Small active antennas like the “mini-whip” (by PA0RDT) are very popular at the moment. They provide good reception of signals from HF all the way down to the VLF band. Here in the Netherlands Van Dijken Elektronica sells a nice kit for this antenna, and since Adrian PA0RDA was interested in this antenna, I ordered this kit and built it.
The kit comes with a high quality circuit board, all the parts, a circuit description, building instructions and some tips to combine this antenna with an SDR dongle (normally used for DVB reception on a PC or laptop). The required phantom power supply is not included, but the documentation includes a circuit description of this very simple thing. This is not a problem, since I’m the proud owner of a PoRG. The case is also missing, you’ll have to build one for yourself by using a small plastic box or some drain pipe stuff.
The circuit is very simple. It starts with a small platter, which is the capacitive antenna. The platter is connected to a J310 FET, which transforms the impedance. The small output signal is amplified by a 2N5109 transistor, with the output on the emitter for low output impedance. The circuit requires about 12 volt DC, which is provided over the coax, so no batteries or extra wires needed, but you will need a phantom power supply, as told before.
Assembling the kit
This kit is very easy to assemble. As always, start with the resistors:
Then add the capacitors, and then the remaining components:
It takes a while to solder the connector, the circuit board has no problem with this, so just heat the pad and connector pin until the solder flows nicely.
Since the antenna needs to be outdoor, you’ll need a weatherproof case. Adrian found a cup with lid, which seemed to be the perfect case. I drilled a hole in the lid to hold the BNC connector. I put the lid onto the cup, finishing the antenna.
I put the antenna in the field and compared it with my available antennas. On the lower bands it works really nicely, on 40m my 1/4 wave vertical is about 1-2 S-points stronger than the miniwhip, and on 20m my 5/8 vertical really overruns this small antenna.
Due to very nearby broadcasting stations on the medium wave band, serious intermodulation occurs, resulting in stations being heard on several frequencies. Still, it is a very interesting antenna and I think Adrian will really enjoy this thing.