To avoid RF returns and/or parasitic coupling with the antenna drop cable, it’s a must to use a Choke filter.
Some examples of Choke are shown below.
We don’t spend time here to explain how to calculate the RF Choke, see on Internet, we need to highlight that:
- Choke is an excellent narrow band filter if the wiring is wrapped in air (see image above n.2)
- Choke with wiring is around to the toroid, the filter frequency is plus flat and for multi band antennas this is preferable (see image above n.1)
- Choke with a lot of spirals the center frequency is in the low frequencies otherwise (few spirals) is on high frequencies
- If you have power cables that come close to the antenna, put a Choke on the power cables too, made with a simple 1.5 mm unipolar cable wound on a toroid model FT1407-43
To create your perfect Choke we recommend using NanoVNA and lots of patience
Remember that the measure must be to do on the shield of the cable Choke using the Port1 (S11 – source) and Port2 (S21 – measure).
See below images.