31 March 2009

Lazy battery pack, an update

Following the previous post, I have done some measurements.

First of all, the DC cable to FT817 does not show unusual impedance. Not the culprit.

Then, since my analog voltage meter is out of service, I have tried a different approach: build a simple XTAL oscillator, supply it with the AA cell and listen to frequency change at different AA cell loads.

Being the human ear faster than any analog instrument, I would be able to detect quick voltage changes.

First doubt: would a standard Colpitts 2N2222A XTAL oscillator work at 1.2V? Yes, it does! As biasing resistors I used 1k5 on the emitter to ground and 10k Vcc to base. Capacitive network in the ballpark of 50-100pF each for a 14 MHz XTAL. Cool! I have just built a XTAL tester that runs from 1.2V up to ... to... 15V and more!

Second doubt: does frequency really change with Vcc variations? And how much? Well, yes, it does! I have not done precise measurements yet, and YMMV, but dropping Vcc from 1.2V to 0.8V the 14 MHz beat on my HF receiver moves 1kHz or so. What?! It still oscillates down to 0.8V! [insert and happy face here]

The resulting circuit has, on one side of the AA cell the oscillator. On the other side I put a selectable resistive load through a normally open pushbutton. The junkbox provided one 1.2ohm and one 0.47ohm 2W resistors, enough for a simple load.

I also hooked up the DVM, and measured across the AA cell (rounded):
  • no load: 1.3V
  • 1.2R load: 1.1V
  • 0.47R load: 0.8V
The beat on the HF receiver confirmed a constant voltage.

With some Ohm's law you can work out that each of these 8 cells has developed an internal series resistance of ca. 0.3ohm. Since my battery pack has 8 cells in series, the pack has an equivalent series resistance of 8 times 0.3 = 2.4ohm.
The FT817 at 5W keydown draws some 2A ... V = RI ... 2.4*2 = 4.8V of drop within the battery pack: no wonder the radio shuts down!

I did a recharge and the situation improved a little, but the pack still does not sustain FT817@5W.

I tried different AA NiMH cells and they do show a lower resistance/lower voltage drop. Strange enough I did not use or abuse that pack. Nevermind, I have learnt few things (highlighted above).

26 March 2009

Strange battery pack behavior

I had built two 9.6V circa 2500mAh external packs for FT-817. One, the older, started misbehaving.

Once charged it shows the usual 9.6+ V. If I connect the FT-817 to it, the displayed voltage is 9.6V rather than 10+V as before. If I hit the morse key the radio shuts down, except for 0.5W output power setting.

I need to check the power cord resistance, because I notice a 0.4V drop on the displayed voltage even with an external 20A PSU.

Then I suspect one or more AA cell gone bad. Now, how do I test them? I am thinking of a resistive load and voltmeter. At 5W keydown the FT817 draws 2.5A or so, that's about 0.5 ohm/3W resistor (testing once 1.2V cell at a time). I might start with a lighter 1.2A load, with a 1 ohm/1W surplus resistor. Resistors with a higher power rating are obviously fine. Let's see what I have in the junk box or recycle bin... Any better idea?

12 March 2009

The shack, storming

Taken from the model of group development ("Forming – Storming – Norming – Performing", see wikipedia), I'd say my shack evolution is now in the storming stage.

I bought a swedish designed 150x75 cm desk with extensible legs and above it a 120x28 cm shelf.

On the shelf I will install all radio-related equipment, such as RTX and power supplies. The desktop must be as clear as possible for morse key, logbook, notebook (if any) and ongoing electronic projects (on the left hand side).

The desktop will be completed towards the left wall with an additional ca. 50cm held in place with a chest of drawers: the total workspace in my laboratorio (listen to SolderSmoke #102) will be 200x75 cm.

10 March 2009

Can you see it?

This morning I forgot to collapse my 30m balcony antenna. So I took a picture from ground level: can you see where it is?

If you read this after having worked me on 30m CW in 2009, this is the antenna I used in that QSO. I was QRP too.

09 March 2009

Design ideas for a QRSS/WSPR 30m TX/RTX

The presence of a strong local QRSS 30m beacon nulls out my chances of successful DX in that portion of the band. So to enjoy these narrowband techniques, including WSPR, I started thinking of a homebrew transmitter.

WSPR support requires:
  • SSB-like modulation
  • linear RF amplification
  • stability
but the design is simplified by the fact that:
  • output sits in max 300 Hz (threehundred hertz)
  • no need for >0.5 W for satisfactory results
Given the above, combined with the fact that I have a 40.5504 MHz canned oscillator (10.1375 MHz * 4), I imagined three solution as depicted in the following picture:

Option 1 should be an SDR transmitter. Not much different in the TX chain from a simpler NE602 Gilbert cell mixer, but has an intrinsic bidirectionality that allows building a transceiver without duplicating stuff. The single XTAL filter removes the unwanted sideband (both on option 1 and 3).
Option 2 should generate an USB signal but requires a I+Q WSPR signal, that should be possible to produce software-wise.

Ah, one more thing: driving a QRPp transmitter with a QRO computer is complete nonsense. I would record WSJT signal in a WAVe file and send it to the TX with a simple mp3 player.

A good Monday morning on 30m

Before retracting the antenna in >50 mph winds (and going to work), this morning I tried some calls on 30m. Managed 3 contacts, including one 2xQRP: me 5W, RA3 with 1W.

I do not complain about dead HF bands. HI.

05 March 2009

My first WhiSPeR

Intrigued by a simple TX for WSPR mode I came across while surfing the net, I wanted to give it a try. Since at the present time I own only a 30m antenna, it was very easy.

I plugged the laptop input to the DATA socket output of my FT-817. Please note: the laptop was battery powered so to avoid ground loops. Proper radio-computer connection requires an insulated interface!

Tuned to 10138600 Hz USB and listened while the WSPR software was silently running on its own (What is it doing? I wondered...). I have a local S9 QRSS beacon just below 10140000 Hz and a lot of RTTY activity was going on (weird for a Tuesday night on a no-contest band).

On the receiver I used the variable PBT to exclude the local beacon and after few minutes the software reported:

090303 2134 16 -12 0.8 10.140133 NJ0U EN71 37 1 12556 -1
090303 2134 12 -18 -0.5 10.140146 F6BIA JN18 33 0 16341 2
090303 2134 18 -13 1.9 10.140206 W1XP FN42 37 0 1 0
090303 2134 24 -5 0.3 10.140267 F4EMV JN18 37 0 1 0
090303 2140 11 -22 0.3 10.140167 N9KCQ EN62 37 0 4180 0
090303 2140 19 -14 1.0 10.140193 F4DTL JN18 40 0 1 0
090303 2140 21 -8 1.9 10.140203 IZ1ERR JN35 37 -1 1 0
090303 2142 21 -11 2.0 10.140206 W1XP FN42 37 0 1 0
090303 2142 23 -9 0.8 10.140245 F5WK JN18 40 0 8 0

Stations were running between 2W (33 dBm) and 10W (40 dBm). I was surprised to receive French stations, that I've never heard on 30m! They were probably backscattering over the Atlantic ocean.

Nice, technically advanced system. A bit cryptic software. I am considering to build a QRSS/WSPR TX out of commonly available parts. Either DSB with a single-XTAL filter or SSB with I/Q generation.