How AM Directional Antenna Proofs Were Done – 60 Years Ago!
AM radio directional antenna systems go way back; into the 1930’s. The FCC evolved a very strict body of rules to govern every detail of the setup and adjustment of those antenna systems.
From the FCC archives comes this proof of performance done by one of the early and well-known broadcast engineers – Harold Rothrock. Active from the earliest days of radio, Rothrock was a broadcast pioneer.
The cover shot is from the FCC file copy – notice the manual typewriter, onionskin paper, and crooked text. That’s how it was done back then. How old is this? All FCC to station correspondence was by telegraph!
Today we take finding AM proof measurement points for granted. Between GPS and high quality, up-to-date maps, its relatively quick and easy! In 1951, not so much. There was no electronic navigation and the sole source of “good” detail maps was the US Geological Survey 15’ quadrangles. If you were lucky there were more modern 71/2’ maps for your location Hundreds of locations had to be hand-plotted on maps that had to be ordered weeks before.
Getting around was miles of dirt roads and farm lanes. Interstate highway construction hadn’t started yet, and there were a few miles of four lane near cities. Aside from the occasional pay telephone, there were no communications with the site. A few broadcast consultants were experimenting with two-way radio, and others used primitive mobile amateur sets to clandestinely stay in touch.
Measurements went to about 20 miles in eight or more directions, two sets, or more, depending on day/night directional patterns. In our era, these measurements undertakings are measured in days. In Rothrock’s time, weeks or months! In fact, Harold’s wife Mary often traveled with him and set up temporary housekeeping at these sites. Many consultants took wives to measurement projects to assist.
The results of the measurement activities, hundreds of values, were analyzed and hand-plotted on groundwave conductivity charts. These revealed the RF radiation of the antenna system along different radials or bearings. No Excel spreadsheets or graphing assistance here. What is done in the office today in a few hours took days of work with a calculator and slide rule. Just in case you’ve never seen these relics, see them below. By the way, all figures had to be copied off these devices onto paper. You were the working memory!
When adjusting an AM directional antenna system, the radiation pattern predicted for the particular tower arrangement didn’t always emerge. That meant this whole, tortuous process often was repeated time and time again until the FCC pattern specifications were proven. Today, method of moments computer calculations allow the RF engineer to model electrical details of the towers and site, and AM reradiation objects, often achieving success on the first try.
Many directional stations are still operating under the same licenses and configurations as they had many years ago. In the 1960’s and 1970’s, LBA engineered, supervised installation, and did FCC proofs for hundreds of stations. Because many of the station which must be considered in cellular AM detuning and AM collocation today, are of the old type, we are uniquely positioned to understand and help wireless operators deal with their idiosyncrasies.
If you would like to look at the whole 60 page, handcrafted 1950 WJPS proof, you can download the PDF HERE.
For assistance in AM colocation or AM detuning, contact 252-757-0279 or sales@lbagroup.com
Since 1963, LBA has also supplied innovative AM and medium wave antenna systems to broadcasters around the world.
