When operational, my experimental 30m QRSS grabber can be seen here. (<– click to open in new tab)
The receiver from my previous post can be used for other weak signal modes like QRSS.
QRSS is transmitting information at very low speeds. At the receiver side this information
can be integrated for long periods, increasing (weak) signal to noise ratio (SNR) significantly.
It’s a public secret that QRO (high power) guys use low power (QRP) techniques
to optimise their top (contest) stations. Besides having sufficient output power it is VERY
important you’re also able to receive low power.
The quest therefore is to improve the RX SNR of your (contest) station.
This is where WSPRP/QRSS comes in.
In order to experiment with the super simple DSB subharmonic receiver I installed
a grabber. A grabber is a piece of software analyzing the audiospectrum using Fourier
transform techniques (FFT).
This allows you to visualize the weak signals because you can’t hear them.
Onno PA2OHH wrote LOPORA (LOw POwer RAdio) grabber software in Python.
After installing python 2.7 and some fiddling I managed to get it running.
While eagerly awaiting the first ‘lopshot’, results were disappointing. Besides some weak
WSPR signals I hardly couldn’t see anything with my ‘quick & dirty’ 30m antenna
consisting of 5m wire running through a window into the garden around 2m above ground.
Because I live in a very noisy environment, my first action was to minimise noise from
the receiving contraption itself. The receiver is connected to a SMPS (I know, not ideal)
and a computer. All their (ground!) connections were fed through 6 hole RFC’s.
Subsequently I decided to lengthen the antenna wire to around 7.5m (1/4λ on 30m)
and use my central heating system as counterpoise.
These simple actions resulted in a dramatic SNR improvement!
It almost resembled my first experience listening to Beverages on 160m. At first
glance you think your receiver is broke because you think you don’t hear anything.
Below is the difference between the 5m wire and the temporary 7.5m wire + counterpoise.
(click on image to enlarge in a new tab)
Believing the SNR algorithms of WSPR, SNR improvements were around +12 dB (!!)
Although the receiver now sounds very quiet (I still here my antenna btw), my amount of WSPR
spots increased flabbergastingly and am now spotting a new league of WSPRers.
I also had to fiddle with the audio level, FFT settings, contrast and brightness levels in LOPORA.
The result was opening of a new 30m QRSS world. When a (WSPR) signal appears very bright,
its SNR is mostly around -6 dB. Most signals are between -20 – 25 dB SNR, or even lower.
My benchmark is the 30m grabber of Steen Erik LA5GOA. Click on this link and see why (of course it depends
on the time of the day, try between 11 – 18 UTC).
Apparently Steen Erik lives in a very quiet environment and must have a good take off, also due to the
nearby sea (salt water!). It’s almost incredible what he’s able to receive with his PA2OHH style
DC receiver (I reckon also with a subharmonic mixer) with ‘own adjustments’.
I mailed him and learned he uses the same setup as Joachim, or vice versa.
LA5GOA’s antenna is a dipole directed E/W.
Below is a picture of Steen Eriks receiver (click to enlarge in new tab).
Less receiver noise.
One of the first things I’ve to do is decreasing the intrinsic receiver noise. Thus, like Onno did,
surpress the unwanted lower sideband. Theoretically this results in +3dB SNR improvement.
Secondly, increase the selectivity using a 10.140 MHz crystal as band pass filter.
Onno measured 800 Hz crystal filter band width. Bandwidth of the usable audio now is around 7 kHz.
Narrowing this to 800 Hz theoretically increases SNR with 10log(7000/800) = ca. +9 dB.
Btw, my WSPR/QRSS audio is around 4.5 kHz due to the LO frequency of my receiver.
In other words, if these two measures are carried out another +10 – 12 dB SNR improvement is possible!
Increasing frequency stability.
The receiver now lies open on the living room table without measures to stabilize it.
E.g. it’s not temperature compensated and I notice around 2-3 Hz/°C frequency drift.
When one of my cats lies next to the receiver (for whatever reason she wants to) LO frequency goes up,
and when she leaves LO frequency goes down ; -)
At this moment of writing 7.5m wire with the central heating system counterpoise is used.
I did not measure the antenna impedance, but from 40m I know that such antennas are noisy.
Also the radiation pattern is lousy due to its low height above ground. It’s looking up to the clouds (NVIS).
