Monday, December 27, 2010

Getting ready for the 4m band

If all goes according to plans/wishes, Dutch radio amateur will soon be allowed to use the 4m band between 70.0Mhz and 70.5Mhz. Trying to avoid the mistake of being surprised and hence not properly prepared for 600m, I started considering options for 4m.

First thing was looking for available surplus/commercial equipment. Unfortunately, in this range, all available surplus rigs are providing FM only. Best candidate so far, the VRC8000; actually, I plan to pick up one, before they are all gone. Advantage of this rig, it will also cover 6m.

I guess, a lot of operation will make use of CW and USB. Since no commercial rig seems available, a transverter could be the best shot here.
There are two CMOS oscillators available which would be suitable for the job converting/transverting to the 30m band: 60MHz (regular mixing) and 30MHz (subharmonic mixing).
Another two CMOS oscillators would enable us to convert the 4m band to the 6m band: 20MHz (regular mixing) and 10MHz (subharmonic mixing).

For being prepared, I will to pick a VRC8000 asap, and also consider to build a subharmonic transverter for the 6m band.

Tuesday, December 21, 2010

AKAI APW20 World Receiver

A new member to the receiver collection, the AKAI APW20. The device grinned at me in my local entertainment shop and for just 75 Euros... too seductive.

I will spare you most of details that can be found on AKAI's website. Here are just some interesting bits and pieces.

On LW, MW and SW, small tuning steps are 1kHz. For SSB a fine tuning pot is present. When rolling over frequencies, no (stupid) mute function interrupts audio.

LW goes up to 519kHz, covering NAVTEX, which I could directly hear from my living room.

Although the receiver has got a connector for an external SW and/or VHF antenna, the unmodified receiver does not switch to an external aerial for neither LW nor MW.

The rotary encoder not only changes frequency, is also can be used for adjusting volume. There is a line-out jack (grabber) and also a line-in (?). The rig is provided with a mute button, which will mute the loudspeaker and phones but not line-out.

Added bonus: built-in thermometer.

The APW20 may not be the best world receiver in terms of ham-radio, I believe however the APW20, in its intended function, is the best world receiver in my collection. I may even reach out and get a second one tomorrow, one for the boat, one for the suitcase.

Sunday, December 19, 2010

600m QRP TX at the Plumbtenna - VFO Range

Did some additional soldering on the 600m QRP TX, which is now foreseen with a BNC antenna socket. Hooked up the Plumbtenna (matching&coupling details) and just went for it.

The first tests, once again, were done indoors, from the ground floor.

Lots of plasma TV lines. It seems clear that the Plumbtenna actually radiates, at least a little bit. The HF3's AGC is clearly pulled (ant: Octoplumb, 2 stories higher). I did a little keying, nothing of significance though.

The spectrum indicate a VFO range from just below 501kHz to 503.2kHz. Fingers crossed that this will fall into the range (hopefully) to be assigned to radio amateur at the WRC12.

Friday, December 17, 2010

600m QRP TX update

It is about time to hurry up. Just a few more days and my 600m permit will have expired. As my energy slowly seems to be returning (don't ask what drained it - the regular reader may have a clue though), my soldering iron heats up more regularly. Today, the last drips of solder are dropped and the 600m exciter (see earlier post) has been given a "power stage", namely a 74HC240 operated at 8V.

Just applied some power, no keying yet... neither an aerial, just a few centimeters of wire. Still my grabber's AGC was pulled (tx-ing from ground floor through two reinforced concrete ceilings).

The spectrum shows DI2AM at 505180Hz. The signal at 503200Hz (1955z) would be me testing.
The HC240 developed slightly elevated temperature. I guess this is normal when running it at 8V. I said, the signal was not keyed, and it wasn't, however, I was handling the PCB, feeling temperature etc, hence the variation in signal strength.
I hope that, later 2nite, I will have hooked up a QRSS keyer to the transmitter and have it wired up to an aerial (I figure that will be the original Plumbtenna).

