Refusing to pay good money for something I can build myself
It's been too hot of late to be grafting unnecessarily, and I'd rather spend time planning than throwing failed prototypes in the bin (though I do my fair share of that too) so I've been thinking deeply about what already exists that I could re-use for a simple low-cost tilt-over mast project. Hours of trawling through both eBay and my many spare parts bins as well as closely scrutinising the endless commercially available offerings followed. I came up with a few fancy but overly complicated designs and then was struck by my epiphany moment.
Don't ask me why, but I've always mused over what cool projects could be constructed with scaffolding and/or the modular Key Clamp fittings range. Yeah, sure - I know I'm weird, but there is just about every conceivable Key Clamp fitting imaginable. If I'm honest this is all born out of my frustrating inexperience with welding stuff. I've got all the welding gear and no idea, but that's for another time.
So I scanned the Key Clamp catalogue and came up with what looked like some suitable parts. I have to retrofit this system to a 1m tall ground post concreted into the middle of my lawn. With the camo tape that's applied it runs to about 44m OD. The fibreglass telescopic mast that I'm using is 48mm OD (very slightly tapered bottom to top, but only by around 1mm). That's about standard scaffold pole dimensions (48.4mm or 1 29/32" apparently). Lucky for me.
There are other ranges of Key Clamp fittings to suit 27mm, 34mm and 42mm tubing if that's what you need, but scaffold size is the most popular.
Before we go any further, when I mentioned the concept of this project on the 70cms repeater net last week, Russell asked me why I was building a tilt-over system for a telescopic pole. Good point. Well, it's because it's going to be semi-permanent and I'm going to weatherproof all the joints, so de-telescoping it will be a major headache and I want to be able to drop it to the ground quickly and easily when Tomasz Schafernaker tells me it's going to get a bit sketchy out there.
Key Clamp handrail fittings type 173 female x2 (sourced from eBay - item number 263969449130 from seller pegasusindustrial)
Key Clamp handrail fittings type 173 male x2 (sourced from the same listing above)
M10 x 40mm bolts with nuts and washers of your choosing
Paint: Hammerite Dark Green Hammered finish (your choice of course, if required)
Shims if required. I purchased eBay item number 351775109744 from seller everyexhaustpart) and introduced it to my angle grinder. The seller offers to make three free cuts for you, but they completely ignored my cutting request. Annoying.
So, here's the plan: A simple bottom swivel, and an equally simple latching mechanism at the top of my (existing) 1m ground pole which will employ a quick release wingnut and bolt.
I'm going to need some split shims to take the existing 44mm OD pole up to the 48mm required for the fittings, so a couple of those were constructed out of stainless steel (48mm OD is a common size for car exhaust tubing). This tube has a 1.5mm wall thickness and therefore an ID of approximately 45mm - near enough for my application.
Some Fettling Required
I ordered the Key Clamp fittings a little bit blind. I didn't know if they would swivel through a full 90 degrees, but it looked close so I took a punt. Turns out I didn't receive the fittings as per the eBay listing photos anyway, but no matter. The reality is that they achieve about 88 degrees, and so I took a little at a time off with the bench grinder until I had the full movement I needed.
I ground away just that part of the male fittings that was coming into contact with the corner of the female part. You can see from the photo that it amounted to around three millimetres - down level with the edge of the protruding tab. While I was about it, I smoothed out the bores to remove the high spots of zinc, just to stop them chewing up the surface of the pole/mast unnecessarily. It was an easy job with some coarse Emery paper.
Never let it be said that I'm anything if not a stainless steel fixings freak and I've built up quite the collection, so M10 x 40mm Allen head bolts and washers were liberated, an M10 nyloc nut and a stainless wingnut too.
A test-fit of everything looked promising, and everything worked as expected without any fouling, which was a relief. Let's face it, you can never guarantee that some unforeseen obstacle won't arise that you just hadn't considered.
Fortunately, this good news meant that the next step was to give everything a coat of dark green Hammerite paint (rust protection where the grinding took place, and to help tone down the visual impact).
I applied one coat in two stages over 48 hours. This stuff is very forgiving, but it's not a quick-drying paint and you'll need some spirit-based brush cleaner. The male parts I painted internally too, as I knew I had enough clearance.
Finishing off, the bottom of the mast was wrapped in a few turns of back electrical tape (just to protect the finish and tighten the fit very slightly) and then the bottom grub screw was just done up finger tight. All the grub screws got a smear of Teflon grease before assembly. The female fitting that sits at the bottom of the fibreglass pole comes manufactured with a large hole in the casting, so water drainage won't be an issue. Shims were fitted to the ground pole and the clamps were tightened down firmly. The stainless shims allowed a really good clamping force to be applied. No chance of them coming loose accidentally. I would have complete confidence mounting a 10m alloy mast in this way as long as the concrete base was up to the job.
The final fitting which clamps to the fibreglass pole a metre up from the bottom was originally just going to be left finger tight too, but at the last minute I decided to engineer a little slipper shim to spread the load of the grub screw and stop it cutting into the pole over time. Just enough to apply some grip and to stop the whole thing rattling in the wind. There's also a photo of this part once it's fitted.
The 49:1 transformer will clamp around the fibreglass mast just above this point. I'll cover that in a future article on the actual antenna build itself.
The Jubilee clip (or 'hose clamp') on the fibreglass pole is to prevent the pole from lifting up out of the base fitting. Not sure how that could happen, but better to be safe than sorry. This obviously won't have any function while the pole is being lowered from its vertical position, but the weight of the pole and the close control of the mast-master means that's not a problem that I've encountered thus far.
Here's the completed bottom swivel. Take care not to overtighten the nyloc nut if that's what you choose to use. I overcooked it and had to cut the whole thing off with the angle grinder as the nylon insert can form a perfect wedge between the threads and no amount of force will free it. This is therefore Take #2.
The 'old' radial wires seen in the last picture will soon be combined with two 4 foot copper earth rods and will be connected to the counterpoise of the matching transformer and will also provide an anti-static ground.
There you have it. If you're not worried about using stainless steel parts or painting the end result, you can get yourself an extremely robust tilt-over mast base for less than £25