Single Antenna System(SAS)

This is a Single Antenna 10kHz-30 MHz receiving system, generally useful from 1kHz to 200 MHz. It is a larger version of the Field Probe and the original PreampA/2m-dipole projects.  PReampA and SAS are low noise antenna system capable of approaching or achieving the ITU "Quiet Rural" regional noise limit over the entire LF-HF range when and where a suitable situation exists. Like the other n6gn OSHW broadband receive system designs it is a highly symmetrical/differential probe. Because it uses a symmetric dipole rather than a monopole referenced to earth or to a radial system, as are many commercially available broad band antenna systems, it can provide much higher rejection of common mode feedline and ground noise that can easily reduce system performance. Also because it is a probe rather than a resonant/matched structure it is an extremely broad band system that can provide effective coverage from audio frequencies well into VHF.

A goal of this system is to approach performance dictated by regional noise limitations as described by Rec. ITU-R P.372-16 rather than by unwanted local noise sources present at a particular location.

Caution: The broadband antenna system described here and on associated pages MAY NOT WORK FOR YOU ! It  cannot operate well in every possible environment. Even with modifications and adjustments being identified and described and with very considerable extra effort there will be situations too difficult to manage. An entirely different receive system approach may be required.

This kit is not a simple solution by itself. Simply obtaining or building all the kit necessary to install the hardware is required but not sufficient. Proper deployment of a complete system is at least, if not more, important than the equipment itself. Not understanding and following this advice may result in wasted time, energy and money !!! Background for this kind of wide-band receiving systems is provided in a Broadband Receive Systems overview. PLEASE read that background and also Deploying a Single Antenna System before beginning the building process.

In the SAS a high impedance, high CMRR preamplifier is mounted inside a 3D printed plastic housing located near the middle of an  insulating  fiberglass mast and fed with standard CAT5 cable as shown below. This antenna relies on the SWTL model of a dipole and through the use of small .5mm diameter conductor (not shown) can allow the nearby CAT5 feedline (also not shown) to run  parallel along the mast for most of the monopole  conductors length before finally exiting near the base. This can be done without upsetting antenna balance and symmetry which would otherwise unbalance the structure and possibly raise common mode noise ingress and decrease the capability.  Steps are taken to avoid shadowing of the monopole's aperture located near its tip.

Scattering & Shadowing

A probe antenna is categorically different from a matched antenna in that no power is extracted from an incident wave.   Although no power is extracted, the presence of such a probe element does deform the nearby  electric field of an incident wave which is to be received.  Put another way, the presence of conductive material causes scattering.  For both matched and probe antenna  types there is a region of influence for such an element generally called an aperture.

Empirical measurement

For the SAS use case, induced current in the CAT5 acting as a feedline will produce re-radiation that counters the impinging electric field near it and casts a shadow that can overlap the probe's aperture. Rerouting the CAT5 or reducing current within it can reduce this interaction.  Empirical measurements have shown that a few percent degradation in WSPR spots is produced when the CAT5 simply exits vertically, parallel and close to the dipole (CAT5 90)  but not far enough away to escape the shadowing as compared to spots produced when the CAT5 comes away at an angle (CAT5 45).  This slight rerouting seems to be enough to cause the interaction to be negligible.    For this reason, a revised mounting technique different from the one shown below is being followed whenever possible.

Mounting and Interconnection

3D printed plastic clips are used along the mast to hold the conductor in proper position. Clamping this way also adds extra assurance that the mast sections won't loosen and collapse.

The SA Preamp interfaces to an SDR or other receiver by way of a ShackBoard  while delivering balanced RF output from one of the 100 ohm twisted pairs. Remaining pairs of the CAT5 are used to supply power, and also  to operate a low-capacitance mechanical relay which can short the dipole terminals to verify CM rejection of the entire system. One switch on the ShackBoard temporarily energizes the B pair while a second switch allows removing  bias to the high impedance input buffers. These are used as diagnostics and verification that the signals being sent to the receiver are indeed differential originating at the dipole and not common mode ingress  in or after the preamp.

The SAPreamp PCB is mounted inside a 3D printed enclosure with antenna wires soldered to pads on the PCB. These wires exit through the enclosure walls, run along the mast through the clips and terminate at the top and near the bottom of the mast. The CAT5 cable exits downward from the enclosure bottom and is clamped by the enclosure cover which has a silicone rubber gasket. The result is a water resistant housing for the electronics.

The preamp design uses a ADA4930 in a transformerless output configuration to drive one pair of the CAT5 cable.