Earth mode VLF DX now 5.1kms

Today I managed to copy my 4W earth mode 838Hz signal at a range of 5.1km. QRSS signals were so strong that I was able to copy by ear my 10wpm CW beacon too! To copy the 10wpm signal I needed Spectran running with narrow bandpass filters. Without the filtering, all that could be copied was 50Hz noise.  At this range the signal level is definitely being helped by conduction through pipes in the roads and the main propagation is not by induction. Lots more information as well as screen shots and recordings on my sub-9kHz webpage.

VLF Earth Mode: 3.6km DX today with 4W

My earth mode VLF DX increased to 3.6km today using QRSS3 and a PC running Spectran. TX was 4W into 20m separated electrodes in the ground. TX frequency was 838Hz. The 80cm loop and preamp receiver were set up in several places well out of the village in several directions and results well exceeded my expectations. Lots of information on my earth mode webpage including screen shots and sound recordings made using Spectran when out in the field.

This screenshot was from the roadside 2.5km south of the QTH. Signals were so strong (30dB S/N) that the signal was audible when Spectran's BPF in circuit (lots of 50Hz hum without).

Stable audio frequency tone generation (for VLF QRSS)

Next week I want to try QRSS3 with my earth mode VLF system to extend the range. As in the test this week,  Spectran software will be used my PC to filter and decode it. For a stable source I'll use a crystal and divide this down to sub-9kHz. Looking around for a suitable simple circuit I found G4HCL's very simple 1750Hz toneburst circuit which uses a 455kHz ceramic resonator (pulled slightly) and a 4060 IC as a divide by 256. With a crystal instead of the ceramic oscillator this should be perfect if followed by some filtering to produce a decent sine wave output. See M1GUR's page for more details.

Earth electrode loop effective area at 500kHz

From some measurements of my earth electrode "antenna" this evening and some estimates of the ERP calculated from received field strengths 69km away, I've received an estimate of my effective loop area including that part that is within the ground. This was calculated by Rik Strobbe OR7T as 70m^2 in total, suggesting some 20m^2 of the loop was within the ground. These were Rik's calculations:

"Assuming

- 50uW ERP

- antenna is traded as a loop (gain = -0.4dBd)

Due to the negative gain (-0.4dB versus dipole) a loop antenna must "radiate" 55uW to get 50uW ERP.

Since the antenna feeding current is 0.15A the radiation resistance is 2.44 milliOhm (R= P/I^2)

The radiation resistance of a small loop is : Ra = 320xPi^4*A^2/L^4

where A = loop area (m^2) and L = wavelength (m)

For 500kHz Ra = 5*10^(-7)*A^2 or A = 1416*sqrt(Ra)

If Ra = 0.00244 Ohm then effective the loop area is 70m^2"

Diagram of my earth electrode antenna for 500kHz and sub-9kHz

This is a hand drawn diagram of my earth electrode antenna for 500kHz and sub-9kHz. On 500kHz WSPR this has managed to TX a signal as far as G0KTN who is 210kms away when using my 5W QRP transverter. Sub 9kHz my 4W TX is audible by ear at 0.5km range on an 80cm loop.  This is a totally "stealth" antenna that is not visible at all to anyone looking at the property.

Screengrab of 1.8kHz earthmode transmission today

Today I modified my VLF TX so it could send either a 10 wpm or QRSS3 beacon signal. I also went out into the fields with my loop/preamp and a PC with Spectran to receive it. For the first attempt with a PC I went 0.3km away from home where the 4W signal from the earth electrode "antenna" could clearly be received by ear. With Spectran I could read the 10wpm CW signal with 20dB S/N in a 12Hz bandwidth, if my understanding of the Spectran settings is right. There is an audio recording here so you can judge the S/N yourselves. This suggests that with QRSS3 and really screwing the bandwidth down I should be able to do much better. My problem is knowing how to calibrate the soundcard etc and clearly I need a more stable source than my free running twin-tee audio oscillator.

Grounded electrode antenna on 500kHz

This evening I tried listening on 500kHz WSPR using the 20m spaced grounded electrode pair "antenna" in the back garden (earth rods 20m apart). To my surprise I decoded G3ZJO and G7NKS. Both stations were using just 1-2mW ERP. I'm tempted to try this "antenna" (actually a loop formed in the earth) on TX to see if anyone can decode me on 500kHz.

The latest VLF receiver

Here is a picture of the latest version of my VLF 1-2kHz receiver for use with my 80cm loop. The version before this (Wednesday) had one less Sallen-Key active high pass filter and was able to copy my 4W grounded electrode transmissions "by ear" at 0.5kms. This latest version now has 2 sections of 50Hz active high pass filtering so should offer better S/N at the limit of range. The audio above 2kHz is attenuated by the passive LPF.

