1. Antenna sizeI tried to simulate two kinds of flag antennas for MW band. One is a normal flag antenna and the other is Twisted Double Delta Flag (TDDF) antenna. I used two simulation software called MMANA-GAL and MMANA (Japanese version) .
I'm not sure the best size for these two antennas. So I adopt that the length is 20 m and the height is 5 m for both antennas considering practical field installation. Fig 1. depicts the size of each antenna. The termination load value is chosen for best FB ratio on 1.2 MHz. Relative permititivity and conductivity for ground are 13 and 1 [mS/m].
Figure 1. The size of antennas
2. Directional characteristics in horizontal planeFig.2 shows directional characteristics in horizontal plane for both antennas. In this figure, FB ratio of the normal flag antenna is better than that of the TDDF antenna. However the half value angle of the TDDF antenna is 104 degrees and the angel of the flag antenna is 158 degrees. This means the TDDF antenna has more sharp directivity than the normal flag antenna.
Figure 2. Directional characteristics in holizontal plane
3. Directional characteristics in vertical planeFig.3 shows directional characteristics in vertical plane for both antennas. In this figure, elevation angle of the TDDF antenna is smaller than that of the normal flag antenna. It seems that the TDDF antenna is suitable for very long path propagation of MW band. In terms of the shape of directivity for back side, the TDDF antenna has more pressed shape compared to the normal flag antenna. This means that the TDDF antenna is more robust for interference by domestic MW radio stations and other noises coming from behind the antenna.
Fig.4 shows antenna patterns for both antennas in 3D view. The antenna gain Ga of TDDF is 16 dB lower than that of the normal flag antenna. To compensate for the reduction of Ga, a preamplifier with low NF and high IP such as RPA-1 of DX Engineering could be applied.
Figure 4. Antenna patterns in 3D view
Figure 5. Low NF and high IP preamplifier
Fig.6 shows simulated frequency characteristics of both antenna impedance and F/B and absolute antenna gain Ga for the isotropic antenna. The antenna impedance varies by a value of termination load and frequency etc. If 50 ohm coaxial cable is used as a feeder, an appropriate impedance matching circuit must be needed. Appropriate impedance matching methods will be discussed later in this blog.
It should be noted that both F/B and Ga of the TDDF antenna is almost proportional to frequency but F/B of the normal flag antenna is inverse proportional to frequency even though Ga is proportional to frequency.
Figure 6. Frequency characteristics for antenna impedance and F/B and Ga