I see many questions About antenna impedance/resonance ect. in simple words about the complicated. Z -complex impedance R – active resistance X – reactive impedance (j – is the imaginary unit, and is used instead of i in this context to avoid confusion with the symbol for electric current.)
Z=R+(-)jx Swr 1:1.0 – 50=50+(-)0
remember swr this is unnecessary indicator.
Antenna Resonance in simple words this jx =0 (and then you see real antenna impedance)
For example: Dipole z=50ohm on 7100, but z = (R)70-(jx)20, and we have for example swr 1,2 But real resonance for example on 6900 where minimum jx = +2.5 and R = 65. Swr on 7100 ~1.2 Swr on 6900 ~1.2 BUT 6900 its antenna resonance.
Such an antenna was developed for professional communication, but then it became popular among radio amateurs.
Depending on the length of the structure, such an antenna can operate in the frequency range up to 2 – 30 MHz, that is, in almost the entire HF range. In this case, any tuning of the antenna to the frequency is not required, it works the same in the entire frequency range.
T2FD antennas are available in various lengths. The most popular lengths are 28 m (regular) and 14 m (short). But there are other lengths as well. The T2FD antenna is 28 m long and operates in the frequency range from 3 to 30 MHz, covering all shortwave amateur bands except 160 m.
But there is also an extended version of the antenna with a length of 56 m. Such an antenna is slightly less popular among amateurs due to its large size. But thanks to its size, it covers the entire HF range and allows you to work both for reception and transmission in all amateur bands.
The efficiency of such antennas is slightly lower than that of resonant dipoles. But on the other hand, a very wide frequency band, the absence of the need to adjust the antenna make it irreplaceable in many cases.
The antenna operates in a wide frequency range (swr is often 1: 5) without pronounced directivity (almost circular directional pattern). The gain of the T2FD antenna practically corresponds to a half-wave dipole (at low frequencies it loses to the dipole). However, the T2FD chart is more extended upwards. It should also be borne in mind that the radiation pattern changes with frequency and at higher frequencies is similar to a flower, but still, most of the radiation is directed upward. It is this fact that makes the T2FT antenna unsuitable for DX, although it does not exclude the possibility itself, since some of the petals are directed at a low angle to the horizon.
Dimensions A and B both vary, depending on the frequency range needed and the need to match the antenna to the terminating resistance correctly. Typical values for the length, A, are between 20 and 26m. The amount of spread, B, ranges far more widely – between 0.4m and 3m. A number of sources provide a formula to work out A and B.
A is typically put at around 100/f metres, where f is the frequency of the lowest band of operation in MHz.
B is sometimes put at about 3/f metres, although both B&W and Buxcomm seem to use a value of about 1.66/f
For operation from 80m upwards (based on a mid-band frequency of 3.65MHz)
A = 100/3.65 = 27.4m and
B = 3/3.65 = 0.8m
R1 – 330 ohm
Advantages of the T2FD antenna
Easy to manufacture.
Wide-range. Works in the entire range of operating frequencies.
Requires relatively little space and low suspension height.
Does not require a separate mast, can be hung from the house.
Good signal-to-noise ratio at reception.
Almost omnidirectional in the horizontal plane.
Does not require an antenna tuner when using 1:4 balun.
Disadvantages of T2FD antenna
Low efficiency. Part of the power supplied to the antenna is spent on heating the load resistance.
Most of the radiation is directed upward. Bad for DX.
Considering the advantages and disadvantages, it must be admitted that this antenna is “limited in capacity” when the radio amateur does not have enough space, a mast or other restrictions. Therefore, it is mainly used as a broadband receiving antenna and for local radio communications.
Dipole its symmetric system with ~72ohm in center.
The dipole is any one of a class of antennas producing a radiation pattern approximating that of an elementary electric dipole with a radiating structure supporting a line current so energized that the current has only one node at each end.
The most commonly used is the center-fed half-wave dipole which is just under a half-wavelength long. The radiation pattern of the half-wave dipole is maximum perpendicular to the conductor, falling to zero in the axial direction, thus implementing an omnidirectional antenna if installed vertically, or (more commonly) a weakly directional antenna if horizontal
A half-wave dipole antenna consists of two quarter-wavelength conductors placed end to end for a total length of approximately L = λ/2. The current distribution is that of a standing wave, approximately sinusoidal along the length of the dipole, with a node at each end and an antinode (peak current) at the center (feedpoint)
How to build half-wave dipole antenna for hf:
1.Need 2 copper wire 1-4mm diameter and λ/4– use this table:
Note that the shortening factor of the wire is not included in the table. So the antenna will be about 5% shorter in reality.
2. Balun 1:1 (without balun you will be have many common mode. Balun need for current symmetry and choke commonmode)