Category: QRP

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.