https://www.cojot.com/wp-content/uploads/2017/06/cojot-logo.png 0 0 Norbert Nawroth https://www.cojot.com/wp-content/uploads/2017/06/cojot-logo.png Norbert Nawroth2021-06-22 08:02:532021-06-22 08:02:53Spacing Considerations of Multiple LPDAs on a Mast
Written by Johan FuhriIf two LPDAs are mounted on a mast, one on top of the other, the coupling between them will be much higher when they are both horizontally polarised (HP) compared to when they are vertically polarized (VP). In a simulation of the LPDA-A0102 using the same spacing between the two antennas, we can see that the isolation between the two VP antennas at 100MHz is about 40.7dB, whereas the isolation between the two HP antennas is only 22.5dB. To put this into perspective, a 5kW (67dBm) signal transmitted from one of the antennas will induce only about 0.43W (26.3dBm) of power into the other antenna when both are VP. When both antennas are HP, a massive 28.2W (44.5dBm) signal is picked up by the second antenna.Doubling the distance from 2m to 4m only increases the isolation by 6dB (and even less at 100MHz), which is not all that much since an additional 2m spacing on a mast is a practical impossibility under most situations. As a rule of thumb, every time the spacing between two antennas is doubled (or halved), the isolation between them increases (or decreases) by about 6dB. This is true unless the antennas are really close to each other (in terms of wavelengths) and is true for both the VP and HP scenarios. It is thus clear that an increase in spacing between antennas is unlikely to be the magic bullet that will solve isolation issues in most practical scenarios!It can be seen that there is significant coupling between the two antennas from 500-750MHz, which is well outside of the frequency band where the LPDA-A0102 is designed to work. The same is true for the frequencies below 200MHz, which is below the designed operating frequency of the smaller LPDA. Thus, just because two antennas do not have overlapping frequencies of operation, do not assume that the coupling will be negligible.