Sunday, 13 February 2011

Ham Radio BFO


Ham Radio (amateur radio) is a popular hobby amongst electronics enthusiasts all over the world. Basically the hobby involves a person in making his own gear consisting of a receiver and transmitter or a transceiver (a receiver and a transmitter in one unit) after procuring a licence from the Ministry of Communications. Home brewing or self construction, an integral part of the hobby, has been sadly neglected in our country, despite the fact that various institutions with governmental help have come into being recently.

Hams aboard can buy the latest transceiver off the shelf at a reasonable price and go on the air immediately. But in India, with a sixty per cent duty involved (now changed?), a commercial transceiver would cost a whopping Rs: 50,000. Hence, it is beyond the reach of an average Indian Ham.

The Indian ham is often handicapped for want of ham gear. To overcome this shortcoming a small receiver and a transmitter can be home brewed with indigenously available components. The total outlay may not exceed a few hundred rupees. Some of you may wonder if this is feasible with out fancy test equipment like oscilloscopes and LC bridges etc. Yes it is possible.

How to get started?

We are all aware that wireless communications means receiving and transmitting messages. We have many modes of HF (high frequency) communications like CW (continuous waves) where the signals are transmitted in the form of morse telegraphy. Generally, radio telephony consists of FM (frequency modulation), AM (amplitude modulation) or SSB (single side band). FM is mostly used for VHF (very high frequency) communications. AM, because of its inherent disadvantages, has become outdated for hams. SSB is the most popular mode used by hams all over the world. It permits them to have long rag-chews (long radio contacts). In addition to this, we have exotic digital modes like packet radio, SSTV (slow scan TV) and satellite communications.

First, let us look into the the earliest and simplest ways of CW and phone communication systems used by most Indian radio amateurs. In our country, we can see much of the amateur activity on 7000 - 7100 KHz (40 meter ham band) and 14000 - 14350 KHz (20 meter ham band). To get acquainted with this hobby, and to sharpen your skills in the communication procedures, you should make it a point to listen to the QSOs (radio contacts between hams) regularly.

The key to the success of an air operation is the possession of a good receiver. A good receiver is mated with a good pair of ears is an unbeatable combination. It is also possible to monitor the conversations between radio hams on an ordinary broadcast receiver. It would be advisable to use a broadcast receiver with three or more bands because it permits better tuning on the small spectrum of ham frequencies.

Only some of these receivers can tune on to 14 MHz (20 meter) band but almost all of them can tune on to 7 MHz (40 meter) band where you find most of the hams from the south chatting away.

Monitoring The Ham Bands

Take a broadcast radio of three bands or more. It can be a valve radio or a two in one.

Next, get an outdoor antenna of any ordinary wire between 20 and 16 SWG with a length of 8 to 15 meters erected at about eight meters or higher above the ground level. The latest transistorised radios may not require this antenna set up as they are very sensitive, but an external antenna is of much help in catching weak stations.

Identifying the band

Now with all set, around 7.30 am tune to the 41 meter band of your radio. Tuning should be done very slowly because of the narrow spectrum of the ham band. A little practice will make you perfect in the art of slow tuning. Now follow the frequencies of the station as given below.

You can easily tune in to Radio Sri Lanka on 7.190 MHz. A little lower down, you can find AIR Madras on 7.160 MHz. Further below you can hear the voice of America right on 7.115 MHz. Just below it is the 7 MHz ham band. Here you will here some AM phone stations saying 'this is VU2' followed by their station suffixes. Now you are bang on the ham bands.

In addition to this, you will hear stations sounding like quackling ducks. These are SSB (single side band) phone stations. To make these signals intelligible, you require to add a BFO (beat frequency oscillator). Make a hairline mark on the dial for permanent identification of the ham bands.

Regular broadcast receivers are meant for reception of AM signals. AM signals consists of an RF carrier wave with two side bands (upper side band and lower side band). These side bands are generated when audio signal is modulated with RF carrier. These carry the actual intelligence. Once the side bands are generated with the RF carrier, it is vestigial, consuming most of the power. So while transmitting, the RF carrier is removed from the signal, and the one side band is also removed as both the side bands carry the same information. But at the receiving end the carrier should be reinserted to make the audio intelligible.

In CW mode, RF carrier is transmitted in the form of dots and dashes in morse telegraphy. At the receiving end this incoming carrier should be mixed with another carrier with a difference of 1 KHz to make the signal audible. Such CW and SSB require re insertion of the carrier at the receiving end.

Beat Frequency Oscillator

A simple free running oscillator oscillating at the receiver's IF (intermediate frequency) beats with the incoming signals in either CW or SSB and the resultant audio frequency goes to the AF amplifier.

The BFO works on any DC supply from 6 to 9 Volts. It can be assembled on any small veroboard. As the circuit is small and simple, no separate PCB design is given. A 2X transistor radio type tuning capacitor can be used to change the frequency back and forth. This unit can be housed in a small battery eliminator box. Power can be borrowed from the broadcast receiver.

BFO's use

Keep the unit close to the radio. Connect a 30 cm long wire at the output point of the BFO and leave it. This wire will serve as an antenna. RF signals radiated by this wire are sufficient for the radio to resolve CW and SSB signals.

Now switch on the BFO, keep the 2X tuning capacitor at the center. Switch on the radio and adjust the core of the transformer on the BFO with a trimming device until you hear a hissing sound in the radio. Now tune the radio up and down. If the hissing sound appears on the MW and SW you are on the right track in resolving the SSB signals. In this condition, tune in to any station and you will find that the BFO signal is beating with the incoming signal.

Resolving the SSB and CW signals

Switch off the BFO. Tune to the ham band and tune into a SSB signal correctly. Now switch on the BFO and slowly adjust the 2X tuning capacitor until you hear a clear and sharp sound. Leave it in that position. At this stage, you can copy SSB and CW signals, occasionally you may have to change the tuning position. To copy AM stations and regular broadcast stations simply switch off the BFO.

A Amarendra VU2AAP EFY MAY 92

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