Further tests on 1kHz using conduction and induction

Today I continued "by ear" testing local induction and conduction communication using my 4W PA at 1kHz with 10m spaced grounded electrodes. Using the 80cm loop, HPF and audio amp I was able to achieve 0.3kms today, slightly less than on Monday. Everything else is unchanged, so I'm wondering if the effective TX loop area formed by the 2 earth rods in the ground changes with soil moisture? I.e. when the soil is damp is the loop smaller? Logically this makes sense. Copy using my 80cm loop and VLF up-converter was disappointing: I was unable to get further than 0.2km. This system needs further work as this should be every bit as good or better than the audio amp receiver.

Also today I checked the resistance of the TX grounded electrode system using an audio oscillator and potential divider method: it measures approximately 50-60 ohms from 1-10kHz, which is not too bad. I also tried receiving my signal 0.2km away using another pair of grounded electrodes spaced by 10m. Although copy was just possible, the signal was buried in mains hum. Mains hum pick-up on the loop is much lower. This again suggests that the main means of communication is induction, not conduction through the soil.

DK7FC's VLF PA

Stefan DK7FC posted the 300W PA used in his historic tests on 8.97kHz on the LF Reflector today. He's been copied by at least 10 stations in 5 countries using this PA with best DX 902kms.

4W TX for sub-9kHz experiments

Attached is a picture of the "transmitter" I'm using for my ground and induction communications tests. It uses a TDA2003 audio IC into a toroidal step-up transformer that can match from 10 to 150 ohms. Not sure of the transformer type which came from an old Pye Telecom PMR radio, possibly an M206, Whitehall or similar (it was used on the audio stages). I haven't yet tried listening with an active probe antenna to see how that would perform. Time is limited, so further tests may have to wait until the end of the month. Best range so far, receiving "by ear" on a loop antenna, with no clever selectivity or signal processing, is 0.35km.

VLF loop and up-converter ready for tests

This is a shot of my receiving loop antenna for sub-9kHz local earth-mode and induction comms testing. The loop consists of 30 turns of wire on a wooden frame. This feeds into my SBL1 based VLF-to-HF up-converter (the small blue box on the frame). The output of the converter feeds into the FT817 receiver.

Japanese Hamsat off to Venus

On May 17 Japan's Space Agency plans to launch a mission to Venus. It will also be carrying a 35cm cubed nano-satellite developed by universities and colleges. This nano-satellite will go into a Venus encounter trajectory and will become the world first university satellite which goes beyond the moon. It will perform technology experiments and test long-range, inter-planetary communication using amateur radio frequencies:

Downlink Frequency: 5840.000MHz, band width 20MHz
Transmission Power: 4.8W/antenna, 9.6W total
Antenna: 2 Microstrip patch antennas
Modulation: AFSK/FM 1200bps during LEO flight
CW 1bps during Interplanetary flight

DK7FC's 8.97kHz VLF grabber active antenna

DK7FC runs a grabber on 8.97kHz so he can monitor signals testing on this VLF frequency. He feeds the signal from this active antenna into his PC's soundcard. This is a sketch of the schematic that Stefan posted on the LF reflector today. He has since added some additional C between the drain and ground to reduce the intermod products from LW/MW broadcasters that otherwise produce a strong line on 9kHz.

VLF up-converters and MOSFETs

Today I built a MOSFET up-converter, converting 7-9kHz VLF to around 20MHz. However, I was disappointed with the results: low conversion gain and sensitivity when using either a 3N211 and a BF981 device. I used an 82mH choke with a 4n7 cap as the tuned circuit on G1 (tapped capacitance for max sensitivity with 50 ohm input) with LO injection on G2. Despite optimising everything I couldn't better the sensitivity achieved with the SBL1 based up-converter that appears on my website.

Filter Calculator (and other calculators)

When designing a simple Sallen-Key active filter yesterday for my VLF receiver I found several useful web resources. One of the best is http://www.calculatoredge.com/index.htm#electronics that has a lot of useful calculators for electronics. I like the fact that when designing filters it asks you, and gives you, values in "real" numbers like nF and kohms. Some calculators ask you to give values in Farads and ohms to the power E-09 and similar and are obviously written by a college professor and not a real engineer who finds it hard to work these out!

Sub-9kHz NoV - still awaiting Met Office OK

I chased Rod at OFCOM this weekend to see if there was any further news on the NoV application to TX below 9kHz. The hold-up is still the Met Office, from whom OFCOM are awaiting approval. I'm carrying on with sub-9kHz earth-mode and induction tests at 4W meanwhile as I can't imagine anyone will object. My transmissions last about 10 minutes and the radiated power is picowatts!

Earth mode and induction testing at 1kHz

Today I restarted my earth-mode and induction communication tests at VLF using with my 4W transmitter and 10m separated earth electrodes. After some false starts, I ended up by field testing a simple receiver consisting of 30t of wire 80cms square into an active high pass filter followed by 3 stages of audio gain into a crystal earpiece. 2N3904 transistors were used at each stage. With the loop over my shoulder I set the TX sending 1 second "beeps" at 1kHz and went walking....

Even with 20-30dB of 50Hz rejection it was still an issue in many places. Walking across the field behind my house, and well away from cables and pipes, the signal could be heard in the earpiece at RS53 to a distance of 0.35km. Mains hum and LW/MW interference (the RX is not in a screened box) rather than signal level  limited further range. This week I'll add more high pass filtering, put the RX in a screened box and make the whole thing more immune from BC breakthrough. As I can hear this signal by ear at 0.35km with essentially no narrow filtering, then I'm confident that twice as far will be possible with QRSS/WSPR and a PC based receiver. Not bad for 4W at 1kHz.