Category Archives: SHF

Upside-down Dishes and Rotary Clamps

Inverted 1.2 metre dish with 10 GHz waveguide feed connected. The 5.7 GHz horn is fitted so a 3 degree elevation change brings that band into play. Having the feed at the top with the dish inverted keeps the rain out of the horn and removes any risk of hitting your head on the arm when standing on the roof rack. Bracing bars are 10 mm carbon fibre tube with quick-detach fittings epoxied into the ends. Elevation strut is a cheap 12 volt mechanical actuator.

The rotator fits on a mast section that tilts over so the dish can be worked on for band changes. It rests on a trestle or “mast scissors” when luffed over. This is the 3.4 GHz setup with a commercial C band scalar choke on a 9cm feedhorn I made for Tony

Rear view of the dish with two ODUs for 5.7 and 10 GHz

Another implementation of the rotary mast clamp. This one has a side entry for use on narrow lane verges. It also has a ground-mounted tilt plate and rotator. The fishing-rod stand allows you to lower the mast but it remains supported above ground for adjustments and band changes.

The second version of the clamp. The brass nuts fit on to captive clamps with 3/4 inch spigots to allow rapid removal of the clamp from the roof rack.

Close-up of the mast clamp with the gate open, showing the rollers and the knurled locking nut

The first prototype clamp

The 2.4 metre mesh dish on Tony’s van-mounted 60ft pneumatic mast

Elevation mechanism I made for the 2.4 metre mesh dish. The arm at the right is for the counterbalance weights

Porthole feedthrough in the mesh surface to allow shorter feeders to be used. Brass insert in the Delrin bush is for the quick-detach carbon fibre feedpoint support rods. Designed to be dismantled while standing on the van roof on a stormy winter night in pitch darkness with horizontal sleet on a Yorkshire Dales hilltop at 54 degrees North.

10 GHz Dielectric lens Feedhorn

I’m working on a simple, waterproof, reproducible design for a 10 GHz dish feed for folks who are taking part in the group buy project to build F6BVA 10 GHz to UHF transverters. This uses a probe launch into a round waveguide machined from solid aluminium. The lens is made from Rexolite 1422, which is a free-machining cross-linked polystyrene with well-defined relative permittivity and a loss tangent of about 0.0004. This one is designed for a rather flat offset dish I have with equivalent f/d about 0.75, but I will be doing some for more common offset dishes

Finished 10 GHz feedhorn with Radiall SMA connector

The body is turned and bored from a bit of aluminium round bar

The flat area is too large on this one, I’ll make it narrower on subsequent versions so I don’t have to shorten those M2.5 screws

Another batch of 122 GHz W2IMU f/d 0.8 feedhorns

Brass W2IMU dual-mode feedhorn for 122 GHz

I’ve just completed a batch of 24 of these W2IMU feedhorns for the 122 GHz band. Thread as usual is M8 x 0.5 mm. Horn ID is 4.03 mm, 27.1 degree internal flare, 2.00 mm reamed waveguide core, rear duplexer cavity for VK3CV boards, four M2 threaded holes, 4.00 mm reamed barrel, 3.98 mm spigot on the waveguide. This is the version 2 with a flat step as the end stop. Rather than relying on the 7.5 mm tapping drill to make the bottom of the threaded section, I now machine that using a centre-cutting M7 end mill. Part number for this version is DBN-122-IMU-0.7-02 and price to bona-fide 122 GHz experimenters is £14

The coupler body, internal thread M8 x 0.75, 14.85 mm diameter
Core of the horn, threaded M8 x 0.5, reamed to 2.00 mm internally
Rear cavity to fit over the chip on the VK3CV board. The barrel is adjusted for best TX and TX performance

WR90 to SMA transition

One of the completed horns ready for testing

I needed a couple of transitions, so decided to try to make a very simple narrowband design, optimised for 10368 MHz with low loss and a good match over a few hundred MHz. I ran up a design with rounded corners to the cavity to make it simple to machine using an 8 mm slot drill. I chose aluminium for the body as it performs well, although without anodising, it is going to need protection from the elements. I used some good quality Radiall SMA four-hole flange-mount sockets.

Although this looks a simple part, the instructions I make for myself show the level of detail.

I’ll publish the measured performance soon. So far, I can get around -23dB across ±100MHz. Once optimised, I will have some of these for sale to bona-fide experimenters. Email for details

CAD model in Fusion360
Modelled through loss and return loss from OpenEMS

Dish Feedpoint Mounts

Tony G8DMU has a 2.4m mesh dish on his van. He uses a variety of feeds, and they are all different diameters. I made up a quick-detach support ring to fit an RF Hamdesign multi-band ring feed, but Tony’s 23cm feed is larger, so I made up some extension blocks and a set of cheeks and spiders to support the different feeds.

The original ring feed, with the adjustable ends on carbon fibre struts, QD sockets and attachment brackets

First step was to extend the ring to fit the large feed

Blocks inserted into the ring to increase the diameter and provide attachments for the spiders and cheeks

I made up a stainless steel internally threaded clamp nut to fit on a stainless threaded bar fixed into the block with Loctite, so I didn’t have to worry about threads in the aluminium getting damaged.

The big 23cm band feed in the clamp ring

Next step was to make support cheeks/crescents to hold the multiband ring feed

Adjustable crescent with a knurled brass head on a pointed stainless steel thread, with a brass insert to spread the force and prevent wear on the Delrin
Fixed crescent, milled from Delrin and threaded M5

The 3.4 GHz feed is the smallest, so I milled a support ring and fitted support rods with threaded ends to fix to the holes in the outer ring. The threads were M5 so they fitted easily through the M6 threaded hole in the clamping block.

Clamping ring and spider for the 3.4 GHz feedhorn
3.4 GHz horn fitted to the main ring.

The 13cm horn is a little larger, and had to be made in two pieces to fit over the backshort.

2.3 GHz feedhorn in the clamp

I milled the rings on my ancient Bridgeport mill using a shop-made fixture plate on a rotary table with a sacrificial plate made from acrylic sheet.

Finally, a photo from Tony of the big dish in use on his van at a portable contest site up in the hills

122GHz free-flange coupler v2

The original flanged coupler has a solid core, so as it is adjusted, the flange rotates. This is fine if it is used with an axisymmetric flange-mounted horn or a dish or lens which can rotate relative to the transceiver board. Where an experimenter wishes to connect the flange to something which cannot rotate, or where adjustment of the polarisation is needed, a new approach is required.

Flanged 122 GHz coupler with spanner flats on the threaded barrel

I separated the flange and central waveguide from the threaded barrel, and reamed a hole through the barrel. That allowed the flange to float and rotate, but it needed a clamp. I decided that a split nut which would fit over a raised ring on the shaft might work OK.

More pics to follow…

23cm Isolator enclosure

Nice simple machining job to enclose a 1.3GHz RFCI drop-in isolator (three-port circulator with a dummy load on the third port) and act as a heatspreader in case of a load fault. I had bought six RFSL2347s direct from RFCI, but only needed three. They can handle 200W CW, 1kW peak and 100W CW dissipation.

I sold two and this one was spare. I ran up a CAD design in Fusion360 to get the dimensions right, but then made it on my manual Bridgeport mill.

CAD drawing with transparent lid
Finished body with N sockets and isolator
With the lid, showing the 8-24 UNC hole for a feedthru capacitor from a diode to detect if there is any dissipation in the load from a mismatch on the output port
Underside of the body. 4.3mm clearance holes allow it to be bolted to a heatsink or chassis