Chapter 32 - Electronic Keyer

 We have touched all important stages of Ham Band Transmitters and Receivers. One thing we left untouched was the electronic keyer. Serious CW operators know that a paddle is the way to make CW easy and fun. Building one for oneself also is not a difficult task except it requires careful soldering and IC mounting. The electronic keyer is configured in such a way the moment one touches the paddle the entire system changes over to transmission. The speed of a keyer also can be set. Those contact problems with Morse keys are also do not affect the performance of a electronic keyer. The pad contacts at the electronic keyer requires no pressure at all.   In C-32/1 see the circuit of an electronic keyer. 

In my experience all those who do SSB transceivers builds keyers too. If I share my personal experience insisting on quality components and beginning a project with all god quality components arranged will save you a lot of time and money. Still failures and repeated efforts are not uncommon. There could not be a Ham home brewer who would not have done at least fifty mistakes. This comment goes to one of the best home brewers in India - Late OM TP Guhan Menon VU2TG. In short every Ham had stories of difficulties to share. 

Working an equipment developed by oneself from bottom is a matter of great pleasure for all Amateurs. The equipment should be compact, portable, operational in battery, sensitive to weak signals, powerful and easy to operate .... individual specifications vary. The truth is that there are Hams who could reach this state of affairs too, though with difficulties. What we were discussing through all those previous chapters was guidelines that could take anyone to such a level of craftsmanship. If this Book could be of any help to the readers the efforts behind this is justified.

Chapter 31

Chapter 31 - Multiband German Quad Antenna.

 As we all have understood by now, the most important stage with regard to a transmitter is certainly an antenna. Unfortunately, I doubt if at least most of the home brewers give due importance to it. If the antenna is not resonant to the frequency, if the SWR is not matched and if the transmission feeder and antenna arms are not quality pieces, there would not be any power to go out into the space. Dipoles stand out among simple and effective of antennas. Another type of antenna which can be used at situations of space limitations is ground plain about which we already discussed.  The length of a round plane radiator element is calculated with the formula =  234/f (MHz) in feet. 

The German Quad designed by a German Ham DL31SA DL31SA also belongs to the list of most effective multi band antennas. The substance is a quad with each arm at 71.75 feet. Total length of the copper wire is 287 feet. Just fix it 30 horizontally at feet above the ground. Further of the terminals are connected to 75 Ohms coax cable, the German Quad Antenna is complete. The gain of this antenna is 6 db more than the usual antennas on use. One attraction is that it can be used in all HF Bands. It is the ground that acts like a reflector.  If the ground conductivity is pretty good the gain could go up to 10 db even. If we are not sure of the ground conductivity, we can substitute it with another quad at the measurement of 5% more than the driven element and fixed below it by 0.15 feet below the wave length. With reflectors like this the efficiency of dipoles also can be increased.  If the proposal is for a German quad for a definite frequency, the length of the total copper wire is 1005/f (MHz) in feet.

In HF bands, all theories are against the concept of mobile antennas. If the length of radiator arms are below 1/4 the full wave length, there will be considerable loss in its radiation resistance. To see the quantity of loss see that if we feed 100 W RF to a 80 meter 8 feet long Whip antenna at perfectly matched a condition, what that radiates from the arms will be just 1 W. According to Indian telecommunication rules, mobile antennas are not permitted in HF Bands. Even at VHF bands mobile antennas require special permission from WPC. Any decrease in the length of radiator arms will affect the bandwidth also. In the 80 meter antenna situation explained above theoretically the bandwidth will be near to 5 KHz. The radiation resistance of a half wave dipole is calculated to be 73.13 Ohms. Arm length is reduced means the bandwidth also is reduced proportionally.  

In most situations special SWR meters are home brewed. When we use such meters there is great possibility for errors. A calibration with ref. to a commercial SWR meter may not be enough. It requires frequent verification.   Before going for else readings the SWR meter should be set for full scale at forward power. 

