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====== SDR Web-based Shared Access Receiver (RX) ======

===== You may already have noticed that... =====

...we have SDR receiving (RX) capability set up. 

  * For educational purposes: for example when holding courses or open-door days and projects.
  * For test purposes: for example to test our RF transmissions (TX).

The SDR receiving end of our shack is composed of 1 receiver.
Our "Kiwi", for High Frequencies i.e. Short Wave, a KiWi SDR v.1.

Work is in progress to add VHF:
Our "Berry SiDeR", for VHF, with a single RTL-SDR attached to a Raspberry Pi (4 Model B) running OpenWebRX, is under construction.

Frequency ranges
  * KiwiSDR: 10 kHz to 30 MHz (VLF-HF spectrum) - with optimal range limited to between 1.6 MHz and 56 MHz due to the Diplexer (see below).
  * 
  * RTL-SDR: 24 MHz - 1766 MHz (HF-VHF-UHF spectrum) - with optimal range between 76 MHz 470 MHz due to the Diplexer (see below).

Both will be connected to the same antenna through a diplexer (which on a shelf, it is next-up).

The concept is "Web-based Shared access":

  * "Web-based": users do not need to install anything on their PC, all that's required is an HTML5 capable browser.
  * 
  * "Shared access": Multiple users can use the same receiver at the same time, and can listen to different frequencies and modes.

The number of concurrent users are restricted on OpenWebRX, typically to five or six for a single 2 MHz RTL-SDR.


OpenWebRX is a python based software that allows to run a web server to which multiple users can connect to the RTL-SDR and listen to it through a web interface.


Short Waves (HF) and Medium Waves (AM) can already be listened to by accessing:

https://sdr.chaospott.de/

http://kiwisdr.local:8073/

Wavelengths of ten meters to one meter (VHF) can be listened to by accessing:
(Under construction - http://openwebrx)

===== About the Antenna =====

Since 2024 a "Mini Whip Active Antenna".
Planned to be replaced with an Eggbeater antenna in 2025, which has a lower noise floor compared to the Mini Whip, it is located on the side of the building, it's best exposition is along the North South axis.

The Eggbeater Satellite Antenna has a Ground Plane and is resonant on 70 cm.
We have tested it and has proven itself to be a great omnidirectional base station antenna receiving antenna for all Amateur Radio, Broadcast bands and many services. 
The ground plane elements under the Eggbeater increases the overall upward gain of the antenna, useful for hearing Satellites and the International Space Station.


===== About the Diplexer =====

DIAMOND MX-62M Diplexer for the ranges between 1.6 MHz and 56 MHz, and between 76MHz and 470 MHz.
Allows to connect one antenna to two receivers.

The diplexer limits the optimal ranges of the SDRs to an overall smaller bandwidth, 
this limitation can be overcome by bypassing the diplexer for the required time, at the cost of using one SDR at a time.


===== About the KiWi SDR SW HF Receiver =====

It comes with a GPS Antenna for precise timing.
The KiwiSDR is a software-defined radio that attaches to a Seeed BeagleBone Green (BBG) embedded computer.
It is a standalone device that allows access through LAN and optionally through the Internet.

===== About the OpenWebRX SW running on Raspberry Pi =====

OpenWebRX is a multi-user SDR receiver that can be operated from any web browser without the need for any additional client software. 
Requires a computer, an SDR device and network access. 

OpenWebRX is designed for public shared access.

OpenWebRX
https://www.openwebrx.de/

Images from 
https://www.openwebrx.de/download/rpi.php


===== Some additional Resources: =====


OpenWebRX: A Multi-User RTL-SDR Receiver with Web Interface

https://www.rtl-sdr.com/openwebrx-an-multi-user-rtl-sdr-receiver-with-web-interface/

https://www.openwebrx.de/about.php



A list of currently active WebSDR servers is on

http://www.websdr.org/


===== Future developments =====

A project for future development could be to design and make a home brew Diplexer with wider bandwidths.
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