Feature Articles

The need for change

My 20m Delta Loop had been in place since April 2019 when the lunchtime net moved from 15 to 20m and had the point fixed about 37 feet up in one of the large Oak trees at the top of my garden.

Since then the Oak trees have grown and new branches and a multitude of leaves and twigs covered up the antenna changing its impedance when wet and pulling the wires out of place.

After some thought I decided to dismantle the triangular Delta Loop and install a single 20m square Quad Loop further away from the Oak trees putting the antenna in the clear yet still facing towards the east coast of the States.

Recycling and renewing

I already had one aluminium mast which supported one end of my 80m Doublet so I strung a new (blue) polyprop rope from this and utilised the original halyard and pulley which had supported the point of the Delta Loop for the other end. This gave me a horizontal rope at 37 feet on which to hang my new antenna.

I then made up a square loop from 2.5 mm stranded plastic covered copper wire (blue) with 17 foot 9 inch sides fitting thimbles at three corners with one bottom corner connected to the 450Ω feedline via a commercial junction piece. The above lengths are the result of my calculations to construct an antenna which would resonate at 14.345Mhz. I pegged out the loop on the lawn with tent pegs to form an accurate square and cut 73 feet of wire from the roll to make up the 20m loop which actually measures 71 feet around.

I then fitted two 20 foot lengths of cord to the thimbles on the bottom two corners to prevent the square Quad loop from moving about and hoisted it all into the air.

Testing

The result is that I now have a vertically polarised square Quad Loop (with no front to back ratio) with the top wire at 37 feet and the bottom about 19 feet off the ground. This faces Bob, N4XAT and David, WB1EAD and puts an S9+30db signal into Barry, SA7GDB (his report to me yesterday). David WB1EAD was still able to read my SSB signal when I turned the output of my FTDX 3000 down to 5 watts output. Bob was also able to work me on CW from his home using his MFJ magnetic Loop even when I reduced power so that satisfied me that the new loop is working well.

Because I choose to feed this loop with 450Ω line rather than coax I am also able to match (but not resonate) the loop and feedline for a 1:1 match on 21, 18, and 7 Mhz giving 4 HF bands from a single length of wire, and you could do this too.

I fitted a commercial 1KW 1:1 Current Balun at the shack end of the 188 feet of 450Ω line I need to reach the loop and the balun connects to my manual ATU via 6 feet of mini 8. I don’t worry about needing 188 feet of feedline since 450Ω line has a loss of only 0.15db per 100 feet at these frequencies. Compare that with the coax feeder you are using.

Why 450Ω Ladder Line?

You could make up one of these square loops and feed it with coax but in this case you’d need to fit an electrical quarter wave matching stub made from 75Ω coax between the loop and your 50Ω coax but then you’d only have a single frequency loop since that set up would not match on other bands.

There you are, something to think about. The wire and ropes used don’t have to be blue but as I’ve said many times before, It’s all about antennas!

73, Ken G3WYN.

I built this unit for my Yaesu FT-871 transceiver. There may not be much interest by other club members, but if anyone wants some more information, I have the schematics and the layout of the PCBs. The unit will supply 3A and 20V maximum.

Low Power PSU

Low Power PSU

Low Power PSU

Low Power PSU

Regards

Alex M0TOT

Please find attached some information on the buggy, ROVer, I built during the various 'lockdowns'. No radio licence required !! There is also a video clip of a slightly earlier version of the vehicle undergoing some kitchen-based testing.

Regards

Alex M0TOT

ROVer Top View, showing FPV camera, decoder and 12V battery

Alex Henderson's wonderful ROVer remote vehicle project

ROVer's LIDAR sensor and running gear on the underside

ROVer's control centre

In the video below, you can also see some early operational testing taking place. The ROVer also has an FPV front-facing camera and 'headlight' LEDs installed. The video clip is in .avi format and is approximately 94Mb in size.

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.

My existing 10m telescopic fibreglass pole


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.


 

Shopping List

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.

The Plan

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.

Key Clamps galvanised handrail fittings

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.

316 grade stainless steel - 48mm OD, 45mm ID

Some Fettling Required

A bench grinder made short work of removing the small amount of material necessary

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.

Stainless steel fixings were used throughout

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.

 

Both top and bottom assemblies painted and ready to fit

Small slipper shim made from stainless steel and curved using a folding machine to fit the pole diameter

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.

Saved some money and had some fun in the process

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.

Anti-lift device, should gravity fail us

Key Clamps fittings mean no welding required!

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

Berni M0XYF

A few weeks ago, I posted an article on an RF choke I had built. I was very happy with the results when using it in the shack on my Cushcraft trapped vertical antenna. Much reduced noise levels, and also slightly improved reception reports when using WSPR and FT8. Not sure exactly why that was, but less RF in the shack allowed better manual ATU tuning I suppose, and also less noise being transmitted with the data signals, I'm going to surmise. So all good and I was ready to run some more objective tests. Then came the antenna change to an End-Fed Half-Wave. Apples should be compared only to apples, so I cleared the partial observation fluff from my head and decided to start any comparative tests from scratch using alternate fruit.

The antenna is not properly installed yet - it's awaiting installation of a second supporting mast, but I couldn't resist seeing if it was useable and it absolutely is. Much better results than the vertical it has replaced. But there was a new problem which I didn't really expect. A heap of RF in the shack. Just what my RF choke was built to avoid. Disappointing. All my USB peripherals began playing up - a sure sign of unwanted RF. I'm not too concerned at this point, as the antenna is going to be raised much higher, and I'm only getting problems when using more than around 60W on bands where the SWR is less than ideal. Still, the plan was always to mount this choke outdoors, and then to potentially install a second one at the transceiver end of the coax as well. This is what my guide and mentor Ian GM3SEK recommends anyway, so I'm just going to push on with my plans at this point.

So step one is to mount this enclosure outside. I had a Male-Female-Female-Male PL259/SO239 join in my feedline anyway, where some idiot cut through the coax with a pair of edging shears earlier this summer, and I didn't want the choke too close to the antenna so this seemed the perfect place to insert it.

Off to the workshop to find, cut and fold a suitable 2mm sheet of aluminium and form it into the above bracket. M5 x 16mm stainless button-head Allen bolts screw straight into to mounting holes on the rear of these excellent Fibox enclosures. The holes are untapped, but M5 bolts self-tap a perfectly adequate thread in the housing and would tighten up way more than the (approx. 3Nm) torque that I used to solidly secure the mounting plate to the back.

One other modification I made to the enclosure before relocating it outdoors was to add a really neat little pressure compensation valve to the underside of the unit. It only requires an 8mm hole.

I tried hard to find these two-way pressure valves in the UK, but to no avail. I had to source them from HF Kits in the Netherlands which is fair enough, but the £13 UPS shipping cost was tough to swallow. They did arrive within a couple of days however, and as I felt bad about the postage being more than the parts, I ordered loads of other HF goodies to make myself feel better...

The valves (part number RND 455-01100 if you want to try and source them elsewhere yourself) are there to solve the potential problem of the pressure differential between the inside and outside of the IP66/67 enclosure as the internal coax and toroids heat up and cool down with normal use. If you just punched a hole in the enclosure, it'll just fill up with spiders and moisture over time, so I've been told.

Another little job completed, and on to the next one.

Berni M0XYF