Antennae For Good Fm Reception And Fm Dx Listening: Radio
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Antennae For Good Fm Reception And Fm Dx Listening: Radio
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The Glass Castle: A Memoir
Alexander Hamilton
R AADDI OI O R
ANTENNAE FOR GOOD FM RECEPTION AND FM DX LISTEN
Read on to know which antenna is good for FM reception, how you can improve FM D reception, how you can make a good, low-cost antenna yourself, and much more D. PRABAKARAN
recti usual gain, increa FM re
S
ignals on the FM broadcast band (88 to 108 MHz) are greatly affected by altitude and terrain. How high the receiver is placed and the landforms around it determine the strength of the signal you receive. For example, it may be difficult for someone located in a valley or on the side of a hill to hear FM stations from a particular direction, even if the stations are relatively closeby. On the other hand, someone living on the top of a high hill can reliably receive FM stations over 160 km away. Atmospheric conditions also cause the signals to fade. This is especially the case if the station is located far away and/or is transmitting at a low power.
Choosing an antenna for FM Before purchasing an FM antenna, consider your location and the problems you need to overcome. The key to good FM reception is the maximum direct signal from the transmitter with minimal signal reflection. If you live in a metropolitan area close to an FM transmitter, you’ll need an antenna with high directivity rather than a high-gain antenna. A good antenna rejects unwanted signals coming from different directions or bouncing off
Fig. 1: A typical FM antenna
The prime FM reception area extends from 3.2 to 40.2 km from the transmitting station. Here, multipath distortion is less of a problem, and a medium-gain antenna with a moderate beamwidth suffices. However, for receiving FM stations from different directions, you need a directional antenna with rotator. In the normal reception area from 32 to 64 km out, you need an antenna with good gain. If you are located more than 64 km out, you are in the ‘fringe’ area. Here, you need an antenna with a high gain. If stations come in at the same or adjacent channels, the antenna must be directional. If the antenna alone doesn’t provide enough gain,
an FM is m enced half-w as a f tenna an FM an ad 3 dB your proxi powe standard half-wave dipole A directional antenn those stations which lie fined antenna beamw beamwidth is a measure and is defined by half p dB) of the angle of the hemisphere. If the angle is antenna beamwidth), the ceive the desired FM broa signals reflected from diff directions, which may in broadcast. However, it m precisely position an ante narrow beamwidth for p of the desired FM broadca
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Antennae For Good Fm Reception And Fm Dx Listening: Radio
Original Title: FM Antenna Uploaded by bagsourav The Selection
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The Glass Castle: A Memoir
Alexander Hamilton
The dipole antenna sh mately 1.5m long and is connecting the ribbon ca receiver. It can be erect the cable to a wall, with of the cable positioned ho the best signal is found. tenna to optimise recepti sults, mount it high in the away from anything else wards the desired directio Fig. 2: Multipath reception
Power cord antenna. Most radio sets come equipped with an antenna consisting of a clamp that is clamped internally or externally to the AC power cord, then connected to the antenna terminal of the FM receiver. The AC power cord acts as an antenna. For best reception, extend the power cord to its full length and place in different positions until reception is clear. Short wire. Some receivers come with a short wire attached to one of the antenna terminals. If your receiver does not have either this or the power cord antenna, try about 75cm (30-inch) length of flat TV wire on one of the terminals of your set and orient the wire for best reception. Take the radio with its antenna to different locations in your home to see whether reception improves. TV antenna. Since FM signals are in the same frequency band as VHF television signals (channels 2-13), most VHF TV antennae can be used effectively on an FM receiver. So if you have a television antenna, hook it up to your FM receiver. If the transmitting stations are located in different directions, you need an antenna rotor for good reception of both TV and FM signals. A UHF/VHF/FM splitter allows the antenna to be connected to both the TV and FM sets at the same time. The splitters are inexpensive and can be found in the electronics accessory rack of most depart-
Car aerials than the average TV antenna on your FM radio. Easy to set up for indoor use, these improve reception of nearby FM stations for apartment dwellers. Indoor TV/FM antenna. These antennae are available in several models priced at Rs 100 to 200. Higher-priced models are usually intended for long-range reception. Outdoor FM non-directional antenna. Mounted on the rooftop of your house or a tower, these can receive signal from up to 160 km. Outdoor directional FM antenna . Outdoor directional FM antennae, mounted on the rooftop of your house or a tower, can receive signals from up to 160 km in the direction these are pointed. FM signals drop out in the city because of tall buildings, obstructions, etc. The signal is reflected or absorbed by buildings. Outdoor directional FM antennae are effective in pulling FM signals into apartment buildings.
