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©2018-2021 F. Dörenberg, unless stated otherwise. All rights reserved worldwide. No part of this publication may be used without permission from the author.


Latest page update: 28 May 2021 (added ref. 6)

Previous updates: 5 February 2021 (added tripod section); 23 December 2018


INTRODUCTION

At my location on the top floor of an apartment building (7th floor, or 8th if you count the US way), I am basically limited to installing antennas temporarily on my terrace. For 80m/40m, full size antennas are basically out of the question. Even reduced-size dipole antennas etc. cannot be installed high enough to avoid being limited to straight-up radiation (NVIS), i.e., local communication. Small transmitting loops (STLs) for 80m/40m work fine when installed close to the ground. However, one side of my terrace is closed off by a very heavy steel pergola (see right-hand photo in Fig. 15 below), and two sides by concrete walls. Raising the STL antennas at least 1-2 m above the pergola (2.5 m high) has shown to considerably increase DX performance in the otherwise "shielded" directions. So, for my STLs and general experiments, I decided to acquire a telescopic mast. It should be extendable to a height of at least 4 m (≈13 ft). Unfortunately, due to overhanging eaves of the roof, I cannot use such a mast attached to a wall of the apartment...


THE GEROH 6M (20 FT) MAST

After some searching (size, quality, availability, price including shipping), I settled for a 6 m tall German army surplus mast. They were manufactured and used from the 1960s to the 1990s. I bought mine in June of 2018.

Basic characteristics:

  • Manufacturer: Geroh GmbH, Germany
  • Model: 6 Klp/F AKAC019
  • Collapsed height: ca. 1.6 m (≈5.5 ft)
  • Fully extended height: ca. 5.9 m (≈19.4 ft)
  • Weight: ca. 18 kg (≈40 lbs)
  • Number of sections: 5
  • Outer diameter of the base tube section: 88 mm (≈3.5 inch)
  • Max antenna weight: 15 kg (≈33 lbs)
  • Max wind speed: 100 km/h (≈62 mph)
  • Max wind load: 100 N
  • Crank revolutions - fully retracted to fully exctended: 15
  • Diameter of mounting hole in top section: 26 mm (≈1 inch)
  • Weight of accessories pouch: ca. 12 kg (≈26.5 lbs)

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Figure 1: Sides of the Geroh 6 meter mast


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Figure 2: Geroh 6 meter mast - partially extended


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Figure 3: Geroh 6 meter mast - side view and cross-sectional view with the lifting/lowering mechanism

(sources: ref. 1, cross-section is adapted from ref. 2 and ref. 3)

The lifting/lowering mechanism looks complicated. Actually, it is quite simple - once you figure it out. There are four separate cable loops - one for lifting/lowering each of the movable mast sections. Obviously, the bottom section of the mast (gray in Fig. 3 above) is stationary. When the winch drum is turned clockwise with the hand crank, the bottom of the next section (blue in Fig. 3) is pulled up by the cable that is attached to the lower right-hand corner of that section. There is a cable loop on the left-hand side of that same mast section. One end of this loop is attached to the bottom of the next higher section (green in Fig. 3). As the top of "blue" section moves up, that cable pulls up the bottom of the "green" section. In turn, this pulls up the bottom of the "pink" section, which pulls up the bottom of the "orange" section. As a result, the four upper sections are extended in unison. That is: not the lowest one first until it is completely extended, then the next one, etc! Also see Fig. 2 above. To lower/retract the mast, the winch drum is cranked counter-clockwise. This pulls down the bottom of the "blue" section. This simultaneously pulls down the other three sections.

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Figure 4: Inside of the crank box

(source: adapted from amateurtele.com, retrieved Jan-2021)

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Figure 5: Bottom and top view of the Geroh 6 meter mast


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Figure 6: Label on my Geroh 6 meter mast


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Figure 7: Content of the accessories pouch of the Geroh 6 meter mast (hammer not shown)

(source: this 2010 thread in the "Allrad-LKW-Gemeinschaft" forum)


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Figure 8: Table of hand-crank revolutions vs. height of the mast top and vice versa

(source and printable sheet: see ref. 5)



ADAPTER STUBS

To mount anything on the top of the mast, you need an insert. The official accessories bag includes such an insert, see the top-left corner in Fig. 6 above and Fig. 9 below. This insert is 22.5 cm long, has a flange with six holes (e.g., for guy ropes), a 3/8 inch threaded hole in the top, and a wood screw at the bottom. It also has two horizontal through-holes: one for the securing bolt at the top of the mast, the other for a securing bolt through whatever is mounted onto the insert.

