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.
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.
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.
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