Car radio aerials are alwa nally. Telescopic rods are mon types of car radio aer include heated rear-wind stubby helical-wound ante the best from your car rad aerial should be: 1. Mounted as high a erably on the rooftop, and engine 2. Fully extended 3. Kept well clear of not folded back onto the r 4. Around 80cm long or longer than this may results
Common radio rec problems
Weak signal-hiss. If your stereo but not on mono,
FM dipole antenna FM dipole antennae considerably improve reception quality. Economical and relatively simple to install, these are built from TV ribbon cable, which is often supplied with hi-fi tuners. A dipole is customarily an electrical half wavelength of wire at the frequency of interest, since the 72-ohm resistive im-
Car antenna
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Antennae For Good Fm Reception And Fm Dx Listening: Radio
Original Title: FM Antenna Uploaded by bagsourav The Selection
fm Full description
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The Glass Castle: A Memoir
Alexander Hamilton
RADIO
ceiving a very weak signal. Hiss is produced because radio receivers need a stronger signal to successfully decode the stereo component of the signal. To eliminate radio hiss, you need a good aerial correctly positioned to pick up the best possible signal. Ideally, it should be mounted outdoors, like a TV aerial. You may add an amplifier to the aerial in order to boost weak signals. If your radio receiver is portable with no provision for plugging in an external aerial, adjust the position of the radio’s own aerial by tilting and swivelling it to get the best signal (see Fig. 4). If this fails, try moving the radio to somewhere else in the room for good reception. FM reception varies a great deal over very short distances, and is better near windows or upstairs than downstairs. Distorted ‘s’ sounds. Multipath distortion is characterised by sibilance, which
antenna. Try to mount the the house screens it from but not from the wanted s Tittering. Overloadin and discordant reception, r ground twittering sounds. curs when a radio antenn strong signal. So if you l transmitter, it will only cr The distortion will be pre reo and mono receptions. loading, plug an attenuato aerial and the aerial socke Adjacent channel cent channel interference channel that is close in f station being listened to. twittering noise in the called ‘birdies.’ This pro apparent on FM stereo. fering channel is very clo i.e. only 50 kHz or 100 noise heard
roof aeria lem, terfer is no the d desir stereo rate adjac
Fig. 3: Basic Yagi antenna consisting of four
is the distortion of ‘s’ and ‘z’ sounds to ‘shhhhh’. It is caused by the transmitted signal travelling to the listener’s radio receiver via more than one path (see Fig.
