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
I’m working on an elevation pivot plate for a large 70cm moonbounce array for a friend. The design uses a 40 x 30cm plate with clamps and alignment blocks to carry GRP and aluminium tubes to support the array and LNA/phasing harness. This is the first stage, making the knuckle and pivot pin and bearing bushes/carriers. The bodies are aluminium, the shaft is 316 stainless steel and the bushes are phosphor bronze. It will have dust caps and grease nipples. The bushes are in two parts with a 1mm grease groove between them
So far, it is looking OK. More to follow.
The original concept for the knuckle is here: http://www.g4dbn.uk/?p=1618 and this is the story so far of the machining,
Aluminium 75 x 100 x 150 mm
20mm stainless shaft with milled flats
50mm stub mast
DB0MU/b is a tremendous signal this evening in IO93NR on 10 GHz. Peaking 65 dB over noise in 12 Hz bins. 591 km/367 miles
And here is a spur from PI7ALK on 10368.0916 at the bottom of the band!
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
One of the main contests of the year for me is the CQ Worldwide 160 metre CW (Morse code) competition at the end of January. Here is a snapshot of the activity between 1810 kHz and 1900 kHz. The section around 1842 kHz is FT8, and there is a little WSPR traffic just above 1836.
I had enormous fun with a compromise antenna and only 5 watts from my FDM-DUO SDR, but I worked plenty of US stations as far as Texas and Ohio. Click the [ ] to show it full-screen
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 firstname.lastname@example.org for details
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.
First step was to extend the ring to fit the large feed
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.
Next step was to make support cheeks/crescents to hold the multiband ring feed
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.
The 13cm horn is a little larger, and had to be made in two pieces to fit over the backshort.
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
I made one of these kits https://www.g0mrf.com/5W%20linear.htm for a ham in the Czech Republic, and sent it in June, but it has never arrived. I made two other kits at the same time, so I machined up a new case from a bar end of some 7xxx aluminium mystery alloy. Went a bit mad with the slitting saw on those fins, but had a lot of fun making it. It will soon be on it’s way to its new home, not far from Prague.
As I was making the case, I managed to snap an M3 tap in one of the holes. After a lot of failed attempts, I managed to mill it out using a new carbide 3.5 mm end mill, only to snap that off deep in the hole because I forgot to allow for the missing bits of the tap, and the mill grabbed when it hit the tapered tip of the tap. Not willing to admit defeat, I drilled a 2 mm hole from below and knocked the remains of the tap and mill out of the hole. The of course, I had a raggy hole too large to tap, so I reamed it to 4.00 mm and made an oversize stainless steel sleeve, drilled and tapped M3, then used the mill and a mandrel to press the sleeve into the hole. Almost an invisible repair…
I machined up some 2.2 mm spacers to put the board at the correct level above the case, and drilled and tapped the output transistor mounting hole at an angle so as not to spoil the look of the fins.
I found that the lid was a bit too close to the output filter toroids, so I milled out a pocket to give more clearance.
I was going to counterbore the holes for the lid mounting screws, but they were too close to the edge, so I just milled out the corners.
After finishing the case and fitting the amplifier PCB, I ran some tests to check the gain and linearity of the amp. It starts to sag a bit above 2 watts output, but will go to more than 6 watts when saturated.
|Input dBm ||Output dBm||Gain dB |
The spectrum of the output has some amplitude noise at -76 dBc or so, which I think is coming from the power supply I was using for tests. I tested for harmonics, but the signal generator (HP E4421B) is putting out a second harmonic at -66dBc, so the -41dBc result is probably not that bad. Third and higher harmonics are well suppressed.
At 9 dBm in, the initial current draw is 1.07 amps at 13.8 volts. I have set the bias at 140 mA. Saturation is at around 7 watts, but continuous carrier at that level is probably not advisable.
I ran it for 10 minutes at 5W and the output transistor only got to about 45 C, ands the heatsink reached 42 C, with no proper airflow.
After 20 minutes at 9dBm in, the gain had dropped to 27.0 dB and the current draw increased to 1.24 amps. Output dropped to around 4 watts after 8 minutes or so.
I checked the temperature of the components using my FLIR One infra-red camera, and found some hotspots
The IR and visible images are offset about 8mm, so the really hot parts are one of the resistors near the PA and the PA RF choke. The output 1:4 transformer and the driver transistor are also quite hot.