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Want a receiver that has excellent sensitivity, good selectivity, will easily resolve CW and SSB signals and even AM if required, that is cheap and very simple to build and only uses one tuned circuit? Of course you do. Sit back and read on - we might just have the very thing for you.
I enjoy receiving letters from readers of Short Wave Magazine, although sometimes they do take up time that I might be spending wielding a soldering iron. Some are fascinating, never more so than the one that ended, "P.S. If you're ever passing through Hong Kong, drop in and try one of my receivers yourself". 1 am still waiting for the opportunity! The letter was from another Anglican clergyman, the Rev. Keith Granger, ex9M2RK and an avid builder of simple receivers. Keith was pleading the case for the TRF receiver having read an item in Short Wave Magazine on a direct conversion receiver. Although the TRF receiver is now thought of as being out-dated, Keith suggested that it should be looked at anew as a simple alternative for the home constructor.
He suggests that a simple receiver, compact enough to carry about and capable of good amateur bands reception on short aerials, can easily be built using TRF techniques. There is only one tuned circuit so complicated homewound inductor matching is not required and several bands can be accommodated on that single tuned circuit. Keith has made a number of small TRF receivers and uses them portable from all sorts of locations to monitor the amateur bands. On a recent visit to the U.K., such a receiver with a 10 foot throw out wire received ZL4 on 7 MHz SSB; W stations on 3.5 MHz SSB and KH6 on 14 MHz SSB. Not to mention a PY4 on 21 MHz CW received on a moving bus in Hong Kong!
Keith rounded off his letter with several circuit sketches of the TRF receivers he has built. Naturally I was interested. I well remember my own experiences with TRF receivers in the 1950s. All valve receivers used with large coils wound onto toilet roll formers. The end of the 1950s was a good sun spot maximum period and the DX (most of it on AM, I remember from that period) received on simple TRF receivers was amazing. So I began to play around with some of the circuit ideas in Keith's sketches.
Front view of the "Kowloon" TRF receiver.
The TRF
Fig 1 The TRF priciple
It occurs to me that there may be Short Wave Magazine readers who have no experience of the TRF receiver. The principle is very simple, in fact until the superhet reared its ugly head, they were the radio receivers. TRF stands for 'tuned radio frequency' and in this type of receiver the only tuned stage or stages are at the radio frequency being received. Fig. 1(a) shows the simplest form of radio receiver. It could be a crystal set ... did you build one? The RF signal is tuned, detected and converted into an audio signal which can be heard on the headphones. Such simple receivers have poor sensitivity and poor selectivity: you cannot hear much and that "much" is often more than one signal at once.
These disadvantages can be improved by a more sophisticated form of detection called regenerative detection. This is shown in Fig. 1(b). The system feeds some of the signal back into the input of the detector stage. As a portion of the signal is passing through the stage again and being reamplif ied, the sensitivity is increased. A tuned circuit has resistance which reduces its 'Q', or "goodness" to we simple-minded folk. The fed back signal also has the effect of reducing these resistance losses and hence the receiver is also more selective. A nice little system but it does have disadvantages. The main practical disadvantage is that the signal at the output of the detector stage is out of phase with the input signal and fed directly back would reduce, not enhance the gain. This is easily solved by reversing the phase inductively before it is fed into the input. This is physically simple because the feedback route can be via another winding close to the input tuned circuit winding, but wound in the opposite direction to reverse the phase. This is positive feedback.
This solution leads to another problem in that, if too much positive feedback is applied to the input, the receiver will oscillate. Not very useful. Because not only does this make copy of the signals very difficult, it damps the tuned circuit - and what is more the receiver has become a little transmitter. (I am told there were those, who, in the 1920s, used to have amateur radio contacts using the receiver only. By giving it lots of feedback with a key in the power supply they were able to use is as a low power transmitter. Interesting - but not advised!) However, these problems can be overcome by providing a control of the feedback to acceptable levels. Since the amount of feedback required is dependant upon frequency, a front panel variable control is usually employed. The audio level from the regenerative detector is low, except for strong signals, so audio amplification is also required. Fig. 1(c) shows a viable regenerative TRF receiver and is the type of receiver described here.
Fig 2 Circuit diagram of the 'Kowloon' TRF
| R1 | 2K7 |
| R2 | 3K3 |
| R3 | 470R |
| R4 | 220K |
| R5 | 4K7 |
| R6 | 100K |
| RV1 | 1K lin. pot. |
| RV2 | 5K log. pot. |
| C1 | see text |
| C2 | 100pF min. cer. |
| C3, C12 | 100µF, 25V elec. |
| C4 | 0.01 µF min. |
| C5, C7, C8 | 0.1µF min. |
| C6 | 47nF min. cer. |
| C9 | 1 nF min. cer. |
| C10 | 47 µF, 16V elec. |
| C11 | 470µF, 16V elec. |
| VC1 | 25pF airspaced variable |
| TR1 | 2N3819 |
| TR2 | BCI71 |
| IC1 | ULN2283 (Cirkit, no. 61-02283) |
| SW1 | 4-pole, 3-way wafer switch |
| SW2 | switch on VR2 |
| L1/L2 | see text |
| Slow-motion drive | 6:1 reduction see text |
| Also: Type J8 Equipment Case from Minjfordd Engineering, Sun Street, Ffestiniog, Gwynedd LL41 4NE (076676-2572); T68-2 and T50-2 cores from Cirkit Holdings PLC, Park Lane, Broxbourne, Herts. EN 10 7NQ (0992-444111), or TMP Electronic Supplies, Unit 27, Pinfold Workshops, Pinfold Lane, Buckley, Clwyd CH7 3PL. | |
"Kowloon" inside view, with all components in place prior to interconnection.
Part 1 - Part 2
G3RJV, Rev. G. C. Dobbs.