Rocky Road

Yesterday, my brain was mush, so I didn’t attempt anything at the workbench. I turned on the radio and tuned around the ten meter band, not expecting to hear much. However, I did hear considerable activity from South America, calling with “MM TEST”, which turned out to be the Manchester Mineira contest. Since the contest originated in Brazil, it wasn’t surprising to hear so many stations from that country. I had my first contacts with Peru and Ecuador, and logged a few with Columbia, Aruba, Cuba, Mexico and a few U.S. stations. I heard Chile and Panama, but didn’t manage to land them. I only caught the last couple hours of the contest and did not try running at any point.

Afterwards, I plugged in the 550 mW rockmite 40 and tried calling for a bit. The signal went through my LDG tuner and then my 43 foot vertical wire in the backyard. The band conditions were not great last night, but the activity level was relatively high.  I tried for about an hour, and stopped around midnight. I figured that I might have more luck the next morning, with fewer competing signals (but also fewer listeners awake). I started calling around 6:30 and went about 45 minutes. Local sunrise was 06:27 EDT / 10:27 Z, so 40m was fading towards the end of that period. I didn’t get any responses, but I was picked up at reasonable levels on the reverse beacon network, by stations from Massachusetts and Georgia.

reverse beacon network listing showing ai4sv picked up from 7 to 20 dB above noise

At one point this morning, my ears were nearly blasted off by a New York station operating on 7.031-something. I cranked the RF gain all the way down on the RM40, but it was still loud. The reverse beacon network registered the station at 51 dB above background in Illinois and Maryland — that’s about 10,000 times stronger than my 500mW signal, so I have to guess that the station was using either a linear amplifier or had one heck of an antenna. Even operating more than 3 khz away, he cooked me.

So, no cigar so far on the higher-power RM, but I am sure that the signal is getting out. Maybe I need to keep an eye on the QRPspot site.

Honey, I blew up the amplifier

Actually, let’s start on a bright note, and then we’ll get to the part involving smoke.

Inside view of the rockmite 40 installed in a mity box
The Rockmite 40

My main reason to build the amplifier was to get a bit more power out of the rockmite. Part of the problem in getting the amp working was also likely low driving power. The basic design of the rockmite uses the prototypical bipolar NPN for the “final” amplification — a 2N2222.  There are variants that use other transistors and get the power above a watt, and there are also some tricks to increase the drive, but the stock rockmite should yield a nominal half watt or so with no mods. So, I took off the cover of the rockmite and poked around, checking all the part values. I had already made one substitution: C8 was decreased in value from 0.1 uF to 0.01 uF to knock the side tone volume level down to something tolerable.

Sure enough, I saw the problem — I had used 47 pF (marked 47J) instead of 470 pF (marked 471) for C15 and C17, which are on the output side. Yes, the “1” and the “J” looked similar. I stuck the right value capacitors in, and power output using a 13.2V supply was 550 m. Not too bad.

Next, I plugged the RM into the Texas Topper. I didn’t power it right away, though, because I was interested to see what sort of insertional loss the Texas Topper introduced when it was not active. Power output was about 400 mW. This probably wasn’t an entirely fair test because the Tx Topper was still on the bench top, with tack-soldered connections to the BNC connectors.

Texas Topper before final install

I removed the extra N4148 from the amplifier, because I figured it probably had enough drive now to work without extra bias. Power output was measured as 2.2W, so about 6dB gain.  While I could live with that (and, in retrospect should have), I was curious what would happen if I bumped the bias back up a bit. The N4148 went back in, raising the bias voltage from about 2.05 to 2.75V. Power output was now 7.7W — 11dB gain. I measured roughly 7Vpp in, 20Vpp out, so roughly in agreement.  Part of the increase in gain might also have been due to switching from using alligator clips to apply power to using a thicker wire terminated on one end with a power pole connector and on the other end with a type N coaxial plug.

So, at this point, I was actually (hah) thinking of introducing a one or two dB attenuation pad, although the idea of burning off power in  a QRP rig feels inelegant. More inelegant, however, was trying to bend the FET forward so the heat sink would fit in the enclosure. When I pushed it forward (yes, with power applied), there was a bit of sizzle and then a bright flash from the LED. I don’t know if something arced before the LED toasted, but I was left with the acrid and no doubt carcinogenic aftertaste of stupidity wafting through the shack.

I inspected the board around the FET and couldn’t see an obvious short. The parts in that area do share some close quarters, and the heat sink is right next to both transformer coils. I took out the LED (shorted now) and yanked the FET. Good thing I had a bag of them on hand….as will soon be even more evident.

With a new FET, a new LED (not quite the same type, but close), and another diode, I was back in business. Everything was fine until I tried to stuff the heat sink into the enclosure. This time, not under power. The problem is, though, at some point, you have to apply power, or the whole thing is just a paperweight. Zot. Sizzle. Flash. Puff.

