How low can you go? Explorations of the MF, LF, and VLF bands.

Last edited: 01.12.2018

I've always been fascinated by ELF, VLF and the lower part of LF, and both the man-made signals and Natural Radio emissions occurring on these bands have always intrigued me. I never had a suitable receiver and antenna for this part of the RF spectrum though.

This changed when some time ago I bought an RSP1A SDR-receiver, and more recently a MegActiv MA305FT E-field probe. This receiver / antenna combination for the first time enabled me to seriously explore the mysterious realm below 100 kHz, all the way down to about 9 kHz (the lower limit of the frequency range of the MA305FT)!

It's really fascinating listening for the first time to the various Time Signal Stations and to the transmissions of the numerous naval stations using these low frequencies to communicate with submerged submarines. On VLF, radio signals are able to penetrate seawater to a depth of up to about 40 metres, depending on the salinity of the water (contrary to higher frequencies which do not penetrate seawater to any significant depth). On ELF the signals can penetrate even deeper, and the Russians are operating a transmitter known as Zevs near Murmansk to communicate with their submarines on a frequency of 82 Hz (yes, Hz, not kHz!). A similar system is (or was?) also in use by the United States on 76 Hz.

The Norviken VLF transmitter (callsign JXN) in Norway can be heard on 16.4 kHz. The station is used to transmit messages to submerged submarines. VLF antennas are huge; the antenna of JXN is made up of three wires spun between two mountains and spanning a distance of over 2 km! (source)

The MegActiv MA305FT E-field probe is manufactured by NTi in southern Germany close to the Swiss border. It's an active antenna of the popular so-called Mini Whip type. Probably the most well known mini whip is the original Mini Whip designed by PA0RDT, and most other mini whip designs are more or less based on the same principal.
Some time ago I already bought a mini whip kit from Van Dijken Electronica, but I never got round to building it. Then, while visiting the yearly VERON Ham Radio Convention in Zwolle early November, I noticed the stand of Bonito with various active E-field (mini whip) and H-field (loop) type antennas for sale. Bonito is closely involved in the designing and testing of the various antennas made by NTi. The MegActiv MA305FT caught my eye, was looking very well built, and was offered for 20 euros below the normal price. I checked the internet and found that the antenna in various reviews was scoring very well. I decided to buy one.

I bought the MA305FT mainly for exploring the VLF, LF, and MF bands, and this is where the antenna really shines, as you'll see later on.

At the University of Twente they have a mini whip antenna in use for their WebSDR. Their mini whip is performing extremely well, and their WSPR reception has become the high standard reference for me for doing rx antenna evaluation. WSPR reception results can easily be compared by checking the spots of the University of Twente (callsign PI4THT) on WSPRnet.org. When doing WSPR reception comparisons, on all bands from LF through to HF 20m (I never checked the higher bands) the mini whip of PI4THT always outperforms my HyEndFed 10/20/40 sloper wire antenna.

When I got home from Zwolle I immediately set up the MegActiv in the back of my small yard at a height of about 3 metres on a PVC pipe placed on a large tripod. The active antenna is powered over the coax feeding cable with the special coaxial power inserter that comes with the antenna. The inserter can be powered in several ways, including from for instance a laptop USB port. The antenna / inserter can be fed with a power source ranging from 5 to 15 Volts DC. I tried powering it with several of the 5 V power banks that I have, but all shut down after a while due to the power inserter drawing very little current and the powerbank thinking nothing is plugged in ( I recently saw a little device for sale at SOTABEAMS which in such a situation will prevent a powerbank from shutting down, and of course such a "keep alive load" could also be homebrewed very easily).

The MegActiv MA305FT active E-field probe. Frequency range 9 kHz - 300 MHz.

Once everything was set up, one of the first things I did was monitoring the 630m MF WSPR frequency (474.2 kHz dial). This looked very promising; the SNRs of the signals received often were not far below of the spots made by PI4THT, sometimes even better! The next day on November 5th I managed to receive the 5 Watt WSPR signal of AA1A in Massachusetts, USA! With this I really outperformed PI4THT, as no MF WSPR spots for US stations were made at all that day at the University of Twente!