There are a lot more types of antennas like Delta Loop, quibical Quad, Yagi Beam, belonging to categories of rotational and non rotational and remote controlled and manual controlled, which we have not even referred at all. Since the purpose of this book is paving the way for earnest enthusiasts who loves the hobby, a deep study is quite in irrelevant. This book is also limited to HF spectrum; this is simply because no much home brewing is seen in VHF. It is the deep wish of every Amateur that HamRadio should grow fast. I should ay that within the last few years the number of Hams have multiplied by more than 20 times. In 1984 I was the 134th licensee in Kerala. 20 and 40 meters are too busy at peak hours in the morning and the evening. It is natural that Amateurs will turn to SSBs. Another possibility is switching over to mostly free lower bands. One discouraging factor is that at 80 M a full antenna requires a free space long by at least 130 feet.  Here, in C31/2 we give you a loop antenna for 80 meters that demands only much less space than the dipole configurations. 

Two pieces of 8 meters long RG8U cables are the main parts of this antenna. The 50 Ohms coaxial feeder cable is connected at the centre of the vertically fixed loop antenna, on top. Both the terminals of the cable RG8/U cable closed by braid (shield) and core. Tuning metal gang should be closer to the point of cable connection.  At the lower  side of the loop RG 8U will be available only by 1.68 meter from either sides. The remaining portion is used for a capacitive action in the antenna. This capacitor is simply a TV ribbon cable of 300 ohms. One wire of the ribbon wire is soldered to one side of the loop and the other to the opposite side. This antenna can be tuned to absolute resonance using a dip oscillator.  

For frequency tuning just trim the open sides of the ribbon wire accordingly. The capacitor on top is also used for SWR matching.  Only problem with this antenna is that there will be a loss of 10 db gain when compared to a full wave antenna.

Chapter 30                                                                    Chapter 32

hapter 30 - Antennas

 Experiments and Researches, observations and inferences ...all are part of the royal hobby of Amateur Radio.  Most of the Amateurs go to bed listening the music of the soldering iron and the first thing they do every day is switching their rigs on. I also was not different and that's why I could prepare this long article on home brewing. To understand this hobby in full it is a must that every Ham swim into all its possibilities.  I think the majority of experiments Hams did could be on antennas. This is one area in which Acharya Jagadish Chandra Bose, who first utilised a radio signal, did not touch.  It can be precisely said that every Ham uses their own unique antennas. Still, that question is there - which is the best antenna? Even we cannot say that antennas are better if higher. There is an optimum point even int he matter of height. 

A forty meter antenna at 20 meter height works fine in features. Whatever be the polarisation of the antenna it won't give satisfactory performance in arm random arm measurements, other than that of exact divisions of wave length, won't work satisfactorily. Below VHF shielded wires or stranded wires or open wires can be used for antenna arms. At VHF and above since there are substantial changes in the velocity factor of insulated wires, when compared to open wires, insulated wires are not recommended.  Also stretchable type of wires are not used for antennas. When selecting a particular gauge we need to remember that the 'Q' factor increases respectively to increasing diameter of the antenna arms. This is applicable while winding any coil too. Simply because of increasing skin effect experienced on the outer part of any as the frequency increases, thicker wires would be better in higher frequencies. A 200 W RF transmitter can use a 28 SWG wire as antenna without any fear of damage to it. 

I think the Indian Hams are particular on having copper wires for antenna arms. Because the power loss increases with higher resistivity and electrical contact is not easy in many conductors as that in copper, most people recommend copper for antennas. This does not however mean that metals like aluminium are not usable here. During Kuwait war an Amateur from Kuwait used an Aluminium antenna and successfully contacted Kerala.  

Whatever, antenna line contacts need something more than simple touch contacts. Even at low power transistors, pin and sockets like PL - 259 and SO -237 are recommended. A few more things are to shared with regard to coaxial cables. They are mostly available in 50 ohms and 75 ohms categories. RG-8 and RG-p categories with thick shielding and stranded wires inside are the best in the market. When ordinary TV cables are used for transmission line around 50% of he power is lost. The higher the frequency higher the loss. That's why even RG 58U, RG 59U are not recommended at frequencies higher than 14 MHz.  In VHF Va RG 58U cables wastes 50% of the output power. 

Cables that have full outer shielding are always the best. If they are silver plated the loss will further be decreased. If the quality of the plastic cover is low the inner copper wires will easily get green due to chemical changes doe to atmospheric interference. The output power reaching the other end of similar cables will be comparatively low. When anyone goes for antennas I want these factors also be considered duly. 