Yeah. So, at this point, I’m out of FETs and thinking that maybe I need to do something more creative regarding the strained relations between the heat sink and the enclosure. The heat sink should be applied right to the metal tab on the FET, which is the drain. The case is aluminum, and at ground potential, so that particular twain shouldn’t meet.

I tend to only order from Mouser when I get enough items on my “want” list to make the shipping work out, so it might be a bit before I replace the burnt out components, but I’m sure I will in the next couple weeks. The board has held up very well to my repeated soldering/unsoldering, and I really don’t have any complaints about the Texas Topper per se. This is more a mechanical issue at this point — all the electronic bits seem to do a fine job of amplifying. I may, in fact, order another one just to play on another band.

Even without the Texas Topper, doubling the output of the Rockmite should make it more usable. I’m looking forward to rolling both it and the TenTec 1320 out next week for the QRP to the Field Event.

Texas Topper QRP Amplifier

I seem to be doing well enough running the Kenwood B2000 at 5W or the TenTec 1320 at around 4 watts, but I haven’t made many contacts with my Rockmite, which on a good day puts out around 250+ mW, but less when the battery runs down a bit. A while back, I had ordered the Texas Topper (a.k.a. Tuna Topper) amplifier (nominally 5W out) from www.QRPme.com. For $25 it’s a good deal. The design lends itself to flexibility and experimentation, allowing the user to choose whether to use onboard/offboard options, a tuna-shaped round or altoids-shaped rectangular form factor, a range of input powers (using fixed or variable attenuator, if necessary), fast/slow switching, and transceiver or xmtr/rcvr configuration.A round Texas Topper printed circuit board, unpopulated

I put the kit together last week, made an enclosure, stuck it in, connected everything up, and … nothing. Well, not quite nothing. My WM-2 wattmeter read about 50mW output. Not good — the amplifier was doing something, but not in the direction that I had hoped.

The amplifier is well-documented on Chuck Carpenter’s website, which provides a parts list, pictures of the board, schematics and helpful advice. From the circuit diagram, it’s apparent that there are two halves to the pc board — one that controls switching through a relay, and the business end of the amplifier that centers on a MOSFET followed by a filter network.  As far as I could tell, nothing was shorted.  In the “receive” state, signals fed straight through from input to output connectors. When I keyed down the rockmite, I could hear the relay click in, and I was able to verify that the input signal was being appropriately routed over to a 4:1 transformer to feed to the MOSFET.A constructed Texas Topper on the bench top for troubleshooting

I checked DC voltages with a DVM, and verified that the bias voltage (determined by the forward voltage drop across an LED that conveniently also serves to let you know power is applied) was 2.05V. The voltage on the MOSFET’s metal tab (the drain) was about the same as battery voltage, as it should be.  Unfortunately, I didn’t have an RF probe on hand for tracing of RF voltages — the probe was lost in the last move. It would be easy enough to build one (see nice plans on W5ESE‘s site), but I didn’t have a suitable diode on hand and apparently Radio Shack no longer carries the 1N34 in stock. No problem — I have something better, although not quite as portable: an oscilloscope.

The incoming signal was about 8Vpp, and 4Vpp after the 4:1 input transformer. I expected to detect something on the drain of the MOSFET, but all I got was hum (maybe just background).  Probing beyond the MOSFET, I didn’t get much. I was stumped at this point, and starting wondering if I had done something wrong during construction.

It seemed to me that there were two likely places that I could have screwed up — in winding the two bifilar transformers (which, I recall I did while watching an episode of “No Ordinary Family”, so maybe I was distracted), or maybe in installing the MOSFET. I had placed a mouser order at the same time as the kit order, so I had a couple extra MOSFETs to play with. Using static-free everything (mat, wrist band, soldering tip, etc) and low heat, I replaced the MOSFET. No change. The kit comes with 22 and 26 gauge red magnet wire for winding toroids. To be extra careful, when I rewound the two bifilar transformers, I used on strand of red, and a strand of another color. Radio Shack does carry a magnet wire set, which conveniently includes 22, 26 and 30 Ga lacquered wire, and the 22 is gold and the 26 is green. The transformers look much better when wound with two color wire, and it’s easy to verify at a glance that the correct wires are tied together and that they all end up where they should. Again, though, no change.

I tried replacing the MOSFET one more time, as I thought that perhaps I had not had the right load on the amplifier when I tested it the first time, but again, no change.

