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Some broadcast radio history

It happened in 1962, I had just begun working at an aerospace company in California. It was at that time, having always had an interest in broadcast radio, that I also took the FCC test to qualify for a First Class Radiotelephone Operator's License. I passed, and as a result, I could legally maintain a radio or television station as a resident engineer. It was backup insurance, should my aerospace career not work out.

Why was I so interested in broadcast radio?

My radio roots go back to my grandfather's radio activities in the 1910s and 20s, long before there was an FCC. His first amateur radio license, 6UO, was issued in 1921. And, my dad became a teen-age ham radio operator in the 1930s, with a call sign of 6GYX.

When Nevada was moved from the 6th licensing area to the 7th, my grandfather was issued W7VO, and dad became W7JOS.

So, I grew up with radio, and though I never acquired an amateur license, I made my own receiver using vacuum tubes (with Dad's considerable help) in the late 1940s, when I was just 8 years old.

With such early exposure to radio, it should come as no surprise that I gravitated to local broadcast radio stations in Reno, NV, looking for part time work when I attended college there in 1957-58. The stations I had an interest in were KOH AM, and KNEV FM, both powered by vacuum tube technology and completely analog audio and transmitting circuits. I did audio remotes to KNEV from the university, and we used analog volt-ohm-milliammeters (VOMs) and vacuum-tube voltmeters to service their transmitters and studio equipment. It would be another two years before I became aware of Fluke.

KOH AM - Reno

During the early fifties, KOH AM, owned by McClatchy Broadcasting, operated a 5 kW, custom-built phase-to-amplitude modulated (Ampliphase) transmitter. A 50 kW version was used at a sister radio station, KFBK AM, in Sacramento, California. I was fascinated with this highly efficient design which replaced a classic design using a relatively inefficient audio modulator requiring half the power capacity of the station's rf output. For the two stations mentioned above, that would mean 2.5 kW and 25 kW of audio, respectively.

Fast forward to last week

Fluke recently introduced the innovative model 233 digital multimeter, which features a removable display unit that can be remotely placed using wireless technology. I asked Fluke if I could evaluate its performance in some interesting situations. One that came to mind because of my past experience was to measure inside the cabinets of broadcast transmitters, which are interlocked for safety reasons due to high voltages and the rf fields which may be present. With the new Fluke meter in hand, I contacted a local AM broadcast station that just began operating what they call "High Definition (HD) Radio," with a 50,000 Watt all solid state digital transmitter.

How different are modern day transmitters from those I had fleeting experience with over 50 years ago. The description of the digital modulation method used in their Nautel XR50 transmitter sounds suspiciously like that used in today's 50 Hp adjustable speed motor drives, but it does so much more. Not only do they do the normal AM modulation using Pulse Duration Modulation (PDM), but they cleverly add additional digital text information in the sidebands for the new digital AM receivers, to display such things as traffic, and program information.

As I looked at the station's website to learn more about this new AM technology, I was amazed to find in their history that their first broadcast license was issued by the U. S. Department of Commerce in 1922, just one year after my grandfather received his amateur 'ticket.' I also found that the station's first license (KFBL) was the next one issued after the Sacramento station I mentioned above. You can read their interesting history here: http://www.krko.com/stationinfo/stationhistory/

So, now, the details of our experiment using the new Fluke 233.

KRKO's new transmitter site is located in the floodplain of the Snohomish River, an ideal location providing a good ground plane for the antennas. As a result, the building housing the transmitter and related equipment is built on a platform mounted on stilts, several feet above the ground.

I climbed the stairs and entered the building with the engineers. We then powered down the transmitter cabinet to be tested and removed the cover plate for the power supply section. The Fluke 233 was connected to read the dc buss voltage at the output of the rectifiers. I then removed the display and the transmitter cabinet cover was replaced, enabling the equipment for operation with the meter inside.

Would it work with 50 kW of RF around?

I wasn't too worried, because the transmitter was operating in the relatively low frequency AM band - 1380 kHz to be exact, where a wavelength is about two football fields long. The situation could be far different at the UHF frequencies used for the new digital television, where a wavelength could be about half that of a dmm test lead. My concern was that resonances might develop in the test leads that would subject the meter internals to high rf voltages. Low pass filters on the input leads might be advisable in such situations, and even then, the meter should have fairly robust shielding as well, such as that which recent Fluke dmms have, allowing for operation in electric fields of up to 3 volts per meter.

On with the test

The power was applied and we observed the supply voltage rise to about 300 Volts. And, I was able to walk anywhere in the room with the display in hand and maintain communications with the meter, which was now fully enclosed in the transmitter's aluminum cabinet. The only problem we encountered was that the magnetic meter mount didn't work on the aluminum control panel. However, there was a steel rack cabinet next to it, so we stuck it there, still convenient enough while operating the transmitter controls.

The experiment was pronounced a success.

A much more common application for most users will likely be in safely testing in power distribution panels and motor control centers, where the new NFPA 70E rules for the use of flash suits might otherwise apply. And, of course, we can all imagine other uses, such as monitoring the difficult to reach test points in and around an automobile or truck.

I'm sure we'll hear many innovative ideas in the months to come.