# A Compact Wideband Active Two-Dipole HF Phased Array
## 3 Design Considerations  
The aim was to construct a two-dipole phased array, employing elements that are as short as possible in order to be easily installed at the desired polarization sense. A good trade-off was to set the length of the dipole elements at 5 m, i.e., 2.5 m for each element with an 8 mm tubing diameter. As a result, both dipoles are short in the lower HF band but close to resonance at 30 MHz. The self-impedances of the short dipoles and the E-plane half-power beam widths (HPBWs) have been simulated from 5 to 30 MHz and are depicted in Table 1 below:  
Figure 5: Oscilloscope output from simulation at 25 MHz based on mutual impedance of the two driven elements of Figure 4 (oscilloscope 0.2 V per division) versus high impedance (oscilloscope 0.5 V per division). Figure 6: The effect of mutual coupling on the 0.25 \(\lambda\) array radiation pattern under ideal and real conditions. The phase errors and current imbalance of the quarter-wavelength two-dipole phased array is neither easy to calculate nor is it a standard design parameter. Therefore, there is no point in presenting charts and equations that concern the calculation of the mutual coupling between the two parallel dipoles. The aforementioned is affected by many complicated factors such as the operating environment, the height above the ground, the type of ground, the polarization sense of the dipoles, the distance between them, etc. That is why the isolation of the two dipoles, to avoid a current imbalance and phase error, must be maximized as much as possible. Since the two dipoles are installed side by side, the active impedance of each driven element due to the effect of mutual coupling will be different, as has been discussed previously by [PERSON] [15]. The active dipoles terminated at a 50 \(\Omega\) combiner should enable balance of the impedance variations, as expressed by the following equations:  
\[\Theta=\text{Tan}^{-1}\frac{X}{(R+50)} \tag{9}\]  
\[I=\frac{V}{\left[\left(R+50\right)^{2}\,+\left(X\right)^{2}\right]^{0.5}} \tag{10}\]  
where:  
\(I\) is the current at the input port of the hybrid,  
\(V\) is the output voltage of the active antenna,  
\(\Theta\) is the phase angle in degrees,  
\(R\) is the real part of the antenna's active impedance, and  
\(X\) is the imaginary part of the antenna's active impedance. For this to be accomplished, a two-stage LNA has been implemented, as shown in Figure 7. The first stage of the amplifier has been designed around the JFET BF245B as a common drain amplifier [16]. The advantage of this unity voltage-gain, low-noise amplifier is that it presents a very-high-input impedance and a low-noise figure across the HF band, which is about 1.5 dB. Based on Equations (7) and (8), the high-input-impedance property of the JFET amplifier serves two fundamental purposes: it reduces losses from the effect of the short dipole's high reactive components, given in Table 1; and, on the other hand, isolates the two dipoles in terms of mutual impedance variations. The role of the second stage, class A small-signal common-emitter amplifier is to compensate for the unity voltage gain of the JFET amplifier and also to match the dipole's terminal impedance at the 50 \(\Omega\) hybrid's input port. The performance of the LNA has been evaluated with a response given in Figure 8. The amplifier gain response is not flat due to the Miller effect, mainly on the second stage of the LNA, as was expected, but can be corrected, if needed, by the use of a simple high-pass filter at the output of the combiner. The input impedance of the BF 245B at 25 MHz has been measured to be around 800 \(\Omega\) using an oscilloscope and the voltage divider method. Specifically, a variable resistor (4K7) was placed in series to a 50 \(\Omega\) signal generator's output. Initially, the probe of the oscilloscope was connected at the output of the signal generator without a load in order to determine the open circuit voltage. The next step was to connect the signal generator at the input of the BF 245 common drain amplifier.