The Texas Topper laid out for testing on the benchtopAfter  I looked at the data sheet for the FET and noted that the gate threshold voltage is listed as a minimum of 2v and max of 4v. The transfer function graph showed the drain current picking up sharply above 4v. My rockmite has lower output than most, and it occurred to me that I might be at the lower end of this amplifier’s design — not enough umph to drive the FET’s gate. To up the bias voltage, I stuck a 1N4148 diode between the stock LED and ground. This bumped the bias from about 2V up to 2.75V. Result: 1.5W output. On the oscilloscope, the waveform was a bit distorted on the FET’s drain, but smoothed out in the filter and was well formed at output.  Going from 250 mW to 1.5W is somewhere around 7dB gain — not quite the 10 dB gain typical for this amp, but a huge improvement over my rockmite’s usual output.

So, now I am playing around a little to see what happens when I run this system with a fully charged battery and play a bit with the bias voltage. Hopefully, I’ll have a chance to try out the rockmite-on-steroids this weekend.

 

 

No QSOs in Boca Raton

This started as more of an IF (interactive fiction — not intermediate frequency) blog, but it does make a lot of sense to consolidate other topics here as well, since most of the time when I have to list a “blog” or “web site” link, I list this one. So, consider the flood gates opened. That probably means a flurry of excited posts followed by intermittent (on a geological time scale) dry spells. Let’s face it: that’s just how I am about updating websites. Maybe postings will be more regular if I can broaden the scope of the posts to just about everything and if the blog posts are useful to me as a sort of lab notebook.

Along these lines, one topic I’d like to document is ham radio trips, which I will conveniently define as any time I am not at home and get to play radio. Since I take a lot of trips for work, and since I almost always take a radio along, there should be a bunch of these.  Very often, the conditions aren’t optimal, and I don’t have a lot of time on trips, so more often that not, I probably won’t make that many contacts, but it’s more the effort than the QSO count.

Rather than recount projects and outings to date, I’ll just start from here forward. This weekend, I spent a couple days in Boca Raton. Scratch that. That’s how it is listed on the map — it is actually Deerfield Beach, Florida.  It’s only a beach if you consider concrete to be beach-like. It’s inland a few miles, and the view was of route 95. The conference I was attending was held at a resort that actually *is* in Boca Raton and overlooks the the ocean, but  I’m travelling at government rate, and that only goes so far.

My room was on the third floor of the hotel, right near the front entrance, which made antenna placement challenging.  On the first evening, I bid my time until there were no cabs or cars in the oval driveway, and then lobbed a 65′ long wire over a palm tree. I had attached it to a water bottle, and A long wire antenna running from the window to a palm treecouldn’t see where it landed after I gave it a toss. There were a couple tense moments when I went looking for it outside and found it dangling 20′ above the driveway, over the heads of some oblivious guests. I managed to yank it back a bit and get it into the bushes. Later that night, after dark, I guyed it down more substantially. Most of the antennas was elevated at about 35′, with the distant end down lower. I ran a 35′ counterpoise around the room and tuned the whole thing with a Hendricks SLT+ tuner.

The rig du jour was the 40m Rockmite, with the 7028.0/7028.9 crystal, running from a 7.2 Ah absorbed glass fiber lead acid battery. A picokeyer was built into the rockmite, and morse code was generated by a Palm Paddle which was duct taped to the battery. Where the wire crossed the metal window frame, I wrapped some duct tape around the wire as padding and then closed the window snuggly. Luckily, the hotel cleaning crew didn’t mind wires sticking out the window, and left the whole thing intact while I was attending my meeting on Saturday.

So far, I have had two — count them, two — contacts on the rockmite, but considering that it only puts out 300mW, that’s not too bad. Both have been from home using my 43′ vertical antenna (which is nothing more than a vertical wire radiator slung into a tree plus a few ground radials). One contact was across town, the other was to Michigan.  In the Michigan case, I was responding to a CQ call, and it was a difficult QSO.

I had no problem hearing other stations calling on 7.028 plus or minus 1 khz. Powerful stations would cut across both of my frequencies, and I had to wait them out. I heard strong signals from Italy, Slovakia, Slovenia, Germany, Croatia, France, and around sunset, from Columbia. Some strong local signals (SC, GA) were zero beat, but I could not work them. I called for a few hours intermittently, but had no responses. Well, maybe some qrz’s, but I am not sure to whom they were replying.

This was the first I had tried out the SLT tuner with the rockmite. Interestingly, the rockmite is powerful enough to illuminate the LED that tells you when the system is resonant. I would have wondered if my signal were getting out at all, if I had not had some confirmation from the reverse beacon network. Apparently, I was just above noise for a few of the stations, but at times, my signal was pretty decent (also, though, taking into considerations that the receiving systems for some of these reporting stations have high gain antennas).

So, no bites, not even nibbles, but considering that the antenna was somewhat of a compromise, I’m not writing off the rockmite. I am, however, strongly considering finishing the Texas Topper amplifier, which would boost the signal nearer to 5W and give me a fighting chance when my antenna options are limited.