25 April 2010

A modification to obtain a DRM receiver?

Back to an old friend, DRM (Digital Radio Mondiale) ?! Perhaps.

I have been given a working Roadstar TRA-220, which includes few short wave bands in AM. With PLL tuning at 5 kHz steps it is not suitable for SSB/CW reception, but it can be adapted for DRM. Moreover that radio has always suffered from some design mistakes, like:
  • volume control slider too sensitive
  • audio volume too loud
  • strong hum on audio
  • loss of presets and clock unless 2xAA cells are fitted in

The radio has plenty of room inside and all components are through-hole. At a first glance the probable cause of hum is evident: 15VAC and audio line to speaker run together to/from the main board.

I have to build a (circa) 455 kHz to 12 kHz downconverter.

The IF system chip is BA4237L; the AM IF filter is marked SFU450B (thus 10 kHz BW at 450 kHz). First decision to be taken: should I pick the IF before or after the filter?

If I pick it after the filter I need a L.O. at + or - 12 kHz, so 462 or 438 kHz: how to make a stable one? (I will use my xtalfind tool to see if any of my surplus XTALs fall close to those values)

If I pick it before the filter I can make a stable 455 kHz L.O. and retune the receiver L.O. so that the resulting difference is 12 kHz. Would the incoming signal be too wide?

Let experiments begin!

19 April 2010

Geko 201 and date bug

Yesterday I could make an excursion to the mountains and took along my Garmin Geko 201 GPS receiver. It fixed to satellites very fast but ... it showed a wrong date!

It was probably caused by a detail of the GPS protocol information, where it carries a "week" information with 1024 possible values. That's almost 20 years. See the wikipedia page about GPS for further info.

Garmin has released a firmware update (currently v2.90 for the Geko 201) that corrects the issue.

14 April 2010

WSPR on 2m, IC706 drift and aircraft scatter

A single WSPR experiment has lead to interesting observations:
  • the IC706MKiiG "as-is" is not suitable for weak signal work on VHF and UHF
  • aircraft scatter is easily observed at 2m too

The picture shows both the IC706 drift during a two-minute WSPR transmission and aircraft scatter on two airplanes (at circa 2 minutes interval, their "usual" distance on the same path).

The enormous drift was present at each transmission slot (1:4 ratio), rendering unuseful the WSPR signal. This was on 2m. A compressed drift amount was later observed on 10m as well, but the signal could be decoded correctly.

Another IC706 problem, not shown in the screenshot, was the misalignment of the device and probably some difference on TX/RX frequency too: the IC706 station had to retune 400 Hz higher to appear into my WSPR receiver window.

It's always good to know your transceiver peculiarities!

09 April 2010

FT817 external display - preview video

As announced two days ago, the external display for FT817 lives! Here is a preview video of what it does at the present time.

The microcontroller board is exactly the one of my FT817 keypad. I only needed to add a connector to source +5V and GND to power the display.

Depending on firmware size, some single-function buttons could be added around the display, such as VFO A/B, split, mode roll.

Comments welcome.

07 April 2010

Shortened VHF beam

So, did the 5 element Yagi-Uda work with 4 elements only? I know the World wants to know!

Yes, it does work. First I checked with an antenna analyzer the new setup (using dir2 and dir3 as dir1 and dir2 respectively) and I measured a nice 1.1:1 SWR at 144.3 MHz.

Then I mounted it outside and tried some real-life contacts. SWR was 1.2:1, probably increased because of the metallic fence nearby the antenna.

Either propagation was better than one month ago or the theoretical 1dB extra gain on my other 4el beam did help a bit: best DX at 227km with 50W SSB. Not bad for a beam on a balcony!

FT817 external display

I received an email asking if I could develop a simple remote display for our beloved FT-817. It would need to display only the operating frequency.

Greg assumed that given my keypad code it would not be a big problem to implement such a gadget, and he was right.

I have the code ready for real-hardware testing. Now I need to get hold of a LCD display.

I will let YL choose the color :-)

03 April 2010

Shortening a VHF beam

In my last excursion in August 2009 I damaged the boom of my homebrew 5 elements VHF beam. The boom broke at the 2nd director hole, at 1.475m from the reflector. What now?

Months have passed and I have come across the broken boom again. I had already planned a longer beam for my portable VHF contests (7 elements, 3m long) so I will not replace the boom.

Instead I simulated what would happen if I shortened the 5el to 4el with MMANA-GAL. The result is encouraging because without changing element lengths I can:
  • install the 3rd director as 2nd director at 1.465m from the reflector
  • have about 7.7dBi gain and 15dB F/B (not the best)
  • keep a good match to the original 28 ohm
The original 2nd director remains unused, or could be used as 1st director to see if in the real world it provides any improvement (in SWR, gain, F/B, whatever).

Not bad! Well, this operation moves the center of gravity a bit behind, so I have to re-position the boom-to-mast adapter or add a little weight in front of the antenna.

I hope I can get it up and running for next Tuesday's monthly VHF activity day.

Note: the original 5 el. is DK7ZB's VHF Yagi 28 ohm with 2m boom.