We’d like a brief AM antenna shortly
Fig. 1: WJMC Studio with temporary antenna
Mike Murrey was hired as an engineer at WJMC (AM / FM) and WAQE (AM / FM) in Rice Lake, Wisconsin, in 1998. He glanced at the 459-foot tower serving WJMC at 1240 kHz and knew it would have to be replaced one day.
Well, that came one day in late 2019 when a crew refused to climb the 63 year old structure. This began a series of events that replaced the tower.
Heavy and persistent rainfall made the preparation of the construction site extremely difficult. Temporary roads were built with rock and gravel so that concrete could be poured at the new tower base and guy anchors. The original concrete could not be used as the towers are now “constructed” so that they can be insured by insurance companies.
It looked like the project would extend to 2020 when the tower crew announced that they would start “NOW” to demolish the old tower. Rather than dismantling the old tower in sections, they decided to cut a guy’s anchor and drop it. (Watch the video.) People were evacuated from the studio / transmitter building and a nearby store to allow the tower to fall. Besides, who would want to work inside while there was a spectacle going on outside?
It had been assumed that there would be more than enough time to set up a temporary AM broadcast antenna, but now there was a mess to make this possible.
Mike’s original plan was to put up two power poles to support a long wire antenna. My experience with horizontal wire antennas is that they are good “cloud burners”, as we say in the amateur radio hobby – HF radiation rises more than it reaches the horizon. I found it that way when I helped another station. The coverage with a quarter-wave wire from the base of the tower to a tree was only a few kilometers. Ouch!
A better choice was to build the tallest temporary vertical antenna possible.
The local utility installed a used 40 foot utility pole that protrudes 35 feet from the ground (see the illustration at the beginning of this article).
The decking consisted of 40 feet of pipe attached to the pole. A wooden dowel was inserted into the lower pipe section to prevent crushing when tightening the mounting screws. There was quite a bit of overlap between rods and tubes. It turned out that the top was only 68.5 feet from the ground.
The rod consisted of four 10 foot sections of iron pipe reduced 1-1 / 4 inches from the bottom to 1/2 inch at the top. To improve antenna efficiency, Mike constructed a “cylinder” at the top of three 10-foot wires attached to nylon guy ropes. These wires were number 10 bare, soft drawn copper. It was the same wire normally used in AM grounding systems. They helped make the antenna’s electrical height a little higher. A number 6 copper strand ran from the metal pipe above the wooden post. The cable was connected to a used / temporary antenna coupling network below. Four 200 foot copper radials were made from the base to the top of the floor. Some half-length radials were also done as additional wire was available on the supply reel. Might use it too.
Things did not go exactly as hoped (Fig. 2).
Fig. 2: Lifting the temporary antenna
Mike attached the pipe to the cylinder while the power pole was stuck into the ground. Then the tube bent almost 90 degrees while it was being brought into position. This required two outriggers to straighten the pole so that it could be guyed. You will see that it was still a little bent in the photo.
Contract engineer Del Dayton of Eau Claire, Wisconsin, was called in to measure the antenna impedance at night (Fig. 3).
Fig. 3: Del Dayton sets up the temporary antenna coupling network
He came up with 38 –J180 and then calculated a draft. The components were then installed and adjusted in a temporary antenna coupling network. It was convenient to pull the original 50 ohm transmission line and connect it to the coupling network.
The downside is that the temporary antenna couldn’t be built 100 feet from the studio and the original tower as originally planned. Instead, it was right next to the parking lot, about ten feet from the studio. This is because trucks were unable to drive over the water-saturated soil.
The location presented its own challenges. Although the tower staff were RF safe, RF got caught in unshielded cables that led to the fax and credit card processing equipment. Mike moved this to another part of the building.
Mike is a US Air Force veteran and was laughed at by his US Navy veteran brother. It seems Air Force folks don’t know how to tie knots in ropes to guy lines. It takes a sailor to get it right!
He requested and was given special temporary authorization by the FCC to cover the situation. He opted for 250 watts instead of the licensed 1000 watts to keep RF in check.
Construction of the new 459-foot tower began, but soon the tower crew moved off the job for three days to work for an off-air television station. The foreman felt entitled to do so because WJMC was actually “in the air”. The station manager and staff were happy because they still had listeners instead of being absent for weeks.
How well did it work?
The station had a usable cover. This intrepid reporter measured the field strength at 14 random points in the listening area using a GPS to document each location (Fig. 4).
Fig. 4: The author takes field intensity measurements
This enabled me to determine the distance to the measurement locations and display it on a curve. This was “Mark’s two-hour AM micro-evidence,” as described in an article I wrote for Radio World in 2003 (read it at www.mwpersons.com/articles/6-4-03-RW-article. html). .
The data showed that the field strength was 33 mV / m over a kilometer with only 250 watts of transmission power. It was definitely better than nothing! It was about 10 mV / m in downtown Rice Lake and about 12 mV / m in residential areas. The population of this small town in Wisconsin is 8,338 people. The half mV / m contour assumed its low ground conductivity of only 4 about 10 miles.
The meter I used was a Potomac Instruments FIM-41. The FIM-21 and PI 4100 are similar instruments commonly used to measure monitoring points on AM directional antenna systems. They are good tools for determining antenna efficiency as you will see in this article.
Before and after
I did the previous antenna resistance measurements back in 1993. It was 108 ohms with a reactance of -247 ohms for an antenna current of 3.04 amps with 1000 watts input. Del Dayton measured the new tower at 43 ohms, -125 ohms reactance, for 4.83 amps at 1000 watts. The details are shown in Fig 5 below, which you can click to enlarge it. Yes, the two towers were the same height.
Fig. 5: Schematic representation of the WJMC antenna system. Click to enlarge.
Many factors can change the characteristic impedance, including tower width, antennas on the tower, isocouplers, lighting chokes, capacity to the tower from the guy lines, and introduction into the antenna coupling network.
I now believe that the old tower had poor electrical connection between tower sections near the top. Yes, this can happen when towers rust. Sections can be electrically disconnected even with tons of downward pressure. It’s hard to believe, but it’s true. Therefore, at least one leg must be welded at the connection points. Find out more about an article I wrote on Radio World in 2012: “Better life through tower welding”. Mike had the crew weld two tower legs as this was convenient for the welder at work.
In Fig. 6, Mike Murrey shows the completed project with new isocouplers and a converted AM antenna coupling unit. The new tower has FM translators for the two AM stations and a replacement antenna for the three full-power FMs.
Fig. 6: Mike Murrey and the completed project
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Mark Persons, WØMH, is a certified professional broadcast engineer and recently received the SBE John H. Battison Award for Lifetime Achievement. His website is www.mwpersons.com.
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