General Coverage Receiver
LW, MW and SW bands

About

This is a compact three transistor regenerative general coverage receiver with fixed feedback. It's based on the principle of the ZN414 only much higher coverage. The sensitivity and selectivity is relative good (especially on the LF and MW bands) as can be expected with this "simple" design. The reception on the broadcast bands, LW(longwave) & MW(mediumwave) needs no external antenna! Just a ferrite rod which can be recovered from an old portable MW/LW radio which tuned from approximately 540 - 1600kHz on MW and 140kHz-240kHz for LW. If you find such a radio you'll wont need to make your own coils for those bands.

An AM radio receiver is fundamentally a very simple device. In its simplest form, a resonant circuit builds up a signal if there is one in space at the frequency to which it is tuned. A crystal (galena and cat-whisker) then rectifies the signal, which reproduces the modulation. All the energy comes from the received electromagnetic wave. A good receiver must combine sensitivity and selectivity. Sensitivity is obtained by amplification in several stages, while selectivity is obtained by a narrow bandwidth of the amplifiers. There is a severe problem if the receiver must tune over a reasonable interval, such as the medium-wave broadcast band from 550 kHz to 1.65 MHz. The filters of the several stages of amplification cannot track well enough as their frequency is varied if the bandwidth is narrow, so one must choose between sensitivity and selectivity in such a tuned-RF receiver. There are other problems as well, such as the variation in selectivity as the circuits are tuned over a wide range.

This radio also covers shortwave bands form 2.5Mc up to 24MHz. All shortwave frequencies needs an external antenna. C1 acts as a very simple tuning unit to avoid the receiver being saturated at higher frequencies (shortwave) when using a longwire or any other "large" antenna that isn't resonant. Using a real antenna tuner would be even better... The sensitivity is not as good as on MW and LW though.

It doesn't need a external power supply as the total current is very low (total 12mA) and can be fed with just 3 (chargeable) batteries.
Transistor T3 has a dual purpose; it performs demodulation of the RF carrier whilst at the same time, amplifying the audio signal. Audio level varies on the strength of the received station but I had typically 10-40 mV. This will directly drive the TDA7052 and drives a 8 Ohm speaker up to 0.5 watt @ 3.6volts.
T1 and T2 form a compund transistor pair featuring high gain and very high input impedance. This is necessary so as not to unduly load the tank circuit. T1 operates in emitter follower, T2 common emitter, self stabilizing bias is via the 56k resistor, the 150pF capacitor and the tuning coil.

P1 set the audio volume.
All connections should be short, a veroboard or tagstrip layout are suitable. The tuning capacitor has fixed and moving plates. The moving plates should be connected to the "cold" end of the tank circuit, this is the base of T1, and the fixed plates to the "hot end" of the coil, the juction of 56k/150p/100n. If connections on the capacitor are reversed, then moving your hand near the capacitor will cause unwanted stability and oscillation.

Switch between the coils (being the frequency bands) with S1. I've put a RCA/Cinch-connector (or better, a BNC-connector) at the band switch to allow experiments with different types of coils for different frequency ranges. The tapping points on the coil allow the set to be tuned to different frequencies by adjusting the position of tap-switch S1.

Frequency coverage
LW : 140 - 240 KHz
MW: 520 -1600 KHz
SW1: 2500 - 6000 KHz
SW2: 5 Mhz - 14 MHz
SW3: 12 MHz - 24 MHz
External coil: use any type of coil for experimental frequency ranges

Schematic

Details - Coils

L1: total of 37 turns of 0.65mm on a 18mm diameter plastic tube of 30 mm height:
taps at 24 turns, 8 turns, 3 turns, 2 turns
L2: 4 mH, 550 turns on a 9/10mm ferrite rod OR MW 'loopstick' antenna scrapped from an old transistor radio
L3: 310 uH, 65 turns on a 9/10mm ferrite rod OR LW 'loopstick' antenna also 'recycled' from an old transistor radio (both L2 and L3 are on the same ferrite core)

The loopstick antenna coil is best wound on a bit of cardboard or plastic tube around the ferrite rod. The coil can then be slid along the rod to adjust the tuning range. Use this to set the low-frequency end of the band. If you need to set the upper end of the band then place a capacitor across the tuning cap and re-adjust the low end of the band again (in schematic L3 150pF).

