TWIN TOWERS

     

          Online August 2023; updated May 2024 by WB2FKO.

Shortly after our arrival in north Florida, our friend Arne N7KA made us aware of the K7RT SK estate in nearby Brooksville that had a 100 ft Rohn-45G tower available. I hired Dan K1TO of A1 Tower Service (now retired) to dismantle and transport it to the new QTH.

I decided to use the ten 45G sections to put up two 50 ft towers. One holds stacked 9-element beams for 144 MHz and the second tower has single yagis for 50 and 222 MHz. Eventually I'll add a 6m LVA (large vertical array: a fixed stack of 4 x 3-el) pointing to the northeast on one tower and a second LVA pointing NW on the other.

VHF DX in Florida is excellent, but it comes at a price: occasional hurricanes and frequent lightning. In an attempt to attain high reliability and integrity, I did commercial-grade tower installations, closely and carefully following the documentation in the Rohn catalog. The two structures are setup identically.

Each tower base is a brand-new 5 ft section of 45G that is placed in a 30 x 30 x 48 inch hole. The bottom of the hole is covered with gravel for drainage. The two bases are separated by about 150 ft and located 85 ft from the shack.

A single set of guys are spaced at 120 degrees and anchored at a 40 ft radial distance from the tower center. One tower uses 7 ft Rohn GAR30 rods with a single eyelet. The second has 7 ft Rohn GAC3455TOP rods with attached 5 hole equalizer plates that were generously donated by Art WA2LLN. I guyed to the center hole.

The ends of the anchor rods are embedded in 36 x 36 x 18 in blocks of concrete reinforced with 1/2 inch rebar, buried at 4 ft. This is sometimes called a "dead man" anchor. I ordered the rebar cut to individual lengths from Metals Depot and assembled the six cages myself using saddle ties. A few bricks keep the cages from contacting the dirt floor of the hole. The photos show the various components prior to and during installation.

 

 

 

A contractor had all the holes neatly dug and ready for the concrete pour in just a couple of hours. I was happy to hand over this part of the project to the pros. Here is a video and second one showing the concrete going into the anchor holes. Each block of concrete weighs roughly one ton.

 

 

The tower sections were all individually cleaned with compressed air and touched-up as needed with a wire brush and cold galvanizing compond spray. I built 10 sawhorses to keep the sections up off the grass and dirt, which made them easier to work on.

Assembly then took place horizontally with the able assistance of my neighbor Hal WA4QLA. We needed slight leg bending provided by cargo straps and plenty of hammering at almost every joint, but eventually all the sections were bolted together. The photo on the right shows Hal standing next to one of the completed towers.

 

 

For reasons not clear to us, each joint in the Rohn design has two different bolt sizes. The only explanation we've been able to come up with is that use of a smaller bolt saves some component cost. The entire vertical load of the tower is maintained by the shear strength of these bolts and whatever static friction exists at the leg couplings.

A Rohn GA45GD bracket assembly is installed at the 42 ft level, ie. 8 ft below the top of the tower. WA4QLA donated 1/4 inch EHS cable for guying. Guys are attached to the bracket with Big Grip preforms, 3/8 inch thimbles, and 5/8 inch galvanized shackles. At the top of the tower is a Rohn BPL45G plate that holds a heavy-duty TB-3 thrust bearing for the mast. All the hardware was purchased new from 3Star and increased the cost of the project significantly.

 

 

The VHF antennas are positioned with Yaesu G-450A light duty rotors. Following the instruction manual, the rotor is located about 40 inches below the thrust bearing, bolted on a Rohn AS455G mounting plate. The plates are pre-drilled for HyGain rotors, not Yaesu. Ron KM4SYT helped me out in his machine shop by drilling holes on each platform for the Japanese metric hardware.

The 5-section towers were lifted into place by B & C Crane Services in Gainesville, Florida. A harness is looped at the top, the crane hoists the tower vertical then carefully swings it into place just above the base (see video). Our very skilled crane operator Curtis was able to align the bottom section with enough precision to allow the ground crew (me, WA4QLA, KM4SYT, and Steve W4IT) to install the last set of bolts. Curtis then took tension off the harness and disengaged it from the tower without the need for any climbing. Both towers were bolted to the bases in under an hour.

