Occasional reader Scott asked for a picture of the inside of a BE AM output tuning network. I figured it might be helpful to make a short post about it.
These things are pretty simple; a T network with a capacitive leg to ground.
This particular unit is for 1230 KHz. I believe the capacitor is frequency determined and they may also use larger inductors for lower frequencies.
The inductors are Kintronic LV-15-20 (15uH 20 amp) and the capacitor is 0.0018 uF CDE 6KV 5.6 amp.
The issue with this particular unit is dirt. The inductors have round metal plates that roll along the inductor coil to make the variable inductor tap. Dirt has accumulated on the coil turns and on the inside of the plates. This, in turn, causes arcing anytime the Tune or Load controls are moved. A through cleaning should take care of the problem.
Working on another old AM station, this one is a simple Class C one tower on 1230 KHz.
The main problem today was this BE AM output network unit between the BE AM1A and the ATU. This site has had some dirt difficulties over the years and the internal parts of this tuning unit arc at full power. I attempted to drive the ATU directly with the transmitter, which was a no-go.
I took a look at the ATU, which is a pretty standard Gates 1 KW ATU from the late forties or early fifties. I have seen perhaps dozens of these things.
My first thought was that over the years, likely due to changes in the ground system, the base impedance has shifted away from its licensed values. However, a quick measurement of the base impedance shows it to be exactly at the licensed value, 17.3 ohms. The tower is 67 degrees tall, so that impedance value is right in the theoretical norm.
I measured the input to the ATU, which showed 38 ohms with about 7 ohms capacitive reactance. I can only surmise that it has always been this way. The transmitter in use before the BE AM1A was a Harris/Gates Radio BC-1G. That model transmitter will drive anything including an open transmission line.
Having the bridge on hand, I decided to retune the ATU for a better match. I put the bridge on the input terminals of the ATU and set it to 50 j0. Using the remote control, I turned the transmitter off and on while making small adjustments to the output strap on the coil until the resistance was 49 ohms with zero reactance. I would have gotten it to 50 ohms, but the strap on the output side of the coil would not stretch far enough to reach the proper spot on the coil.
Now the transmitter will run into the ATU directly at full power with about three watts reflected. The BE AM output matching network unit has been removed for cleaning and repairs. I will reinstall it once those repairs are completed.
Yesterday I took, what I hope to be, my last walk across Pleasure Beach Island in Bridgeport, Connecticut. The task at hand was repairing the antenna array for WICC. There turned out to be several issues which were addressed in turn.
The trouble started when the feed line between the ATU and the tower became disconnected during a storm. That consists of a 1 inch copper pipe extending from the ATU feed through insulator up to a brass plate suspended between the four tower legs by hard drawn single 0 copper wire. The feed line separated at the brass plate which, unfortunately, is approximately eighteen feet in the air.
The feed line was repaired, but not effectively. By the looks of the picture, the brass plate never got hot enough to accept the solder.
After the feed line was re-repaired, other issues became apparent. The base impedance of the tower was still off and the array was still way out of tolerance.
It was noticed that several bypass capacitors on both of the tower lighting chokes where blown open. Those where replaced and the tower lighting chokes where checked for shorted turns. While it is always nice to replace burned out parts, this did not correct the problem.
Finally, we were back at the base of the tower with the defective feed point and a decided to grab the pipe and give it a good shake to see if it came apart again. It did not, but then I realized that that tower was supposed to be back in the circuit and I did not receive any RF burns for my carelessness.
We dug into the ATU and discovered that the input capacitor was marginal and there was a large crack in it. The output capacitor seemed to be completely open. The base current that we were seeing on the base current meter was being induced by the other tower. It all began to make sense.
The parts were ordered and shipped and I made another trip out to install them myself.
Thus, on this particular day, I had my tool bag, an OIB-3 with fresh batteries, my cordless drill, drill bits, and three type 294 mica capacitors. I took the drill because the new capacitors were quite a bit larger than the old ones, so I needed to move the stand off insulators to remount them.
The walk from the end of the dock to the transmitter site is approximately 900 meters or 0.55 miles, according to google maps. On a nice day, it is a pleasant walk. On not so nice days, it can be less so. It was foggy with light drizzle. Not enough to get wet right away, but enough to get slowly soaked while working on the ATU repairs.
With the new capacitors installed, I needed to adjust the array back into tolerance, which didn’t take too long. I made a short video of the station running at full power showing the antenna monitor readings for both the day and night patterns. Then packed up and headed back to the dock.
I wanted to take a set of monitor points, but the FIM-41 had been moved to another location. That was fine, I was getting pretty uncomfortable in my wet clothes, so I headed home.
One of those things that I have written about before, but seems to be common these days as older AM towers need to be replaced. One of our clients had just such a tower. Erected in 1960, the hollow leg stainless tower was rusting from the inside out. When the tower crew came to put up the translator antenna, they discovered that there was a hole in one of the legs and climbed back down.
The tower condition was somewhat known about, there were braces installed several years ago at certain levels to keep the tower standing. The new owner had planned to replace the tower eventually, so those plans where moved ahead.
A temporary utility pole was installed near the transmitter building and a wire was strung to another customer owned pole about 170 feet away. At 1,490 KHz, that proved to be a pretty good length. The issue with these medium wave temporary antennas is always the height above ground. In order for the radiation resistance to be somewhat reasonable, the antenna needs to be at least 1/8 to 1/4 wave length above ground. That means a minimum of 78 to 157 feet at 1,490 KHz. The utility pole installed is 35 feet AGL.
Thus, the wire antenna has a fairly low resistance, with loads of inductive reactance. Something on the order of 20 ohms, +j480. Since this is temporary, we reused the existing ATU that was designed for the series excited tower. With a capacitor installed on the incoming wire to cancel out some of the inductive reactance, a simple T network was configured to match the 50 ohm transmitter output to the 20 ohm antenna.
In the end, we were able to run about 400 watts into the wire, which covered the city of license fairly well. While the new tower was being erected nearby, we had to reduce that to about 100 watts to protect the tower workers from the hazards of non-ionizing radiation.
The new replacement tower has been constructed. It is the exact same height as the old tower, but has a twenty foot pole on top instead of a normal tower section. The pole was installed to mount the translator antenna. In addition to that, there will be other wireless services installed on this tower.
WKNY will have a six wire skirt installed in the next few days. As this tower is close to 160 degrees at 1,490 KHz, the skirt can go anywhere from 60 to 120 degrees up the tower.