Topband: Fwd: Interesting story of mixing of LF & MF & ground systems

[Bill Tippett]

Hi Bill

One of the guys here ran across this somewhere and forwarded it to me.
 It really is an interesting read, and the conclusions apply very much
today, etc - one of the ways the 10 KHZ broadcast "birdies" happen.

73

Robin WA6CDR



*A Messy Ground Radial System Can Cause Radiation of Spurious Signals*

By Walter Maxwell, W2DU

In 1948 I was the consulting engineer for the proposed first AM
broadcast station in Mt. Pleasant, Michigan, that resulted in a
construction permit and license for WCEN, 500 watts on 1150 KHz. Using
a National HRO receiver, I was performing a hands-on search for an
available frequency for the new station, when I encountered an
interesting and unusual signal that was entirely out of place in the
AM broadcast band-a CW Morse-code station illegally transmitting
five-letter-word code groups at 30 words per minute. The illegal
signal was S9 +40 dB on 1297.8 KHz, producing a 2200 Hz beat-note with
the 1300 KHz frequency of WOOD, Grand Rapids, thus producing a an
audible CW signal with the receiver BFO disabled.

The format of the coded messages appeared to be military, IDing as
NSS. We know that NSS is the flagship station of the U.S. Navy in
Annapolis, MD, but in the AM broadcast band? It appeared that either
an NSS transmitter was producing a spurious emission, or a station
using NSS as a fraudulent call sign was operating clandestinely in the
AM broadcast band. I deemed it important to find out which.

As a former FCC monitoring officer at the Allegan, Michigan monitoring
station, the next step was to report the situation to the Allegan
station. Although Allegan was 90 airline miles away, the monitoring
personnel there could not hear the spurious signal, even though it was
S9 +40 dB at Mt. Pleasant. I let the FCC monitors hear the signal
through the telephone, but they still heard nothing on their receivers
tuned to 1297.8 KHz. Thus the signal must be of local origin near Mt.
Pleasant, and not from NSS. However, to be on the safe side, FCC
notified the Navy of the spurious signals, and the NSS operators began
combing all their transmitters for spurious signals, and found none.
The situation is now becoming even more strange.

I then copied five minutes of the coded text and sent a copy to the
FCC, who relayed it to NSS for comparison to their transmissions. The
situation is now both perplexing and frustrating, because the text I
copied on 1297.8 KHz agreed exactly with a transmission that had been
made by NSS. How could that signal have been transmitted on 1298.7 KHz
if no spurious signals were emanating from NSS? But it's obviously not
a fraudulent station. What then?

A partial answer came shortly thereafter. As I resumed the search for
a useable frequency for the new station, I proceeded downward from
1298.7 KHz, going through 1280 KHz and hearing WFYC, Alma, 1000 watts,
15 miles away, also S9 +40 dB. But on continuing further downward I
immediately came across another S9 +40 dB thumping CW signal. I
switched on the BFO and discovered the CW was also a five-letter-word
coded transmission at about 30 wpm, the same as NSS. I retuned to
1298.7 KHz and the NSS signal was also there, as before. So I cranked
up a second receiver to monitor both frequencies simultaneously.
Surprise! Both frequencies were showing identical simultaneous
transmissions. I then measured the frequency of the lower-frequency
signal-1262.2 KHz. Voila!

The higher CW frequency was 17.8 KHz above WFYC's 1280 frequency and
the lower CW frequency was 17.8 KHz below WFYC's frequency. A quick
reference to the Berne frequency listing showed NSS assigned to 17.8
KHz. This situation now appeared to indicate something very wrong
going on at WFYC. The low-frequency world-wide ground-wave signal from
NSS was apparently somehow mixing with the signal from WFYC, and
producing the 1297.8 and 1262.2 KHz sum and difference frequencies.
But what non-linear device in WFYC's system could perform that mixing?
Don't know, but I reported this new information to the FCC and that
was the last I heard of the situation.until.

Fast forward now to 1955. I was now employed as an electrical engineer
at the RCA Laboratories, the David Sarnoff Research Center in
Princeton, NJ. An assignment took me to Washington, D.C. to attend the
annual conference of the NAB, the National Association of
Broadcasters.

President Dwight D. Eisenhower gave the keynote address. However, one
of the technical forums was presented by Jack Young, Chief Engineer of
the RCA Broadcast Division. His topic was on the solution of mutual
interference between two broadcast stations in Los Angeles, KFI and
KNX.

It seems that in a section of the Los Angeles area it was impossible
to hear one of these stations without hearing both simultaneously-when
tuned to 640 KHz for KFI, both KFI and KNX were heard, and when tuned
to 1070 KHz for KNX, both KNX and KFI were heard.

Young was assigned the task of determining the cause of the
interference and eliminating it. To make a long story short, he
discovered that there were ancient and rusty oil well derricks in the
affected area. Currents from both KFI and KNX transmissions were being
induced in the oil well towers, and the rusty joints were acting as
mixers for the two frequencies, producing both the sum and difference
frequencies, as well as re-radiating both signals on their original
frequencies. When the derricks were removed the interference stopped.

So how is this incident relevant to the NSS problem? Well, at the
conclusion of Young's presentation I had the opportunity to talk with
him, and because of the similarity of the problems, I told him of my
discovery of the NSS signals appearing in the AM broadcast band. Talk
about coincidences! He was surprised and excited to learn that I had
discovered the NSS problem at WFYC, because he was the one assigned to
determine the cause of the problem. He had never been told how that
problem originated, or how the problem had been discovered.

He then explained that he had found the ground radial system under the
WFYC antenna a horrible mess. Cold solder joints throughout, and far
ends of the radials hanging loose in the water of the nearby Pine
River, establishing the non-linear mixer condition that resulted in
the sum and difference frequencies being generated between the NSS and
WFYC signals.

Cleaning up the radial mess ended the appearance of the NSS signals in
the AM broadcast band, thus concluding an interesting journey.