Reader Letters

Unlicensed microwave secondary to licensed radios
Thanks for the article in Mobile Radio Technology on unlicensed microwave ("Unlicensed Microwave: Blessing or Curse?," Stephen Bartlett, October 2000). I feel it’s important to fully educate radio technicians on the planning, installation and operation of these devices.

One thing I have not seen in any article regarding these devices is their secondary status to licensed radios, especially in the 2.5GHz bands. Unlicensed spread spectrum devices operate under Part 15 of the FCC Rules and therefore cannot cause and must accept harmful interference to and from licensed users.

Here’s the problem. Unlicensed spread-spectrum radios can operate from 2,400MHz to 2,483MHz under Part 15 of the FCC rules. Video links licensed under both FCC Parts 74 and 90 can be found from 2,450MHz to 2,483MHz. As the unlicensed links hop between 2,450MHz and 2,483MHz, they can cause interference to the wideband video links. This interference is typically seen as horizontal "hits" jumping up and down through the video signal.

When the licensed users finally track down the interfering transmitter, which is usually a difficult task in itself, the unlicensed users are usually mystified how their "interference free" devices can be causing any problems. The unlicensed users are usually very upset when they learn that their transmitters must be turned off to cure the interference—even if the licensed user constructed their facilities after the unlicensed user. Per the FCC rules, Part 15 devices are not, and cannot be, given the interference protection that licensed users have.

As a Part 74 frequency coordinator, I am concerned about the interference to licensed users as many of these Part 15 devices are installed by marginally qualified users who either don’t know or understand the FCC rules. I suspect the manufacturers and sales representatives of these devices "forget" to tell potential clients of their secondary status, fearing that information could "blow the sale."

You did a wonderful job at pointing out some of the interference issues between Part 15 spread-spectrum devices. Unfortunately, you forgot to mention their secondary status to licensed links. Help me get the word out.

—Karl Voss
Broadcast Auxiliary (Part 74) Frequency Coordinator
KPNX-TV
Arizona
Kvoss@kpnx.com

Installing roof-mount antennas on new vehicles
I’d like to read/hear how two-way mobile installers have coped with the challenge of installing and wiring roof mount antennas on some of the 2001 automobiles containing "cloth-over-foam-glued-to-the-roof" liners, e.g. Ford Crown Victoria.

—Richard Shima
FM Communications
Mayfield Heights, OH
Rshima@att.net