weat Abno cause in th This ally atmo is ex elements when creases dramatically to p warmer, drier atmosphe poor due to interference VHF (FM radio and TV)
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Antennae For Good Fm Reception And Fm Dx Listening: Radio
Original Title: FM Antenna Uploaded by bagsourav The Selection
fm Full description
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Fig. 4: FM DX set-up
Japanese engineers named Uda and Yagi. The Yagi antenna is a narrow-band antenna designed to work only on FM channel. It has the best gain for its size, and a correspondingly narrow main lobe (beam). For the highest gain or discriminating against an interfering signal 20°-40° azimuth off the desired signal, use a Yagi antenna. Figure out the centre frequency and bandwidth you wish to receive. The antenna’s gain rises slowly up to the de-
500-watt, 5.1dB yagi FM antenna
sign frequency and falls off sharply thereafter. FM is usually transmitted over 87.5108 MHz, so the centre frequency is about 100 MHz. Basically, the antenna consists of one
The Glass Castle: A Memoir
the director elements. The reflector reflects RF energy and the director directs RF energy. Typically, the reflector element is 5 per cent longer than the driven element and the director is 5 per cent shorter than the driven element. The antenna parameters (element lengths and spacings) are given in terms of wavelength, so an antenna for a given frequency can be easily designed. The lengths of various antenna elements are related to the frequency (f) as follows: Reflector length=
150 f (MHz)
Driven element length=
143 f (MHz)
First director length=
138 f (MHz)
Second director length=
134 f (MHz)
Use a folded-dipole as driven element and feed it with twin-lead line. A folded dipole has an impedance of 277 ohms, which is a good match to 300-ohm twin-lead line. Of course, the presence of other elements slightly reduces the input impedance, but there is still a good match. No balun is required, as twin-lead is a balanced transmission line. Being narrowband, the Yagi antenna can work well only at or near one frequency, unless you are willing to make a few compromises to broaden signal response. It’s pretty simple to
Alexander Hamilton
Suggestions for im FM DX reception
1. FM radio is much like FM reception as you w ception. 2. At home, put up a g rooftop FM antenna. 3. Use a fully shielded c ribbon cable, from you set and connect it to t the set using appropria 4. Consider a rotor to ai the desired radio statio 5. Buy the largest FM a ford. The bigger the a ter the reception! 6. Avoid indoor antenna ject to drifting per move about the room terference from PCs, TV and other appliances. 7. The ‘T’ type ribbon ant most hi-fi sets is gross it receives only the str nals. 8. Clock radios, portables without terminals for e don’t perform well at h are quite close to the have an unusually clear 9. Most cables are manufa attention to FM recep connect your FM radio tem and instead use th tenna for good receptio 10. Look for car radios that reo switches. The bes monaural mode. 11. Some car stereos h reception. The diversity antennae on the car and pares reception on bot better one to the radio 80-90 per cent of the fence distortion in a mo
make yourself a Yagi out elements or aluminium tu to ‘cut’ it for a particular f For a 50-ohm feed, th provides a gain o The F/B ratio is 1 your doubts, refe tenna before fab stalling the anten
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Antennae For Good Fm Reception And Fm Dx Listening: Radio
Original Title: FM Antenna Uploaded by bagsourav The Selection
fm Full description
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The Glass Castle: A Memoir
Alexander Hamilton
RADIO
VHF Slim Jim Antenna for 2m Band Here is a back-fed, vertical, folded-dipole antenna for the 2m band (144-146MHz) that is inexpensive and easy to build compared to ground-plane vertical antennae and Yagi antennae. In India, the frequency band allotted for amateur radio operation in VHF is 144-146 MHz. Generally, antennae work well only when placed over a good ground system. The success or failure of an antenna system depends on RF ground. Poor grounds cause antennae to operate at less than the best efficiency. In fact, it is possible to lose between 50 and 90 per cent of the RF power by heating the space under the radiation lobe instead of transmitting into the air. The antenna is a vertically-polarised, omnidirectional, free-space antenna offering a gain of about 1.8 dB. It is unobtrusive and has 50 per cent higher radiation efficiency than a ground-plane antenna due to a low radiation angle. As the antenna has