Monday, April 24, 2017

rtl-sdr direct sampling on linux

There's a nicely put together rtf-sdr receiver box available on Ebay from China for about AU$45 with the direct sampling modification for HF already done for you and separate inputs for HF and VHF+.

There are several sellers so shop around.

You can see it on the right here and the box above it. (Click to enlarge)

Internally, it's an RTL2832U.

On my recent weekend visit to Kevin, VK2KB, I helped to get it going under Windows using SDR# software and this prompted me to get mine going at home under Fedora linux.

One mistake I've made before is that some USB cables seem to come wired only for charging and don't wire up the other USB connections. I wasn't seeing the device in lsusb and when I tailed the log (which, incidentally, has changed under Fedora since I last looked, to be sudo journalctl -f), there was no chatter as the device was plugged in. Trying alternate cables fixed this.

It turns out that GQRX finds the device just fine. To switch on direct sampling from the Q input, you set it like this:

CubicSDR saw the device but couldn't receive until I blacklisted the built-in TV reception driver by creating a file /etc/modprobe.d/blacklist.conf and entering blacklist dab_usb_rtl28xxu in it. Then I rebooted.

After that, the CubicSDR device config looks like this:

And it receives 40m SSB like this:

(There's not much on at this time of day).


To answer Carlo's comment below. Here's the wiring from dongle to board (marked in red).

Google reader alternative - tt-rss review

I still miss Google Reader which went away in 2013. It was an efficient way to catch up on all the RSS/Atom news feeds I subscribe to in a web browser.

The key feature I need is that whatever I use remembers what I've read and can be accessed from multiple devices that stay in sync.

When Reader went away I subscribed to FeedWrangler which works well but lacks a decent web interface so must be accessed from a macOS or iOS app such as Reeder.

Recently I've installed Tiny Tiny RSS, also known as tt-rss on a Fedora linux computer in my home network. The installation guide is good but I ran into a few things that were not right on Fedora 25.

I chose to use mariaDB (mysql clone) as the database although they recommend Postgresql.

From memory, the issues I encountered were:

  • PHP couldn't connect to the database due to secure linux. 
  • httpd couldn't write to the tt-rss web directory for config and cache writing. Needed # setsebool -P httpd_unified on
  • The name of the mysql driver for php is different, I needed # dnf install php-mysqlnd
  • # dnf install php-pdo php-gd php-mbstring php-common php-apc
To get my feeds imported, I exported the OPML file from FeedWrangler and imported it into tt-rss by going to Actions, Preferences, Feeds tab and there's a long horizontal panel called OPML that's a button for choosing the file to import.

The user interface is nice and clean and I particularly like how it responds to up and down arrow keys to roll through the stories in the scrolling panel.

Updating feeds

The authors suggest leaving a process running to do the updates (it sleeps periodically) but I prefer a cron job as follows:

*/30 * * * * /usr/bin/php /var/www/html/tt-rss/update.php --feeds --quiet

Anyway, so far I like it a lot and my plan is to eventually move it to a virtual server in the cloud. It looks like you can aggregate all of your unread items into a new feed and presumably read that from an app while out - this could possibly replace the commercial service I'm using at the moment.

Saturday, April 15, 2017

QRPver pocket sized transceiver review

The QRPver is a US$80 1W QRP transceiver that is perfect for running digital modes such as PSK31. It connects to a computer via analog audio cables and has internal VOX so there's no need for an interface box or serial rig control.

It's about the size of a pack of cards as you can see in the photo on the right. 

I'm using it with Fldigi running on Fedora with a generic USB audio dongle. I did need to use the Sound application to increase the audio level a bit to get the VOX to trigger when I transmitted.

Note that the audio sockets are stereo sockets with left and right tied together so you do need to use stereo leads or you'll short audio to ground.

I ordered my QRPver for 14.070 (there are a range of bands and frequencies to choose from). 20m PSK31 was very lively a few years ago but at the moment seems rather abandoned for some reason, perhaps everyone's running JT65?

First contact was pre-arranged with Stephen, VK2BLQ who is pretty near by. Today VK5WOW popped up being operated by Grant, VK5GR who made a few comments about the signal not being all that clean and also it drifted about 20-50Hz during a longer over.

The QRPver draws just 20mA on receive and 650mA or so on transmit so it would be great for portable digital operation. The receiver seems sensitive enough and while not a perfect signal on transmit - something that might be because I need to tweak the level a bit or possibly there's some RF getting in to the audio - I think it's a great gadget for the travelling ham.

