Oak Hill Microwave Station

Oak Hill is a distinctive microwave site located on Communications Hill in San Jose, California. Many will recognize Oak Hill for the unusual concrete tower, which sits above an underground building that is semi-hardened at most. Much about the history of Oak Hill is obscure, including the reason for the unusual construction. Functionally, Oak Hill was a microwave station for connectivity to the San Jose CO and likely also exchanged microwave traffic with an L-carrier cable that runs north-south through California.

Points of Interest

id	symbol	latitude	longitude	description
1		37.285095º	-121.8577º	Site Location

This list is probably incomplete and confusing, the routes seem to have changed over time. A mailing list post from Wayne M H also gives the following. I am not familiar with the maps referenced.

The map I have that seems post-1979 shows Oak Hill with a MW path to
Starr Canyon (aka "Hogsback" and was moved from the San Jose office in
1975), Walpert Ridge, Redwood City (1974 along the path), Loma Prieta
(1975 along the path), and possibly others to the San Jose office.

Another map, or diagram rather, with "AT&T Communications" at the
bottom has Oak Hill with a Lenkurt path to Sunnyvale (Now a PacBell
switch), a DR11 / 609E(?) path to Redwood City (Current AT&T Office),
lightguide to San Jose 02, WLEL(?) to San Jose 02, and TD2 / 609E to
Loma Prieta, and the 11GHz Lenkurt dual path to Starr (For Pacheco
Pass).

The history of the Oak Hill site is not well documented, but LinearBob WA6WHT sent an explanation to the ColdWarComms list that goes like this: AT&T originally intended to add floors to its downtown San Jose office (on Almaden Road) and mount microwave antennas there, but expansion of the San Jose Airport lead the FAA to prohibit the added height. Instead, AT&T selected the Communications Hill site, which was already known by that name due to a Santa Clara County emergency communications facility nearby. This became known as “Oak Hill” within AT&T, and the microwave facility served mostly as a “remote” radio site for the San Jose downtown office.

Based on LinearBob's story, construction of Oak Hill would have been sometime in the late 1960s or 1970s. Indeed, historic aerial images bracket it between 1968 and 1980, and a tangential mention in a San Jose planning document puts the date of construction at 1972.

Microwave IF signals were conveyed between San Jose and Oak Hill over 4-5 miles of coaxial cable, allowing most of the mux equipment to stay at San Jose and perhaps explaining the relatively small floor area and power plant at Oak Hill. This was a system called “WLEL” or “Wireline Entrance Link,” used for short-haul carriage of microwave supergroups.

Oak Hill didn't just serve microwave, the 1979 route map appears to depict a north-south L-carrier cable passing through. Other mailing list posts suggest that this cable was rerouted to appear at Oak Hill during the later part of the 1970s, as AT&T continued to relocate more equipment to the remote site. It is not thought that there was ever power feed equipment at Oak Hill, due to its close proximity to the San Jose CO, but Oak Hill may have had equipment to exchange groups or supergroups between the cable and microwave.

The tower at Oak Hill is an unusual concrete design, unique but not unprecedented considering vaguely similar towers at some CO sites like Queene Anne Hill in Seattle. The reason for the unusual and interesting tower remains unknown. Another oddity of Oak Hill is the building, which was built by cut-and-cover or had soil piled against it later so that it is semi-underground. The roof remains exposed, but the walls are covered by sloped earth. This does not appear to be a hardening measure, especially since the building lacks blast doors or vents.

Both the tower design and semi-underground building may have just been accommodations to local aesthetic objections. Bernal Heights seems to have been similarly part-buried, likely for the same reasons.

When the Oak Hill site was originally built, it was isolated on the hill. The residential development around the tower is a later project.

The underground building leads to an interesting arrangement where microwave feedlines leave the building through an underground gallery that runs directly beneath the tower, where feedlines turn upwards. It's actually quite elegant compared to the hardened microwave sites.

• Long-Lines.net (including interior photos)