Antenna height is an important factor for HF communications. There are new antenna masts on the market, and, on rare occasions, a used mast may also appear on the marketplace. One such model is the RDM-12 antenna mast. The RDM-12 was a military mobile station mounted on a Praga V3S truck. The mast was transported on the roof of the V3S in a folded position. It is extended using a crank and mechanical gears.
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The Base of the RDM-12
The upper section of the mast with the rotator and bearing
I managed to purchase the mast in December 2019, including the upper section with the rotator and bearing. There was visible corrosion on the surface, but mechanically it was in good condition. The mast then sat waiting until summer for restoration to begin. Before starting, I read the available information from the OM7KW and OK5AJA websites. Števo OM7KW spent a week sanding the mast by hand. So I fitted an angle grinder with a sanding disc and set out to sand it down, assuming I could finish it in a day.
Grinding and Coating the RDM-12
The reality, however, is that this method can only be used to grind the outer surface of the tubes; the grinding wheel cannot reach the internal structure. That’s where the steel brush came in—a tedious three-day job under the scorching sun.
Painting the mast
And this is what the painted RDM-12 mast looks like
Close-up of the painted mast
A straight segment of the RDM-12 mast before applying the primer
Original corrosion on the mast
Painting is only slightly easier than sanding. I applied the primer by spraying. The top (two) coats were applied with a brush—a flat brush. Unlike the original paint, I chose a shade known as “blacksmith’s black.” All of this work took a total of about 40 hours.
Excavation Work and Anchoring of the RDM-12 Mast
The base of the mast was planned to be located approximately in the center of the lot, with anchoring at the lot boundary. Although it would have been preferable for the anchor cables to extend directly from the foundation in the ground, this was done to allow for ease of movement in the garden. A mini excavator was therefore used to dig four holes in the corners of the property and one in the center. Thick-walled 60 x 60 mm profiles, 3 meters long, were placed in the corner holes. By fall, I had managed to coat and set in concrete only two of the profiles. They are embedded in the ground to a depth of approximately 1.1 meters. The concrete was mixed in a wheelbarrow. This work took about 10 hours.
First Spring Tasks and Tips for Mast Footings
As soon as the spring weather allowed, I set the remaining two profiles in concrete. The interior of the profiles was filled with dry concrete. This is because if water gets into the profile and freezes, it can destroy even a solid profile and the concrete foundation. Once it hardens, the dry concrete does not serve to reinforce the structure but rather to fill the interior. This part of the work also took about 10 hours.
Support structure, winch—hoist
Thick-walled profiles and segments of the support structure
I was inspired by OM7KW’s insights regarding the so-called hoist. Two 60 x 60 mm thick-walled profiles, each 4 meters long, were used to create the support structure, including the Strend Pro HW-100-800 winch. This required drilling with large drill bits, which was quite physically demanding. The hand drill I had available for this wasn’t the most suitable, and I even broke three HSS drill bits (of lower quality) in the process. The structure was then sprayed with primer and again with two coats of topcoat. I embedded the hoist in about one cubic meter of concrete. The installation also includes a pipe into which the RDM-12 mast with a joint is inserted. A grounding rod is also installed. The work took about 15 hours.
Concreting the support structure for the mast—the hoist
Stones from the garden were placed in the excavation
All that remained was to prepare the top section of the mast with the rotator. It required some repair work, but it was relatively simple. Of course, this part of the mast was also sanded and painted like the other parts. It took 5 hours.
Anchoring the Mast
I didn’t want to underestimate the importance of choosing the right anchor ropes. After a long search online, I decided on materials from DX Avenue. And not just the ropes, but the clamps as well. Everything is very well illustrated on their website, and it’s a good source of inspiration for what’s needed for proper anchoring (eyelets, clamps, tensioners). This is a Deltex rope, which, especially in the long term, should be of higher quality than commonly used anchor ropes. Deltex feels extremely strong, though also a bit stiff. Selecting the ropes and accessories took about 3 hours.
Anchoring the RDM-12 Mast
With that done, I could proceed to hoist the RDM-12 mast and prepare the anchoring. It’s easier to lift an empty mast and measure out the anchoring than to lift a loaded mast. All the more so since I did all the work by myself. When lifting the mast, I used a base made of concrete blocks and a ladder as a support. However, extending the mast revealed a planning error. The handle for the lifting mechanism couldn’t be attached because it was blocked by the hoist. I had to use regular socket wrenches to extend the mast bit by bit and then secure it. This part of the work was both physically and time-consuming. There are a lot of nuts on the guy wires. But better safe than sorry. All of this work took about another 10 hours.
Moxon antenna at the top of the tilted mast
Mast raised but still retracted
The Slovak flag was also hoisted on the mast
This was followed by raising the mast. At the top of the mast will be a Moxon tribander, and lower down, an arm with a pulley that allows wire antennas to be quickly raised into the air. The preparatory work—that is, mounting the antenna, running the cable, and preparing the anchor ropes—took about 4 hours.
This was followed by the final extension of the mast. The weight of the top section, the rotator, and the antenna is already noticeable. The extension took about 3 hours.
The fully extended RDM-12 mast
In total, the work on the mast amounted to 100 hours. That is a significant amount of work. This negates the advantage of a general overhaul of an older mast compared to purchasing a new one. A new section of a lattice mast costs approximately 100 € per meter, but such a mast would not be extendable or tiltable. The RDM-12 is one of the older military designs of outstanding quality and will serve excellently for amateur radio purposes.