no ground planes, it is very easy to place. The name Slim Jim comes from the slender construction. The Slim Jim vertical angle of radiation is almost parallel to ground, so the maximum radiation is where it is needed: straight out and all round. With all ground planes, including those with radials an entire wavelength long, the vertical angle radiation is tilted upwards at an angle of 30 degrees or more. This gives the Slim Jim a gain over 5/8th wave of 6 dB when measured parallel to the ground! The antenna is fed on the base, which causes no problem with the connection between the feeder and the antenna. The feeder impedance is 50 ohms. Slim Jim can be made of 10mm/12mm dia. aluminium pipes or a 300-ohm cable. The distance between the two parallel elements is not critical and neither is the length if it is made of 10mm pipes. Isolate the two halves, i.e. the half-wave and the quarter-wave halves, using pertinax or teflon. Use of a ‘J’ type matching stub (J integrated matching, or JIM) that facilitates feeding the antenna at the base overcomes the problem of interaction between the feeder and the antenna. The feed impedance is 50 ohms With a correct feed, the standing wave ratio (SWR) is less than 1.5 all over the band. The antenna can be constructed for perma- Fig. 6: Slim-jim antenna nent use or portable use. Basically, it is an end-fed folded dipole operated vertically. The matching stub provides a low-impedance (50-ohm) feed point at the base and couples to the antenna section at high impedance at one end. As with all folded dipoles, the currents in all legs are in phase, whereas in the matching stub the currents are in opposite phase, so little or no radiation occurs from the antenna. For a correct match, the voltage SWR is much less than 1.5, and remains so across the band. A slip sleave made from copper can be added to the element above the gap for tuning purposes, although the average length of the gap and spacing between the elements is 7.6 cm at 72 MHz and 2.5 cm at 220 MHz. No part of the antenna should be grounded to the tower or mast. For mounting, use a PVC pipe. Make sure the space between the tower and the antenna is one ‘freespace’ quarter wavelength. The antenna should stand upright on a railing or something else, clear of metal water tanks,
distorts the signals. So us fiers with very low noise in the coax lead-in. Boost the signal at the the weak signal better ov fier noise. Use outdoor a all signals are weak withi which the amplifier wor antenna mounted amplifie figure below 3 dB. If nearby signals (two or other FM signals) are amplifier with filters amplifier’s activity to a free of the strong signals. W distortion limit of an am observe signals that were gin with. These new signal two or more real signals stations at the same time. has a gain control, these rapidly as you get into dis
Dipole antenna
A dipole antenna consid the FM reception. To ma you need about 2.5 metre ductor cable and some ordinary lamp or speaker c but ideally you should cable—the kind used to c to a rooftop antenna. First, strip one end expose about 1.3 cm of ea the end you attach to your Next, measure out 1.5 met this end and tightly w electrician’s or duct tape around the cable at this exactly 89 cm from this t cable. Now use a knife or rate the two wires back to the tape is wrapped aroun not remove any of the pl You now have a ‘T’ shape shown in Fig. 5. This is tec dipole antenna which is tu at the middle of FM band
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Antennae For Good Fm Reception And Fm Dx Listening: Radio
Original Title: FM Antenna Uploaded by bagsourav The Selection
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The Glass Castle: A Memoir
Alexander Hamilton
R AADDI OI O R
ANTENNAE FOR GOOD FM RECEPTION AND FM DX LISTEN
Read on to know which antenna is good for FM reception, how you can improve FM D reception, how you can make a good, low-cost antenna yourself, and much more D. PRABAKARAN
recti usual gain, increa FM re
S
ignals on the FM broadcast band (88 to 108 MHz) are greatly affected by altitude and terrain. How high the receiver is placed and the landforms around it determine the strength of the signal you receive. For example, it may be difficult for someone located in a valley or on the side of a hill to hear FM stations from a particular direction, even if the stations are relatively closeby. On the other hand, someone living on the top of a high hill can reliably receive FM stations over 160 km away. Atmospheric conditions also cause the signals to fade. This is especially the case if the station is located far away and/or is transmitting at a low power.
