What's wrong with this
picture? Believe it or not, that is how my former USAF buddy, Jim Flinn, of
found this connection for an AM broadcast station that he and his team were called in to fix. Per Jim,
"Before we started you couldn't hear this 1000 watt AM radio station five miles out of town. After we
were done it could be heard 50 miles away." Paying advertisers probably were not happy about the 5 mile
range since their audience would have been reduced considerably. It is a wonder that the transmitter
even survived what must have been an atrocious mismatch.
The photo is not detailed enough to
show the intricacies of the center conductor connection, but the ground/return/shield connection is
nothing more than a piece of copper wire bolted to the coax feed cable shield. So, in addition to the
awful return loss, the potential for PIM
(passive intermodulation) generation is enormous. I'm guessing if a spectrum survey of the tower output
had been conducted, it would have shown a huge blob of crap all around the carrier that would have warranted
a severe violation issuance by the FCC. A lot of times, those kinds of problems are first noticed and
reported by listeners of AM radio that pick up interference on adjacent channels. OSHA would have had
a fit over the shock hazard presented by the open connections
Doing some back-of-the-envelope calculations shows just how
bad it could have been. The reported range improvement is a factor of 10. That equates to a power difference
of 20 log (10) = 20 dB. In terms of power, that is a factor of 10 ^(20/10) = 100. So, if we assume 1000
W into the antenna cable yields a 5 mile range with the crappy connection and 50 miles with a good connection,
that means roughly only 1/100th of the transmitter power is being radiated. So, only 10 W out of 1000
W is being pushed through that kludge, and 990 W is being reflected!
Reflection coefficient (Γ
) is sqrt (Preverse
so we have Γ
= √(990/1000) = 0.994987.
VSWR (or SWR) = (1+Γ
):1, so we have
VSWR = (1+0.994987)/(1-0.994987)
Whoa! That's a serious mismatch. Like I said, it's hard to believe the transmitter and/or
tuner was not destroyed. Jim replaced the mess with a nice set of connectors designed for the cables.
I somewhat hesitated to publish this because the identification of the radio station will be revealed,
but it is a good object lesson on how just because an electrical connection might appear sound, that
is no guarantee that it is - particularly from an RF perspective. Based on this and other instances
I have seen both on Jim's website and those of others, there are a lot of ill-maintained stations around
Oh, Jim called last Friday afternoon to wax nostalgic about the old days; that is
what prompted this story. It had been about 32 years since we last talked upon departing from Tech School
(although we did write a couple times thereafter.)
Now for a little background on Jim and me.
We first met in USAF technical school at Keesler AFB, Mississippi, in January of 1979. We were both
in the Air Traffic Control Radar Repairman class (AFSC 30331). Jim hearkened from the Saginaw, Michigan
area; I crawled out of
, Maryland. We both had a lot of exposure to electronics and electrical circuits
prior to joining. Jim's uncle owned a Radio Shack, so that fed Jim's taste for things electronic. He
was also an avid radio broadcast guy, and was the first person I know that had a TRS (aka Trash) 80
computer. I left a job as an electrician and a hobby of electronics to enlist.
We both graduated
with the highest GPAs in our classes (he was a week ahead of me), partly because of previous knowledge,
and partly because neither of us was a big partier. That doesn't mean we never pushed the limits of
good GI behavior, just never far enough to get into trouble or otherwise affect academic performance.
While at Keesler, we along with a few other friends managed to totally rebuild the engine on his VW
Bug, have battles (not on base) with arsenals of bottle rockets and firecrackers, make a few trips to
New Orleans and Pensacola, FL, help organize and pull off a really great toga party for the entire barracks
(imagine Animal House with crew cuts), and we even built a couple stereo power amps and other minor
circuits as extracurricular activities.
There were plenty of real characters there at Keesler.
I remember one guy in our radar class (we called him Sully) who could barely pass the exams, but had
every skit from the Cheech & Chong records memorized. He had all the parts down pat, and was such
a clown that the Air Force instructors (the younger ones) sometimes allowed him to get up in front of
the class and go through his routine. One of the funniest ones he did was the one with the room full
of pilots being briefed Kamikaze mission - anyone remember that?
Jim and I also spent about two
weeks seeing to it that the floors outside the squadron chow hall had a glass-like shine to them each
morning before the troops showed up for breakfast. We had to get up at about 3:00 in the morning to
use those big mechanical buffers to first strip the floor of yesterday's wax, then apply new wax and
buff it to a high sheen. It took about 4 hours, but the reason we volunteered was because unlike the
other airmen waiting for classes ("Sets") to begin, we did not have to work full 8-hour days.
