QRP Rigs I've Built

I can't claim to have the best collection of small QRP (low power) ham rigs, but I have built a few. I thought I would share some of them with you.

2 QRP radios

MFJ Cub (MFJ-9380) 3.5 MHz Transceiver

This is a small rig that does not take long to build or tune. The manual is easy to read and provides a place to check off each part as you install them. It has a lot of surface mount parts, but MFJ has them installed and soldered by a machine before you start. As long as there are no errors with that process, every thing else is easy to solder. I had a few problems with the parts, but MFJ sent me replacements within a day of getting my emails. This rig uses the common 2.1 mm power jack and 3.5 mm phone jacks. It does use a phono jack for the antenna, but there is an option to put in a BNC jack. A list of the issues I found when building the rig are here.

My Cub is pretty easy to use. I did get chirp reports from my first few contacts, apparently my little Heathkit power supply isn't stiff enough. I don't have a chirp when I run the Cub off a small gel cell battery. The receiver has an agc and audio levels are plenty loud enough if you turn up the volume. It has a 3 crystal IF filter. Transmit power is good enough to get to North Carolina today. I finished this rig just before Christmas 2009, so I've only had 23 QSOs so far with it.

SW-40+ 7 MHz Transceiver

This is a nice little rig that was relatively easy to build and tune. The manual is easy to follow and is very readable. The rig is fairly simple, so the schematic is easy to read. The controls are simple, just a tune control and a gain control. The receiver is a superhet with a 3 crystal IF filter. There is no agc. There is no speaker, you must use a headset. The frequency is VFO controlled but it seems very stable. Note that this kit does not come with a case. Small Wonder Labs sells a case kit, I bought the case kit.

The only problem I had when I built this kit was that the VFO would not tune as high in the band as I wanted. So I removed one turn from the VFO coil (a toroid) and that got me about a 34 KHz tuning range from 7.015 to 7.049 MHz.

I did have another problem with this rig, when I took a lightning hit on my 80 meter antenna. This rig was connected to a small dummy load at the time, I thought nothing would we wrong with it. When I tested it, it transmitted all the time. Turned out a diode (D5) in the keying circuit had shorted.

My first QSO was with WB4DCW in Jasper, TN. I have 230 QSOs in the log from this rig. They stretch from California to the Balearic Islands near Spain. I've used it on field day off batteries (class 1E) and in CW Sweepstakes.

Ten-Tec 1320 14 MHz Transceiver

This rig is more complex and more difficult to build. It also has a manual that is more difficult to read (looks like a photocopy of a photocopy on some pages) and the schematics are particularly difficult to read. The receiver is a superhet with a 4 crystal IF filter and it has an AGC (Automatic Gain Control) and RIT (Receiver incremental tuning). The VFO tuning is very touchy and it seems like the calibration changes every time I check it. A very nice case is included in the kit price.

I had a number of problems building the rig. I have 12 notes about assembly problems and I had many of the problems noted in the N5ESE web page. In particular the transmit parasitic oscillation was a real problem. I did not observe this when the transmitter was running into a 50 Ohm dummy load. It only happened into my antenna. N5ESE's fix did not work, but pushing the output RF transformer (T1) down towards the output filter toroids worked for me. Your milage may vary. Remember to check the output of any QRP kit you build to make sure it looks clean, especially into an antenna.

I don't use this rig as much as the SW-40+, mainly because of the loud thump in the receive audio when you press the key down. N5ESE has a fix for that, I need to try it. Other than that the receiver performance is good. Transmitter output is a bit lower than I expected. I have worked 68 QSOs with this rig, most of them in the 2005 CW Sweepstakes and other contests.

Ramsey QRP-80 3.5 MHz Transmitter

I built one of these a while ago. They don't sell the 80 meter version any more. I never did use this on the air, but it was fun to build. I think I would have been more likely to use it if I had bought the case for it.
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page last modified January 1, 2010