Antenna Tuning Unit
The base antenna tuning unit (ATU). Ignore the flare from the camera flash. RF enters the enclosure via the SO-239 at lower right. The 1/4-20 stainless ground post is at bottom center and connects to the ground rod and radial system. The control cable connects at lower left via a weathertight Hirose connector. Output is through the ceramic pillar insulator at the top.
The three relays at left are for switching tuning networks in and out. They are 12v coil DPDT relays with 10 amp contacts. The bottom relay switches between pass-thru (the feedline connects directly to the antenna) and the top of the tuning network coil. The middle relay taps the coil (blue wire) at the appropriate point for 15 meters in the de-energized position. The variable capacitor at upper right resonates the coil at 15 meters. Another (red) wire leads from the energized position for future use. The top relay is not used. It's likely that this network will be re-designed somewhat in the future, most likely for additional bands.
Prior to opening the box for this picture, the box was sealed for approximately two years and survived two winters, including one where the box was completely buried in snow. I expected that there'd be corrosion and water inside, but the box was as dry and clean as the day I closed it up. Chalk one up for Carlon boxes!