Choosing an antenna for FM Before purchasing an FM antenna, consider your location and the problems you need to overcome. The key to good FM reception is the maximum direct signal from the transmitter with minimal signal reflection. If you live in a metropolitan area close to an FM transmitter, you’ll need an antenna with high directivity rather than a high-gain antenna. A good antenna rejects unwanted signals coming from different directions or bouncing off
Fig. 1: A typical FM antenna
The prime FM reception area extends from 3.2 to 40.2 km from the transmitting station. Here, multipath distortion is less of a problem, and a medium-gain antenna with a moderate beamwidth suffices. However, for receiving FM stations from different directions, you need a directional antenna with rotator. In the normal reception area from 32 to 64 km out, you need an antenna with good gain. If you are located more than 64 km out, you are in the ‘fringe’ area. Here, you need an antenna with a high gain. If stations come in at the same or adjacent channels, the antenna must be directional. If the antenna alone doesn’t provide enough gain,
an FM is m enced half-w as a f tenna an FM an ad 3 dB your proxi powe standard half-wave dipole A directional antenn those stations which lie fined antenna beamw beamwidth is a measure and is defined by half p dB) of the angle of the hemisphere. If the angle is antenna beamwidth), the ceive the desired FM broa signals reflected from diff directions, which may in broadcast. However, it m precisely position an ante narrow beamwidth for p of the desired FM broadca
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Antennae For Good Fm Reception And Fm Dx Listening: Radio
Original Title: FM Antenna Uploaded by bagsourav The Selection
fm Full description
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The Glass Castle: A Memoir
Alexander Hamilton
The dipole antenna sh mately 1.5m long and is connecting the ribbon ca receiver. It can be erect the cable to a wall, with of the cable positioned ho the best signal is found. tenna to optimise recepti sults, mount it high in the away from anything else wards the desired directio Fig. 2: Multipath reception
Power cord antenna. Most radio sets come equipped with an antenna consisting of a clamp that is clamped internally or externally to the AC power cord, then connected to the antenna terminal of the FM receiver. The AC power cord acts as an antenna. For best reception, extend the power cord to its full length and place in different positions until reception is clear. Short wire. Some receivers come with a short wire attached to one of the antenna terminals. If your receiver does not have either this or the power cord antenna, try about 75cm (30-inch) length of flat TV wire on one of the terminals of your set and orient the wire for best reception. Take the radio with its antenna to different locations in your home to see whether reception improves. TV antenna. Since FM signals are in the same frequency band as VHF television signals (channels 2-13), most VHF TV antennae can be used effectively on an FM receiver. So if you have a television antenna, hook it up to your FM receiver. If the transmitting stations are located in different directions, you need an antenna rotor for good reception of both TV and FM signals. A UHF/VHF/FM splitter allows the antenna to be connected to both the TV and FM sets at the same time. The splitters are inexpensive and can be found in the electronics accessory rack of most depart-
Car aerials than the average TV antenna on your FM radio. Easy to set up for indoor use, these improve reception of nearby FM stations for apartment dwellers. Indoor TV/FM antenna. These antennae are available in several models priced at Rs 100 to 200. Higher-priced models are usually intended for long-range reception. Outdoor FM non-directional antenna. Mounted on the rooftop of your house or a tower, these can receive signal from up to 160 km. Outdoor directional FM antenna . Outdoor directional FM antennae, mounted on the rooftop of your house or a tower, can receive signals from up to 160 km in the direction these are pointed. FM signals drop out in the city because of tall buildings, obstructions, etc. The signal is reflected or absorbed by buildings. Outdoor directional FM antennae are effective in pulling FM signals into apartment buildings.
Car radio aerials are alwa nally. Telescopic rods are mon types of car radio aer include heated rear-wind stubby helical-wound ante the best from your car rad aerial should be: 1. Mounted as high a erably on the rooftop, and engine 2. Fully extended 3. Kept well clear of not folded back onto the r 4. Around 80cm long or longer than this may results
Common radio rec problems
Weak signal-hiss. If your stereo but not on mono,
FM dipole antenna FM dipole antennae considerably improve reception quality. Economical and relatively simple to install, these are built from TV ribbon cable, which is often supplied with hi-fi tuners. A dipole is customarily an electrical half wavelength of wire at the frequency of interest, since the 72-ohm resistive im-
Car antenna
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Antennae For Good Fm Reception And Fm Dx Listening: Radio
Original Title: FM Antenna Uploaded by bagsourav The Selection
fm Full description
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The Glass Castle: A Memoir
Alexander Hamilton
RADIO
ceiving a very weak signal. Hiss is produced because radio receivers need a stronger signal to successfully decode the stereo component of the signal. To eliminate radio hiss, you need a good aerial correctly positioned to pick up the best possible signal. Ideally, it should be mounted outdoors, like a TV aerial. You may add an amplifier to the aerial in order to boost weak signals. If your radio receiver is portable with no provision for plugging in an external aerial, adjust the position of the radio’s own aerial by tilting and swivelling it to get the best signal (see Fig. 4). If this fails, try moving the radio to somewhere else in the room for good reception. FM reception varies a great deal over very short distances, and is better near windows or upstairs than downstairs. Distorted ‘s’ sounds. Multipath distortion is characterised by sibilance, which
antenna. Try to mount the the house screens it from but not from the wanted s Tittering. Overloadin and discordant reception, r ground twittering sounds. curs when a radio antenn strong signal. So if you l transmitter, it will only cr The distortion will be pre reo and mono receptions. loading, plug an attenuato aerial and the aerial socke Adjacent channel cent channel interference channel that is close in f station being listened to. twittering noise in the called ‘birdies.’ This pro apparent on FM stereo. fering channel is very clo i.e. only 50 kHz or 100 noise heard
roof aeria lem, terfer is no the d desir stereo rate adjac
Fig. 3: Basic Yagi antenna consisting of four
is the distortion of ‘s’ and ‘z’ sounds to ‘shhhhh’. It is caused by the transmitted signal travelling to the listener’s radio receiver via more than one path (see Fig.
