Single Antenna System

This is a Single Antenna 10kHz-30 MHz receiving system, generally useful 1kHz to 200 MHz. It is a larger version of the Field Probe and the original PreampA/2m-dipole projects - a 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 site 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 ground or radial system like 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 the ITU rather than by unwanted local noise sources due to a particular location.

Caution: The broadband Antenna Systems described here and on associated pages MAY NOT WORK FOR YOU !  They 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 this process.

A high impedance, high CMRR preamplifier is mounted inside a 3D printed plastic housing near the middle of a 7m or 10m telescoping fiberglass mast and fed with 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) allows the nearby CAT5 cable (also not shown) to run along the mast, separated only ~50-100mm from the conductor and 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.

Clips are used along the mast to hold the conductor and the CAT5 in proper position. If running a feedline this close to the antenna seems contrary to what is believed about parallel transmission lines, please review the supporting theory presented in the broadband receive antenna systems overview.

Steps in the inside diameter of the clips allows them to fit at mast section intersections. Part of the diameter fits over the lower and larger section while the remainder fits over the smaller section above it. Clamping this way adds extra assurance that the mast won't loosen and collapse.

The SA Preamp interfaces by way of a Shack Board PCB while delivering balanced RF output from one of the 100 ohm twisted pairs. Remaining pairs of the CAT5 are used to supply power to operate a low-capacitance mechanical relay which can short the dipole terminals to verify CM rejection of the entire system and to supply power. One switch on the Shack Board temporarily energizes the B pair while a second switch allows turning off 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 either at the antenna or after the preamp.

The SAPreamp is mounted inside a 3D printed enclosure with antenna wires soldered to pads on the PCB. These wires exit through the enclosure walls 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.