Amateur Radio Association of Lanka

Home-brew low cost dual conversion 2M Receiver.

Many people aspiring to become Amateur Radio Operators (Radio Hams)first encounter difficulties in obtaining amateur radio equipment for two reasons. One is the cost of commercially made radio equipments and the other is the non-availability of these items in Sri Lanka.

My idea was to develop a low cost receiver for the 2M band, with parts that can be obtained from the local market.

Many circuit diagrams were available but most of them had several drawbacks, mainly due to non availability of components locally and the difficulties encountered in tuning up due to the complexity of the circuits.

I experimented upgrading/modifying several commercial FM receivers up to 144MHz but the results were not very promising due to the reason that they were designed to receive Wideband FM. These also use single conversion methods. So, the stability and the sensitivity were not enough to get a weak narrow band signal.

This receiver I am talking of uses a Sanyo LA1186 IC as its front end. It's a very sensitive IC, which will work even over 174MHz. I have used the built-in vary-cap diode of this IC to tune the Local oscillator and a simple fixed LC circuit for the RF input. This LC circuit is wide enough to pass the full 144 to 146 MHz Ham band when tuned at the center frequency. By running the local oscillator of this IC at a frequency 10.7 MHz below the receiving frequency, the receiving frequency can be converted to 10.7MHz (First IF frequency.)i.e.: to receive the Yatiyanthota Repeater, which is at 145.625MHz, run the oscillator at 134.925MHz.

The 1st IF is fed to the Motorola MC3361 IC which is having Dual IF stages. By using a 10.245MHz crystal at the oscillator of this IC, the incoming 10.7MHz signal is converted to 455KHz. (MC3357 can also be used. But I found that MC3361 is twice as sensitive than MC3357 and has less external parts)

A white 455KHz IF transformer is used as the quad coil to detect audio. Normal 10.7 MHz Pink or Orange IF can be used to couple two IC's. A low cost LM386 audio amplifier can be used to drive a small 2-inch speaker which will give out ample audio. The squelch facility is built in to the MC3361 and it works excellently by grounding the by-pass pin of LM386. I made this receiver on a copper strip board and it worked but had several stability problems. 4S5RM - Ranjith made a PCB for this circuit diagram and the final product is a very stable and very sensitive 2M receiver. (LM386 Circuit Diagram is not included, but any small audio amp will do fine with this)

I compared this with my Standard C168A hand held and I can say this is nearly equal to its performance! I connected this to a 9V battery and connected a 5/8-lambda whip to this and the results were quite impressive as I could copy the YATI repeater without any problem. When the same antenna is connected to my Hand-held at the same location, it only gives 2 out of 10 bars on its signal strength meter!

I wish to thank 4S5WP Wasnatha who helped me in tuning the unit and for encouraging me to use vary-cap-tuning method, although many HAM's are worried about it'd stability. 4S5RM also designed an excellent PCB worthy of praise. Already 4S5RM has turned out several of these units for fellow Hams and even done an excellent job turning out an enclosed aluminum cabinet in a presentable finishing.

I downloaded data from Sanyo and Motorola sites and its freely available for anybody on PDF format. I hope this will help many HAMs who would like to listen on the 2M band and it is also ideal as a fixed receiver just to keep listening to the repeater

4S7NMM - Nalaka Mutukumarana