In the Very Early Days of Wireless a TRF (Tuned Radio Frequency) Radio Was the Next Step

TRF RADIOS

In the very early days of wireless a TRF (Tuned Radio Frequency) radio was the next step up from a crystal set. It offered amplification of both the radio frequency and the audio so that more stations could be received and the sounds would be loud enough to power a loudspeaker.

TRF sets are quite simple and require battery or mains power. In the 1920' intil the 1940's the glass valve was the component used for amplification. A valve is quite a large device, about the size of an eggcup, the filiaments glow red hot and the radios would get quite warm consequently they also use quite a lot of power.

A TRF radio has its limitations and was superceeded by the Superhet, which is a principle used in all modern receivers. It was not until the introduction of the transistor in the mid 1950's that radio sets could be made smaller and truly portable and consume much less power.

The TRF radios described here all use transistors, though the circuit layouts are fairly similar to some simple valve TRF radios.

THE LADYBIRD

I was introduced to the construction of TRF radios as a child with the Ladybird book 'Making A Transistor Radio' written by Rev. G.C. Dobbs and published in 1972. The book describes how to make a very simple receiver and then progresses in stages up to a 3 transistor TRF radio that uses a technique called Regeneration (also called Reaction) which uses the first Radio Frequency transistor 3 times by implementing regeneration. Regenaration is positive feedback , similar to the howl-around produced when putting a public address microphone too close to the loudspeaker. In the radio the amount of this feedback has to be very carefully adjusted to just the right level, just before it begins to howl, in this state the radio is at its most sensitive and is very selective.

Unfortunately the design uses old Mullard OC71 and OC45 germanium PNP transistors which are no longer made. I am sure that equivalent silicon NPN could be found with some research and used in the circuit with some adjustment to component values. I still have my original radio that I built in the 1970's and it still works a treat, in fact I use it to listen to the football in the garage.

The Ladybird book decribes a novel solderless technique for construction, using a wooden 'breadboard' and brass screws and screw cups to trap the leads of the components. I have since rebuilt the radio into a smaller case and soldered the components to tagstrip.

Here are some illustrations of the radio. Photo showing the 'Breadboard' method of construction.

The regeneration amount is controlled by the 10pF trammer capacitor.

The OC45 transistor amplifies the tuned radio signal once, and then again. The diodes change the radio signal into an audio signal (detect) and the transisor then This is complete working circuit digram. amplifies this too.

The two OC71 transistors add further audio amplification to drive a loudspeaker.

Above: My own re-housed TRF radio, with components soldered together The Completed Radio onto a tag strip.

RADIO IN A MATCHBOX In 1975 the idea of being able to build a complete working radio that would fit inside an ordinary matchbox seemed absolutely amazing. Today it is run of the mill, but this article from Everyday Electronics of Sept 1975 is really interesting. It uses the Ferranti ZN414 integrated circuit which is no longer available, but the direct replacement MK484 can be used with confidence and is available from Bowood Electronics.

I have built this radio and another one using the ZN415 IC which includes an additional stage of audio amplification.

Have a look at the ancient article A Matchbox Radio below, as stated the MK484 IC (This one uses the ZN415 IC - see further down) can be used in place of the ZN414, the 500pF trimmer may be still be available from Bowood Electronics or you may have something similar in your junk box. The circuit could still be made up using a miniature tuning capacitor salvaged from a discarded Chinese pocket radio.

(I have drawn in a pair of IN4148 diodes which are not absolutely necessary, but act to stabilise the voltage. Disregard my other pencil jottings.)

Even simpler would be to use the circuit on the right which omits the BC107 transistor and uses a simple Crystal earpiece to listen to the radio. It would make it even easier to assemble into a matchbox.

Also have a look at the circuit below, which uses the ZN415 integrated circuit which includes additional amplification, enough to directly drive a pair of walkman headphones. The two 32 ohm earpieces must be arranged so that they are wired in series to give the necessary 64 ohm load. The ZN415 makes assembley even easier.

Circuit digram of the Ferranti ZN415 single chip radio. This circuit can also be used for the later ZN416 and ZN416E integrated circuits - if you can find one.

The ZN415 / ZN416 / ZN416E is, like the ZN414, also discontinued by Ferranti, but you may be able to find one from somewhere, there may even be a replacement IC, but I have not come across one.

