Building on the work of PA0NHC en LZ1AQ (and as a kick-off in this blog) a design for a broadband magnetic loop antenna amplifier. It consists of two printed circuit boards. The first for the actual amplifier, and a second, as an RF/DC splitter placed near the receiver. I moved a year ago, which roughly coincided with my retirement. Two reasons to pick up my old hobby of amateur radio again after years. It would have been no different at my old house, but what immediately struck me once I had installed a dipole antenna was the extremely high interference level. Apparently a sum of all solar panels, power line adapters and LED lighting drivers that are commonplace today. All in all it produces a quite interesting radio spectrum but pretty unusable for HAM radio. All this misery didn't seem like the most appropriate reason to honor all my new neighbors with a first visit. Before saying goodbye to a once beautiful hobby, the aim was a last attempt to find some useful (FT8) signals in this huge mountain of QRM / EMC / EMI / RFI.
Most interferences from the immediate environment manifests itself in the E-field (the electric field) and much less in the H-field, the magnetic component of the electromagnetic field. Signal strength is not the primary concern on the shortwave bands; but the signal-to-noise ratio all the more. In that case, an antenna with large dimensions does not make much sense. Also for de 80 en 160 meter band a loop with a diameter of about 1 meter is sufficient and a larger one hardly yields any profit. The electrical component of the field can be kept outside as much as possible by shielding the loop, for example by using an RG213 coaxial cable whose shielding is interrupted only in the middle. The shielding can even be omitted entirely by using a differential amplifier which is aimed at transferring the current through the loop and at the same time suppressing the voltage on the loop as much as possible. In that case a copper or aluminum tube can be used. In addition to its compact size, the magnetic antenna also has the advantage of a strong directional effect, so that man made noise can be eliminated as much as possible.
In the past I already gained experience with shielded brass loops in professional (OAR) direction finding equipment and it seemed like a nice idea to pick up that thread again and put some things into practice again. The intention is to continue to use the dipole for transmission, but to fall back on this H-field antenna for reception. An alternative, to use the loop for both transmitting and receiving is possible, but I've decided against it. This is because the loop must be tuned in that case. That is constructively a lot more difficult to realize because of the high voltages involved and the need for expensive variable high-voltage (vacuum) capacitors. In addition, it results in a horribly small bandwidth. And that, in turn, immediately excludes the use of SDR (software defined radio). And, the dipole remains excellent for transmitting. A little more or less interference in the immediate vicinity makes little difference!
A search on the internet taught me that many useful designs can be traced back to either the Wellbrook (or Wellgood), the Norton Lossless Feedback Amplifier or the work of PA0NHC en LZ1AQ. My interest was mainly in this latter design, although it struck me that the success of it has led to a considerable commercialization in which the transparency - against the spirit of amateur radio - has certainly not improved. I have therefore chosen to use the original design as a starting point and tried to create a greatest common denominator that responds as well as possible to the developments (and derivatives) that have arisen since then.
- I have tried to make a PCBt design that keeps all doors open as much as possible, especially on the critical parts: the transmission line that can be used, the impedance transformers and choice of ferrites, the baluns (common mode chokes) and the transistors.
- No irreversible choice for UTP/STP cable and its related RJ45 jacket (of which I have encountered many complaints due to long-term corrosion and poor shrinkage of RJ45 connectors from the original design, but instead an "agnostic" KF141r terminal block. Reasonably reliable for this purpose and extremely flexible, with the immediate option of using RG58, RG174 or RG196, whether or not supplied with a separate power cable or fed over coax.
- Host a range of alternatives to the transistors by supporting three (!) different PCB footprints. The 2n2222A alone is available as PZT2222A (SOT223), PMBT2222A (SOT23), PN2222A (TO92), 2N2222A (TO18). I have merged the SOT23 and SOT223 footprint into a new ine, while TO92 and TO18 (through hole) transistors can be used in a 4-pole TO72 footprint. Sockets are also available for this.
- All transmission line transformers and (optional) common mode chokes (CMCs / baluns) in sockets so that the PCB does not suffer when these parts are exchanged for the umpteenth time. Ultimately, they can still be placed directly in the print.
For all backgrounds of the I refer to original publications. I mainly focused on finding a nice middle ground between the commercially available products and the spins I encountered on Veroboard.
Magnetic Loop Interface
There is not much to say about the accompanying interface. It is also equipped with a KF141 terminal block to be able to use different (Coax, UTP or STP) cables. There is an RF/DC splitter in case it is chosen to feed the amplifier over the coax. Furthermore, a broadband transformer to deal with the impedance matching between the antenna cable and 50 ohms, also on this side there is the option for common mode chokes, both in the signal and power lines. Finally, a zener diode and fuse for simple overvoltage and reverse voltage protection. The BNC Connector in the photo is still missing pending shipment from China!
Below you can find both schematics, the print layouts, 3d PCB animations and some photos. It also contains some examples of transformers with different footprints, the location for the TO72 transistor sockets, the ring cores and 'double aperture cores' (pig's noses), powder iron cores and air coils at the entrance.