I actually spent about 5 weeks buffing floors because the USAF hosed me. Here's how. The first 6
weeks of classes consisted of Basic Electronics, where lessons began with Ohms Law and an introduction
to electronic components. It progressed through to the point where the graduation project was the successful
completion of an astable multivibrator circuit using two transistors and a handful of Cs and Rs. You
had to design, build, and troubleshoot it yourself. A test was given at the end of each week. Well,
there was an option to test out of all Basic Electronics classes by taking all the tests and building
the circuit. The payoff would be getting into the "Sets" a month earlier and therefore out of Keesler
and onto my permanent duty station that much sooner. I was done in 3 days. The next morning I reported
for TDY (temporary duty) to the squadron maintenance NCO, one Mst. Holloman. After doing odd jobs for
a few days, he offered me the floor waxing detail. How could I refuse? He said my orders to begin radar
classes (about 30 weeks worth) would be forthcoming. So, I waxed floors. After a couple weeks, I was
still waxing floors, and that's when Jim came on to help. About two weeks passed and Jim got his orders
to start classes. Others had come and gone the same way. I finally went to the squadron commander and
asked what the heck was going on. It was just a simple SNAFU, I was told. I started class the next week.
But, I was not surprised at the USAFs propensity for SNAFUing. My "guaranteed"
job when I enlisted was as a weather equipment specialist, where I would have maintained weather measurement
and reporting equipment (weather phenomena is also an interest of mine). About 4 weeks into Basic Training,
I was called into an office where a guy with lots of stripes on his sleeve (intimidating to a new recruit)
explained to me that he could not figure out how that job was promised when an opening never existed.
The good sergeant told me not to worry though, because he had lots of other good jobs for me. He handed
me a list to choose from. Let's see, administration clerk, paving maintenance specialist, dental hygienist,
military policeman... When I dared to suggest that I really wanted something having to do with electronics,
he grunted and told me that the jobs on the list were all that was available. Somehow I managed to garner
up the courage to remind him that my enlistment contract stated if my guaranteed job was not available
through no fault of my own, that I had the option of separating from the service. I scored 100% on my
entrance exam (which didn't require a genius IQ), so being qualified was not an issue. He did not appreciate
my boldness. Sarge left the room and came back with another list. This one included jet engine mechanic,
aircraft loadmaster, and... wait for it... air traffic control radar technician. The rest, as they say,
Yeah, I'm rambling a bit, but some people like reading this kind of stuff.
The saga continues. A few weeks prior to graduating from the radar course, I received orders to
report to an Air National Guard base in Duluth, Minnesota. I was Regular Air Force, but would be maintaining
a mobile radar unit used by the ANG there. It was a perfect assignment since I preferred the cold weather
over being in the South. Jim received his orders for K. I. Sawyer AFB on Michigan's Upper Peninsula,
not real far from his home. K.I. Sawyer closed back in the mid 1990s, BTW. I was making all kinds of
plans for what I would do in Duluth, including take classes at the local college to work towards an
engineering degree. About two weeks from the end of tech school, I was informed that my orders had been
red-lined (cancelled) and that new orders would be issued. Graduation came and went - along with all
my friends - and still there were no new orders for me. It was back to waxing floors again. Finally
new orders arrived - for the 5th Combat Communications Group
(5CCG) at Robins AFB, Georgia! Ugh, hot, humid weather. Strike 3.
the afternoon of the day before
through the area, and drove through Mobile, Alabama just a few hours before the eye passed through it
early the next morning. I was the last person that the MPs let go through the gate. If I hadn't gotten
out that day, I would have spent a couple weeks helping to clean up the mess on base and out in the
Biloxi area. I know because another guy (Allen
) came into our radar shop a year later who had arrived at Keesler just a couple weeks before
the hurricane hit. He and all the other students spent about three weeks doing clean-up.
last 30-some years, Jim has built quite a reputation as a commercial radio equipment specialist. He
has repaired and rebuilt many stations around the U.S. He has also done some stints as a radio talk
show personality, going by the name
. Always the go-getter, entrepreneurial type, Jim even travelled around the country
conducting Guerilla Marketing seminars.
There's a lot more I could tell you, but that will have
to wait for another day.
These images have been chosen for their uniqueness. Subject matter ranges from historic events, to really cool phenomena in science and engineering,
to relevant place, to ingenious contraptions, to interesting products (which now has its own dedicated
Featured Product category).
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Posted August 22, 2011