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INTRO & UPDATE
This circuit is coil-less portable shortwave radio that's easy to build and works very well.
Here shown only as a minimal design with 2 transistors, is a very good regenerative radio receiver.
(WARNING! No Xtal or Resonator device on EC! This simulation therefor uses a coil and adjusted caps in the colpitts oscillator)
PERFORMANCE
The receiver is an excellent performer allowing the reception of AM, SSB, CW & Digital modes:
1) High selectivity, sensitivity & stability (due to the ceramic resonator)
2) You may use a short whip antenna or a simple one or one and a half length of flex wire
3) Low weight & very portable (I Built the little radio into a small hinged tin 11 cm X 8 cm X 2.5 cm. Other metal housings would also work well)
4) Low power - even with the headphone amplifier the receiver only uses about 12 mA! This is well over 60 hours use when run from a PP3 9 Volt battery!
5) Good tuning range of approx 100 KHz (From 7000 KHz to just over 7100 KHz - This covers the whole CW/ Digital mode part of the 40 meters band and a part of the Voice section) The modification shown at the centre with the Lamp bulb depicting the ceramic resonator has a fine tuning pot & a 1N4007 as a Varicap diode; this makes the receiver a pleasure to use!
6) Very usable with a 2 transistor NPN/ PNP low power audio headphone amp (check my circuit list)
NOTE: This design is the result of hours of experimentation, spent building and modifying circuits soldered onto copper clad board using 'Dead Bug', 'Messy' or 'Manhattan' style prototyping technique.
(My style of prototype build is not really Manhattan. No! it's really messy using punched circular lands cut from old copper clad board, but it works well. I recommend this build technique; it's a good way to do things with RF. The main idea is build it first, test & modify it until it's working how you want... then build yourself the neat version)
The receiver is designed to be highly portable yet very sensitive. It is fixed band; this design is centred on the 40 meters or 7 MHz band, which is controlled by the choice of resonator used. Equally, a 3.5 MHz resonator could be selected and this would correspond to the 80 Meters band.
CIRCUIT DESCRIPTION
The first stage is a simple wide band RF preamp. The signal is fed into the regenerative stage; which is essentially an oscillator that is frequency controllable.
The second stage is the main engine of the receiver, functioning as frequency discriminator, filter, oscillator and even beat frequency oscillator. Because this stage is essentially an oscillator, it acts as an amplifier with a potentially huge gain at the selected frequency.
Even though the receiver is very simple, this design is far from rudimentary and is capable of world-wide reception at high sensitivity. It uses only a small whip antenna and should not be used with a big HAM radio antenna.
As explained, this circuit should use a resonator and is designed to be a coil free radio. However, due to the current lack of Xtal or Resonators in EC, I have replaced the resonator with a typical LC colpitts style oscillator for the purposes of the simulation. The original capacitor divider and ceramic network are depicted inside the colpitts oscillator; the resonator is shown here as a lamp bulb...
UPDATE!
The section of circuitry with the lamp bulb depicting a resonator has been updated.
The 150 pF cap shown here should be a variable capacitor. I have successfully used the low cost type described as polyethylene film tuning capacitors for AM radios. Because these variable capacitors only have 180° of rotation, fine tuning is quite difficult. For this reason, I added a 1N4007 rectifier diode which is used here as a varicap diode. There is a 100nF cap after the 100KΩ resistor; this is important as it decouples any noise picked up from the off-board potentiometer.
This varicap arrangement works very well and provides a few KHz fine tuning. I chose a 20KΩ pot to supply the variable voltage to the top of the diode. No big deal, any value below 100KΩ and above 10 KΩ will work here. A linear taper should be used.
The original idea for the resonator regenerative receiver was from VK3YE an Australian HAM radio enthusiast and electronics mega-boffin. In his YouTube videos, he describes a number of versions of this circuit running from 12 volts, as is the norm with most QRP (low Power) HAM radio circuitry. However, the simulation has been brought down to 9 volts, as I wanted to make a small portable radio that would run from a PP3 9V Battery; in practice this works fine.
UPDATED REGENERATIVE CONTROL
After some use and a check of other designs, I modified the resistors around the regeneration pot. The 1 MΩ resistor between the base of the oscillator and the wiper of the pot is now increased to 5 MΩ. (lets about 100 - 150 nA pass). Also the top lug of the 100 KΩ pot is now fed by a 270 KΩ resistor. This presents just above 2.5 Volts to the top of the pot.
Why the changes? It makes the whole receiver much more sensitive and the regeneration control is now far more usable.
I have no idea why this is the case and I'm still working on an explanation but it is far better than before! If anyone knows why this might make the regenerative oscillator stage more sensitive then please tell me?
Check out the YouTube video by VK3YE
https://youtu.be/iYap-hCLD7Y
Enjoy!
(Again I look forward to all objections, anecdotal suggestions and mental insults even by people who have never used a soldering iron in their lives, yet insist that they are engineers)
73!
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