Over on YouTube the Scanner and Sdr Radio channel has uploaded a video comparing four different brands of HF wideband loop antennas using an SDRplay RSPduo. The loops he tested include the cheap Chinese MLA-30 (~$40), the Cross Country Wireless (CCW) loop ($70), Bonito ML200 (~$442) and the Wellbrook 1530LN (~$305).
The MLA-30 was slightly modified with the cheap coax removed and a BNC connector added. Each of the antennas used a wire loop with diameter of approximately 1.6m, except for the Wellbrook which has a fixed size solid loop of 1m.
The tests compare each loop against the Wellbrook which is used as the reference antenna. In each test he checks each HF band with real signals on the RSPduo and compares SNR between the two antennas.
The results show that the two expensive antennas, the Bonito and Wellbrook, do generally perform the best with the lowest noise floors, but surprisingly the MLA-30 actually performs very well for it's price point, even outperforming the Wellbrook reference on SNR in some bands. We note that some of the improvement may be due to the larger 1.6m loop size used on the MLA-30, compared to the 1m loop on the Wellbrook.
Also we note that it can be hard to compare antennas in single tests, because the differences in antenna radiation patterns could be favorable for some signals, and less so for others, depending on the location.
SDRPlay have recently published a video demonstrating how the new RSPduo diversity feature in SDRUno can be used to cancel local interference. The SDRplay RSPDuo is a 14-bit dual tuner software defined radio capable of tuning between 1 kHz - 2 GHz. It's defining feature is that it has two receivers in one radio, which should allow for interesting phase coherent applications such as diversity. The RSPDuo's diversity feature allows us to either combine two antenna signals together for an up to 3 dB increase, or for removal of an unwanted noise source via subtraction of signals.
In the video they show a broadcast AM signal that has it's SNR reduced by being on top of a local electrical noise source. The use a Bonito Mega-dipole on tuner 1, and a Bonito Mini-whip on tuner 2. The Mini-whip appears to receive the local interference stronger, so can be subtracted away from the Mega-dipole's signal with the diversity function. The result is improved SNR, and the noise is almost entirely cancelled.
There are 2 very practical applications for diversity software. The first is MRC (Maximum Ratio Combination) Diversity which, in order to be effective, needs two antennas presenting the same signal with some degree of diversity. Then there is this second impressive application which is becoming more and more useful due to the growing number of domestic sources of interference.
This is possible in an RSPduo, due to the coherent nature of the combined tuner streams being presented to the computer for processing.
Using Diversity in SDRplay's SDRuno to Cancel Local Interference
A couple of days ago we ran a post noting that the Airspy HF+ WRTH review was now available for public viewing. Now thanks to Jon Hudson of SDRplay for letting us know that the SDRplay RSPDuo review from WRTH has also been released for public viewing (pdf). The SDRplay RSP Duo is a 14-bit dual tuner software defined radio that is capable of tuning between 1 kHz - 2 GHz, with two separate 2 MHz bandwidths tuned to anywhere within that frequency range.
The review provides an overview of the RSPduo noting it's various features and discussing the SDRuno software. They also note that diversity reception would be an excellent application for a dual tuner SDR, but SDRuno does not support this feature as of yet. In their tests they also mention how they found very few overloading problems.
The RSPduo is the latest product from SDRplay, and it's main defining feature is it's dual RX channels. The two channels allow you to simultaneously listen to two stations that may be at opposite ends of the spectrum. Of course the same could be done cheaper with two separate RSP1A devices, however the advantage of the RSPduo is that the two channels are phase coherent. This opens up a lot of technical possibilities such as active noise cancelling/spatial filtering of an interfering signal or unwanted RF noise.
The basic idea behind phase coherent noise cancelling is that you have one antenna pointed towards your signal of interest, and the other pointing towards the interfering signal or coupled to the unwanted noise source. The unwanted signal/noise can then be subtracted from the signal of interest, resulting in a cleaner signal. This can only be done if the two channels are phase coherent like they are on the RSPduo.
Over on YouTube and his blog, ICAS Enterprises has been experimenting with noise cancelling on his RSPduo [Part 1] [Part 2]. The blog is entirely in Japanese, but all the relevant screenshots are in English, and we have provided Google Translated links. SDRplay have also provided a description of the process on their blog. The idea is to use Linrad to process the sound output of the two RSPduo channels from SDRuno. Linrad has capabilities that allow you to subtract two signals from one another.
In his experiments he receives a known 50 MHz CW beacon, and then injects an interfering test signal into the system. Then using Linrads 'signal adaption' feature he is able to completely remove the interference.
SDRplay have released a new product at this years Hamvention called the "RSPduo". The product uses the same technology as their previous RSP models, but this time includes a phase-coherent dual tuner architecture which allows you to tune to two completely separate 2 MHz bands of spectrum simultaneously. The RSPduo retails for US $279.95 + shipping. Their press release reads:
Today at the Dayton Hamvention, SDRplay is announcing the launch of a new Software Defined Radio product – the RSPduo.
The RSPduo is a radical new addition to the RSP line of SDR receivers from SDRplay. Architecturally, it is different from any previous RSP in that it features dual independent tuners, both piped through a single high-speed USB 2.0 interface.
The SDRplay RSPduo is a dual-tuner wideband full featured 14-bit SDR which covers the entire RF spectrum from 1kHz to 2GHz giving 10MHz of spectrum visibility. Initially using Windows based ‘SDRuno’ supplied by SDRplay, you can simultaneously monitor two completely separate 2MHz bands of spectrum anywhere between 1kHz and 2GHz.
Superficially the RSPduo looks identical to the highly popular RSP2pro and will be able to operate in a very similar way. However, it also allows a completely new and exciting set of usage scenarios such as:
1. Simultaneous monitoring of two widely spaced bands – e.g. 40m (HF) and 2m (VHF) 2. Mixing and matching applications simultaneously – e.g. ADS-B and ATC scanning 3. Phase and time coherent demodulation of two receivers
Scenario 3 is very difficult to achieve with two separate USB devices because of the uncertainty of USB latency. The RSPduo overcomes this limitation because all traffic goes through a single USB interface, thus enabling the possibility of the development of various types of diversity demodulation such as: spatial, frequency and polarisation which can bring huge benefits in terms of improved performance.
As well as adding a second independently controlled tuner, which in itself, offers a whole new set of exciting usage possibilities, the SDRduo features 14bit ADCs and a completely re-designed RF front end. These changes provide better RF selectivity and even more dynamic range, offering outstanding performance under extremely challenging reception conditions. The combination of performance and features makes the RSPduo our highest spec RSP yet and sets a new benchmark in the sub $300 SDR market.
Lots of great new videos are appearing on YouTube showing the Dual Tuner RSPduo in use. Just search for RSPduo in the YouTube Search bar.
While it appears that it will be a useful tool for multi-band monitoring, what we're looking forward to most are the phase coherent applications. Thanks to its phase coherence and predictable USB timing implementation, the unit should be very useful for passive radar and spatial filtering techniques, and we're hoping that SDRplay will be working on software for at least the latter. Multi-channel phase coherent applications are probably going to be one of the the next big developments in the SDR space so it's good to see low cost hardware with this potential coming out now.
We'll be reviewing the unit ourselves within the next month, so keep an eye out for that post!