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1924 Alexanderson machine transmitter in the air!!!




 
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Author Topic: 1924 Alexanderson machine transmitter in the air!!!  (Read 1122 times)
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N1BCG
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« Reply #25 on: December 24, 2018, 10:11:02 PM »

Now that we couldnít be further from the next SAQ transmission, I just got the nutty idea to build an op amp mixing circuit that uses an audio transformer to couple RF from the antenna to the + input and connect an audio oscillator to the - input.

The idea is to set the oscillator to a frequency 600 Hz off the desired station in order to create an audible beat note.

I might breadboard it in the next few days to see if thereís any merit to the idea...
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AJ1G
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« Reply #26 on: December 25, 2018, 05:20:48 AM »


The idea is to set the oscillator to a frequency 600 Hz off the desired station in order to create an audible beat note.

I take it from your comment that the app you downloaded to use with the computer sound card doesnít provide the means to demodulate CW, FSK, or SSB signals. Seems like a major shortcoming considering itís intended purpose was specifically for reception of a CW signal.
 
Why not just loosely capacitively couple the beat frequency oscillator to the signal path? At such a low frequency you could use something like an HP-200 audio oscillator, and use the output level attenuator to present a beat signal amplitude that doesnít overwhelm the received signal.

I use this technique with my E. H. Scott SLRM to improve its performance on CW and SSB.  The SLRM in fact does have an internal BFO, but it has a very low injection level into the 12H6 detector, as there is no direct connection of the BFO signal by any sort of coupling capacitor in the original as-built design. I suspect this was deliberate by design so that the set would meet the Navyís incidental radiation specs to qualify it as a super low radiation (SLR) shipboard receiver. I use a twisted wire gimmick cap to loose couple the output of a URM-25 sig gen at the nominal IF frequency into the grid of the last IF stage. The band spread at 465 kHz on the 25 provides for really smooth setting of the beat note, and careful setting of the output level permits using it with the AVC on without excessively driving the receiver gain down when listening to a wide range of incoming signal levels. Itís also useful to crank up the injection level when I want to use the SLRM as a local off-air  monitor of a transmitted CW or SSB signal.
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Chris, AJ1G
Stonington, CT
N1BCG
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« Reply #27 on: December 25, 2018, 11:49:37 AM »

I take it from your comment that the app you downloaded to use with the computer sound card doesnít provide the means to demodulate CW, FSK, or SSB signals. Seems like a major shortcoming considering itís intended purpose was specifically for reception of a CW signal.

The SAQrx.exe is a great app and includes the USB mode and selectable bandwidth filtering. It's ideal for listening to SAQ, but that means for only a few minutes twice a  year and it's top frequency is 22kHz. That's fine for tuning in 17.2 kHz, but somewhat limited for exploring higher VLF frequencies.

SAQrx v9.8 can be downloaded here: http://dl1dbc.net/SAQ/SAQrx/SAQrx_0-98.zip

Instant listening on a European Web SDR makes things as simple as possible:

http://websdr.ewi.utwente.nl:8901/?tune=17.20cw

The RSP1A SDR tunes down to 1 kHz (!) and offers lots of mode and filtering choices. That's what I used for testing audio transformers, and the spectrum display and signal level metering helped tremendously in the comparisons of the various transformers.

You bring up a good point about signal levels. The audio oscillator injection point should be at a higher level compared to the antenna input. Most op amps are dual section so theoretically one half could be used for antenna input amplification with transformer isolation/filter/matching and the second half for unity gain mixing. The output would then be line level for driving an audio amplifier. The downside is the lack of bandwidth control so I imagine it would be a lot like tuning in CW using a direct conversion receiver (like the HW-7 I had!).
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AJ1G
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« Reply #28 on: December 25, 2018, 12:39:33 PM »

Clark, following your suggestion, I cleared out the BCB spurs from my NooLec RTL SDR and upconverter.  Feeding my 75 meter dipole in with a .01 bypass across the input cleared out the BCB junk and I could hear and see a whisper of NAA at 24 Hz but the signal level was just above the electrical noise floor of the upconverter.  Using an amplifier of opportunity, the one available in my Ballentine Labs AC VTVM, which has an amplified signal output jack, I cranked in some gain and now NAA is banging in with and SNR about 25 dB above vs system noise floor.  I can also hear several other VLF signals that are down in the noise and only faintly tracing on the waterfall.  Should be able to improve the SNR with added gain but I am at the limit of the Ballentine at this point and itís time to head out to Christmas activities!  Again, thanks for the tip.


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Chris, AJ1G
Stonington, CT
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