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.
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