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.
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.
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