. With the noise blanker, though, it was a case of living with it or turning the radio off.

A difference between the signal overload and the noise blanker artifacts is that the former will exhibit equally strong signals on either side of the actual frequency, e.g -24db, -18db, -12db, -18db, -24db at regular spaced intervals of a few hundred hertz. The noise blanker artifacts were equally spaced at 220hz apart but with asymmetrical signal to noise ratios, with the better signals being below the actual frequency.

The trick with a display like this is to pick the strongest signal, and even with the noise, the compromised antenna, and the artifacts from the noise blanker I was able to work several stations. That's one of the great things about the JT modes - you can make contacts no matter what the circumstances.

No comments:

Post a Comment