With the advent of using wire verticals around our towers to make a phased array system, it has become apparent that the tower can affect the array's performance, usually for the worse. Many times I am asked about detuning a tower and how to do it. This short article is the result of trying one commonly cited way that did not work, and doing research on how the broadcast industry does it, and how we finally, successfully, detuned a tower with a 160-meter 4-Square array.
The tower in Figure 1 belongs to W5IZ, and it is a 200 foot tower with an 80-meter beam on top and four cantilevered Phillystran guys that hold up a 160-meter 4-Square system. Each vertical is one quarter-wavelength tall and is fed at ground level with buried radials. The 4-Square system is an Array Solutions optimized box that uses what has become known as the Lahlum method of tuning. (The namesake is Robye Lahlum, W1MK - see his two articles "Phase Adjustment Technique for a 4-Element Square Phased Array" and "Phase Correction for a Quadrature Hybrid-Fed Antenna Array" in the May/June 2005 issue of NCJ). The object of this method is to increase the front-to-back ratio and gain of the array by using optimized phase settings of the elements.
We installed the system and noticed the F/B of the array was down from the predicted pattern, which should have been on the order of 30+ dB. We were only seeing about 12 dB of front to back using a signal source that was carefully placed to give us a good reference to adjust the array. By measuring the currents in the tower legs we could see there was significant amounts of current as viewed on an oscilloscope (that had a current probe around a leg of the tower) when we transmitted on the array. This was not predicted in the NEC model of the tower we made. What to do? We needed to detune the tower.
Doing some Internet research, we found that one solution was to create a loop around the tower at some midpoint. Then, we could drop a wire down below it at 40-60 feet and put a capacitor in series to tune the circuit to resonance. We tried this at various heights and we spent a lot of time without success. The tower was just not going to be detuned using this method no matter if we resonated the "loop" by increasing or decreasing the currents in it. We still had significant currents in the tower and this disturbed the pattern.
I knew the broadcast industry often grappled with this problem, so I started doing more research about AM broadcast detuning systems. This is a serious issue in the broadcast industry. An AM broadcast station has to maintain a pattern based on FCC regulations. If the pattern is changed by a newly erected cell tower, for example, the cell tower owner is required to detune his tower so as not to affect the pattern of the AM broadcaster. There are several companies that make "tower skirts" to accomplish this.
As drive around you may notice that some cell towers have these skirts around them. This is a sure sign that they are located near AM broadcast stations - and that their presence has disrupted the stations' patterns.
I called my friend Goose Steinglass, W8AV, who is a broadcast engineer, and he agreed the best way to detune a tower was to skirt it. With the W5IZ tower he recommended to skirt the whole thing to make it "disappear," versus just detuning one quarter-wavelength of it.
Figure 2 shows a schematic diagram of a typical detuning skirt. It usually has 3 or more detuning wires running the full length of a tower. The top wires are electrically attached to the tower. The lower end of the skirt is insulated from the tower, tied together in a halo around the tower and attached to a detuning LC network to adjust the skirt to be nonresonant at the desired frequency. This is a much more rigorous application to build versus the Method 1 approach, but that's what we had to do. We built a 3-wire skirt using water pipe standoffs at the top, midway down (which were insulated), and at the bottom.
At the bottom of the tower the three wires were tied together and held down from the top with turnbuckles and an insulator to electrically isolate them from the water pipe standoffs. I built an LC network just out of trial and error with #12 AWG magnet wire. We brought down one wire from the skirt and tied it to one end of the coil. I scrapped insulation on top of the magnet wire to allow me to adjust a jumper from the coil. I also had a handful of RF capacitors.
For the complete version of this article as published in the NCJ, view the pdf version.