T-antenna
The 'T'-type antenna is most easily understood as having three functional parts: The wires of the top load are often arranged symmetrically; currents flowing in the oppositely directed symmetrical wires of the top hat cancel each others' fields and so produce no net radiation, with the same cancellation happening in the same way in the ground system.
However the ease of just laying wires on the ground or raised a few feet above the soil, as opposed to the practical challenge of supporting top hat's horizontal wires up high, at the apex of the vertical section, typically means that the top hat is usually not built as large as the counterpoise.
Further, any electric fields that reach the ground before they are intercepted by the counterpoise will waste energy warming the soil, whereas stray electric fields high in the air will merely spread out a bit more into loss-free open air, before they eventually reach the wires of the top hat.
The left and right sections of horizontal wire across the top of the 'T' carry equal but oppositely-directed currents.
More current is required in the vertical wire to charge and discharge this added capacitance during the RF oscillation cycle.
[5] The horizontal top load wire can increase radiated power by 2 to 4 times (3 to 6 dB) for a given base current.
Similarly, a receiving T-antenna can intercept more power from the same incoming radio wave signal strength than the same height vertical antenna can.
In antennas built for frequencies near or below 600 kHz[b], the length of an antenna's wire segments is usually shorter than a quarter wavelength[c] [ 1 /4 λ ≈ 125 m (410 feet)[c] at 600 kHz[b]], the shortest length of unloaded straight wire that achieves resonance.
The power is maximum in a horizontal direction or at a shallow elevation angle, decreasing to zero at the zenith.
The effect of poor ground conductivity is generally to tilt the pattern up, with the maximum signal strength at a higher elevation angle.
To efficiently drive current into a short transmitting antenna it must be made resonant (reactance-free), if the top section has not already done so.
The RF ground is typically constructed as a star of many radial copper cables buried about 30 cm (1 foot) in the earth, extending out from the base of the vertical wire, and connected together at the center.
The equivalent circuit of the antenna (including loading coil) is the series combination of the capacitive reactance of the antenna, the inductive reactance of the loading coil, and the radiation resistance and the other resistances of the antenna-ground circuit.
[12] The multiple-tuned flattop antenna is a variant of the 'T'-antenna used in high-power low-frequency transmitters to reduce ground power losses.
[7] Although the vertical wires are separated, the distance between them is small compared to the length of the LF waves, so the currents in them are in phase and they can be considered as one radiator.
[7] The antenna was used in the powerful radio stations of the wireless telegraphy era but has fallen out of favor due to the expense of multiple loading coils.