0.5km on earth-mode with 4W

This evening I achieved my best results so far with 1kHz earth mode TXing on a 20m base electrode pair and receiving on an 80cm loop and listening by ear. Even this range I think can be bettered with yet more 50Hz rejection. With a PC at the receiving end I think 1km is certainly possible now.

More earth mode experiments at 1kHz

This evening I extended my TX electrode spacings by connecting one end to the copper pipes in the house, almost doubling the effective TX baseline to about 20m. With this, my "earth mode" signals were much stronger and I was able to cover further than before in a "by ear" walk-about test around the fields locally.  Ultimate range tonight was 0.4kms using 4W (probably less as the TDA2003 was hot and probably turning down the power) and receiving on an 80cm 30t loop, active HPF, small AF amp and crystal earpiece.  Range was limited by 50Hz mains hum and background sferic noise. I must retry this set-up with my HF up-converter and FT817 this week, which has good rejection of 50Hz (and its harmonics) as well as a narrow CW filter.

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.

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.

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.

Further 8.97kHz earth-mode tests

All being well, I'll be carrying out some more earth-mode (through ground conduction current) WSPR and QRSS tests on around 1kHz and 8.97kHz in about 10 days time. My aim is to achieve a new personal DX record for the mode using about 4W. My previous best distance is 0.3kms, but I'm pretty certain that with QRSS or WSPR this can be bettered. Remembering an earlier post, 18dB more system gain is needed to double distance using this mode of propagation.  This can be achieved by raising TX power or effectively improving the RX sensitivity, in this case mostly by better filtering and signal processing.

Thoughts on VLF Earth-Mode and Induction DX

Earth-mode (through the ground conduction) and induction communication (mutually coupled coils) have an inverse cubed attenuation with distance. Think about this: I reached 0.3km with earth mode for an audible CW signal with around 4W RF and a simple 5m base receiver and 10m base transmitter. To double this range, all other things being equal, means increasing power by 18dB to 250W. To double range again means raising the power to kWs. Conversely, improving the detectable sensitivity threshold by 18dB (increasing the electrode spacings at each end, using WSPR or QRSS, etc) reduces in power needed for a given range dramatically: my 4W signal could be detected at 0.6kms or a just a 63mW signal could be detected by earth mode at 0.3km. In reality, especially in urban areas, water pipes and cabling may help "propagation" and achievable ranges may be greater. DX is relative, especially with conduction and induction at VLF.

A WSPR first on 8.97kHz

This afternoon I set up my "earth-mode" station using a pair of electrodes about 10m apart in the garden, one near the house and one at the bottom of the garden. Using my 4W audio amp driven from the WSPR software in the PC I transmitted a WSPR signal on 8.97kHz. With a separate PC at the other side of the house and with 1m of wire as the "antenna" plugged into the mic socket of the PC I went listening for my WSPR signal.  Sure enough, there was my earth-mode transmitted signal at -15dB S/N. Distance was only about 10m but at least the TX, WSPR system and ground electrodes work, so I now can go out into the fields behind my house to look for my WSPR beacon using a decent, sensitive 8.97kHz RX next time. Strictly I need my sub-9kHz NoV to do this test, but as nothing is radiated I doubt anyone will care.

As far as I know this is the lowest frequency on which WSPR has ever been successfully used.

Communications below 9kHz

This has just been posted on the LF reflector by DO1KHS. I wonder how many other people have experimented with communications at radio frequencies below 10kHz? If so, do share your experiences here. See also my website on the subject at http://sites.google.com/site/g3xbmqrp/Home/10khz

Hello,

Some years before, some experimental licences for ham radio below 9 kHz were issued in germany. Meanwhile this frequency range is free as the Bundesnetzagentur has confirmed last year.

Transmitter is an Präcitronic GF62 Level Generator actually at 8,79 kHz. PA is an old homebrew NF-Amplifier with 2x 2N3055 and about 40 Watt at 4 Ohm into a 100V ELA transformer. Antenna is a T-Antenne of 10m with 20m top capacity and poor ground.

Receiver is a portable sferics RX that I built some years before (AATIS). Unfortunately its whip antenne is highly influenced by trees and houses.

The fact that I can hear (!) my beacon in normal CW up to 100m in an city environment with a highly unmatched antenna makes me believe, that in a better environment and with a matched antenne and with help of ARGO one can make tests over longer distances. But even from my QTH in the city the ODX ;-)) surely can be improved.

The mean problem now ist how to match the antenna . . . - or using ground antennas vor TX, RX or both.

vy73 Horst