Over on his channel Tech Minds has uploaded a new video where he tests the N1201SA / PS100 vector impedance analyzer. This is a US$160 antenna analyzer from China that allows you to measure the VSWR of your antennas. In the video Tech Minds explains a bit about VSWR, and goes on to show the unit in action on several of his antennas.
Compared to the NanoVNA V2 these units seems less useful with a smaller frequency range, and are also more expensive. The unit is also only 1-port, meaning that it can only do S11 measurements and cannot analyze devices like filters. But on the other hand it does come in a metal case with a battery and has a fairly easy to understand and use interface.
Easily Check Your Antennas Tuning - VNA N1201SA / PS100
As Outernet is currently having a sale and selling their their moRFeus product at only US $99 (see next post for details - or simply use coupon code "rtlsdrblog" on their checkout - valid until Saturday 09 May 18), we thought that we'd show an interesting use for the moRFeus when combined with an RTL-SDR.
Outernet's moRFeus is a signal generator and frequency mixer that can be controlled either by it's built in LCD screen, or via software on a Windows or Linux PC. It can generate a clean low phase noise tone anywhere between 85 to 5400 MHz. Because it can be computer controlled it is possible to use moRFeus as a tracking generator for characterizing filters and measuring antenna SWR. A tracking generator is just a signal generator that can be set to output at the same frequency that the measurement receiver is tuned to.
In the past we've posted a tutorial showing how to use a wideband noise source for measuring filters and antenna SWR. However, if available, a tracking generator is usually preferred over a noise source. A wideband noise source outputs high power at all frequencies, and so can easily overload an RTL-SDR causing reduced dynamic range and accuracy in measurements. This is especially the case when measuring bandstop filters as they pass all frequencies, apart from a small blocking band. Since so much noise gets through to the dongle, dynamic range is reduced.
This post shows how to use the moRFeus as a tracking generator together with an RTL-SDR for making RF measurements. This could be called a scalar network analyzer. The set up uses GQRX and a Python script, but in the future it is possible that someone may develop a standalone app.
Since the output of the moRFeus is quite strong, an attenuator is required to keep signal levels low enough to not overload the RTL-SDR.
The cheapest RF bridge we've found is available on eBay for about $7. With an RF Bridge you'll need a 50 Ohm dummy load as well to connect to the 'REF' port. Directional couplers seem to work more accurately however, and second hand minicircuits ones can often be found on eBay. A $2 TV 'tap' is also a directional coupler, and may also work, although we have not tested this.
In this tutorial we're using the method first described by 'LamaBleu' in his post to the Outernet forums. The method uses Linux and involves reading power levels from the RTL-SDR by using GQRX and it's remote telnet connection capabilities. The telnet command "F freq" can be used to change frequency in GQRX, and the command "l" can be used to read out the current power level in dbFS.
To control moRFeus we use Outernet's official "morfeus_tool", which is a command line based tool.
A basic Python script was written to set the frequency in moRFeus and GQRX at the same time. After a 500 ms settling time the power level is measured and recorded in a CSV file, then the script iterates to the next frequency. We iterate at 1 MHz intervals.
Thanks to Cary Wang for submitting news about his new Kickstarter for a $200 open hardware VNA called the xaVNA. The xaVNA is PC USB based, has a guaranteed frequency range of 140 MHz to 2.5 GHz (typical 137 MHz - 3.5 GHz), and is supposed to be a lower cost alternative to other low end PC based VNAs such as the PocketVNA and miniVNA Tiny. In addition to the lower cost, the xaVNA is advertised as being superior to its competitors as it has less trace noise and no warm up time is required, as well as the hardware and software being open source.
A VNA (Vector Network Analyzer) is a tool that can allow you to easily measure things like the SWR curve of an antenna, the characteristics of a filter, or the loss of a coax cable for example. So it is a very useful tool to have around if you are building home brew antennas or filters for your SDRs. Compared to a standard antenna analyzer, a VNA gives you complex impedance/phase information as well, making it possible to design better circuits and antennas.
Currently the closest competitors on the market are other PC based VNA's like the PocketVNA ($430) and the miniVNA Tiny ($580). The main advantage of the competitors so far is that they go down to the HF frequencies, but a stretch goal of the Kickstarter is to create an xaVNA that goes down to 10 MHz.
At the moment they only have a functional prototype with the second iteration soon to be ready. The delivery date of a completed unit is expected to be around April 2018. But as with any crowdfunded project remember to only back the project if you are willing to lose your money as there is no guarantee that a product will actually be delivered on time, or even delivered at all.
The LimeSDR is a new transmit capable software defined radio with a 100 kHz – 3.8 GHz frequency range, 12-bit ADC and 61.44 MHz bandwidth which is currently seeking crowdfunding. At the time of this post there is about four days left to reach the $500k goal, and it is only 80% funded. To try and reach their funding goal they have released another batch of discounted units which cost only $249 USD. After the crowd funding campaign the price will rise to $289/$299 USD. If the LimeSDR is not funded in time, they write that the project will unfortunately be put on hold and it’s future may be uncertain. We believe that this product is shaping up to be a very good TX/RX capable SDR, like the HackRF and bladeRF, but much better overall and for the same or even lower price.
Recently they also released some new updates that show off some LimeSDR features. In a post previously featured on our blog beta tester Alexandru showed how he was able to get the LimeSDR to transmit DVB-S2 HDTV. In later updates they showed how the LimeSDR can be used to:
Create a Vector Network Analyzer. With the help of a directional coupler the LimeSDR can also be turned into a Vector Network Analyzer to measure parameters such as gain, insertion loss, return loss and VSWR.
Build a remote radio head. By mounting the LimeSDR near the antenna and streaming the data back over an IP link, coax feed losses can be eliminated.