I needed inserts for my large "magnetic loop" and small "magnetic loop" STL antennas, and a small dipole with antenna rotator. My STLs are mounted on PVC tubing with standard 63 mm and 50 mm outer diameter (OD), respectively. Note: this type of tubing is specified with a reasonably precise OD, and a not-so-precise wall thickness and resulting inner diameter (ID). This means that the upper part of the inserts must be custom turned to a diameter that provides a slip fit with the actual PVC tubing! The inserts are made of aluminium, for reasons of cost, weight, and ease of machining. The M12 flat washers are "off the shelf" from a hardware store. They prevent the PVC tubing from sliding down, onto the mast. The upper and lower part are bolted together with a piece of M12 threaded steel rod. I used some 2-component blue Locktite® glue to prevent them from loosening.

For my lightweight rotator, the insert is made of very high-strength machinable POM (polyoxymethylene), that is available under various brand names. I used standard off-the-shelf rod diameters. Again, a piece of M12 threaded steel rod connects the two POM rods. No glue, as it does not stick to POM.

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Figure 9: Design drawing for two of the adapters that I need


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Figure 10: Parts for the aluminum and POM/acetal/delrin mast adapter inserts


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Figure 11: Aluminum mast adapter inserts - assembled




GUYING THE MAST

The mast has two sets of three anchor eye-loops for guying ropes or cables: one set of three at the top of the base section of the mast, the other at the top of the fourth section of the mast. The anchors are evenly spaced around the mast ( = 3x 120°). This is illustrated in the following diagrams:

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Figure 12: Guying instructions

(source: ref. 3)

During field operations, the guy ropes/cables are fixed with large ground spikes. This is not an option on my terrace. I use two eyebolt anchors in concrete walls adjacent to the terrace, and a very solid post of my pergola - all three anchor points are at the same height:

gehro 6m mast guying

Figure 13: My three anchor points for the 3+3 guying ropes


List of materials for my guy ropes and anchors: 

  • 36 m double-braided 5mm special polyester rope (Dyneema®) with low elasticity/stretching (1.5%) and high tensile ( = breaking) strength: 1100 daN ≈ 1100 kg (when new). I got this at a local boating supplies store.
  • This rope is cut into six sections. As my mast is not centered between the three anchor points, all rope sections have a different length.
  • The three bottom ropes are tied to a carabiner at the respective anchor points on the walls - a tying knot must be used that does not slip under varying load! The mast end of each of these ropes is looped through an other carabiner (i.e., not tied!) , and then folded back. The folded back part is long enough to be able to pull on it, and fix it to the rope with a cleat. I place my cleats typically at least 50 cm from the carabiners. Note that the these three ropes must be adjusted several times, until 1) the mast is perfectly vertical and, 2) the ropes are tight enough, such that the mast does not move around when you pull on it in any direction!
  • For the three top ropes, the opposite is done: the ropes are tied to a carabiner at the respective anchor points on the mast, and looped through a carabiner at the wall anchor points. These ropes must be long enough to go to the top of the mast when the mast is fully extended, and be folded back to a cleat.
  • 6 Camcleat® line-locks (type CL276 for 3-6 mm rope/line)
  • 2 eyebolt anchors (8 mm bolt, M6 thread, stainless steel)
  • 9 carabiners (6 mm diam. stainless steel)

gehro 6m mast guying

Figure 14: Parts & material for guying the mast at two levels - rope, carabiners, cleats, eyebolt wall anchors


The bottom of the mast has a 3x3 cm ( = 4+ cm diagonal) "knob" protrusion, see Fig. 4 above. It fits the triangular metal "Mastfuß" base plate that is part of the standard accessories kit (see right-hand side of Fig. 5 above). Of course, the bottom of the mast must be immobilized. On my terrace, I cut a 43 mm diameter hole at the center of one of the 50x50x5 cm concrete tiles of my terrace, with a concrete hole saw:


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Figure 15: Guyed mast with my 80-20m "magnetic loop" antenna, cranked up to 3 m and 4 m above ground

(note: the protrusion at the bottom of the mast is inserted into a hole in the concrete tile)


STOWING THE MAST

Originally, two brackets were used to mount the mast against next to the door of the radio container on the back of an army truck (e.g., a 2 ton Unimog). See items 1 and 2 in Fig. 3 above. The top one is a collar bracket:

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Figure 16: Several views of the standard upper mounting bracket for this mast and installation on the back of a BW radio container

(source right-hand photo: panzerbaer.de, retrieved February 2021)

I stow the mast against a wall on my terrace, so I only need the collar bracket:

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Figure 17: My mast - parked upright against one of the walls of my terrace



FOLDABLE TRIPOD FOR THE MAST

It is a "a bit" of hassle for a single person to take the mast from its clamped stowed position, carry it to the installation position in the middle of my terrace, and attach the lower three guying ropes without the mast falling over in the process. Also, to leave the retracted mast on the terrace, the mast must remain guyed with the lower three ropes across the terrace. This blocks access to the terrace.