All the way we were laking about antennas that require big space. A Ham who is destined to remain confined to a small room cannot ever be satisfied with any of them. Magnetic loop antennas are just for them. If you can make a ring of 3.1 dia. 10mm aluminium hollow rod, a magnetic loop antenna is possible. Filling the rod with smooth sand particles, using a drum of equal diameter to coil the tube and heating the tube ..... all helps to bend the pipe without damage, unless you have with you a bending mechanism. A 12' long pipe is enough for this. The ends of the pipe should be cut off so that the distance between ends shall be 3/4". Fix this loop vertically on any non- conductor so that the open ends come above. Connect the terminals to a 150 PF variable air gang capacitor, which should be fitted mechanically as close as possible to the terminals. 

When tuning the gang, be careful not to make body contacts with the metal parts of the gang. You may use a knob. Fix an RF socket close to the opposite point of the open terminals, where the shield of the cable is to be connected. The live connection comes somewhere at a point away by 6 to 7 inches from any terminal of the ring. However this point varies from ring to ring and confirm using a SWR meter. This wire from the RF socket should be fixed in a semi circle. This ring is advised to be fixed on a wood piece or an insulated material that an be moved. The mounting details are decided by the maker. It is circular polarisation that is used here. This can be used either  outdoor and indoor and at any power. See fig. C-30/1. 

This is usable at any HF bands. The disciplines require for multi band antennas had been discussed before. Since a phase shift of 180 degrees between magnetic electric waves, electrical noise is almost fully eliminated in this design. If its' features, especially that of its' 'Q',  are used appropriately this design could be very effective for Receivers.

Another antenna of low space is inverted 'L' antenna. See fig. C-30/2. 

This antenna also asks for more heights. Live connection of the 50 ohms cable is connected to the horizontal arm and the shield at the cable junction shall be connected to the arm going towards the ground, which should be strongly earthed, using more than one radials. Both the horizontal and vertical length of the live arm need not necessarily be equal. Let the distance between ground wire and live wire in the cable connecting point be 1/2 inch. The word insulator here refers to anything from plastic to all available non conductors, but the best always will make a difference in the output also.  

In C-30/3 the schematic diagram of a ground plane antenna for VHF transmission is shown. This is usable at all Ham bands above 14 MHz. 

Not only the vertical driven element but also the radials to all four sides shall be simple wires in ordinary use. The vertical driven element can be copper or tube aluminium. If it is plane wire a copper reaper of the same length can be used to keep it always erect. The insulator for the driven element is fixed at the centre of the square base metal plate. The live point of the feeder cable goes to the driven element while the shield is connected to the metal base plate to which all four wires from its corners are connected.  The one major difficulty with a ground plane is that  it has a dead zone of 100 to 400 Kms after a live zone of 150 kms.  150 kilo meters, there could be a dead zone of 100 to 400 Kms. For Dxing, this is one among the best antennas. The impedance of this omnidirectional antenna is 50 Ohms. Each of its radials are 5% longer than the driven element length. The base plate can be fixed on a mast pipe. The angle of the radials are advised to be 45 degrees. The length of the driven element is 1/4th of the wave length. There are lot more types of antennas still un introduced, that are in use. 

Chapter 29                                                Chapter 31

Chapter 29 - Transmitter Antenans

 Even with a similar transmitter and antenna, the report received and the contact experience need not exactly be the same especially when contacting Dx stations. It was already told that not only the peculiarities of the atmosphere but also the height of the antenna are determining. Directional qualities of low level antennas will be low and the radiation angle will be high.  Only low radiation angle signals go to more distance.  But at low angle radiation the dead zone effect will be higher. Away from line of sight of antennas the next point of the same signal will be where the nearest reflected wave reaches the ground. Th area in between these two points where no signals from that particular antenna is available is called the dead zone.  See fig: C-29/1

The effect of sunspots on propagation also are relevant. The study on the effects of sunspots are still continuing. Those black spots on the face of the sun which keeps changing in size and number, keeps changing the magnetic activity of the sun - thus invariably influencing radio propagation in the earth's atmosphere.  Simply because it repeats its activities at an eleven year pattern, this sun spot activity also is called 'eleven year cycle'. 

