Page "Heliograph" Paragraph 4

The heliograph had some great advantages.

It allowed long distance communication without a fixed infrastructure, though it could also be linked to make a fixed network extending for hundreds of miles, as in the fort-to-fort network used for the Geronimo campaign.

It was very portable, did not require any power source, and was relatively secure since it was invisible to those not near the axis of operation, and the beam was very narrow, spreading only 50 feet per mile of range.

However, anyone in the beam with the correct knowledge could intercept signals without being detected.

For the Boer war, where both sides used heliographs, tubes were sometimes used to decrease the dispersion of the beam.

Conversely, the narrow beam made it very difficult to stay aligned with a moving target, as when communicating from shore to a moving ship, and the British issued a dispersing lens to broaden the heliograph beam from its natural diameter of 0. 5 degrees to 15 degrees for that purpose