TowerskirtNCJ.pdf

How to Detune a Tower

By Jay Terleski, WXØB

With the advent of using wire verticals

around our towers to make a phased ar-

ray system, it has become apparent that

the tower can affect the array’s perfor-

mance, 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 op-

timized box that uses what has become

known as the Lahlum method of tuning.

(The namesake is Robye Lahlum,

W1MK—see his two articles “Phase Ad-

justment Technique for a 4-Element

Square Phased Vertical Array” and “Phase

Correction for a Quadrature Hybrid-Fed

Antenna Array” in the May/June 2005 is-

sue 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 pre-

dicted pattern, which should have been

on the order of 30+ dB. We were only see-

ing about 12 dB of front to back using a

signal source that was carefully placed to

give us a good reference to adjust the ar-

ray. By measuring the currents in the tower

legs we could see there was significant

amounts of current as viewed on an os-

cilloscope (that had a current probe

around a leg of the tower) when we trans-

mitted 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.

Method 2

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 broad-

cast 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 re-

quired 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 you drive around you may notice that

some cell towers have these skirts around

them. This is a sure sign that they are lo-

cated near AM broadcast stations—and

that their presence has disrupted the sta-

tions’ patterns.

I called my friend Goose Steingass,

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 elec-

trically attached to the tower. The lower

end of the skirt is insulated from the tower,

Figure 1—The 200 foot tower at W5IZ,

with an 80-meter Yagi on top and a 4-

Square system around it.

Method 1

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.

Figure 3—W5IZ’s tower with the

detuning skirt (you have to look closely

to see the wires around the tower).

Figure 2—A typical detuning skirt.

NCJ

November/December 2005

Figure 4—The insulated bottom of the skirt. Note the halo

wires that tie the skirt wires together.

Figure 5—The LC network used to used to detune the tower.

It’s made of a coil of enamel wire and an RF capacitor.

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. See Figures 3 and 4.

At the bottom of the tower the three

wires were tied together and held down

from the top with turnbuckles and an in-

sulator to electrically isolate them from the

water pipe standoffs. I built an LC network

(see Figure 5) 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 measurement we used a 500 MHz

oscilloscope and placed a current probe

in the wire that was feeding the coil from

the skirt. The trick is to minimize the cur-

rent in the skirt, or the voltage read on

the scope.

The current probe can be made simply

with just some hook-up wire. Place five

or more turns around your network wire,

or just enough turns to easily produce 1-

2 volts on your scope when transmitting

into the array with 10 W or more. The

‘scope I used easily picked up the trans-

mit signal. As a reference, we adjusted

the scope down to the millivolt scale and

we could see AM broadcast signals mov-

ing the trace. Al, W5IZ, lives way out in

the country and has no nearby AM broad-

casters, so these signals were in the

range of 2-3 millivolts. We knew we prob-

ably could not make the 160-meter sig-

nal that weak, but we could try.

Adjusting the coil and capacitance val-

ues I found a sweet spot for a tap point

on the coil that minimized the 160-meter

signal being transmitted. I was further able

to reduce the signal by changing differ-

ent values of capacitors. I ended up us-

ing a 100pF cap soldered to the coil to

form a T-network. Al and I were amazed

that yes, indeed, the signal on the skirt

was down at the same level as the AM

broadcast stations. Now to see if it

worked!

We set up the measurement system

and, lo and behold, the F/B had jumped

up 18 dB or more. I readjusted the

4-Square system and we saw 30 + dB of

F/B.

The moral of the story is towers can

significantly adversely affect the vertical

phased arrays we hams build around

them. The good news is we can fix the

problem by carefully building industry-

standard skirts. But you get what you put

into it, and trying to cut corners just may

not pay off in the long run.

November/December 2005

NCJ

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