Plotting Signal Strengths with Spectrum Lab

Spectrum Lab has a feature which allows one to bracket a signal on the waterfall display and watch its signal strength as a strip chart recording.  This feature is called "Watch List & Plot Window" and is found in the "View/Windows" menu at the top of the display.  Here's the set-up screen:

Set-up screen for SpectrumLab signal plotter.

It's a bit tedious to learn to use and involves a set of commands, functions and expressions reminiscent of BASIC which allow a signal on the waterfall display to be selected and plotted.  But the good news is that only a few instructions are necessary for making a simple strip chart recorder.  I use two expressions:  noise_n(f1,f2) which plots the band noise in the range between f1 and f2 which are two audio frequencies on the WF display and peak_a(f1,f2) which yields the peak signal between two frequencies.  Up to 6 signals may be selected and plotted at the same time.

The noise_n function divides the range between the two specified frequencies into 256 bins and averages the 1/4 lowest values to obtain the band noise.  The likelihood of including actual signals in this average is very low unless there is broadband interference present such as from RTTY or JT65.

The peak_a(f1,f2) expression brackets the signal of interest, divides it into 256 bins and selects the largest value to plot.  This makes a varying QRSS fskcw signal appear as a continuous line since the mark and space are always within the brackets.  Wolf thinks of everything.

OK, so why the interest in plotting signal strength?  One reason would be to watch for changes in your signal while doing some antenna comparisons, for example.  Another and the one in which I'm interested is to study QSB.  There are three causes of QSB:  multipath interference, Faraday rotation and ionospheric focusing.  In multipath the signal is reflected from two or more layers and arrives at the rx antenna to interfere destructively and constructively.  In Faraday rotation the polarization of a linear transmitted wave rotates as it passes thru the Earth's magnetic field and arrives at the polarized rx antenna at a state other than of the tx antenna, ie a vertically polarized transmitted wave can arrive at the rx as another polarization, including eliptical.  Focusing is cause by the shape of the ionosphere which can vary from convex to concave.

Scientist and engineers who study propagation have utilized simultaneous recordings of the vertical and horizontal vectors to detect the Faraday effect and the qualitative nature of the overall variations to separate the three effects.  It ain't easy and requires sophisticated computing capability and understanding of propagation, definitely a job for professionals.  However, that doesn't keep we amateurs from having fun with the subject, making our own observations and attempting to guess which effects are occurring.

My favorite reference on QSB is the dissertation of Dr. Yau at the University of Adelaide who studied the effect on OTHR signals in the 6 to 10 MHz range.  There's a good discussion of hf propagation there plus a details of the receiving apparatus and computational techniques.  Imagine getting a PhD in QSB!

Here's an example of QSB recordings I've made on some 30m signals:

Example of strip chart recording

The Red trace is the band noise, Yellow is KD4PBJ near Chattanooga, TN and Green is AK4T near Atlanta, GA.  

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Measurement of 30m Noise Background at W4HBK

In my last post I described a method I use to disengage the AGC of my TS-440 so that accurate measurements of signal levels can be made.  Last Winter I tried to measure the background noise on my receiver but found that AGC compression was a major problem.  Now that I have that ironed out I've tried again to measure the background noise, this time with success.

My system consists of an Inverted V antenna into a Kenwood TS-440 receiver whose audio output goes to a computer sound card and then the Spectrum Lab software.  All signal levels to which I refer are as measured on the SL display.  I define background noise as the signal level of the noise above the noise floor of my receiver which is determined by replacing the antenna with a 50 Ohm resistor.

I used a little know feature of Spectrum Lab called "Watch Window"   to record my data. WW allows you to select a specific part of the spectrum to be recorded as a strip chart, e.g., amplitude vs time.  I described this in a previous post called "New Toy" back in March.  Several expressions are available to measure both signals and noise.  The noise expression is,

     noise(freq1, freq2)

which measures the noise level between f1 and f2 and eliminates the effect of discreet signals using the following scheme:
   1.  an array of amplitudes from the last FFT calculation is sorted into order of increasing amplitude.
   2.  The amplitude of the lower quartile value is then returned as an estimate of the mean noise level. 

With the frequency range set to cover 10140.0 to 10140.1 and numerous QRSS signals present it ignores them and measures just the noise....like magic  You can read about this in more detail in the SL help file.  The only problem I've had is with strong RTTY and JT65 signals which cover the entire QRSS spectrum but this is rare and easily recognized..

I recorded the noise background for a 24 hour period so see how it varied with time of day.  The results are shown in Figure 1.  The spikiness is due to static crashes plus occasional clicks and pops from electrical equipment.  I can hear nothing that sounds like power line noise or crt emission, etc....just the steady crunch of sferics.  Thunderstorm activity was actually mild during the measurement period with just a few small storms no closer than 25 miles, indicating to me that the sferics were propagating in from afar.

Figure 1.  Noise vs Time of Day Measured @ W4HBK

The time of maximum noise was just around sunset at about 35 to 40 dB above the NF.  Minimum noise occurred around noon at 20 to 25 dB above the NF.

For comparison I obtained the noise estimate from VOCAP, Figure 2.  The trend for time of day is close but I measured a max to min range about twice as large as VOCAP, 15 dB vs 7 dB, respectively.  My measurement is for a particular day whereas the VOCAP estimates are based on extensive observations used to determine averages.

