RTLSDR success on 23cm

The RTLSDR dongle came just before a weekend, meaning I could spare half and hour playing with it. The first kitchen table test with the stock antenna showed that it could receive local beacons in 2m, 70cm and 23cm (in line-of-sight). When I moved to the shack on the other side of the building, beacons were hidden and the DVB-T antenna was not enough anymore.

So I quickly cut a 23cm dipole and soldered it to the end of a short TV cable. With this antenna moved inside the shack room the local beacon became visible and audible again. But there was also another signal up there!

At 1297.000 MHz local HAMs were testing a new repeater. In the SDR# screenshot above you can see an FM QSO, with "Operator 2" having a very narroband signal: with a couple of clicks I fixed his S/N and his voice was loud and clear.

I sent the screenshot to the repeater designer and I got a positive feedback about this SDR setup. I also used the repeater frequency to insert the needed frequency correction into the software (+61 ppm in my case), otherwise frequencies don't match.

Having worked for users of microwave transverters, which once upon the HAM time had troubles locating their respective signals over the wide RF spectrum sea, an SDR application like this would greatly simplify "meeting" for a microwave QSO!


When is the next 23cm contest?! (tomorrow!)

USB speed, physical vs virtual (VirtualBox)

Pen drive directly attached to the host.

While the DVB-T RTL USB stick is travelling towards my shack, I am trying to understand if I can install all the software in a virtual machine instead of my powerful host system.

Main concern: would the VirtualBox USB emulation be fast enough for the raw bandwidth required by the RTL stick?

I did some tests using a new USB pendrive. On the host system the benchmark suite peaked reading at 14.8 MByte/s. This is probably enough for the RTLSDR application.

Same pendrive, benchmarked in the virtual machine.

Next I moved the USB stick to the VirtualBox guest using the standard settings. Sequential reading reduced to 7 MByte/s, and all other values were scaled down.

Pendrive in the guest with USB2 enabled.

With a bit of googling I discovered that VirtualBox (4.1.16) has an extension pack that provides emulation of USB 2.0 EHCI. Downloaded, installed, rebooted: results were disappointing! Speed reduced even further, therefore the new driver probably only adds a hardware compatibility layer and not additional speed..

Nevermind. 7 MBps could be enough for running the RTL-based SDR in a virtual machine. If not, I will take the risk of installing everything on the host and enjoy VHF/UHF scanning.

UV-3R receiver chopping

At last my UV-3R was subject to the receiver chopping effect that was described on the Yahoo group few months ago.

On the way home I met with a local HAM, then we drove our cars in the same direction, still talking over the usual UHF repeater. Our antennas were 5 to 20 metres apart in the traffic. When he was transmitting on the repeater input frequency, 5 MHz away from my VFO, I could not hear the output signal: I had to switch in reverse (next memory ;-) ) and listen to his direct transmission. My UV-3R was heavily chopping, probably because the receiver was overloaded.

When our paths diverged the reception on the repeater output gradually improved, with the chopping becoming more and more rare.

On the other hand listening to his transmission frequency did not cause any chopping in my receiver. I wonder if other name brands receivers would have behaved differently.


For those interested. I was using the UV-3R with a flexible 19cm dualband antenna on it. The fellow HAM had a V/UHF dualbander with an external antenna at 5W.

R1235 bypassed

After much thinking and no time to test the polyfuse solution, I opted to bypass the burned R1235 resistor in my FT817 with ... another resistor.

I picked an axial 16 ohm 1/4W which was at hand and soldered it in parallel to the original resistor. One lead goes directly to the MiniDin8 socket (PCB solderside). The other lead is held with an ugly blob of solder at the junction of R1235 with the bypass capacitor. I used heatshrink pipe to insulate resistor leads.

One remark about re-assembling the FT817. If you need to move/remove the bottom side PCB, the job is much easier if you remove also the top cover. That's because the mask around the microphone socket is then free to move.

No pictures taken this time.

I might get back inside the 817 and do a proper replacement, but now I am able again to continue the development of my Interactive Frequency Reader with Knob (that's display, keypad and knob all in one!).

Raspberry PI: I'm in

This morning I filled in the reservation form for a copy of Raspberry PI ARM/GNU Linux box (in other words, a small computer).

One of the two worldiwide distributors did not send me a confirmation emaiLL. The other ReSeller wrote to me that boards will be shipped out in a first-come-first-serve fashion, when they become available. There was no mention of a possible date.

Yaesu FT-8x7 CAT-to-Bluetooth dongle, a user's manual

I have composed, in the form of an online slideshow, a user's manual for my CAT-to-Bluetooth dongle (no download, no registration required). It shows technical specifications, words of caution wisdom and screenshots from the WinXP Bluetooth pairing process.

Use your keyboard left and right arrow to move between slides.

VHF/UHF SDR from a DVB-T dongle

Many people on Internet are talking and documenting "GNU radio" ability to create a SDR out of a USB DVB-T TV tuner. [I apologise for too many acronyms in one sentence.]
A 20US$ device (Ezcap EZTV668 in my case) is supposed to tune, display and decode transmissions between 65 and 1700 MHz. Sampling resolution is only 8-bit and there is no front-end filtering for this direct conversion receiver (can be added externally, of course), but this will let me have a peek at 23cm band.
Also it could be useful as a poor man's spectrum analyzer in my lab.

Who knows? Time - and experimentation - will tell (me).

Well, first it has to come from Far East.

Locating R1235 on FT-817

Two weeks ago I proposed a "play safe" procedure for testing the health FT-817's ACC port (Vcc line only). If R1235 is blown, what can be done? First let us find out where it is.

Remove all power sources to the FT-817, including the internal battery.

Unscrew the top cover (note, this procedure is the same of the "optional filter installation") and gently unplug the speaker cable: a shiny PCB full of SMD components is exposed. Well, that's not the correct side for R1235.

FT817ND without top cover.

Remove the five screws that hold the board (red circles in the picture above), the flat cable near the DATA connector (yellow rectangle) and the two coax connectors (yellow circles). Now the PCB can be lifted and rotated 180° above the front panel.

Check out the video to understand where you have to look for R1235 on the bottom side of the top board.

 

And here is an annotated picture of the area of interest:

Area of interest to locate R1235

It is tiny, isn't it?

A Vcc line shorted to ground though this 10 ohm resistor for a couple of seconds increased the resistance to about 7000 ohm. Voltage without load is the same of the supply, but as soon as some current is drawn the voltage reading drops.

Fix? Stay tuned.