Reception of the 630m WSPR signal of AA1A with the RSP1A and the MegActiv antenna

On VLF and the lower part of LF a lot of signals were received with excellent strength, mostly time signal stations and the naval stations mentioned earlier, but also various telecontrol signals and for example the RTTY weather reports from the Deutsche Wetterdienst DDH47 on 147.30 kHz.
In the 1990s I used to be an avid LF NDB DXer, and I really enjoyed rediscovering this part of the radio hobby wth the RSP1A and the  MegActiv. New to me in the field of NDB DXing are the DGPS stations, many of them actually being the old closed down maritime NDBs. I also had excellent reception of various coastal stations with NAVTEX weather and navigational warnings on 518 kHz, and I see some real DX potential here for the dark winter months. With MultiPSK I was able to decode the DDH47, EFR telecontrol, DGPS, and NAVTEX signals.

I'm looking forward to the next Morse transmission of the historical station SAQ Grimeton in Sweden on 17.2 kHz which I hopefully will be able to pick up with the setup described in this blog. The last time I tried it with a wire antenna and I failed. SAQ Grimeton was one of the reasons why I really wanted to improve VLF reception.

Reception with the MegActiv MA305FT on HF so far has been a little disappointing. I expected the reception on HF with the MA305FT at least to be on par with my HyEndFed, but on 40 and 20m WSPR and FT8 the HyEndFed was the clear winner. Compared to 40 and 20m WSPR reception of PI4THT the MegActiv just could not compete at all.
This doesn't mean the MA305FT can't do better on HF though. It could all be a matter of finding the best location and setup for the antenna. To work properly a mini whip type antenna should have the coax shield close to the antenna connected to an earth electrode. I don't have such an earth electrode available yet. At PI4THT they don't have an earth electrode, but the roof on which their mini whip is located contains a lot of metal which serves as the antenna's earth. I might install an earth electrode later on, and more experiments are needed to say something meaningful about the performance of the MegActiv on HF.

The MA305FT opened up. With a jumper an FM broadcast band notch filter can be switched in.

I've heard some people say that an SDR and an active antenna don't match very well, due to the antenna causing overloading very easily. Also I've heard people say that an active antenne like the mini whip is very prone to picking up the omnipresent electrical noise of an urban surrounding. I didn't notice any of this being much worse than with the other antennas I have in use though.

With the recent exploration of the RF spectrum basement, a new radio hobby door has opened up, and I got inspired to continue with more VLF, LF, and MF experiments. I would like to experiment with using special software and a PC soundcard as VLF receiver  (a soundcard with a sample rate of 48 kHz can receive radio signals up to 24 kHz*). One day I'll also built the mini whip kit and see how it compares to the MegActiv.
And who knows, maybe the future sees me transmitting WSPR on 630m MF myself, using the MF Solutions transmit converter described here and here (in combination with my QRP Labs U3S), and an earth-electrode antenna as in use by G3XBM and as described here.

And still the RF spectrum below 9 kHz and Natural Radio remains unexplored. Someday I will also buy or built myself a natural radio receiver.

* I was mistaken earlier and wrote 96 kHz, it should be of course 24 kHz.


See also:

https://en.wikipedia.org/wiki/Very_low_frequency
https://en.wikipedia.org/wiki/Extremely_low_frequency
http://www.vlf.it/zevs/zevs.htm
https://en.wikipedia.org/wiki/Noviken_VLF_Transmitter 
https://www.fenu-radio.ch/en-index1.htm#MegActiv
https://www.vandijkenelektronica.eu/nl/57-antenne-s-en-antennebenodigdheden
http://websdr.ewi.utwente.nl:8901/
https://www.mwlist.org/vlf.php
https://www.efr.de/
https://en.wikipedia.org/wiki/Non-directional_beacon
https://en.wikipedia.org/wiki/Differential_GPS
https://en.wikipedia.org/wiki/Navtex
https://alexander.n.se/the-radio-station-saq-grimeton/saq-transmissions/?lang=en
http://www.472khz.org/
http://www.pa3fwm.nl/technotes/tn09d.html
http://www.pa3fwm.nl/technotes/tn07.html
http://dl1dbc.net/SAQ/Mwhip/pa0rdt-Mini-Whip.pdf
http://dl1dbc.net/SAQ/Mwhip/Article_pa0rdt-Mini-Whip_English.pdf