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Field Probe Pre-amplifier

This project is for a small handheld portable field measuring device, particularly for locating, identifying and measuring noise levels in conjunction with a TinySA or (especially) a  TinySA "Ultra" hand-held spectrum analyzer. It is intended to help identify and understand a particular candidate antenna site for amateur radio reception.

It is a member of a class of receive antenna systems using non-resonant and therefore broadband antennas. Included in this are the Hybrid Antenna System which uses a similar PreampA electronics for the Loop-over-Earth and Beverage "Traveling Wave" antenna types as well as the Single Antenna receive system.

High Impedance Field Probe pre-amplifier Board.

Here's a portable arrangement using  the TInySA "Ultra"portable spectrum analyzer with a short dipole

Calibration

When used with a  properly calibrated receiver or spectrum analyzer  it can be a useful tool for measuring local receive system noise temperature and identifying some of the contributors that raise it.

Calibration is based on a theoretical model for a short dipole, length L in meters, providing impedance  Z = Ra + jX
Z = 20*((pi*L/lambda)^2) - i(120/(pi*L/lambda))*(ln(L/(2*a))-1)
Understanding Ra to be a source of thermal power kTB = -174 dBm/Hz at K=290 Kelvin in B = 1 Hz bandwidth.

This source is modeled as input to the FP preamplifier which results in the output power shown in silkscreen on the FP PCB for reference.
An assumption is that the dipole elements are set to equal length for a total dipole L of 1 meter and held vertically polarized.

As a handheld tool it is very useful to examine SNR as polarization and azimuth are varied.  The device has high axial ratio, that is, an incoming vertically polarized wave can easily be nulled by going to horizontal polarization.  Sometimes local and near-field noise results in highest SNR of DX signals occurring at neither of these polarizations, even recognizing that there is often significant decrease in levels due to earth absorption when oriented horizontally which also tends to push the arrival angle overhead.

TinySA Ultra BW is set to 300 kHz for a 0-30 MHz span resulting in a 10*log10(3e5) = 55 dB offset such that kTB will display as -174 + 55 = -119 dBm. The TinySA can be set to display Marker noise power directly in dBm/Hz.

A starting point for settings might be:

The FP PCB has a graph of the ITU  "Quiet Rural" noise level so that  measured noise may be read directly from the TinySA in comparison to both ITU and thermal noise levels as location is changed.

Assembly

Since there is no separate enclosure, once the assembled board arrives solder the 9V battery connector/pigtail to  Battery +/- wire holes on the board.  The only assembly required is to TyWrap a standard 9V battery to the back/bottom side and attach the two telescoping monopole antennas.  The calibration curve will apply only to a specified total dipole length and vertical polarization when the board is held about 1m above the earth.

Test

Verify that the test points marked,  Vp measure as marked.

For better  quick  viewing of the design, download the Kit file from the Source Link below, unzip it and drop the .sch or .pcb file onto KiCanvas from a web browser. This should permit examination of the current schematic and PCB layout. As an alternative, install KiCad, download the FP Source file (an archive),  open KiCad and  select "Unarchive" from its File:  menu.  This should bring up the entire design, both schematic and PCB and allow viewing as well as modification.

Use

A first purpose of this probe is to identify areas for siting a permanent receive antenna system with regard to minimizing local noise sources.  This involves surveying potential candidate mounting locations for minimum unwanted, often near-field, noise sources.  To the degree that a user has any degree of freedom locations that appear better or worse may be identified.  As a reference methodology, setting the telescoping monopoles to .5m each and holding the FP vertically polarized 2m above the earth seems a reasonable reference orientation.  

Once a likely antenna site is selected on the basis of minimum noise within the sub-spectrum of interest between approximately 1 kHz and 200 MHz that site may be further examined by seeking to identify sensitivity of the noise level measured to polarization and azimuth. This may best be done by focusing on the signal-to-noise ratio (SNR) of a propagated signal, e.g. a NIST Time/Frequency standard station or other easily visible signal coming and changing the FP orientation to maximize SNR.

Because the FP has high axial ratio, because it predominantly linearly polarized along the axis of the monopoles, and because other sources of signal ingress have been eliminated by the high CMRR of the design, it is possible to get a high degree of nulling of a linearly polarized interferer which compromises the SNR of the desired signal.  In this way, seeking to find a polarization and azimuth the optimizes not signal strength but SNR a better understanding of preferred orientation for a permanent installation may be discovered.  This type of "noise nulling" can produce impressive improvements in SNR in situations where there is only a single interferer.  In some situations this is many 10's of dB. 

Sources of elliptically polarized interference may need to be mitigated with a more complex system such as the Polarimeter described elsewhere. If there are multiple interferers presenting similar magnitudes even though neither may be eliminated a best compromise for communications SNR may be found. 



Material List

What you will need to build this hardware

Item Description

Provider

Source Code

Notes

Approximate Material Cost

(excludes setup fees and shipping)

Assembled FieldProbe PCB

Download Field Probe Kit

Download Field Probe Source 

 pending better calibration curve, verify C2156927  TLE2426QDRG4Q1rail splitter selection
 US$ ?

Telescoping Antenna (2)

Amazon

US$12

9V Battery&Connector



TyWrap (2)





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