This is why early 2021, I decided to design and build an other kind of support: a tripod. As the name suggests, a tripod has three legs. I made the leg sections out of boards of treated wood that are hinged. The hinged legs are easily folded up and stowed. The legs are spaced by 120° around the mast, and are aligned with the direction of the guy rope anchor points on the mast and the guy ropes. The angled part of each leg is bolted to one of these anchor points. Three short sections of treated wood are bolted against the horizontal parts of the legs. This makes for a sturdy, rigid construction that is easy to (dis)assemble - which takes about 5 minutes.

The only critical part when building this tripod, is the exact length of the legs that are angled upward, and the cuts at the upper end - see the blue rectangle inset in Fig. 17. This must be such, that the mast ist perfectly vertical when installed. Fortunately, the legs are flexible enough, that the upward legs can be lowered next to the mast, for taking size measurements. And remember: always measure twice and cut once!

I am very pleased with the result! The mast + tripod is also easy to drag around on the terrace. Of course, the guy ropes that are attached to the top of the mast must still be used when the mast is not retracted!


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Figure 17: My tripod for the mast


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Figure 18: My tripod - collapsed/folded-up and disassembled

(for size reference: the concrete tiles measure 50x50 cm = 20x20 inch)

When I put the mast on the tripod all by myself, I simply prop up the three angled legs with short pieces of scrap wood:

gehro 6m mast

Figure 19: My tripod - angled legs propped up


Parts list for my tripod design:

  • Pressure treated wood boards; I use Class 4 wood (suitable for permanent outdoor exposure), and Class 1 or 2 (straight, few if any nots or flaws). One of the standard board dimensions at my local do-it-yourself store is 240 x 7 x 4.5 cm (LxWxH). I used five such boards, made of green pine (€5 each, 2021 pricing). The boards are planed (F: raboté, D: gehobelt), so they are straight, and will stay that way.
  • 3 sections of 120 cm - these are the horizontal parts of the legs. One end of each board has a 120° pointed tip.
  • 3 sections of 170 cm - these are the angled parts of the legs. The upper end of each board has a vertical slit for the 9 mm wide rope anchor on the mast. A 6 mm hole is drilled across this end, for an M6 bolt through the rope anchor hole.
  • 3 sections of 40 cm - these are the parts that stabilize the "Y" of the joined horizontal parts of the legs. They are attached to the horizontal parts of the legs with M10 bolts.
  • 3 steel hinges - I used hinges with 65x25x2 mm plates
  • 18 self-tapping 5x35 mm wood screws, for attaching the hinges
  • Hardware for tying the wooden braces to the horizontal parts of the legs:
  • 3 sections of M10 threaded bar, each 19 cm long (I cut up a standard 100 cm long bar)
  • 9 flat washers, M10
  • 9 hex nuts, M10. Each bar has two nuts on one end (tightly tightened against each other) and one on the other end. The length of the bars is such that a socket wrench can be used.
  • Hardware for attaching the angled parts of the legs to the mast anchor points:
  • 3 bolts, M6x80
  • 3 flat washers, M6
  • 3 wingnuts, M6 (preferably lock nuts)
  • Miscellaneous:
  • waterproof anti-seize grease (F: graisse antigrippant, D: Anti-Festfress-Paste).

The bottom of the mast has a 3x3 cm protrusion that fits the "Mastfuß" base plate that is part of the standard accessories (see Fig. 3 and 6 above). I used a 4 cm diameter hole saw to cut a hole at the center of the "Y" of the horizontal legs. If you make this tripod but want to cut a square hole, to prevent the mast from rotating: note that neither the sides, nor the diagonals of the square protrusion are aligned with the "Y" formed by the anchor points on the mast, or with the hand-crank gear box: it is about 11° rotated!

Stainless steel bolts of this size are rather expensive, so I used galvanized steel bolts. Outdoors, they do tend to corrode quickly. So I applied anti-seize grease to the ends of the bolts that only have a single nut on it. I don't mind if the end with the two nuts corrodes and seizes the nuts: they are supposed to stay on the bolt!

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Figure 20: Anti-seize grease applied to the bolts


Disclaimer: if you build my design, I assume no liability for design flaws, construction or operation faults, and any associated damage!

I use the mast pimarily with my 80-20m Small Transmitting Loop antenna (a.k.a. "magnetic loop"):

gehro 6m mast

Figure 21: My tripod - with my 80-20m STL antenna on the partially extended telescopic mast

(day without any wind, guy ropes not used)


REFERENCES



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