 |
K3KY's QRP Page 10,000 miles per watt on 40 meters- do you think it is impossible? Others have done it and so can you! |
|
In 1982, I built WB5DJE's 40 meter cw transceiver from
April 1980 Ham Radio Magazine. It has a hot direct
conversion receiver, one of the best designs I have
seen. It is insensitive to AM double sideband signals and it
reduces static levels and eliminates tunable hum and
microphonics.
This unique circuit must be heard to be
fully appreciated.
Although DC receivers do not have single signal reception
(signals are heard on both sides of
zero beat), this one is quite sensitive and selective.
The lack of an AGC circuit has not been a problem,
though I do spend some time riding the gain control, of course.
It is more than adequate for all normal conditions except when
the band is extremely crowded or a contest is on-
then a heterodyne circuit with
sharp crystal filters would be essential. Current drain is
about 25mA from a 12V supply, including an LM380 speaker
amp not in the original design (more on peaks for loud
volume settings).
I also modified the VFO to tune with a silicon capacitance
diode and a 10-turn pot. It is very stable and smooth-tuning.
Transmitter output is 1 watt from a 2N3019 TO5 transistor
with a small heat sink.(17 Feb 2011: KK7B has designed some phasing type direct conversion receivers that actually do provide single signal performance. I recently built his microR2 rx from the October 2006 QST article, and was quite impressed with it.) Recently (Nov 2000), I pulled this radio out of storage and put it on the air again. I have chatted with hams all around the US, but I like to chase DX, and have worked mostly overseas stations. The results have been most gratifying. Europeans are contacted routinely while that path is in darkness, especially during the hours just before their sunrise when signals are strongest. I have also worked into Australia and New Zealand- that is 10,000 miles per watt. I do have a 2el 40m yagi up 120ft (37m), so that may not seem quite 'sporting' to some veteran QRP operators. I am getting some true 599 reports from Europe, however, so they should certainly be workable with 1 watt and a dipole at half the height or less. A Bobtail or Half Square antenna would be a marvelous companion to this little rig for the QRP DXer. The Bruce array is also well worth considering. I decided to build a small power amp to boost my signal when conditions are marginal for 1 watt.. The FET amplifier design by WA2EBY in Mar/Apr 1999 QST Magazine was just what I was looking for. It requires 1 watt drive and produces 40 to 75 watts output on 160 through 10 meters, depending on the band. Stage gains this great are not usually obtainable with bipolar transistors, but are easy with this simple but elegant circuit. My amp went together smoothly and works beautifully. It uses a pair of inexpensive IRF510 power FETs in TO-220 cases. Although designed to run from 28V, it will also produce usable output from a 12V supply. So far I have about 25 watts output at 13V, as anticipated. A 28V power supply is being built, and I expect to see 50+ watts output on 40 meters with it. Increasing power from 1 watt to 25-75 watts is about a 14-19 dB increase, or roughly 3 to 4 S-units (5dB). That can be a very significant improvement in readability. This was a fun project! It was my first experience with surface mount components. The design is not for beginners, but hams with some building experience should have no problems. It does require some coil winding and soldering of chip components. It is a great little amp- well designed. I highly recommend it! (17 Feb 2011: I'm now planning to integrate the microR2 Rx with the 1W QRP Tx built in 1982 and the 25W FET amplifier built in 2000. This will result in a complete, portable, battery operated station in one enclosure, for both home use and various outings. Add in a Curtis CW keyer, W7IUV RF preamp with switchable attenuators, a W3NQN passive audio CW filter, and various other accessories, and it should be a very nice radio for casual use.)
View photo of K3KY 1982 QRP station (27K) View photo of WA2EBY FET Amplifier (18K) Please excuse the fuzzy photography- I just got a digital camera and I am still learning how to use it. :o) |
| Send Comments to K3KY | K3KY's DX Toolbar | PREV |
| HOME | K3KY's DX Toolbar | QRP Station Photo | NEXT |
|
|
