I have done several of these posts in the past, but it always seems to be of some interest, so it bears repeating. AM antenna systems are not black magic. They are actually pretty easy to understand if the fundamental knowledge is in place. Medium Wave frequency wavelengths are fairly large compared to other broadcast frequencies. Thus, the components are larger.
The three basic components of an AM antenna system are the tower, the ATU (antenna tuning unit) and the transmission line (AKA Coax). The tower is the radiating element and they come in a variety of flavors; uniform cross section guyed, self supporting, series excited, shunt excited, etc. A series excited tower has a base insulator and is fed directly from the ATU. A shunt excited tower has a grounded base and uses a skirt or folded monopole design to transfer the RF to the main radiating element. This design has an advantage as the tower can be used for other wireless and broadcast services.
The antenna work in question for this project is WINE, 940 KHz, Brookfield, CT. The skirted tower is used for WRKI. It also has two way and cellular clients. The issue is instability of the WINE antenna system, which is likely due to improperly attached shorting wires between the skirt at the tower. Over the years, the impedance of the skirt has gone way up. The tower itself is 152.1 meters (499 feet) tall, or 170.3 electrical degrees. The skirt length is about 82 electrical degrees and it is shorted at about 72 degrees. There have been several papers written about folded monopoles for Medium Frequency (AKA AM or Standard) broadcast service. The recommendations state that for best performance, the short to the tower should be between 62 and 90 electrical degrees. Since the existing system falls in that range, there must be other problems with the antenna skirt and or shorting wire to the tower.
If one looks at this diagram, that configuration should look something like a gamma match, often used on dipole and yagi type antennas. A gamma match can be thought of as a stub of transmission line which is bonded to the radiating element at some favorable wave length corresponding to the desired radiation resistance. This is one of several configurations for folded monopole antennas and this type is most often seen on towers that support other wireless service antennas such as cellular and two way systems which are installed above the skirt.
There are a few interesting data points when looking at these type of antennas. First is the ratio of the diameter of the skirt over the height of the tower, or D/H. The larger this ratio is, the better the bandwidth characteristics of the antenna system are. This makes sense, when you think about it. In this instance, the tower is 151 meters (495.4 feet) tall and the skirt is 3.3 meters (10.83 feet) wide, thus the ratio is 0.0218.
The licensed base impedance if 234 ohms with a good amount of inductive reactance. When Sprint and T-mobile changed their configuration on the tower, that impedance shifted dramatically. The existing skirt is in fairly rough condition. The bottom ring that connects to the ATU is made out of copper tubing. It is attached to the skirt wires with steel saddle clamps, all are rusted and all of which are lose and can slide around. At some point, the tubing filled up with water, then froze causing the tubing to split open. At the top of the skirt, the jumper wire looks suspicious and the top ring does not go all the way around. The shorting stub to the tower looks like it is made out of battery jumper cable. I purchased new cross wire clamps and found some spare copper weld skirt wire at another site. Both the bottom ring and top ring were replaced as well as the shorting stub to the tower.
After the repair work was done, I had the tower crew reattach the short slightly below the last skirt to tower bonding point. In that position, I found the impedance went way up. Thus, going lower was going towards a resonance point. I had them move the short up to the former shorting point and remeasured and found the impedance was 235 ohms, only 1 ohm off from the previously licensed values.
Initially, I thought it would be nice to find a better position for the shorting stub and get a lower base impedance. This would make the whole antenna system work better (improve bandwidth, stability, etc). However, there was a set of guy wires above the bonding point. The tower crew would have had to disassemble the top ring to move above the guy wires. We were running out of daylight and weather so I had them lock everything down where it was. On a station running an all sports format that has no listeners and does not make any money, it does not make a lot of sense to spend gobs of money and time to rebuild the ATU for a new base impedance. When I got the impedance back to within 0.11% of the licensed values, it was time to declare victory and go home.