9527169 You can find information online about building a bandpass filter. Searching "hellocq.net" should yield some results. Building one yourself without specialized equipment and experience can be challenging. There are also commercially available bandpass filters and multi-band antennas. For example, a seller in the 5th district of Taobao (China) sells an antenna with model K730. The principle is that you add a 20-meter bandpass filter to a 20-meter array (essentially, it's a device that cuts off signals at a specific frequency). Then, you connect another section to the array, resulting in a 40-meter array. This has two advantages: first, a single array can resonate at two or more frequencies, and second, the filter after the bandpass filter acts as an inductor, effectively shortening the physical length. This is a practical solution when space is limited. However, it reduces both gain and bandwidth. If you have sufficient space, it's best to use a full-sized antenna. For multiple bands, you can connect arrays in parallel.
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