NDB CAS

Back to an old passion: VLF NDB hunting. But this time, "live".

Right next to a swimming pool I noticed a weird tower with a capacitance hat and typical white and red stripes. Landing airplanes to Torino Caselle (LIMF) airport passing just over it. Online resources and maps locate the CAS NDB right there.


A detail of the capacitance hat. Don't ask me their length.


A more poetic view of the instllation.

FT817 CAT interface, a clarification on the wipeout warning

So far I have noticed very little interest in my keypad. I hope it is not because of the explicit warning about possible radio settings wipeout, but let me clarify things.

The keypad does not send potentially dangerous data. It sends and reads data as ANY OTHER computer software does. The fact that the keypad uses FT817 "proprietary" commands/extensions does not increase the risk of losing soft calibration settings.

Data (or, more generally, "information") may corrupted on the transmission path by external causes and thus be improperly received. This may happen both with my keypad and any other CAT control software.

Since I do not use CAT control programs I do not know if their documentation issues an accurate warning of total wipeout risks.

I prefer to tell the whole story and provide a preventive measure to recover from such an accident.

So, why hesitate?!

Surplus source for CAT cable

Since soldering MiniDin8 connectors is a tough job, I have found out that in the past there were two commercial applications using these cables:

• Sun SPARC and 3/80 keyboards
• Apple Macintosh RS-422
  

According to a websearch ("sun keyboard pinout"), the Sun keyboard cable has two couples of shorted pins. Fortunately they are not vital for the FT817 keypad application: just eradicate them (pick the right ones!). I've done it and it works.

The same website shows a more friendly pinout for the RS-422 cable, that would be ready to use right away.

One problem: how to find these cables?

FT817 keypad - a suggested assembly

This is the first (and last) keypad I made not for personal use. It is on a veroboard/perfboard and takes about 5x5 cm. I have used a 4 way connector for the cable towards the ACC port and 8-way pin headers for the keypad. I prefer this latter solution to my development board that uses 8 independent wires.


This way the keypad sits on top of the circuit, effectively reducing the overall footprint to a bit more than the keypad itself.

For the reset button I installed only two pin headers, if the owner-to-be will feel the need to add one. The LED is wired permanently to the board.

The cable is 1m long, and did not show any communication error during workbench tests.

These pictures are provided as a guideline for you, homebrewers out there.

Please note that I am not available for building a keypad for anyone for less than 100 Euro, shipping included, advance payment. I hope this amount is high enough to encourage you into homebuilding this keypad. If you have troubles locating some parts, see the keypad homepage for some directions.

Inside Terminator's eye

Don't panic. After you've seen the optical receiving system, the transmitter will look horrible.

The plate is a CD-ROM player cover, bent upwards to hold a clothes peg. The transmitting LED is loaded there. The 488 Hz tone is generated with the usual XTAL+divider chain that drives a 2N2222A (the metallic cylindrical "thing").


In the center of the board there is a current generator (TO-220 transistor, the white cube, two red LEDs in place of a zener) with a jumper to provide two drive levels.

No tilt/turn controls were mounted since the path didn't require any precise beaming. Moreover the LED has a wide beam, so a simple tilt screw might be enough. No need for micrometric adjustments.

Terminator's eye

Finally we've made it! Simone and I could arrange a first complete equipment test.

The OPT201 receiver had been boxed (plastic) and prepared with a 31.8mm standard mount for telescopes. The MCW transmitter was simply mounted on a CD player cover (no picture available yet).

Since I could easily provide a circa 200m path, instead of looking for a dark grass field at <0°c, we decided to stay near the warmth of my place. When Simone looked out of the window was about to ask where the transmitter was. Then he said: "Oh, gosh, there it is! It looks like Terminator's eye!" A 10mm red LED looks pretty bright at 200m. I would compare it to a red light on the back of a car (5W@12V).



Simone brought the "antenna": a tripod and a modified half binocular (dia 50mm, F 300mm). Being experienced at looking for stars he pointed it in the right direction in a couple of minutes, then replaced the ocular with the receiver. Given the strength of the signal we tried staying inside and reducing ambient light (produced by a CFL, by the way) to minimize reflections on the window glass.
The 488 Hz tone was extremely strong and steady, effectively masking out any AC line harmonic. The screenshot below shows more than 50dB over background noise (look at the upper part). The screenshot was taken while my daughter was playing with the laptop keyboard and touchpad sitting on her Ikea drawing desk.


Then I recorded few seconds of signal for postprocess. Since the transmitter was square-wave modulated, many other harmonics were visible:


Note how weak even harmonics are, while odd ones are rather strong, in all the available recorded bandwidth.

Since Simone still had some time available we tried aiming at cars and stars (actually planets, Jupiter and Venus), but nothing could be heard.

Next test will be on a longer distance with some modulated information. Either one way in beacon mode or two-way, if a second CW operator is found. The quest for a proper field is open!

FT817 keypad - real memories, not presets

It is still beta-code, but the keypad is now able to remember your favorite frequencies rather than offer you my factory programmed presets.

The ATtiny2313 has 2kB of program memory, 128bytes of RAM for program volatile variables and 128bytes of EEPROM for long term variables.

In the latter memory space, a program can write data that will survive power off of the microcontroller. This means the program can read something and store it for future use.

In the keypad application, the beta code allows to read the currently selected VFO frequency/mode and write it to a long-term memory area. Then with two keystrokes you can QSY back there. Even days or months after the keypad has not been in use.

Freq/mode information take 5 bytes, so we could store up to 128/5 = 25 memories. Since key combinations are limited and managing them "costs" code memory, I will probably implement these sequences:

- # followed by * followed by any other key assigns current freq/mode to that key
- # followed by any key recalls the assigned freq/mode, except for "#*"

So 15 custom memories will be available. Please note that only frequency and mode will be stored, nothing more. Not even the narrow filter setting.

One thing I noticed. In the BCB band (88-108 MHz) where WFM is the only allowed mode, recalling a memory will hang the radio requiring removal of power (not power-off, total unplug even from internal batteries). This is because I first set the mode, then the frequency. Doing viceversa might result in a different frequency if a change from SSB to CW is performed. Should be worth spending some code to set frequency, mode and then frequency again to avoid these situations, shouldn't it?

[that's why it is still beta code...]

FT817 keypad - automatic whistle

Two more functions have been tested:
- up/down scan (mimics microphone up/dn buttons), with stop too
- "quick tune" for automatic antenna tuners

The up/down scan allows you to scan through the band at the normal scan speed. Takes two buttons, and a third if you also want a "stop" control. Otherwise scanning can be halted with a quick PTT press, or a touch on the morse key.

The "quick tune" sets the mode in FM and keys the PTT for 5 seconds (need more? need less? no problem!), then returns to receive in the original mode. With this function you must take care of the output power level and antenna choice (front/rear) in order to preserve your RF finals' health. No more no less how you'd do it by hand. This function can be assigned to a key or, for safety reasons, to a submenu (ie. within presets or mode combinations).

These two functions can be loaded on customized firmware and replace other functions: you still have got 16 keys and 2k limit on the program!