There had been seasons in which there were no sunspots. During sunspots those ultra violet radiations from the sun create strong magnetic hurricanes on the earths atmosphere. Black outs where no radio transmission is practically done are the result of these magnetic bursts.  

Anything in the vicinity of the transmission field is subjected to change as if a thing in a microwave oven. For human beings close to the feed point, continuous interaction results in heating of internal organs. if he heat crosses 2 degrees Celsius experiments have proved that it could end up in physical disabilities.  If a man remains close to a 400W power 7 MHz transmitter antenna continuously for a hour, the level of heat it causes in the body can be calculated to be 1 degree Celsius. This is enough for partial blindness or deafness. The more the frequency the more will be the magnetic energy the body absorbs. Even if the VHF transceivers are very low in power,  continuous usage keeping it close to the body might cause to generate enough body heat that could be badly damaging our body functions. 

Whatever be the type of antenna used, the length of the feeder cable could make transmission losses and it is always advised to keep the length as low as possible. Antennas are designed at a style in which the antenna feed point comes directly above the transmitter. The easiest of antennas is the single wire antenna, except that  a matching circuit and trans-matching arrangement is necessary for it. One big problem here is the 'hot chassis', the problem created by RF energy that are found at various points on the chassis. Unless the chassis is strongly earthed using appropriate accessories, it may end up in damage of various components. Still single wire antennas have the quality of adaptability to different bands. 

It is not simply space availability or limitation based issues that compels for inverted 'V' and sloper dipole antennas. Both are vertically polarised and are better in ground waves and long distance contacts, compared to horizontally polarizsed antennas.   

If the impedance of Sloper antenna is 75 Ohms the impedance of the inverted V is 50 Ohms. Always a 90 to 120 degree feed point angle is recommended.  In either cases the formula to find the length of a dipole arm is = 468/F (MHz)=L (feet). When an antenna is tuned to other meter bands through trans-match, efficiency will be low at all higher frequencies than the one to which the antenna is tuned.  

Any wire is usable for an antenna, provided it should not be subject to chemical and physical changes in the due course. Normally copper wires of 12,14 and 16 SWGs are ideal.  If it can be mounted without breaks and damages and protected against natural obstacles, 26 SWG is enough for a low power transmitter. Stranded type or single wire type also make no much difference in its gain.  If the available space is very limited there are certain modification possibilities - antenna arms can be bent. Inductors and capacitors connected in series to antenna arms also leads to frequency resonation and a trans-matching circuit to bring impedance match too together makes it complete.  

There is also the tradition of fitting loading coils in the centre of  both the arms to bring the antenna into an impedance matching situation. All these short cuts but increases the 'Q' of the antenna and reduces the bandwidth. Make 60 turns at one and half inch dia. with 16 SWG wire leaving one foot at one end and eight feet at the other end. Connect the feeder cable at the one foot side keeping terminals at half inch distance (just like we do in horizontal dipole. Fix the other ends of the arms (eight feet long part) as shown in C-29/3. This is the details of a 40 M antenna. With any ATU illustrated already, this will also give good results, with slight decrease in gain. 

Study of antennas is quite interesting. Basically it was mere enthusiasm that led Louis Varney (G5RV) to the invention of the very popular multi band G5RV antenna in 1946. This antenna is very popular in the United States and is found best for 20 meters. It should be noted that this antenna can be erected as horizontal dipole, as sloper, or an inverted-V and with a  trans-match, it can be operated on all HF amateur radio bands (3.5–30 MHz). 
In effect this is a dipole only with each arm measuring 51 feet each.  If that much open space is not available, each arm can be bent in 90 degree at 31 feet to hang the remaining 20 feet vertically down. Height is vvery critical for G5RV Antennas.  Even if this works in 25 feet too, 3o feet is the ideal recommended height. it is 300 ohms TV ribbon cable that is used for feeder cable. The length of the feeder cable is fixed at 29 feet 6 inches. If the height is low the lower part of the feeder cable can be bent slightly. Because the length of this cable is critical in impedance matching at different bands, this part of G5RV antenna is called matching stub. This antenna can be erected in inverted 'V' pattern with a 30 feet high pole in the centre. If it is open wire that is used as matching stub the suggested length is 34 feet. This matching stub is connected to a trans-matching arrangement.  After trans-matching antenna is connected to the transmitter through an SWR meter. That portions that is not matching stub can be 75 ohms coax cable, ribbon wire or open wire.  Compared to a dipole a 3 db more gain is  marked for G5RV Antenna. 

