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 antenna 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 pieces 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..

A newer version of the original very broadband design is now provided in the Material List.  This allows optioning for either High Impedance or Medium Impedance preamplifier inputs.  Both options are followed by CAT5 feedline and the ShackBoard version 2.

How this Receive-Only Antenna System is DIFFERENT from traditional Antennas

In order for the Single Antenna system (SAS) to become a useful and valuable solution capable of achieving excellent results at a particular location it is important to recognize some ways it is different from a traditional antenna.

Fundamentally the High-Z  SAS operates as a probe NOT as a conventional matched antenna.

A matched dipole antenna has "joined monopoles" where current passes through a tip at the center to the other element. It is operated as a resonator with reflected energy from each tip re(reflecting) from the other. A probe antenna is one with unterminated monopoles where no current is coupled from one monopole to the other or into a termination. It is not a mechanism that transfers power from or to an incident wave in space. For this reason it is only used for receiving.

The SWTL understanding of a dipole offers a different idea of what is occurring in this case of two colinear elements operated as a probe.  For this usage, the monopoles are still considered as 377 ohm SWTLs "probing" a region of space but because there is no current shared between the two monopoles the structure does not operate in the same manner nor have exactly the same pattern. When viewed from within a 73 ohm environment a traditional matched halfwave dipole can be viewed as a resonator having a Q of ~10 . This Q prohibits its use as a well-matched multi-decade bandwidth  antenna. In comparison, two monopoles separated and acting as a probe are not terminated so have a Q of zero, if the concept of Q even applies when no power is supported. No significant current ever flows from these elements into the circuits that follow. No significant power transfer is involved. Active circuits which follow provide energy to produce output power rather than the incident wave being detected.

For a probe antenna like this one, the differential voltage at the center is the same as the differential voltage between the tips. As the electrical length of the structure increases, the  apertures at the tips that are associated with coupling to a wave in space separate. Voltages presented at the  central high impedance SAPreamp input first increase from the short dipole case where there is significant aperture overlap to a maximum at large electrical length where there is complete separation. As they become non-overlapping, the voltage alternates between a maximum and zero as the phase of the voltage from a boresight intercepted wave at each tip changes. Thus, although the pattern varies wildly as the length is increased and apertures move apart,  the total effective aperture increases only to twice its initial value. The main lobe alternates from a maximum at boresight for odd half-wavelengths to minima near even multiples of a wavelength. At these even multiples the main lobe splits and has one or more maxima.

A simplified block diagram of an entire broadband SA & SDR system follows. This includes not only the physical antenna conductors but simplifications of the active electronics involved.

 

High Impedance Options

In the  high impedance option  a 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.  Operation from VLF through VHF. operation is achieved  due to the very high impedance OpAmp unity gain input stages.  Even though the attached dipole is electrically extremely short at the lowest frequency limit of the system, the high impedance allows relatively efficient transfer of the small signal voltage to the buffer amplifiers while offering suitably low noise floor that permits approaching the targeted ITU limits.

This system utilizes  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.  In addition, steps are taken to avoid shadowing of the monopole's aperture located near its tip.

Medium Impedance Options

The Medium Impedance option does not provide the same lower frequency limit.  It can still operate up through VHF and it can do this while providing as much as 10 dB improvement in strong signal environment protection.  Input signals up to about 2 V p-p  may still be tolerated without overload. When used in fully differential mode, all the way through the ShackBoard circuitry, it can provide even lower noise performance than the high impedance mode.  This is due to the fundamental specifications of the ADA4930 line driver when compared to the ADA4817 buffers present in the high impedance mode.  So while it does not have coverage down to VLF it is  able to tolerate antennas and environments producing three times the signal voltage.  The Medium Impedance mode can work with the same vertical dipole as the high impedance mode and may also be optimum for use with Traveling Wave antennas such as the terminated Beverage, terminated dipole (balanced Beverage), Loop-on-Earth and some Vee antenna designs.  All these are  inherently high-pass in nature so in practice the lack of VLF/LF response may not be a serious limitation.

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. The SAPreamp and Shack Boards are interconnected with standard CAT5 cable.