MORE REACTION

The ZN414, ZN415 and ZN416 radios described above are straight TRF radios that use no reaction (regeneration). A popular design pre-dating the ZN' circuits was the HAC circuit. HAC stands for Heard All Continents, and when used as a short wave radio that is certainly possible. It is very sensitive and quite selective too. I have built one and recieved Japan, America, China, Australia, Poland and Turkey, amongst many others. This circuit was very kindly sent to me by a Short Wave Magazine Reader.

As with many such circuits of its day it used plug-in coils that were made by Denco Ltd of Clacton. The coils are no longer available, but it is perfectly possible to wind your own. I have provided the coil winding details below. The Trusty HAC Circuit

The HAC TRF Radio Internal view of the HAC TRF Radio

MULTI-BAND COIL WINDING DETAILS

The old HAC circuit detailed above is very easy to construct, however the design relied on the use of the DENCO (Clacton Ltd.) Green Range of coils which are no longer available. It is quite straightforward to wind your own coils and the diagram on the right shows how and the table below details the number of turns and what guage of enamelled copper wire to use.

Take a short piece of 10mm diameter ferrite rod (about 50 to 100mm long) and make a paper sleeve to wrap around the rod (for easy repositioning of the coil assembly once in use). Over this paper sleeve wind the first coil (L1) and secure the wires 2 and 5 in place using some Sellotape. Over the top of L1 wind the second coil (L2) and secure the wires 3 and 4 in place. Finally alongside the first windings wind the third coil (L3) again securing the wires 8 and 9 in place with a little bit of Sellotape.

When wiring the coil into the circuit ensure that the wires are connected in the correct place according to the numbers shown. Be especially careful not to get the wires of the The diagram above (top) shows how the coil is regeneration coil L2 swapped over as the physically wound onto the ferrire rod former while regeneration (reaction) effect will not the circuit diagram shows the importance of the occur. lead connection numbering and how the coil is wired into the tuning circuit. With a cursory glance at the schematic diagram and the drawing of the coil opposite you might expect that wire 3 from the regeneration winding (L2) would be physically next to wire 5 of the main tuning coil (L1) however wires 3 & 4 are effectively swapped over because the feedback from the Drain MPF102 field effect transistor is out of phase with its Gate and so would be out of phase with the radio signals in L1 if applied to the 'top' of the coil near the wire 5 end of L1. Since regeneration is effectively controlled positive feedback it must be applied In-Phase with the received radio signal.

COIL WINDING CHART: BAND Range L1 L2 L3 Wire (Approx) (turns) (turns) (turns) s.w.g. SW1 4 - 15 8 4 28 14MHz SW2 10 - 30 6 4 2 22 MHz MW 5.2- 50 18 15 32 1.6MHz The number of turns for each winding on the coil and the guage of enamelled copper wire used are just suggestions that happen to work on my radios quite well. Other guages of wire may be used and the number of turns adjusted until the required range is obtained. In any event some experimentation with the number of turns will always be required.

L3 is not too critical as it is just a coupling winding for the aerial. L1 determines the range of frequencies covered by the set and the number of turns can be increased to tune lower ferquencies or decreased to tune higher frequenciess. The most critical winding is L2 and some experimentation will almost always be required here. You may find that you can obtain reaction across most of the band but not all of it, in which case add a turn or two. Alternatively you may find that reaction cannot be reduced sufficiently at some frequenciess, in which case remove a turn or two. I found the Medium Wave coil the most difficult to get just right and had to adjust the position of the windings relative to each other to obtain satisfactory results

MY FIRST SW TRF RADIO:

Here is the circuit for my first Short Wave t.r.f. radio. As you will see it is very similar to the HAC described above and uses the same type of coils, but with just two transistors it will only power an earphone. I spent many hours with this set as a kid listening to short wave stations from all over the world on a crystal earphone. Most listened to broadcasts on this simple little radio were Swiss Radio International, Radio Netherlands, Radio Sweden, Radio Canada, Radio Moscow and The Voice Of Turkey - and many, many more. I called it the TWO TANSISTOR WONDER - because it was!

TWO TRANSISTOR WONDER

The very old and rather battered but trusty little Two Transistor Wonder. The case is original but the knobs have been replaced with more modern ones. (Note the several holes remaining in the front panel from previous modifications). The 'gubbins' inside the Two Transistor Wonder