weat Abno cause in th This ally atmo is ex elements when creases dramatically to p warmer, drier atmosphe poor due to interference VHF (FM radio and TV)
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Antennae For Good Fm Reception And Fm Dx Listening: Radio
Original Title: FM Antenna Uploaded by bagsourav The Selection
fm Full description
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Fig. 4: FM DX set-up
Japanese engineers named Uda and Yagi. The Yagi antenna is a narrow-band antenna designed to work only on FM channel. It has the best gain for its size, and a correspondingly narrow main lobe (beam). For the highest gain or discriminating against an interfering signal 20°-40° azimuth off the desired signal, use a Yagi antenna. Figure out the centre frequency and bandwidth you wish to receive. The antenna’s gain rises slowly up to the de-
500-watt, 5.1dB yagi FM antenna
sign frequency and falls off sharply thereafter. FM is usually transmitted over 87.5108 MHz, so the centre frequency is about 100 MHz. Basically, the antenna consists of one
The Glass Castle: A Memoir
the director elements. The reflector reflects RF energy and the director directs RF energy. Typically, the reflector element is 5 per cent longer than the driven element and the director is 5 per cent shorter than the driven element. The antenna parameters (element lengths and spacings) are given in terms of wavelength, so an antenna for a given frequency can be easily designed. The lengths of various antenna elements are related to the frequency (f) as follows: Reflector length=
150 f (MHz)
Driven element length=
143 f (MHz)
First director length=
138 f (MHz)
Second director length=
134 f (MHz)
Use a folded-dipole as driven element and feed it with twin-lead line. A folded dipole has an impedance of 277 ohms, which is a good match to 300-ohm twin-lead line. Of course, the presence of other elements slightly reduces the input impedance, but there is still a good match. No balun is required, as twin-lead is a balanced transmission line. Being narrowband, the Yagi antenna can work well only at or near one frequency, unless you are willing to make a few compromises to broaden signal response. It’s pretty simple to
Alexander Hamilton
Suggestions for im FM DX reception
1. FM radio is much like FM reception as you w ception. 2. At home, put up a g rooftop FM antenna. 3. Use a fully shielded c ribbon cable, from you set and connect it to t the set using appropria 4. Consider a rotor to ai the desired radio statio 5. Buy the largest FM a ford. The bigger the a ter the reception! 6. Avoid indoor antenna ject to drifting per move about the room terference from PCs, TV and other appliances. 7. The ‘T’ type ribbon ant most hi-fi sets is gross it receives only the str nals. 8. Clock radios, portables without terminals for e don’t perform well at h are quite close to the have an unusually clear 9. Most cables are manufa attention to FM recep connect your FM radio tem and instead use th tenna for good receptio 10. Look for car radios that reo switches. The bes monaural mode. 11. Some car stereos h reception. The diversity antennae on the car and pares reception on bot better one to the radio 80-90 per cent of the fence distortion in a mo
make yourself a Yagi out elements or aluminium tu to ‘cut’ it for a particular f For a 50-ohm feed, th provides a gain o The F/B ratio is 1 your doubts, refe tenna before fab stalling the anten
81 views
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Antennae For Good Fm Reception And Fm Dx Listening: Radio
Original Title: FM Antenna Uploaded by bagsourav The Selection
fm Full description
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The Glass Castle: A Memoir
Alexander Hamilton
RADIO
