A priori: when operational, my 30m grabber can be seen here (<- click to open in a new tab)
I decided to write a new post, otherwise the previous posts become too long,
knowing that the average reader is too impatient and wants his information quick! ; -)
During the last days I tried to improve my 30m QRSS receiving contraption in
several stages. The estimated overall SNR improvement from the first setup
is around +32 dB, consisting of the following measures:
1. changed 5m of wire to 7.5m wire + central heating system as counter poise (+12 dB)
2. changed laptop power supply + isolated antenna coupling loop + RFC’s (+14 dB)
3. isolated audio + DCTL antenna (+6 dB)
Yesterday Peter PA3EXL gifted me a 10.140 MHz crystal from an old (1978) CB-radio.
I changed the input circuit of the receiver by replacing the air wound coil + trimmer
with a modified small adjustable inductor.
I found this inductor at my radio club. It was rated 4 – 8 uH. The metal can was temporarily
removed. Using a magnifier the 22 original windings were reduced to 16 windings with
a tap at 3 windings. With the remaining wire I wound a 2 windings isolated input coupling
loop at the ‘cold’ side of the primary coil.
Measured inductance of the primary coil was ca. 3.4 uH. With 82 pF parallel ca. 3 Vpp
was obtained with -47 dBm 10.140200 MHz injection (see also previous post).
More than 1 Vpp improvement! Of course this is due to the higher Q of the new input circuit.
The previous input circuit needed around 200 pF for resonance at 10 MHz.
Subsequently the 10.140 MHz crystal was mounted and with a series trimmer trimmed
tuned tothe desired pass band. The resonance peak looked sharp and the optimal series
capacitance was measured 21.7 pF. The trimmer was replaced by a fixed 22 pF capacitor.
With -47 dBm reference signal I measured ca. 2 Vpp. A loss of 1 Vpp, due to the crystal and series cap.
A picture of the current setup is depicted below (click to enlarge in a new tab).
The receiver audio timbre was totally different. Different in a sense it was more quiet
(i.e. less noise) but also sharp high tones were audible. Just like in my young days when building my
first crystal receiver. So, could it be possible that I may have some AM detection?
Perhaps it’s still a good idea to add the balance potmeter in the Polyakov mixer?
Well, I added a 100 Ω potmeter and it makes no (measurable) difference.
While trimming the balance potmeter and listening to the receiver audio I initially couldn’t believe my ears.
With the antenna connected and tapping with the screwdriver on the PCB board I could hear ticks!
I went to the kitchen, got the kitchen towel roll and shouted almost my lungs out
(which is VERY loud! ; -), through the roll over the receiver.
The recording (somewhat filtered and amplified) can be downloaded here.
Besides shouting ‘testing 1,2,3′ and ticking you hear two bursts, but also IK3NWX/B on 10.1373 MHz.
I believed attenuating that significantly with the 10.140 MHz crystal?
Apparently not enough . . .
Perhaps the overall SNR performance is too good and the receiver is therefore susceptible
to microphonics? The only ‘mechanical part’ in the receiver is the crystal in the oscillator block.
Btw, if you read this and use a 5.0688 MHz oscillator block or crystal too, try if you detect microphonics?
After a while I was able to identify GM4GKH’s traces with the 7.5m wire + central heating counterpoise.
With the DCTL antenna these traces became more clear, however I also had around 6 dB less audio
and got the feeling that other signals were weaker. With crystal modification and DCTL antenna MIC
volume settings now have to be MIC boost = 20 dB and slider around 60 to have around 3dB ‘WSPR noise’.
If I may believe the dB scale in Windows, the overall noise reduction is also confirmed by audio volume settings.
The current result is depicted below left, judge for yourself.
Below right is a LA5GOA ‘benchmark lopshot’ in the same time frame.
Warning: The more you watch these pictures the more you see which may not be there! ; -)
That evening I copied several US stations and saw my amount of unique 30m WSPR spots/24hr increasing
even more. Two days ago I was on par with LA5GOA/RX2, yesterday before the crystal mod around +10.
At this moment of writing (1200 UTC 5 Nov 2015) the difference is around +20 uniques (!)
With this number I seem on par with LA9JO.
My next goal is PI4THT, ON7KB or ON7KO , at this moment of writing I have averaged -15 uniques.
Other benchmarks are GM4SFW or DK6UG having +40 uniques. I reckon for this result I have to
move the receiving setup into a more quiet environment.
My overall conclusion after a few days of experimenting and measuring: this super simple receiver,
together with all noise reducing measures, in a suburban environment with lots of noise sources,
is a good performer and is competing in the EU top of 30m WSPR receivers.