Wednesday, April 12, 2017

Switching from Ubuntu to Fedora Linux

Ubuntu has been great for me over about the last ten years. Prior to that I was a RedHat/Fedora user but when I joined a company that was all Debian based I made the switch and learned the joy of apt.

Recently I've run in to some hitches with Ubuntu. It throws up crash handlers from time to time. The ham radio applications offered in the official archives are often very old and recently I found that gnuradio installed with apt runs but crashes in use. (Building from source fixed this).

The announcement that Ubuntu is abandoning the Unity desktop and switching to Gnome prompted me to give Fedora another go (it's already on Gnome).

Things are smoother since last time I looked. On macOS the installer is downloaded in the form of a "media writer" application that downloads the image you need and creates a bootable USB key - much nicer that the dd business I've been doing.

Gnome is very slick these days and on the three machines I've tried so far everything mostly worked out of the box. On an HP laptop it didn't include the Broadcom wireless driver but that was fairly easily fixed. While not much is installed by default, I quickly wanted the nano editor. Amazingly hackrf was built in.

I'm quickly up and running with fldigi, wsjt-x, gnuradio, gqrx, and all the normal unix goodness. The work done by Ubuntu has raised the standard of all desktop linux distributions (not to mention the whole cloud and virtualisation world) and I'll miss Ubuntu for sure.

Fedora notes for an Ubuntu user

Instead of apt (or apt-cache and apt-get) there's something called "dnf" that replaces "yum". For some reason I find it hard to remember "dnf" and sometimes have to type yum so it will remind me.

sudo dnf update
sudo dnf search xxx
sudo dnf install xxx

I've eventually figured out that DNF stands for "Dandified Yum" but that fact is missing from the man pages. Under the hood it's Linux and all the tricks I've learned are the same regardless of the distribution.

Saturday, April 08, 2017

First transceiver with built-in FreeDV

Fantastic to see Chinese ham radio gear for HF increasingly appearing. This one "RS-918SSB HF SDR Transceiver" caught my eye because the operating modes listed are: "SSB(J3E),CW,AM(RX Only), FM, FREE-DV"

That's the first time I've seen a radio with built-in FreeDV.

From the site:

Main Functions

Spectrum Dynamic Waterfall Display

Multiple Working Modes: Receive Mode, Transmit Mode, TUNE Mode, VFO Mode, SPLIT Mode

DSP Digital Signal Processing Noise Reduction

Automatic Notch Filter

Humanized Interface Color Display

Receive Fine Tuning Function, Changeable MIC Gain Value

VCC Power Supply Voltage Indication Table

Transmit Signal Strength Display Table

Multifunctional Instrument: SWR Standing Wave Ratio Meter, AVD Audio Frequency Meter, ALC Signal Modulation Meter


Frequency Range - RX:1.8-30MHz TX:All HAM HF BANDS

Operating Mode - SSB(J3E),CW,AM(RX Only), FM, FREE-DV

TX Power - 5W (Standard, DC 13.8V) , 15W (Full,DC 13.8V)

Receiving Sensitivity - 0.11~0.89μV(RFC 50-20)  

Minimum Frequency Step - 1Hz

Operating Voltage - DC9~15V

Antenna Impedance - 50Ω

Frequency Stability -  ±1.5PPM @ Power on 5 Minutes (Standard) ; ±0.5PPM if Optional TCXO Used

Product Dimension (W × H × D) - 215×74×62mm(Mounting Bracket Included)

Weight - 623g

No idea of price yet and it looks pretty clunky but great to see FreeDV built right in.  (If there are others please let me know).

Sunday, April 02, 2017

Built a 2m J Pole out of copper pipe

Just for fun I tried receiving WSPR on 6m. I'm using a half G5RV but at the end of a long run of RG58 so losses at 50MHz are likely to be significant. Quickly VK2HC was received and Peter emailed to say hello.

I mentioned my poor 6m setup and Peter urged me to try a copper J Pole design for 6m. Only when thinking about it at the hardware store did it dawn on my that the length of over 4m wasn't going to support itself and so I chickened out and decided to try the same but for 2m.

I purchased lengths of 15mm copper pipe, an L piece and a T piece along with a small gas blowtorch. The Ham Universe J pole calculator gave me the lengths. Unfortunately I mistakenly cut too short and had to extend later but that wasn't difficult - Lesson: always cut antennas a bit long at first.

Soldering the joints is done by heating with the blow torch until solder melts on contact and then I let that "wick" in to the joint. Seems to work very well. Hose clamps were used to attach the coax (some waterproofing is going to be needed).

Front of house antenna farm: 1090Mhz antenna, 6m slim jim (on a squid pole), 2m J Pole (left to right):

It tunes up pretty well and gives an SWR below 1.1.