building – Radio Ramblings

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 finished antenna. I did not give it a proper housing since I intend to use it indoors only. Maybe later I will build it into a PVC pipe or something like that. The zip tie on the end is for mounting.

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.

Not a winner in a beauty contest, but works well. The two green chokes are nicely visible towards to the PSU plug. I left out the resettable fuse. Maybe I will add it later.

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.

SWLing with the new antenna. It works pretty good. 🙂

[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

Impedance matching between a transmitter and an antenna is essential. You can even kill your PA if the reflection is too high and much of the energy is reflected back. But it can be important to match your antenna with your receiver, too, in order to reach the best possible reception.

So I decided to build my own tuner (instead of buying one) for my receiver experiments. My choice was the ubiquitous T-match, because it can match wide range of antenna impedance, and it is simple to build.

Since there are no significant power requirements against the tuner, I could use variable capacitors with any voltage rating. So I bough two of those they use in pocket radios. The exact value does not matter too much until it is in the 400-600 pF range somewhere.

I built the coil using a big yellow Amidon core I bought a long time ago. The winding can be calculated using toroids.info. In my design I made a tap after each 6th turns, there are 11 terminals in total. The resulting inductance range is adequate for my purposes.

One of the benefits of this construction that the terminals can be soldered directly to the switch, no extra mounting or wiring necessary. It greatly simplifies the overall construction and needs no extra mounting and wiring.

I built the whole thing into those die-cast alloy cases. It is a great EMI shield, does not allow dust to get in, and very sturdy, so the whole unit can be used even outdoors on a field day or a hiking. It is not waterproof, of course, but it was not the goal anyway.

I went for 50 ohm BNC connectors, because most of my cables and receivers use that.

The components are a tight fit, so I had to be careful to put the holes exactly to the right place, which was not always easy. There was no room for much play. There were only millimetres between the components and the box. Also, the axles of the switch and the capacitors made it nontrivial to mark the precise location of the mounting holes.

It cannot be seen, but it stands on those self-adhesive rubber feet. It is a really nice addition, it greatly hepls to keep the unit in place when a bit heavier cables are attached. It also protects the paint on your receiver if you put the ATU on the top of it.

This little gadget works pretty well. I tested it using a random long wire and a RTL SDR receiver. Seeing a wide spectrum is very useful, you can see how the noise floor changes as the resonance sweeps through as you adjust the tuning. The tuner adds quite good signal boost in most cases. It also helps reducing receiver overload, since it acts like a (not too sharp) preselector, suppressing strong signals out of the band.

Links

[1] What does an Antenna Tuner do?

Tag: building

Building the PA0RDT Mini-Whip

T-Match Antenna Tuner for Receiving