Wednesday, 5 November 2014

Perseus Deep Search

It's always interesting to use one of the many 'audio viewing' software programs, such as 'Argo', 'Spectran' or Spectrum Lab, driven with receiver audio, to dig deep into a section of the spectrum using very narrow bandwidths. The narrower the bandwidth becomes, the greater the signal-to-noise ratio, increasing sensitivity in effect. The use of digital signal processing software can create the extremely narrow milliHertz  filtering needed to view signals buried deep in the noise...the deeper the search (20db, 30db or more into the noise), the longer the time needed to build up the visual display of signals that would be far too weak to detect aurally.

Recently, rather than using Argo to view a slice of spectrum I set up the Perseus waterfall display to have a deep-look at 1240kHz. This is one of the broadcast-band's 'local' channels and one that BCB DXers refer to as a 'graveyard channel'. Almost all stations on the various graveyard frequencies run a maximum of 1kW day and night. According to the Medium Wave List, there are presently 166 stations in North America operating on 1240kHz...one of the reasons that 'DXing the graveyards' is so interesting.

With this in mind, I recently took a mid-afternoon listen. Just one station was audible to my ear, likely one of the stations to my south in Washington state. Centering Perseus on 1240kHz, the waterfall was set to display a ~50Hz slice of the spectrum...1240kHz +/- 25Hz. The screen below shows the display, ranging from 1239.976kHz to 1240.024kHz. After letting the waterfall visual display slowly build up, it revealed 26 separate carriers. The remaining signals, although propagating to my location, were too weak to be detected aurally.

   
26 midday carriers visible on 1240kHz
Checking the MWList once again, it shows only 11 stations to be within expected daylight groundwave distance from here...Washington, Oregon and B.C. Evidently there is some mid-afternoon skywave involved or I am seeing some extended groundwave from Idaho, Montana and California. MW List shows a total of 27 possible candidates, assuming extended groundwave, so it would seem that most of them are making it here in daylight.


Next I switched to an even narrower filter, at twice the previous resolution, visible on the top-half of the waterfall above. This displayed a ~25Hz spectrum slice, still centered on 1240kHz. Although finer resolution is evident, there appears to be no additional signals except for the three new arrivals slowly fading in at the right as sunset creeps closer.


The screen above was made approximately one hour after sunset (looping E-W) and shows the same 25Hz slice centered on 1240kHz. There are ~70 carriers visible by now, with most of them fading. Some transmitters appear to be rock-solid while most exhibit a cyclical drift, no doubt the result of some form of crystal temperature stability attempt. I'm guessing that the rock-solid ones are using more modern synthesizers for frequency generation.

Listening on 1240 during this period reveals a boiling cauldron of audio, mostly unreadable until one station fades-up and becomes intelligible for a short period before fading to be replaced by another. Sitting on this frequency at the top-of-hour identification time can often catch a few idents with careful listening.


Even comparatively empty 540kHz reveals 22 different carriers, the only audible one being CBK in Watrous, Saskatchewan, the brighter trace at 540.002kHz. The one at the right looks as if it may have just come on the air for the evening. It's possible that many of these are low powered traffic information stations (TIS) running at 10 watts.

4 comments:

K2KI said...

Nice write up Steve. I love digging into the dirt to find "pearls". I have a couple of Softrock IF version receivers connected to both my TS-830 and my TS-690SAT. It's amazing what you can find!
73...
Bob de k2ki

Anonymous said...

....MW List shows a total of 27 possible candidates, assuming extended groundwave, so it would seem that most of them are making it here in daylight.,,,,,

Good job Steve. Can you hear these stations ineligibly? ...or can you just see them?

Gyuda, swler

Steve McDonald said...

Gyuda...thanks for your interest. At night, when listening, you can tell only that there are multiple stations all on the same frequency. Usually one will drift to the top for a bit and become readable before fading, only to be replaced by another. This narrow bandwidth deep look just shows how many signal carriers are actually there...sometimes approaching 100 on this particular frequency (1240kHz) showing that the band is propagating even though each individual signal cannot be demodulated. It also shows the stability of some of the transmitters as you can see.

Steve McDonald said...

Hi Bob - it is interesting for sure...not sure how this can be used in any real practical way other than to stand back and say "wow, look at all of the signals making it out west tonight". It would be great to be able to match up frequencies to actual stations...which could be done but would take a lot of listeners and a lot of time. It's basically the same principle used for QRSS CW mode on LF but keying the carrier to identify the source. I suspect most of these signals, even without local QRM would still be too far into the noise to be heard aurally.