All material on this site (c) Peter Parker VK3YE
So you want to build a QRP rig? It doesn't matter what band it operates on, but you want it to provide the most fun for the least outlay (and fruitless CQ calls). You are not worried whether you work DX or local stations, but your signal should be strong enough at the other end to have a ragchew should this be desired. It needs to be stable yet frequency-agile, fairly simple and have a reasonable receiver. The parts to build it must be readily available and not cost a bundle.
If that's your idea of the ideal first QRP rig, the following suggestions will appeal. Construction costs range from about $30 (for an 80m QRP CW transmitter) to $200 or more for a multiband and/or multimode transceiver. However, these costs can be reduced if you use salvaged components and home made boxes.
Best Project #1
If you're in SE Australia, and can operate during daylight hours, 40 metres is probably QRP's best band. A VXO CW rig for this band is all that is required for good results.
Pick your crystal frequency carefully. 7010 to 7025 kHz will be fine for a crystal controlled (ugh!) or narrow range VXO. Or if planning a wide range VXO (recommended) 7025 to 7030 kHz is suggested. That's because VXOs pull down rather than up from the crystal frequency. As for crystal size, use a HC6/U (larger type) or two smaller size (HC18) in parallel for best pulling range. Two 7030 kHz crystals sold cheaply by Expanded Spectrum Systems are particularly recommended and can give up to 30 kHz shift.
The transmitter section could use a simple transistor two crystal 'super VXO' circuit (capable of 20 kHz or more coverage) followed by a buffer, then a driver and finally a power amplifier. A 2N3053 or BFY51 would give close to 1 watt output. However, 2-4 watts may be available if a BD139, IRF510 or 2N3553 PA is used instead. A medium sized PNP transistor can be used to key the rig by switching the PA transistor's collector or drain supply.
This simple transmitter could be used alongside a 'black box' receiver or transceiver to get on air quickly. A direct conversion receiver section could be added later. This could be an NE602 or 4-diode balanced detector followed by a few transistors as an audio amplifier will drive a crystal earpiece or high impedence phones. Add an LM386 if you need to drive either a speaker or low impedance phones. One stage of op- amp audio filtering (700-800Hz) will suffice for local operating during the day, while a second stage is essential for night time operating and DXing.
This setup will give regular daytime contacts up to about 1000 km, with the occasional 3000 km contact possible at dusk or during the evening.
Best Project #2
Those who prefer nights would be better off constructing equipment for 80 rather than 40 metres. While night time QRP contacts are possible on 40m, they are sometimes harder to make, especially in low sunspot years.
Though designs abound for crystal-controlled equipment, and a two or three transistor crystal controlled transmitter on 3.530 MHz can be fun to use, the availability of 3.58 MHz ceramic resonators puts frequency agility within everyone's reach. A pulling range of approximately 100 KHz is possible with a ceramic resonator. However, for ease of tuning (especially without a vernier reduction drive) it is best to restrict the coverage to the CW portion of the band only.
A CMOS IC, a VMOS FET (eg VN10KM) and a PNP keying transistor can form a practical 1-2 watt 80m CW QRP transmitter. Designs abound on the web. A suitable receiver could be along the lines of that described earlier for the 40m transceiver.
Best Project #3
Maybe you already have a QRP rig for 80 metres, but want capabilities on the higher bands. If so, a two or three band QRP transmitter or transceiver is a good project, particularly if QRP DXing is your aim. You can buy crystals for useful CW frequencies for most bands up to 28 MHz. Or use a crystal on say 7020 or 7030 kHz and multiply up from there to get 20 and 15 metres. Again you'll need frequency agility so make it a VXO.
Band switching is the hardest thing to arrange here, and three or four bands is appreciably harder than two band coverage. A keying circuit, a power amplifier (possibly preceded by a driver stage) and a set of pi networks for the appropriate bands complete the project. Refer to the G-QRP Club Circuit Handbook or Solid State Design for the Radio Amateur for examples of designs that you could use.
Best Project #4
Those wanting voice could try a simple ceramic resonator-controlled DSB direct conversion transceiver for 80 or 40 metres. Such a rig could include a bipolar transistor Pierce variable ceramic resonator oscillator, a NE602 balanced modulator and a 741 speech amplifier. The receiver could use similar components.
For frequency agility use a VXO, a ceramic resonator oscillator doubled from 3.5 MHz, a circuit using a 7.2 MHz ceramic resonator or a a free-running VFO. DSB rigs are exceptionally rewarding projects and are highly recommended.
Best Project #5
If you're in Melbourne or anywhere where there's regular 160m AM activity, a small AM rig for 160 metres can be a lot of fun. Either modify an ex-marine 2 MHz transceiver or build from scratch. Such a rig may use a commonly-available (and cheap!) 1843 kHz crystal, have two or three RF stages in the transmitter and use a modulator such as an LM386. For a receiver, a modified pocket transistor radio could be put into the same case as the transmitter, making a nice compact transceiver.