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Приветствую Вас, Гость · RSS 16.01.2017, 15:47
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Theory of the DK7ZB-Quadlong-Antennas

Theory of the DK7ZB-Quadlong-Antennas

The main advantage of a 2-Element- Quad is the great 3-dB-azimuth-angle. This advantage disappears, if you mount director elements in front of the antenna. The additional gain comes mainly from the reduced azimuth-angle and the antenna parameters become similar to a Yagi antenna. Therefore it makes absolutely no sense to build Quads for one band with more than two elements. Friends of Quads do not like to hear that, but in reality a Yagi with the same number of elements as a multi- element Quad has the same gain and bandwidth and you waste material and windload with such constructions! Of cause a 3-Element-Quad for shortwave is a good choice, because the spreaders can carry the wires for more than one band and you get an interesting multiband-DX-antenna for home-brewers.

But how can we get the advantages of the 2-Element-Quad and can increase the gain? Let us take a look to the classic Quad (pic. 1). If you prefer a good F/B and greater bandwidth, the gain is 4,7 dBd and F/B is 25-30 dB with a high impedance of 110 Ω. Increasing the gain to 5,7 dBd means very low bandwidth, a bad F/B of 6-8 dB and a lower impedance of 40 Ω . With a good compromize we do not reach more than 5 dBd gain.  

The first step is a simple modification. The original single loop with a circumference of 1 λ and a side length of 0,25 λ has only a gain of 1,3 dBd and an impedance of 130 Ω. The Quad element is a stacked system, but the distance between the two parts is to low and the current maxima are to close together. We can increase the gain by changing the Quad to an "Oblong”. The stacking distance will be greater, but the radiating horizontal parts of the loop become smaller. The best compromize between the two influences and the bandwidth is a relation of 0,15 : 0,4 λ (pic. 2). The gain of this loop is now 2,6 dBd and the radiation resistance drops down to 35 Ω. This is the reason why the elements should not be wires with too much loss. See table 1.


Table 1: Data of 2-m-Oblongs with 6-mm-Elements, lengths in lambda

Circumference Hor. Vert. Impedance Gain
1,13 λ 0,186 λ 0,378 λ 50 Ω 2,29 dBd
1,12 λ 0,153 λ 0,406 λ 33 Ω 2,62 dBd
1,11 λ 0,136 λ 0,420 λ 25 Ω 2,77 dBd

In a second step we replace the reflector loop by two streched tubing elements. Gain and pattern are better than with a loop and the mechanical problems for building the antenna are much easier to solve. So we come to the "DK7ZB- Quadlong” (pic. 3). The gain is 1 dB above the classical Quad and with 6,45 dBd as high as with a 3-Element-Quad, but all other data are better (see table 1) and the construction is much easier. The impedance is now 28 +/- j 0 Ω and a simple feeding with the "DK7ZB-Impedance-Choke” is possible. We need only 2x 75-Ω-coax in parallel with an electrical length of λ/4 (pic 4). This cables are wound to a choke for a balanced feeding, like it is shown for the DK7ZB-28-Ω-Yagis.


It is possible to add a second loop and a third reflector element. So we come to the Double-Quadlong (Pic. 5). The gain is 7,5 dBd and the impedance is 28 , too.

A very good antenna is the 4x-Quadlong in Pic. 6 with 5 reflectors. The gain increased to 9,3 dBd the azimuth-angle is still 72° and an impedance of 50 is easy to feed. These constructions are to high for the shortwave bands or 50 MHz, but for 2m you get excellent antennas.

The distance of the reflector element in all cases is 0,15 - 0,16 λ.

A comparison of all antenna parameters is listed in the table below. 

A description of the two antennas is on the page with the 2-m-Quadlongs

Table 2: Comparison of some data for 144-MHz-Quad-Antennas

Type Gain F/B 3-dB-angle Az. 3-dB-angle El. Impedance
Single Quad 1,3 dBd 0 dB 84,2° 131,8° 130
Single Oblong 2,6 dBd 0 dB 87,8° 80,1° 35
2-El.-Quad 5,3 dBd 12 dB 70,6° 88,9° 50
3-El.-Quad 6,3 dBd 12 dB 65,2° 79° 50
DK7ZB-Quadlong 6,4 dBd 21 dB 71,8° 70,0° 28
Stacked DK7ZB-Quadlong 9,5 dBd 23 dB 72,4° 30,0° 2x25
DK7ZB-Double-Quadlong 7,5 dBd 19 dB 71,6° 54,8° 28
DK7ZB-4x-Quadlong 9,3 dBd 17 dB 71,8° 36,7° 50
Категория: Антенны КВ | Добавил: URC (10.01.2011)
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