RadCom April 2024, Vol. 100, No. 4

April 2024 25 Technical N is the number of turns, D is the diameter and G the length of the coil, both in mm. I used a 300mm diameter fermentation bin on which to wind my 137kHz inductor. Plugging this diameter into the equation, and playing with the numbers, suggested it was going to need something like 400 turns spaced over a length of 500mm as a starting point. This wasn’t feasible as a single layer, so some careful double-layering of part of the coil was needed, with plenty of good insulating tape. A variometer was included, consisting of 15 turns wound and glued onto a 250mm diameter plastic picnic plate. This was mounted inside the former with a spindle added so it could be turned to either oppose or add to the main coil by rotating its plane. The variometer gave an adjustment range of around 500 m H, sufficient to get exact resonance once the number of turns on the main coil was optimised. Back in 1996, small vector network analysers (VNA) were unheard of, so much experimentation was needed to get the system to resonance. This was achieved by feeding the loading coil and antenna system, after the radials and ground had been installed, using a test signal at 73kHz delivered from a MOSFET audio amplifier. Turns were then adjusted while looking for a peak in RF current using an RF current monitor such as that in Figure 3 . This RF current-monitor design is based on a current transformer, with the antenna feed forming a one-turn primary on a toroid, and is still a favourite amongst LF operators, making for rapid tuning. After much fiddling, and the adding and removing of turns, the antenna current could be peaked at the wanted frequency. Several years later, the process was repeated for 137kHz, using a similar fermentation bin which needed around 150 turns of 1mm wire spread over 270mm for 6.4mH inductance. This was still long before VNAs were available to the average radio amateur. The ground and radial system Any vertical antenna must operate against a low- loss ground. This has to terminate the electric field lines from the vertical antenna in a manner having as low a loss as possible. The usual technique, with a single vertical radiator, is to use radials, placing as much copper on the ground around the base of the vertical element as possible, and ideally extending out to at least a distance equivalent to the antenna height. However, for a tee antenna, much of the capacitance to earth comes from the top capacity hat running horizontally, from one end of the garden to the other at my QTH. Since the electric field lines from this travel vertically downwards, several radial wires were placed directly underneath this part, with many other wires radiating out. As my garden consists of heavy clay that is quite conductive, maximum use was made of this by adding multiple earthing rods connected to the ends of the radial wires, as well as directly to the junction or feed point at the base of the loading coil. I used four ‘proper’ 1.2m rods close in and, to supplement these, shorter lengths of copper water pipe were bashed in as far as they would go – typically half a metre – at the ends of most of the radials. Once completed, the entire earthing and radial system was bonded to the house metalwork in a PME system using three separate 6mm 2 conductors [1] . The DC resistance measured using a 12V battery and ammeter between my earthing system and the house metalwork before bonding measured 4Ω. As we’ll see, this was a meaningless figure at RF. Antenna resistance and efficiency When the antenna and ground systems had been completed, antenna measurements could begin. Antenna current measurement was already in place using the current probe, and a second diode RF voltmeter was connected to measure the drive voltage at the base of the antenna. Knowing both antenna current and voltage, the antenna drive resistance at resonance can be determined directly using Ohm’s law. This drive resistance is made up from a combination of loading-coil loss, earth resistance, and a small amount of radiation resistance. Radiation resistance, R RAD , and RF current is what determines the radiated power, so by knowing the RF current, the transmitted power can be calculated from P TX = I 2 .R RAD . The radiation resistance in ohms of a vertical antenna is directly related to the antenna’s effective height, H EFF , by the equation R RAD = 1580.(H EFF / λ ) 2 , where H EFF and λ are both expressed in the same units (eg in m). An effective height for this 7m high antenna with large capacity top hat can be guessed to be in the region of 6m, thus giving a radiation resistance of 1580*(6/4100) 2 = 0.0034Ω at 73kHz. At 137kHz, the calculated R RAD for this antenna rises to a whopping 0.013Ω. and at 475kHz to 0.14Ω. Using measured RF current and voltage, it was found that the effective drive resistance of the antenna, once it had been carefully adjusted to resonance, was in the region of 220Ω at 73kHz, way above the DC resistance measured to the house metalwork. This is all loss resistance made up from coil losses, earth RF resistance and proximity effects of local trees terminating the antenna electric field. Subsequent measurement at 137kHz showed a loss resistance of 100Ω and at 475kHz of about 25Ω. This effective resistance is way above the value for R RAD and is in series with it. Of the RF power that is delivered to the antenna system, nearly all of it goes into heating the ground, and a bit to heating the coil. Only a very tiny fraction is radiated, and this can be determined from R RAD / R TOTAL . At 73kHz, this equates to 0.0034Ω /220Ω = 0.000015 or an effective antenna gain of -48dB. At 137kHz, the effective gain rises considerably to -38dB. However, at 475kHz, the 0.14Ω R RAD and 25Ω loss resistance suggest a much-better gain of -23dB. Andy Talbot, G4JNT andy.g4jnt@gmail.com FIGURE 2: A picture of the 7m-high tee antenna in use at G4JNT for 137kHz and 475kHz. Two vertical conductors, spaced by 100mm, are used to get the equivalent of a thicker conductor to increase capacitance. The top capacity hat of three conductors, spaced by 150mm, runs the full 12m length of the garden. Extensive ground radials are placed under this run, several terminated with earthing rods. FIGURE 3: RF current monitor used to adjust a loading coil for resonance, and for assisting in determining antenna load resistance. This is Figure 6.1 from the Low Frequency Experimenter’s Handbook , RSGB, published 2000. A more-recent equivalent can be found in [3].