AmateurRadio.com

For reasons that will become clear in a future post or series of posts, we’ve been busy lately—don’t worry, it’s a good thing.  I did manage to sneak away from my regular (usually desk-bound analysis) work into the lab and tune-up an eBay-special cavity filter for 903 MHz with our new network analyzer.  It’s really amazing how you can dial these things in if you have the right tools.  This one is a 3-cavity filter about 10x8x3 cm.

Passband insertion loss is about 1.2 dB.  Harmonic rejection at 1.8 GHz is 70 dB down.  I’ll take it.

I also have some eBay-special helical filters that were advertised to be for 432 MHz.  So, I need to cobble up some carrier boards to try them out.  Another day, another project.

Teaching Evan the basics of radar signal processing with this baby-block 7-bit Barker code and its matched filter.

This is a pretty simple modification that converts a SoftRock Ensemble RXTX SDR RF interface board from “common-IF” (RX and TX share the same antenna port) to “split-IF” (RX and TX have separate ports).  Split-IF is the standard for high-performance transverters on the 50-, (70-), 144-, 222-, and 432-MHz amateur bands.

So, I had a look at the RXTX schematics (here, here, and here) and considered the following options for where to break the RX and TX portions of radio:

The purple dots were the first option.  Unfortunately, these locations on the actual printed circuit board were not easily accessible to miniature coax and this process would involve significant surgery to perform and restore the modification.  The second alternative I considered was the red Xs…jumpering over the BS170 PA transistors.  By this time, I was looking for a way to avoid butchering the original circuit too much.  So, I elected for option three, which was to install a second T/R switch at the antenna jack (golden circle).  This had at least two advantages, the first of which was being minimally invasive.  The big advantage, though, is that the radio could be operated at its design output and run through a fixed or step attenuator on the TX side to the transverter.  This meant that I would be assured of having a reasonably clean signal to work with if I configured the radio correctly.  It also meant that I could operate the radio as designed if I simply disabled the the second T/R switch and connected the antenna to the default port.

So, here is the modification that I came up with using parts I had in my junk box.  KB9YIG ships the Ensemble RXTX with a couple of spare BS170 FETs that can be used in place of the 2N7000.  I just had a lot of 2N7000s and thought I’d save the BS170s in case I ever burnt one in the RXTX.

The interface to the transverter is through a DB9 connector.  PTT for this transverter is +12 volts (as done with the TS-930S) on pin 6.  Pin 1 is ground.  Pins 2 and 3 are shorted together in the transverter cable connector to enable the modification in the SoftRock.  The right portion of the schematic with the relay driver and Omron G6Y relay is based on the T/R switch from W1GHZ’s “Multiband Transverters for the Rover” that I decided not use on the microwaves when I found a cheap source of SMA coaxial relays.  An I2C decoder would be great to put band data on the other pins to select a transverter (or transverter cascade for the microwaves).

The whole mess works great from a switching standpoint.  When I key the PTT on Rocky or PowerSDR, it switches the TX line and keys the transverter.  I’m a little ambivalent about the quality of transmitted signal that’s actually coming out.  I think it’s good enough if you live in a sparsely-populated area, but I have a lot of VHF neighbors that I’d rather not upset.  So, I need to do some more testing on this—a lot of it is getting the RXTX and PowerSDR configured correctly.  It appears that the I/Q channels are flipped on my sound card (Audigy 2 ZS) between transmit and receive.  I don’t know if that’s a wiring error in my breakout box or whether it’s normal.  Spectra to come at some point…

A few photographs of my latest tinkering—a SoftRock Ensemble RXTX.  This unit provides all of the hardware necessary to have a software-defined radio (SDR) transceiver that uses a computer soundcard for ADC and DAC.  It is available here as a kit.  They are kitted in batches of 20-100 every few weeks and usually sell out within 24-48 hours.