The impedance-matching symmetric feed line (ladder-line or twin-lead) can be either 300 Ohm (8.84 metres or 29.0 feet) or 450 Ohm (10.36 metres or 34.0 feet). As is in general the case for all electric antennas, the height of the G5RV above the ground should be at least half of the longest wavelength to be used. There are many variants of the G5RV antenna.

Low angle radiation that is very much required for bands above 7 MHz is the speciality of G5RV Antennas. Another version of G5RV is available at half the size of the measurements given here. Here, the length of the matching stub also will be half. For better G5RV, the ribbon wire matching stub is soldered to the arms at the centre with each wire end coiled at 9" length.  

Chapter 28                                                        Chapter 30

Chapter 30 - Antennas

Experiments and Researches, observations and inferences ...all are part of the royal hobby of Amateur Radio.  Most of the Amateurs go to bed listening the music of the soldering iron and the first thing they do every day is switching their rigs on. I also was not different and that's why I could prepare this long article on home brewing. To understand this hobby in full it is a must that every Ham swim into all its possibilities.  I think the majority of experiments Hams did could be on antennas. This is one area in which Acharya Jagadish Chandra Bose, who first utilised a radio signal, did not touch.  It can be precisely said that every Ham uses their own unique antennas. Still, that question is there - which is the best antenna? Even we cannot say that antennas are better if higher. There is an optimum point even int he matter of height. 

A forty meter antenna at 20 meter height works fine in features. Whatever be the polarisation of the antenna it won't give satisfactory performance in arm random arm measurements, other than that of exact divisions of wave length, won't work satisfactorily. Below VHF shielded wires or stranded wires or open wires can be used for antenna arms. At VHF and above since there are substantial changes in the velocity factor of insulated wires, when compared to open wires, insulated wires are not recommended.  Also stretchable type of wires are not used for antennas. When selecting a particular gauge we need to remember that the 'Q' factor increases respectively to increasing diameter of the antenna arms. This is applicable while winding any coil too. Simply because of increasing skin effect experienced on the outer part of any as the frequency increases, thicker wires would be better in higher frequencies. A 200 W RF transmitter can use a 28 SWG wire as antenna without any fear of damage to it. 

I think the Indian Hams are particular on having copper wires for antenna arms. Because the power loss increases with higher resistivity and electrical contact is not easy in many conductors as that in copper, most people recommend copper for antennas. This does not however mean that metals like aluminium are not usable here. During Kuwait war an Amateur from Kuwait used an Aluminium antenna and successfully contacted Kerala.  

Whatever, antenna line contacts need something more than simple touch contacts. Even at low power transistors, pin and sockets like PL - 259 and SO -237 are recommended. A few more things are to shared with regard to coaxial cables. They are mostly available in 50 ohms and 75 ohms categories. RG-8 and RG-p categories with thick shielding and stranded wires inside are the best in the market. When ordinary TV cables are used for transmission line around 50% of he power is lost. The higher the frequency higher the loss. That's why even RG 58U, RG 59U are not recommended at frequencies higher than 14 MHz.  In VHF Va RG 58U cables wastes 50% of the output power. 

Cables that have full outer shielding are always the best. If they are silver plated the loss will further be decreased. If the quality of the plastic cover is low the inner copper wires will easily get green due to chemical changes doe to atmospheric interference. The output power reaching the other end of similar cables will be comparatively low. When anyone goes for antennas I want these factors also be considered duly. 