Figure2.  VOCAP Prediction of Noise @ Receiver

I plan to repeat the measurement several times to better characterize this time of year and then do the same for other seasons.  Can't wait for a cold Winter night to see how low we can go.

de w4hbk

Improvements to my air conditioning system as observed on the ON5EX grabber

Several nights ago I had my mept on 40m and recorded it's reception on the ON5EX grabber using SiteShoter then stitched the grabs together to see how I did over the long term.  I have described this technique previously on this blog.  Here's what it looked like:

So, what's this got to do with my air conditioning?  When we first moved to this QTH the air condx would cycle on and off on a hot summer day but as the years went by it got to the point of running continuously 24/7 and our electricity bill rose accordingly.  Over the winter we had an energy audit courtesy of the local power company and the auditor recommended we have our ductwork checked for leaks and add more insulation in the attic.  We followed his advice and as the temps began to climb into the upper 90's (37 C) earlier this month it was a relief to note the air condx was once more cycling on and off.  I meant to make a few measurements but haven't had time so far....until the night I recorded the above data.

My mept at the time was a QRP Labs kit which I have found to drift with changes in room temperature and I can see that drift on the ON5EX grabber. It shows the air condx to come on for 10 minutes then off for 10 minutes.  The change in temperature is several degrees F based on a previous calibration also describe on this blog.

Consider the irony.  I have without intending to do so remotely sensed the performance of my air conditioning system via a facility located in Belgium.

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WA5DJJ's new mept

Dave, WA5DJJ, has brought together some ideas to produce the ultimate QRP Labs mept modification that is stable in both time and frequency to make it adequate for image stacking.  To jog ur memories, it is possible to stack successive grabs using an astrophotography program called Rot'N'Stack to pull really weak signals out of the noise. To do so the message must appear at exactly the same place on the grab from one to the next, i.e., must be stable in both time and frequency. The SNR improves as the square root of the number of images stacked.  I'll leave it to Dave to describe his mods but basically it involves using the 4-component temperature controller I described in Part 2 of my series here on this blog and the change to a PIC keyer designed by Mike, N0QBH.  This latter mod allows a 32 kHz clock xtal to stabilize the timebase.

Last night I recorded the signal from Dave' new rig and the results were nothing short of phenomenal.  Over a period of 4 hours he drifted only 1 Hz, which as I told him could easily be my rx...the wx is wonderful now and we're leaving the windows open in the shack resulting large temperature changes.  The times base was virtually jitter free and started exactly on time every 10 minutes.  Figure 1 shows his signal as recorded on my 5 hr grabber. 

Figure 1.  5 hour grabber showing Dave's signal

Each of the short pulses represents a 10 minute grab as normally seen on my regular grabber.  I used 25 of the 10 minute grabs, going from 0717z to 1114z, in the Rot'n'Stack stacking program to produce the averaged image shown in Figure 2.  Even though Dave's signal was strong and didn't need stacking to bring it out of the noise, this provided an acid test of his rig's stability in both time and frequency.  The degree to which it is off will appear as blurring or jitter.  The timing is virtually perfect and what little frequency blur there is is caused by the 1 Hz drift over the recording time.

Figure 2.  Stacked average of 25 grabs

In addition to Dave's signal, several surprises popped out of the noise.  VK6JY is using a stabilized Hewlett-Packard signal generator which responded quite well to the stacking process.  There was no hint of N4FRE or ZL1EE even on the 5 hr grab which has a much better SNR than the 10 minute grabbs.  Both have highly stabilized signals via atomic clocks and/or GPS.  FRE usually drops completely out this time of night and if any of his signal was left it was completely obliterated by P29ZL.  It's ironic to be using sophisticated software and equipment to extract a 'local' station out from under one so far away (with an antenna  up in a banana tree no less).  However ZL's mept is not stabilized in time and thus averages at the same time FRE's builds up.  QRSS does have it's moments.

Congratulations to Dave on his new rig.  It looks like all the mods can be added directly to the rig so it will still fit in an Altoids tin.  Now if everybody adopts those mods then they too can pop out of the noise when stacking is applied.

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P29ZL

The signal of P29ZL caught my attention soon after it's appearance on my grabber last month for two reasons.  First, it appears almost every night for about 5 hours, very strong and easy to copy.  But equally interesting is the extreme multipath/Doppler effect which is always present for at least part of the time.  Tim's mept, from the WA5DJJ Labs,  runs 250 mW to an inverted V hung in a 25 foot banana tree.  Figure 1 is a 5 hour grab from a few nights ago and is typical of what I see every night:  note in particular the considerable fuzziness of the signal between 1000 to 1100z.  Figure 2 is from the 10 minute grabber and shows the effect in more detail.  Other signals from out that way, most notably VK6JY show just a bit of broadening but nothing like that from P29ZL.

Figure 1 - P29ZL on the W4HBK 5 hr grabber, 12Apr11, 30m 

Here is my understanding of the multipath/Doppler effect.  As the signal passes through the ionosphere it may be partially refracted back towards Earth but a part may continue on to find another refracting region for a second signal, an so on.  At the same time multiple hops can occur with the results that a number of signals can arrive at the receiving antenna.  But for a Doppler shift to occur these various regions must be moving relative to one another.  Without this relative motion the signals arriving from the various paths would be at the same frequency but differing in phase to cause fading due to interference.  Other phenomena such as ionospheric irregularities and turbulence can also contribute a continuous broadening....visualize an eddy where some of the electrons are moving towards and some away from the path of propagation.  I've noticed that most writers of the scholarly articles end up stating how complex is the ionosphere....as the kid next door says to me sometimes. "Well, Duhhh!"

Usually the M/D effect is relatively simple with just 2 noticeable frequencies and I see this almost every day on one signal or another as the band is changing around sunrise/sunset.  What makes the fuzz on P29ZL's signal unique is that it's a daily occurrence with a near-continuous broadening, usually towards a higher frequency which would indicate the path length is decreasing.  Sometimes one or two distinct frequencies appear out of the continuum.

Enough for now about this most interesting signal but I will be keeping an eye on things and doing a bit more reading.

de w4hbk