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In his previous contributions to this web site, Prof. Trainotti mostly dealt with the technical and scientific side of Part 15 AM. In this segment, he deals with the art of short MW antenna construction. It should be remenbered that Prof. Trainotti is a consultant on short antenna projects for licensed MW and LW stations. Thus, his recommended practices look a lot like what would be expected for commercial antennas. The Professor’s view is that the best practices should be used for Part 15 because of the exceptional difficulty in getting an efficient radiator.
I asked, in particular, about his concern about the high potential point between the loading coil and and the base of the monopole, considering that the RF voltage there is typically less than 100 Vrms, and can’t get higher than a few hundred volts rms, even for the most efficient Part 15 AM system possible. (Incidentally, if we were allowed to apply 1 W to the 3 meter antenna, the loading coil could induce a voltage of several kV.) Prof. Trainotti said that, even if the antenna voltage is not extremenly high, the losses at the high potential point have to be minimized.
For this installment, I am presenting Prof. Trainotti’s own words with little editing:
“I would like to clarify that the Tuning Coil and Matching System must be installed at the Monopole Antenna Base and connected to the artificial metallic ground plane.
“The hot antenna lead and the tuning coil must be connected through a thick feed-through isolator, because this is a high potential point, and the connection must be as short as possible. The antenna must be installed in a space as free of obstructions as possible, and the antenna rod or tube has to be isolated from ground at a height of 0.1 to 0.2 meters from the ground plane over a thick base isolator. The tuning coil must be installed in the metal box, supported by isolators. The input tuning coil lead is of low potential because this is a low impedance point that depends on the coil Q and the ground plane loss resistance.
“It would be convenient to install the low power transmitter inside this box too. In this case, only the DC or AC power cable, and the Audio cable, have to be run between the tuning box and the house, and these cables should be run underground, if possible in a grounded metallic pipe, thus avoiding any RF induction into the power or audio cables. The metallic pipe must be connected to the metallic ground plane. Cable shields should be connected to the metallic ground plane at several points so as not to create induction current loops.
“Don’t forget that this kind of antenna produces maximum radiation along the surface of the earth, and the field strength is quite high close to the antenna base.
“Care must be taken if the area in which the antenna is installed can be flooded, to avoid water at the antenna base. In this case, the metallic ground plane can be made as kind of a mound, to raise the antenna feed point above the level of the soil. The mound must be as flat as possible, with an elevation at the antenna base of 1 to 2 meters above the soil level. The mound is made of ordinary earth. The metallic ground plane is placed along the surface of the mound. The antenna base can be installed over a small concrete pylon in the middle of the mound.
“As you see, a lot of work must be done even for a very simple antenna like this one, but this is the only way to get an efficient radiating system in these poor conditions.”