Nother Idea. Or two.
World-field EM Detector
From Tesla's notes are the specifics of magnetic fields. It turns out, humans do in fact cause minor fluctuations. And in large enough groups, these can in fact be detected.
By a combination of setting up cables underground, and creating data gathering nodes. And by setting up a World Field EM Detector in our city.
We can create a map of the EM field the earth gives off. In turn, we can detect when humans and/or electrical equipment appear in sufficient numbers. And then...
Then we can strike them with the Beetles atop the walls, and destroy that group.
As for how it actually works, the WFEMD sends out an alternating magnetic signal. This signal interacts with the area around it, and creates a pattern of the magnetic field in question. As this occurs, the nodes also give off the signal. The data gathered is transmitted to the main node in our city, and assembled, before being sent to the other nodes for any people in those buildings to work with.
This creates the stabilized map of the EM pattern. And in the presence of humans and electronics, this pattern is warped. When enough are gathered, about a hundred or more signals, this warp can be seen. Moreover, emotions tend to create different warps as well, though for now, simply knowing they are there is important.
To prevent all our forces from giving false positives, they are assigned a magnetic resonator, which crates an alternating current through an electromagnet. In sufficient number, this produces a very different warp pattern than those without it. It's a lot more uniform for one. So they won't be mistaken for enemy forces.
Radio Packets Broadband EM-Radio
It is well known some things work better when colder than others. Meet the EM-Radio, which using electromagnetic waves, can send voice through it's wavelengths to other EM-Radios. So long as they are powered and tuned to the correct frequency, they will receive sound from the same frequency, and send it along those waves.
In this manner, runners no longer need to run to tell of enemy positions. Instead, they can act as proper scouts. It will also help with communication for commanding our troops.
It primarily works by working with the native electromagnetic field of Earth, allowing a lot of range for almost no setup. Though Booster Towers could be made in the future to help bounce messages along the Earth's Magnetic Field to the proper recipients. The current ones only have 4 frequencies that can be used in the general without connecting to the other frequencies.
Perhaps the best part is that the electromagnet we are using only gets better as it gets colder. So messages will always be clear. Though it does not do well in the heat.