S1: 6 position switch

Transistors
T1, T2, T3 = BC547 NPN

Specs

• Frequency range: 140 kHz - 24 MHz (LW & MW needs no external antenna)

• U = 3...12 volts *Battery or external power operated

• 3 x 1.2 volt chargeable batteries

• I = 12mA @ 3.6volts

• Output LF power: 500mW @ 3.6volts, 1Watt @ 12 volts

• External coil connector

• PL259 connector for external antenna (used for the shortwave bands)

• In/OUT connector - used for external LF amplifier, headphone

• IN/Out connector - OR can be used as an little LF amplifier (TDA7052) with an LF input source (when SW1 is at position 0 (using no ext.coil)).

The receiver sensetivity and selectivity is more then fair on LW and MW bands. However, the higher you go in frequency (> 2 Mhz) the less sensitivity and selectivity the radio will have. this could be solved by using a selective pre-amp between the shortwave bands (S1: position 3,4,5).

top view
Inside view 1	Inside view 2

Dial scale

AM radio is broadcast on several frequency bands:

Long wave is 153–279 kHz; it is not available far into the Western Hemisphere, and European 9 kHz channel spacing is generally used (historically frequencies as high as 413 kHz were used but currently there are no terrestrial LW broadcasters above 279 kHz).
Medium wave is 520–1,610 kHz. In the Americas (ITU region 2) 10 kHz spacing is used; elsewhere it is 9 kHz. ITU region 2 also authorizes the Extended AM broadcast band between 1610 and 1710 kHz.
Short wave is 2.3–26.1MHz, divided into 15 broadcast bands. Shortwave broadcasts generally use a narrow 5 kHz channel spacing.
The allocation of these bands is governed by the ITU's Radio Regulations and, on the national level, by each country's telecommunications administration (the FCC in the U.S., for example) subject to international agreements.

Long wave is used for radio broadcasting in Europe, Africa, Oceania and parts of Asia (ITU regions 1 and 3). In the United States and Canada, Bermuda and U.S. territories this band is mainly reserved for aeronautics, though a small section of the band could theoretically be used for microbroadcasting under the United States Part 15 rules. Due to the propagation characteristics of long wave signals, the frequencies are used most effectively in latitudes north of 50°.

Medium wave is by far the most heavily used band for commercial broadcasting. This is the "AM radio" that most people are familiar with.

Short wave is used by audio services intended to be heard at great distances from the transmitting station. The long range of short wave broadcasts comes at the expense of lower audio fidelity. The mode of propagation for short wave is different (see high frequency). AM is used mostly by broadcast services — other shortwave users may use a modified version of AM such as SSB or an AM-compatible version of SSB such as SSB with carrier reinserted. In many parts of the world short wave radio also carries audible, encoded messages of unknown purpose from numbers stations.

Frequencies between the broadcast bands are used for other forms of radio communication, such as baby monitors, walkie talkies, cordless telephones, radio control, "ham" radio, etc.

AM radio signals can be severely disrupted in large urban centres by concrete bridges with metal reinforcements, other Faraday cage structures, tall buildings and sources of radio frequency interference (RFI) and electrical noise, such as electrical motors, fluorescent lights, traffic signals, or lightning. As a result, AM radio in many countries has lost its dominance as a music broadcasting service, and in many cities is now relegated to news, sports, religious and talk radio stations although some musical genres — particularly country, oldies, nostalgia and ethnic/world music — survive on AM, especially in areas where FM frequencies are in short supply or in thinly populated or mountainous areas where FM coverage is poor.

Also more of my homebrew projects:

.ON6MU Homebrew projects
.Radioamateur related projects

.ON6MU Ham mods
.Modifications of transceivers


73"

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