A second set of Big Grips and eye-to-jaw turnbuckles attach the guys to the anchors at ground level. A Loos PT-2 tensiometer allows straightforward adjustment of the turnbuckles. Each turnbuckle is tightened to reach 10 percent capacity of the 1/4 inch EHS cable, corresponding to about 600 lbs of tension.

This is my first experience with guyed Rohn 45 and I have been unable to detect any motion or swaying when climbing it. The right photo below is the view looking to the north at a height of about 42 feet where the guy bracket is attached. You can clearly see the filled trenches for the grounding radials of the second tower.

 

 

Grounding

Lightning hits are probably inevitable, so a ground system attempts to dissipate strike energy in the earth in immediate proximity to the tower. My layout followed the guidance in the ARRL Grounding and Bonding handbook by N0AX. Each tower leg is connected to a buried 6 gauge bare copper wire to form a radial that extends 40 ft to the guy anchor. Depth is 8-12 inches below the surface.

The three radials are electrically connected by an 18 ft diameter wire ring buried at the same depth. 8 ft ground rods are located at each intersection, which separates the inner rods by the desired direct path of about 16 ft. A ground rod is also placed at the far end of the radial, adjacent to the guy anchor and connected to the EHS cable. Another ground rod is clamped to the radial midway between the ring and anchor. There are 9 ground rods per tower with positions marked with an X in the layout sketch below (not to scale).

I toyed with the idea of digging the trenches myself by hand, but gave up after 15 minutes in the hot summer sun. A local electrical contractor had the needed motorized trenching tool along with some assistants who made short work of the job. Unlike me, they don't work for free.

 

 

I used a Bosch hammer drill with an appropriate adapter to drive in the ground rods. I had long anticipated this step and made sure there was a power outlet installed on the outside back wall of the shack. Even with the drill, the last 4 feet proved to be quite stubborn. Sinking all 18 rods consumed the better part of a day.

Connecting the ground wire was complicated by the presence of several fire ant colonies in the trenches. I incurred many nasty bites during the process, but eventually got all the radials wired to the rods. It is well known that when copper is clamped directly to galvanized tower legs, oxidation will occur. I used specialty clamps with steel shims available from DX Engineering. At the far end of each radial, the wire emerges from the earth in a conduit and gets clamped to the guy anchor. There are purpose-built clamps for this but they are ridiculously expensive; I was able to get a decent deal on six from Electrical Parts. Refilling the trenches with all that excavated dirt was also much harder than I expected so I was glad to get the manual labor over with.

 

 

The towers are far enough from the shack and each other that there is no benefit to tying their grounds together or connecting to the station perimeter ground. The inductance of these relatively long wire runs would generate huge potentials in a fast-rising lightning strike. This is explained well in the ARRL grounding book.

Antennas

The masts are lengths of 1.5 in galvanized pipe (OD 1.9 in) that were acquired from Metals Depot. An M2 6M5X 50 MHz yagi is located on the north tower at about 53 ft. At the top of the mast at a height just under 60 ft is an 11-element LFA (4.7m boom length) for 222 MHz (left photo, below). The LFA was designed by Justin G0KSC and purchased as a kit from his company InnovAntennas.

The south tower holds stacked 9-element beams for 144 MHz (M2 2M9SSB) at a vertical spacing of 9 ft 6 in (right photo, below). The height of the top yagi is about 62 ft. Because alignment is critical, I assembled antennas and feedlines on the mast at ground level. Phasing lines and 2-port power divider are from Directive Systems.

 

 

The mast was already installed on the north tower when the crane lifted it into place, so I hired a bucket truck crew to clamp on the the 6m yagi followed by the 222 MHz LFA. The 144 MHz mast/antenna assembly was heavy and awkward, but the bucket crew was able to hold it, then raise it to the top of the tower. The left photo below shows me on the tower guiding the bottom of the mast into the rotor clamps. I then climbed to the top of the tower and tightened the bolts on the thrust bearing. I'm wearing a DBI-Sala ExoFit climbing harness, AFP dual fall arrest lanyard, waist-mounted positioning lanyard, engineer boots, and hard hat. Everything was under the watchful eye of Steve W4IT on the ground who provided fine-tuning of the azimuthal alignment. The right photo shows the stacked 2M9SSB yagis in place as seen from the top of the north tower. The fences in the distance separate our neighbors' horse pastures.