All the way we were laking about antennas that require big space. A Ham who is destined to remain confined to a small room cannot ever be satisfied with any of them. Magnetic loop antennas are just for them. If you can make a ring of 3.1 dia. 10mm aluminium hollow rod, a magnetic loop antenna is possible. Filling the rod with smooth sand particles, using a drum of equal diameter to coil the tube and heating the tube ..... all helps to bend the pipe without damage, unless you have with you a bending mechanism. A 12' long pipe is enough for this. The ends of the pipe should be cut off so that the distance between ends shall be 3/4". Fix this loop vertically on any non- conductor so that the open ends come above. Connect the terminals to a 150 PF variable air gang capacitor, which should be fitted mechanically as close as possible to the terminals. 

When tuning the gang, be careful not to make body contacts with the metal parts of the gang. You may use a knob. Fix an RF socket close to the opposite point of the open terminals, where the shield of the cable is to be connected. The live connection comes somewhere at a point away by 6 to 7 inches from any terminal of the ring. However this point varies from ring to ring and confirm using a SWR meter. This wire from the RF socket should be fixed in a semi circle. This ring is advised to be fixed on a wood piece or an insulated material that an be moved. The mounting details are decided by the maker. It is circular polarisation that is used here. This can be used either  outdoor and indoor and at any power. See fig. C-30/1. 

This is usable at any HF bands. The disciplines require for multi band antennas had been discussed before. Since a phase shift of 180 degrees between magnetic electric waves, electrical noise is almost fully eliminated in this design. If its' features, especially that of its' 'Q',  are used appropriately this design could be very effective for Receivers.

Another antenna of low space is inverted 'L' antenna. See fig. C-30/2. 

This antenna also asks for more heights. Live connection of the 50 ohms cable is connected to the horizontal arm and the shield at the cable junction shall be connected to the arm going towards the ground, which should be strongly earthed, using more than one radials. Both the horizontal and vertical length of the live arm need not necessarily be equal. Let the distance between ground wire and live wire in the cable connecting point be 1/2 inch. The word insulator here refers to anything from plastic to all available non conductors, but the best always will make a difference in the output also.  

In C-30/3 the schematic diagram of a ground plane antenna for VHF transmission is shown. This is usable at all Ham bands above 14 MHz. 

Not only the vertical driven element but also the radials to all four sides shall be simple wires in ordinary use. The vertical driven element can be copper or tube aluminium. If it is plane wire a copper reaper of the same length can be used to keep it always erect. The insulator for the driven element is fixed at the centre of the square base metal plate. The live point of the feeder cable goes to the driven element while the shield is connected to the metal base plate to which all four wires from its corners are connected.  The one major difficulty with a ground plane is that  it has a dead zone of 100 to 400 Kms after a live zone of 150 kms.  150 kilo meters, there could be a dead zone of 100 to 400 Kms. For Dxing, this is one among the best antennas. The impedance of this omnidirectional antenna is 50 Ohms. Each of its radials are 5% longer than the driven element length. The base plate can be fixed on a mast pipe. The angle of the radials are advised to be 45 degrees. The length of the driven element is 1/4th of the wave length. There are lot more types of antennas still un introduced, that are in use. 

Chapter 29                                                         Chapter 31

Chapter 28 - Simple Dipole Antennas

Even though it may seem that erecting an Antenna is an easy job, the truth is not that. Those who know that a good antenna picks up even the weakest of signals are few. All antennas used for transmissions are best for receivers too but not vice versa. In transceivers the same antenna is used for both transmission and reception. In most multi band low power transmitters, the suggested antennas are not one than absorbs all the output power. It means that a well tuned antenna makes wonderful difference. But only very few experiences this miracle. 

Any antenna includes, tuners, matching circuits, feeder cables and radiators. Almost all QRPs run on the simple half wave dipole antenna. It is easy to build and efficient in performance. When referred to earth, Antennas can be said to be polarized. Antennas are made in different types of polarization - vertical, horizontal, circular etc. Horizontal Dipole antennas belong to horizontally polarized category. The type of recommended polarization depends upon the frequency in which it is used. Antennas made for HF transmissions are not at all usable in VHF ranges and vice versa. The radiation pattern of signals are that different. It is considering such delicate features too that they are divided into LF, HF VHF etc. In HF itself the radiation pattern is different to each frequency. 
That's why from 14 MHz on height also turns critical. Before choosing a particular type of antenna, know  details like the expected points and areas where we mean to reach the signal, including the financial and technical capacity of the user. There is no limit to the possibilities open. 