I originally had little interest in doing a full SDR transceiver (preferring receivers only) until it occurred to me that I could modify the RXTX as an IF strip for VHF/UHF transverters like many have done with the FlexRadio Systems rigs.  So, I have developed a modification that is minimally invasive to the operation of the RXTX.  At build time, you choose a nearly octave-wide “super band” for operation.  This one was built for the 20-30 MHz band, covering the 15-, 12-, and 10-meter Amateur bands, as well as the 25-28 MHz IFs that I (will) use with present and planned transverters for the 6-, 2-, 1.25-, and 0.70-meter bands.

I’m not sure I have the enthusiasm to build another SoftRock.  There are a lot of trifilar transformers to wind.  The final result looks pretty good and seems to work.  I’m going to put it on the spectrum analyzer soon to see how clean the output is and how I should balance the drive for the transverters.  Then, I will also include a write-up of how the modifications are done.  (I promise that writeup is the same place as the control circuit schematic for the 50-MHz transverter!)

Here, you can see the W1GHZ relay board for doing split-IF T/R switching.  If the transverter control cable (DB-9) is disconnected, the SoftRock switches to common-RF for regular 15-/12-/10-meter operation.

I’ve picked up a couple of Mirage “brick” amplifiers over the past few years.   I’ve also rewired the DC cable on each and every one.  Here’s the latest.  AWG 10 to AWG 16 transition?  Seriously?  I know it technically doesn’t matter too much for a short run, but these amplifiers suck down a lot of current and I just don’t see cutting corners on that.  Maybe I’m a purist…

After about six years of sporadic effort, I finally finished the W3NQN filter set by building the 10- and 15-meter filters over the past two weekends.  Now I can operate SO2R with impunity on all bands.  Several band combinations are still problematic, but those are problems that filters can’t solve.

Because the geometry of our home does not permit me to have a walk-behind station desk, I had to attach the filters to the mounting board by laying on my back with a right-angle drill.  That sort of thing is always a bit exciting!

Last night, as I have been doing lately in both the evenings and mornings, I was trawling the bands with a SoftRock (a dual-band v6.0 built for 40 and 80 meters—my first SoftRock) and Rocky.  Without fail, the waterfall enables me to see something interesting, which brings me to the point that I really need to integrate these receivers into my station.  But, I digress.  The really tantalizing, fascinating signals are the weak ones, especially when they are weak and unusual.

The subject weak and unusual signal is shown in the figure headlining this post.  Rocky’s cursor shows the approximate bandwidth of the CW filter (250 Hz, if I recall correctly) and the center frequency (7026.25 MHz).  I tuned it in and caught a CQ from “AA1T…”  Recalling that I head read about Mike, AA1TJ‘s, Das DereLicht transmitter built mostly from parts scavenged from a dead CFL, I suspected it was him.  After a moment the QSB came up and I caught the ‘J.’  Thanks to the fact that the SDR is not fully integrated into the station, I scrambled to plug my 15-year-old Small Wonder SW-40 in and get it online.  Mid-scramble, Sarah handed Evan off to me for a diaper change, but I did manage to snap the screenshot above.  By the time I returned to the shack awhile later to close things down, AA1TJ had disappeared.

Through the (ubiquitous) magic of the Internet, I sent Mike an apologetic e-mail QSL, which yielded a nice response.  It turns out that he was trying a 40-meter version of the first transistor transmitter described in the amateur literature.  The design for 146 MHz by K2AH appears in March 1953 QST.  If you are an ARRL member, pull up those old QSTs in the archives and read about it.  K2AH is on the cover of February 1953 QST with the same transmitter.  Mike figures he was making 20 mW, or 20650 miles/Watt on the path from his station to mine.  The more remarkable thing is that he was using a 1956-era point-contact transistor (the same type as in the K2AH article).  Fascinating stuff!  Hopefully, we’ll connect for a real over-the-air QSO sometime soon.

Thanks, Mike, for making my day by doing something interesting and sharing it on the air!