 

 

When the 2m yagis were first installed, I equipped them with brand-new, custom phasing lines from M2. Unfortunately, the N-connectors on these coaxial cables failed after just a couple of months running high power. This required an expensive and time-consuming repair: a bucket truck had to be hired again to bring the stack down for troubleshooting, a new pair of matched phasing lines were ordered from Directive Systems, followed by assembly and testing, then hauling it back up with a bucket truck a couple of weeks later. The lesson here is if you're operating QRO and not an expert cable builder, choose your feedline supplier very carefully!

All these antennas were originally setup at the former QTH in DM65 and moved with us to Florida. I had considerable difficulty dealing with intermittent high VSWR on the 6M5X that took many weeks to resolve, as described below.

Modifications to the M2 6M5X 5-element 6m Yagi

During the 2024 January VHF contest, the VSWR of the 6M5X went haywire while running high-power in a meteor scatter sked with K5QE. This destroyed the expensive LDMOS power transistor in the M2 solid-state kW amplifier (eventually repaired by John at Island Amplifier). After a few hours of troubleshooting, the source of the high SWR was isolated to the antenna and/or the brand-new flexible coax loop at the top of the tower.

When the antenna back on the ground, no obvious issue was found. The SWR as measured by the nano-VNA was erratic leading me to suspect something was amiss with the UHF connectors. Over the course of several iterations and many weeks, I replaced the rotor loop coax, T-match, and balun. The response looked decent at ground level, but when mounted up on the tower, the VNA sweep again revealed very intermittent SWR, jumping up into the range of 3:1--4:1. The odd behavior was independently confirmed with an antenna analyzer.

I could "cure" the poor SWR by connecting to the transceiver and applying a few Watts of 50 MHz RF power for a few seconds. The transceiver was instantly happy and even stranger -- re-connecting the VNA now showed a decent sweep. This good match would persist for perhaps 10 minutes to several hours, but eventually the SWR would go high on the VNA. In addition, rain would drop the SWR as measured by the VNA to an acceptable level in real-time. When the water evaporated, however, a poor SWR returned.

As long as I was pushing a few Watts into the antenna, there was no appreciable reflected power. But I couldn't risk using the amplifier without having a reliable load on it. I brought the antenna down again and carefully sprayed water on the individual elements while watching the VNA. When I got to the reflector, the SWR dramatically improved. This isolated the problem to the reflector element clamp. Taking it apart revealed what appeared to be significant oxidation or burning in the channels. This photo shows one side partially cleaned and the other with a blackened surface as I found it. This thin dielectric film was causing intermittent conductivity across the length of the reflector element. Note that the 1-inch 8-32 SS screws don't provide a reliable electrical path into the clamp as they are secured by nylon locknuts.
                

 

Not sure why this corrosion developed. I did not use any anti-oxidation compound when assembling it, at least not here in Florida. I don't recall using any paste when I first built it in 2007 when living in New Mexico, although that was so long ago I may have forgotten. There is no mention of it in M2's assembly instructions, so I'm pretty confident I wouldn't have applied any.

I removed all the elements, cleaned them with a wire brush along with the clamps, and re-assembled using Ox-Gard paste. I researched this subject a bit as there are many anti-oxidation products available. Ox-Gard appears to be a good choice as it has zinc particles for aluminum-aluminum connections and is also designed for outdoor use. After refurbishing the antenna, the VNA sweep showed excellent SWR, very close to the factory specification. I followed independent suggestions by N2CEI and WA4QLA and installed shorting straps at all 5 element clamps using stranded copper wire; the modified reflector clamp is shown in above photo. There is a bend in each strap to allow access to the boom mounting screw. I hope these shunts will maintain continuity if corrosion in the channels re-appears. W4IT recommended filling the exposed channel fillets with black RTV to discourage rainwater from accumulating there.

My attempts to model the antenna radiation patterns can be found here.