A deep study on all types and kinds of Antennas do not come within the limits of this book.  But it will be unfair to leave the most used types dipoles untouched or well explained. The impedance of such antennas cannot clearly be assessed to be just 75 Ohms. The impedance changes according to the length of antenna arms, distance from tree branches and other conducting mediums, height from the ground and peculiarities in constructing it... all join to decide the overall impedance of the load. Even signal characteristics itself changes the antenna impedance.  the one who knows the importance of antenna matching either changes the antenna impedance or changes the output impedance. There is still one more possibility - use a matching circuit in between.  Even if the SWR reading is found at the lowest point, it may not mean that the antenna is fully resonant to that particular frequency. Also it is not practical to insist that the Antenna should be absolutely matching to the HF Transmitter output.  But this liberal attitude is not at all advised in VHF and above, even if trimming antenna arms are difficult at high antennas.  

The formula for finding out the total length of a dipole antenna in feet is 492/F(Mhz).  In most situations the antenna is designed to be used at an whole range, say 7000 KHz to 7100 KHz in 40 meters. There the frequency considered for calculation will be 7050 KHz. Here, maximum gain will be served at 7050 KHz and a proportional gain decrease will happen as the operating frequency moves away from 7050 KHz.  This is because the 'Q' of the antenna is high. At high 'Q' antennas  'Q' bandwidth also will be much less. This could lead to mismatch in lower Higher bands. One method to increase bandwidth is to increase the circumference of the antenna wire. If we do so, capacitance at every unit length will be go up while inductance will come down. The change this makes in the 'Q' aspect is higher than the difference it makes in the Ohmic resistance and radiation resistance of the wire. This means that the 'K' factor also is to be considered while increasing the size of the antenna wire.  

There , the formula becomes 492 X K / F(Mhz). At higher edges in HF itself, this should be the formula to be used. Although the antenna may be an electrical half wavelength, or multiple of half wavelengths, it is not exactly the same length as the wavelength for a signal travelling in free space. There are a number of reasons for this and it means that an antenna will be slightly shorter than the length calculated for a wave travelling in free space.

For a half wave dipole the length for a wave travelling in free space is calculated and this is multiplied by a factor "A". Typically it is between 0.96 and 0.98 and is mainly dependent upon the ratio of the length of the antenna to the thickness of the wire or tube used as the element. As said, the 'K' factor is approximately 0.97. This also means that for absolute matching the full length of antenna arms found out through the formula needs some cutting, for maximum impedance matching. 'K' is the factor that shows the necessary difference, in the formula.

In C-28/1 the picture of a 7 MHz half wave dipole antenna is given. in the centre, distance between dipole arms is half inch. It is good to water proof the centre separator and the joints. Only if the dipole height is in perfect divisions of the operating signal wave length or near to it, the impedance also come closer to the proposed value. Simply because at 14MHz and above it is Dx communication that is intended antenna height is decided to match low radiation angle. 

Even at 7Mhz radiation pattern varies according to antenna height. In general case, the radiation pattern is such that maximum radiation happens towards arm sides (broad side) of the antenna. The radiation pattern while the antenna height is at half the wave length or full wave length is not similar to what we already shared. That is why antennas at wavelength height is equally efficient for both long distance and short distance communication. If horizontal dipoles are bidirectional, the same when becomes vertical turns omni directional (all directions).

In C-28/2  there is the details of a half wave dipole antenna using the ribbon wire used for TV antennas. If this is used for receiver only, the capacitor tuning is not necessary. The attraction of this dipole is that perfect matching at SWR 1:1 is possible.  The cable from antenna to the equipment should be vertically hanging. Length adjustments in coiling is not recommended. The value of the tuning capacitor used in between may be close to that of a metal gang capacitor. It may be used here. The length between capacitor and final transistor is output is = impedance of the transistor divided by the frequency = - feet.  For example, in 40 meters it is 75/7(MHz) feet. Both the ends of this horizontal dipole antenna are fixed to insulators at either sides in the atmosphere. This antenna belongs to folded dipole class.

