You don’t need much for listening to the VLF band, only SAQrx and a simple amplifier connected to a small probe – that what we call an active E-field antenna. If you first want to know how this type of antenna works in general, read the notes on active antennas.
We will use an op-amp as the active element of the antenna so everything reduces to a handful of components (see Fig. 1).
Fig. 1: The principle circuit diagram of our homebrew active antenna
This is an amplifier with the E-field probe connected over a coupling capacity of 56 pF to the non-inverting input. A resistor of high value (here 3.3 M) provides the necessary dc bias. Together, they form a high pass with a cutoff somewhere near 1 kHz.
resistors R1 and PT1 set the gain of the amplifier, simply calculated by
The VLF signal leaves the circuit via the capacity C2. The value is not crucial, so take the value you can provide.
The heart of our simple circuit is a TL071, a low noise op-amp with JFET inputs. To be honest, the whole circuit is more an impedance converter than an amplifier. It transforms the high impedance of the probe connected to its high resistance input to its low-ohm output.
Op-amps are designed for using dual power supplies. If we want to use a single supply (i. e. one 9 V battery or so), we have to add some components to generate a virtual midpoint (R3 and R4, C3). Besides, we add C4 and C5 to achieve some noise cancelling on the supply lines.
Fig. 2: The complete schematic diagram for the simple active antenna
You can build the whole thing on a breadboard or on an experimental pcb.
Now we have to connect our impedance converter circuit to the outer world, i. e. the VLF electromagnetic field. We do this with a capacitive probe to the electric component of the field.
Phew, sounds like an elaborate design.
It is just a metal plate of say 25 cm² or so. Or a piece of copper pipe used for water supplies in the house. Take what you can find (see Fig. 3 for an example), solder a wire to it and connect it to the input of the TL071.
Fig. 3: An E-field probe as combination of a metal sheet screwed on a piece of copper pipe
This active antenna design is a Dünnbrettbohrer-version of the PA0RDT Mini Whip antenna. Roelof, PA0RDT, uses a copper plane of the pcb itself as E-field probe. If you decide to make a permanent layout one day, you can do that, too.
Make a connection between the output of your active antenna and the microphone input of your computer soundcard. Choose “System Control” / “Sound” and make sure that the microphone input is not muted (!) and the level is not put to zero. Then start SAQrx.
Make sure that your body is not charged electrostatically, before you try the following! Touch something grounded first (blank parts of your radiator for example).
When you touch the probe or the microphone input, some reaction in the SAQrx window should occur: the baseline should move due to powerline hum or apparent peaks should become higher. If there is no reaction, even when touching the microphone input itself always check the sound settings of the operating system (Windows System Control) first!
With our homebrew antenna working basically, we now must change into an environment where power line hum and noise from switching mode power supplies, computers and home appliances is not dominant. We have to go out. If you have an attic with windows that can be opened, go there.
For serious reception tests, the active antenna should be placed outside the house and as high as possible. So if you have attic windows, go upstairs and place your active antenna near or (better) outside the window.
Now you should notice several military transmitters in the SAQrx window (see Fig. 4). If you are using SAQrx V 0.98 or higher, there should be a call sign information when you move the mouse cursor over the apparent peaks.
Fig. 4: Lots of navy stations in the 48 kHz band pass; note SRC on 40.400 kHz, it is using the same antenna as SAQ and has to be turned down for SAQ transmissions
There is a wide field of experimenting once you have started to construct active antennas. I am living next to Langenberg radio, a medium wave transmitter, and I had to add a filter to prevent the medium waves to interfere with reception. I did this using an active :-) filter.
I call my antenna design “Etherprobe” aiming at the outdated concept of the ether as medium transporting electromagnetic fields. Looking at the Etherprobe schematics, you will note that the E-field probe is connected to an isolating buffer amplifier before entering the filter stage. Amplifying is done in the last stage. The virtual voltage midpoint is generated by the remaining part of a quad TL074. Additionally, there is some Schnickschnack with a comparator and two LEDs to control the state of the battery power supply. [ Download EAGLE files ]
With L1 added, you can operate the Etherprobe with the remote power supply of the PA0RDT Mini Whip.
Have fun improving your own design!