Saturday, December 4, 2010

I/Q-SDR Local Oscillator

Just an idea, have not tried it yet... Inspired by YU1LM, I thought of a frequency independent method for creating the I/Q phase shift of 90 degrees.
You may remember my sub-harmonic approach at half the operating frequency. Here a RC network took care about the 45 degrees phase shift. An RC network is ideal for a 45 degrees shift, since R=XC; this was the trick in the sub-harmonic case. However, such an RC network is only accurate in a narrow stretch of frequency.
In YU1LM's designs, a similar RC network is used for oscillators on the operating frequency and on twice the operating frequency.

Twice the operating frequency is a very appealing thing actually. Frequencies are not so terribly high in comparison to the traditional four times the operating frequency method. However, said last mentioned method creates exactly the shift required, due to the purely digital character of the design.

So, why are twice the operating frequency local oscillator so interesting? Very simple, we have QRP crystals for every band. The QRP frequencies are traditionally on the higher end of the CW portion of a band. Divide that such a frequency by 2 will get us about in the middle of the "regular" CW range of the band one octave lower. Examples:
  1. 28.060 / 2 = 14.030
  2. 14.060 / 2 = 7.030 (here we actually hit the QRP frequency)
  3. 7.030 / 2 = 3.515
  4. 7.040 / 2 = 3.520
  5. 3.686 / 2 = 1.432 (3.686MHz is a cheap standard crystal)
With a sampling rate of 48kbps, this will cover approximately +/- 24kHz about the center operating frequency, and thereby a substantial portion of the CW ranges.

I hope, that I could come up with a pure digital design that will function independently of the frequency it is used at, i.e. no analogue frequency shifting.
Have a look at the concept (there may be details missing in the schematics!):

How is it supposed to work?
U1A (XOR) forms the typical Pierce type crystal oscillator.
U1B is wired as "driver" and is supposed to provide some pulse shaping. Could be that U1B better should be an inverter, meaning, the input which is grounded here, could be wired to 5V.
U1C is an inverter, thereby creating a phase shift of 180 degrees.
U1D is a driver, keeping the original phase. It seems not necessary on the first glance to have this driver, however, it is important to compensate for the delay created in U1C.
U2A (D-type flip flop) and U2B are configured to divide the incoming frequency by 2. Dividing the signal frequencies by 2 means that the 180 degrees phase shift created by U1C and U1D will be just 90 degrees at the frequency divided signals.

Added bonus: I/Q reversal could easily be realized by swapping the roles of U1C and U1D by means of a simple dual toggle switch.

I figure, this design could actually be relatively universal, since Pierce type oscillators are rather forgiving what required passive components is concerned. All the rest is just digital ups and downs, ergh, highs and lows, I wanted to write.

Wednesday, December 1, 2010

NIKKEI NRB10 or a Retro QRP Enclosure

I should not be left alone in electronics stores, I guess. The temptation of buying one of these was just to great, even if the price of about €20 is not really calling bargain.

Here what the receiver looks alike

I figure, the ideal front configuration for a multi-band CW QRP station. Looks cool too! The switch on the right-hand side has got 4 positions: OFF - FM - AM - AUX. Some ideas for that switch:
  • OFF - 80m - 40m - 20m (multi-band CW transceiver)
  • OFF - AM - LSB - CW (75/80m single band multimode transceiver)
  • OFF - RX - TX - TUNE (75/80m AM transceiver)
  • OFF - 2.7kHz - 1kHz - 500Hz (single band CW-TRX w/ several filters)
The left-hand volume potentiometer would very likely keep its function.

Nice bit on the kit, the main tuning knob is equipped with a vernier drive!

The red and green LEDs could serve all sorts of purposes... and also the scale back light could be used for something.

What about the back side of the radio? Again, perfect for QRP! Have a look:

NRB10 back cover

First of all, the material of the back cover seems good workable soft plastics. More interestingly however, enough sockets for all sorts of things. Some thoughts:

  • keep the RCA for the speaker, since this is already done
  • use the mains power cord to connect to the 12V station supply
  • replace the 75Ohm socket by a 50Ohm BNC socket
  • headphones will remain as is, it is nice to have the correct symbol printed on the cover
  • REC OUT also could be used as such, for connecting the rig to a computer
  • AUX will serve as a KEY in or a MIC/PTT

I can't wait to have that box operable, sitting on my living room table. Hmmm, probably, I should write less and solder more ;-)