Chapter 27                                                    Chapter 29

Chapter 27 - SWR Protection

 

Whenever we discussed about RF amplifiers, we were simultaneously referring to the importance of stage matching too. This is not simply a problem of stage loss where for ex. when a 10W signal is coupled to another stage, only 2 W reaches the other. Just like a ripple in a bowl returns back to the centre after touching the sides or like light being reflected from a mirror, we need to assume that the lost power returns back to the source. Here, the power  that  moves forward is called forward energy and the power that returns is called reflected wave energy. Also we need to know that full transfer is not at all possible even at the best optimum arrangements and care. Only when all the applied signal is  fully absorbed by the following stage that absolute cancellation of reflected wave energy happens along with retention of stability in the stage output. At such situations the output impedance and the input impedance will be fully matched.  The more mismatching is there the more will be the percentage of reflected wave.  

Every component or part used in a matching configuration is determining in deciding the final impedance matching level. Suppose that the out put of a transistor is the source and the antenna is the load and even if we have calibrated the output and input impedances to perfect matching, the quality difference of the connector used in between is enough to spoil all the charm. At the same time the length of the transmission line (that connects the transistor out with the antenna) does not make much mismatch, provided it is not coiled anywhere. We are talking about a situation in which the out and input impedances are the same. Where there is reflected wave, a standing wave pattern is formed on the transmission line. It is the ratio between the lower value and the higher value of this standing wave voltage that we call VSWR (Voltage Standing Wave Ratio).

Standing wave ratio (SWR) is a measure of impedance matching of loads  to the characteristic impedance of a transmission line or waveguide. Impedance mismatches result in standing waves along the transmission line, and SWR is defined as the ratio of the partial standing wave's amplitude at an antinode (maximum) to the amplitude at a node (minimum) along the line (see fig. C-27/0A).

The SWR is usually thought of in terms of the maximum and minimum AC voltages along the transmission line, thus called the voltage standing wave ratio or VSWR. For example, the VSWR value 1.2:1 denotes an AC voltage due to standing waves along the transmission line reaching a peak value 1.2 times that of the minimum AC voltage along that line. The SWR can as well be defined as the ratio of the maximum amplitude to minimum amplitude of the transmission line's currents, electric field strength, or the magnetic field strength. Neglecting transmission line loss, these ratios are identical.

If it is a transistor that is used in the final sage and if the output stage is mismatched, it won't be working in its maximum efficiency; at the same time the transistor may not also work long. Those who use transistors in the final can use SWR protection circuits that stops the stage function at critical situations. This is why it is said that at high power circuits a matching circuit is necessary. At the same time the resistance capacity of valves are very high; a valve may withstand such mismatching conditions for prolonged time. Load according to a transmitter is anything that is capable of absorbing RF power. Even though the commercial SWR meters show the exact standing wave ratio, SWR meters can be easily home brewed also, with which we can learn the intensity of the reflected wave.   In fig.  C-27/1 the circuit of such a gadget is shown. 

The RF power from the transmitter is let through a 4" long coaxial cable having the same impedance as that of the out put and input. Since the shield of the cable works like electrostatic shield, only one side of it shall be grounded. The toroid used here can be any toroid that works in RF ranges. The coil should have 14 turns of 24 SWG wire.  If the deflection seen in the meter is very high change coil connections.  

Reading the SWR matching level is no protection; and it also does not mean that maximum power is radiated through the Antenna. What we want is an Antenna tuner that could reduce the SWR and make Source-Load matching possible. 

Fig. C-27/2 gives the details of a simple Antenna Tuner (ATU) that can be used with low power transmitters. Another ATU circuit that had been successfully tried wit high power transmitters are given in fig. C-27/3

As shown in the picture, either the arrangement should be such that switching is possible to each turn of the coil or a soldering and testing at each turn pattern can be followed. In word sense what we require here is a variable roller inductor, which may be available from junk/flee markets. Another ATU circuit close to C-27/3 is shown in C-27/4. 

The construction details of a simple SWR meter that shows standing wave strength at forward and reverse conditions is shown with details in fig.C-27/5. We have given here only a short account on VSWR and ATU.

Chapter 26                                                       Chapter 28