Building a (vertical) deltaloop introduces two assets: a) the antenna is a closed loop <– less noise,
b) the take off angle is relatively low <– less interference from (strong) nearby signals and good for DX.
This may result in at least additional +3dB, but more likely +6 dB SNR improvement.
Last, but not least, try to look for a nearby quiet place to install the receiver. I live in a busy city
with lots of interference like Power Line Communications (PLC), LED-lights, plasma screens, etc.
When the receiver is installed in a quiet environment +15 dB SNR improvement will be a (very)
conservative estimation. It might be an idea to use Beverages (or BOGs) there, but a preamplifier
is inevitable then.
Let’s wait and see . . .?
Update: I made a DCTL antenna for 30m but it initially seemed no success.
That is, no SNR improvement was visible, only lower signal levels.
I discovered that the power supply of my sampling laptop generated some noise.
Replacing the power supply with another one resulted in less noise (around -6 ‘WSPR’ dB).
Antenna input coupling was changed to an isolated coupling loop, just like Onno did.
I still could discern antenna noise and had to increase the ‘audio input slider’ in Windows.
The microphone audio input is used and is now 100% (without ‘MIC boost’ or AGC),
resulting in around 4 dB ‘WSPR noise’.
Below the temporary input coupling loop is depicted. I experienced less audible noise when
the ‘hot side’ of the antenna (yellow clamp, green clamp is GND) is connected to the
‘cold side’ of the coupling loop, i.e. that side which is more near to the cold side of the input coil.
The other way around more noise was audible. With my temporary antenna I tend to believe:
“Less audible noise = better SNR” ; -)
Before these two modifications (power supply & coupling loop) I had +16 – 20 ‘WSPR’ dB noise with
100% MIC volume. Above on the right a ‘lopshot’ showing more and more signals.
If I may believe the WSPR SNR algorithm my SNR further improved with 12 – 16 dB so the
(sub) total SNR improvement since the grabber is up amounts 24 – 28 dB (!?)
That evening I was one of the few EU stations copying ‘early’ US stations and was ‘competing’
with some EU WSPR ‘big guns’ on receiving several US and Asian 30m stations.
Promising? Yes and no. For example, I discovered PI4THT (Twente WebSDR using a Miniwhip
antenna) was able to receive the same stations with sometimes 20 dB higher SNR’s (!!)
The next day I took some additional measures:
1. Tried the experimental DCTL again.
2a. Connect both L + R channel of the soundcard to the receiver as I am not sure whether WSPR
and LOPORA sample ‘in stereo’. If this is the case, then on one channel only (audio) noise is present
which adds to the so called quantization noise of the soundcard.
2b. Isolate the audio path with a 1:1 audio transformer.
The results are visualized below (click on images to enlarge in new tabs).
In the above left picture Hell Schreiber traces of GM4GKH IO77WL become visible with the DCTL.
Apparently the overall SNR of my receiving contraption was not good enough when I tried the DCTL
the first time?
Reconnecting the 7.5m wire with counterpoise delivers more signal but results in lower SNR,
to such an extend that GM4GKH’s Hell traces disappear. These are the ‘highlighted’ parts in the spectrum,
in which the signal of my GPS locked signal generator @10.140000 MHz are also visible.
In this snapshot the reference signal may look wobbly, however this is the receiver LO, not my signal generator!
The influence of 2a + 2b may be seen at first glance. The right picture above looks darker (click to enlarge).
Audio settings between the left and right picture were equal.
Judge for yourself, but I reckon the overall SNR improvement of 1. and 2. combined is at least around 6 dB.
I.e. from seeing nothing (no GM4GKH with 7.5m wire) into seeing something (+ 3dB)
and being able to identify it (another 3dB) = 6 dB.
It could be that the difference in radiation patterns of the wire + counterpoise vs. the DCTL are
responsible or perhaps propagation. However, after these two measures I’m able to see GW4GKH’s traces
and other signals still look ok.
Therefore I estimate the total SNR improvement since the grabber is up to 6 + 24 – 28 = +30 – 32 dB !!
Bear in mind this is still the DSB receiver, i.e. no 10.14 MHz filter crystal and no Weaver SSB demodulator.
Another interesting fact is that at this moment of writing I am six unique WSPR spots
ahead of my benchmark (LA5GOA/RX2) in the last 24 hours.