引用 虎哥:
可能你没仔细看，他这个桥的构型能到多低主要靠磁环，图片中的长度到10M左右问题不大。而且不是传输线耦合，典型的不等臂电桥，设计靠经典电路理论。当然这个有窍门，没照他的做也是对的，现在想来，如果照他的做……

你看他的
“Design
Directional coupler”
那一节，文字和图片都是描述微带线耦合器的。
这是从里边摘出来的文字“Simulated directivity is 20 dB at low frequencies and 30 dB at high frequencies. Coupling decreases very rapidly at lower frequencies due to compared to wavelength. Coupling at low frequencies can be improved by making a multi-stage coupler. I decided to not make one, because it would have required more PCB area and even single stage couplers are pretty big. As a result of it dynamic range at low frequencies is going to be poor. I'm more interested at high frequencies so reduced accuracy at lower frequecies doesn't matter too much.”