Since SWL is in one of my main interests, but I never had enough space to build a full-sized receiving antenna system, it seemed like a good idea to build something which takes up significantly less space. One option was a magnetic loop, but it still needs some space, and some extra equipment to build it (soldering copper pipes together with a blow torch is not entirely compatible with my current accommodation). Not to mention that a magloop needs constant adjustment in orientation and also tuning. When I saw that the most popular SW SDR receiver stations on websrd.org uses mini-whip active antenna, I decided to give it a try.
I rebuilt PA0RDT’s active antenna [1], improved by PA3FWM [2], but i did not bother to make a properly etched PCB, I just used a breadboard. The receiving surface is a piece of sheet aluminium, 0.5 mm thick, held by two screws at the edge of the board. The build was quite straightforward. The connector is a 50 ohm BNC, which carries the signal towards the receiver, and also the power to the antenna unit.
The circuit worked nicely, but achieving good reception needed some extra effort and tweaking in the power supply box, and some trial and error to find the right location to the antenna.
As the original article states it, finding proper grounding is vital for good reception. I do not have an opportunity to put the antenna outside the house, so local QRM is quite a concern.
One of the most important thing was to find the right location for the antenna. Interestingly, it is not as intuitive as it seems. To put it as high as possible is a good rule of thumb, but there might be also some hot spots within a room at locations near well grounded objects, like radiators, metal structural elements, etc., so it is always a good idea to try all the available locations thoroughly.
An important thing to consider is that you have to look for S/N ratio instead of absolute signal strength, especially if it installed indoors. The active antenna has a well enough gain to easily overdrive a receiver if the probe surface is large. I have an FRG-7, which is not so great in terms of large signal handling. It is easy to find a spot where the S-level is high, but it is much harder to find one when you can actually hear weak stations.
Another issue for me was feeding the unit. The mini-whip is quite susceptible to pick up local noise coming through the feed line from the receiver end. In order to reduce this, I have done a few countermeasures. The first and most important thing was to connect the outer shield of the coax to a good grounding point close to the mini-whip. The best grounding point I could find was the valve of the radiator under the window of my room. I also added those noise suppressor clip-on cores to the coax, after the grounding point. I made a few turn coil from the coax as well, to further increase the the choke effect.
I also modified the original feeding unit by separating the antenna ground from the receiver ground by installing a wideband RF transformer to the receiver side, and connect the receiver cable via an ground isolated BNC connector [3]. It reduces the noise coming from the direction of the receiver and also breaking the ground loop. I also added two chokes to the connection towards to the +12V power, to both the positive and negative wire, so the shield of the power unit is not grounded through the +12V PSU any more. I connected the shielding of the power unit directly to the ground.
After all these modifications, the overall noise is much lower than before, though there is still some 50Hz modulated noise on some frequencies, and I could not find the source of it yet, even if I turned off all the possible noise sources nearby, one by one.
In general, the antenna gives quite good reception, despite of it is located inside my room. I found a final place for it where it (usually) does not overdrive my receiver, and the noise level is reasonably low.
[1] The pa0rdt-Mini-Whip ©, an active receiving antenna for 10 KHz to 20 MHz,http://dl1dbc.net/SAQ/Mwhip/Article_pa0rdt-Mini-Whip_English.pdf
[2] Simple and better circuit for MiniWhip antennas, Pieter-Tjerk de Boer, PA3FWM, http://www.pa3fwm.nl/projects/miniwhip/
[3] Antenne active PA0RDT MINI-WHIP,
http://f-10255.pagesperso-orange.fr/antenne/act/miniwhip/mini.htm
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