VHF Slim Jim Antenna for 2m Band Here is a back-fed, vertical, folded-dipole antenna for the 2m band (144-146MHz) that is inexpensive and easy to build compared to ground-plane vertical antennae and Yagi antennae. In India, the frequency band allotted for amateur radio operation in VHF is 144-146 MHz. Generally, antennae work well only when placed over a good ground system. The success or failure of an antenna system depends on RF ground. Poor grounds cause antennae to operate at less than the best efficiency. In fact, it is possible to lose between 50 and 90 per cent of the RF power by heating the space under the radiation lobe instead of transmitting into the air. The antenna is a vertically-polarised, omnidirectional, free-space antenna offering a gain of about 1.8 dB. It is unobtrusive and has 50 per cent higher radiation efficiency than a ground-plane antenna due to a low radiation angle. As the antenna has no ground planes, it is very easy to place. The name Slim Jim comes from the slender construction. The Slim Jim vertical angle of radiation is almost parallel to ground, so the maximum radiation is where it is needed: straight out and all round. With all ground planes, including those with radials an entire wavelength long, the vertical angle radiation is tilted upwards at an angle of 30 degrees or more. This gives the Slim Jim a gain over 5/8th wave of 6 dB when measured parallel to the ground! The antenna is fed on the base, which causes no problem with the connection between the feeder and the antenna. The feeder impedance is 50 ohms. Slim Jim can be made of 10mm/12mm dia. aluminium pipes or a 300-ohm cable. The distance between the two parallel elements is not critical and neither is the length if it is made of 10mm pipes. Isolate the two halves, i.e. the half-wave and the quarter-wave halves, using pertinax or teflon. Use of a ‘J’ type matching stub (J integrated matching, or JIM) that facilitates feeding the antenna at the base overcomes the problem of interaction between the feeder and the antenna. The feed impedance is 50 ohms With a correct feed, the standing wave ratio (SWR) is less than 1.5 all over the band. The antenna can be constructed for perma- Fig. 6: Slim-jim antenna nent use or portable use. Basically, it is an end-fed folded dipole operated vertically. The matching stub provides a low-impedance (50-ohm) feed point at the base and couples to the antenna section at high impedance at one end. As with all folded dipoles, the currents in all legs are in phase, whereas in the matching stub the currents are in opposite phase, so little or no radiation occurs from the antenna. For a correct match, the voltage SWR is much less than 1.5, and remains so across the band. A slip sleave made from copper can be added to the element above the gap for tuning purposes, although the average length of the gap and spacing between the elements is 7.6 cm at 72 MHz and 2.5 cm at 220 MHz. No part of the antenna should be grounded to the tower or mast. For mounting, use a PVC pipe. Make sure the space between the tower and the antenna is one ‘freespace’ quarter wavelength. The antenna should stand upright on a railing or something else, clear of metal water tanks,
distorts the signals. So us fiers with very low noise in the coax lead-in. Boost the signal at the the weak signal better ov fier noise. Use outdoor a all signals are weak withi which the amplifier wor antenna mounted amplifie figure below 3 dB. If nearby signals (two or other FM signals) are amplifier with filters amplifier’s activity to a free of the strong signals. W distortion limit of an am observe signals that were gin with. These new signal two or more real signals stations at the same time. has a gain control, these rapidly as you get into dis
Dipole antenna
A dipole antenna consid the FM reception. To ma you need about 2.5 metre ductor cable and some ordinary lamp or speaker c but ideally you should cable—the kind used to c to a rooftop antenna. First, strip one end expose about 1.3 cm of ea the end you attach to your Next, measure out 1.5 met this end and tightly w electrician’s or duct tape around the cable at this exactly 89 cm from this t cable. Now use a knife or rate the two wires back to the tape is wrapped aroun not remove any of the pl You now have a ‘T’ shape shown in Fig. 5. This is tec dipole antenna which is tu at the middle of FM band
