I0QM 1:1 coax balun test

A query was raised on the RSGBTech forum about an odd-looking balun.  It consists of two pieces of coax, one a quarterwave and the other three quarterwaves long.  The inners are joined at one end to the feeder, the sheaths are strapped and connected to the feeder sheath.

At the other end, the sheaths are also strapped.  Each of the inners goes to one side of a 50 ohm load. I think it might also be worth strapping the output sheaths to the input sheaths and folding the whole balun up.

The original notes by I0QM from 26 Feb 1977 are based on a sketch by I4BBF from the early 1970s and are here:  http://www.iw5edi.com/ham-radio/files/I0QM_BALUN.PDF

The mode of operation is a combination of the classic 4:1 balun using a half-wave line to provide a 180 degree phase shift, and two quarterwave transformers, each terminated with half of the 50 ohm balanced load.  A 25 ohm resistive load on the end of a quarterwave of 50 ohms coax is transformed to 100 ohms.  Those two are in parallel, so the feedpoint sees 50 ohms.  The clever trick is using longer bits of coax to do the transformation. Neat.

I made one to test things out.  I used RG213 (VF=0.66) with one leg 2115mm (0.75λ at 70.2MHz) and the other 705mm (0.25λ at 70.2MHz).

I connected an isolated 50 ohm 250W flange-mount resistor between the two ends and stuck 10W at 70.200 MHz into it. I used my Bird 4410A with a 50-200MHz  x10 slug. The reflected power was so low even on the 0.1Wx10 range that I reversed the connections to be sure it was really that low.

Between 68 and 72 MHz, the reflected power was too low to measure, certainly under 50mW. The load resistor is a Florida RF flange mount which is good to several GHz.

Looks like a useful rediscovery for feeding balanced 50 ohm loads.

Soldering in a hurry….


10 watts forward at 70.2MHz

Nil reflected power at 10W forward at 70.200MHz

You can of course also extend the coax after the output terminals, but you would need to use 25 ohm coax or two identical-length 50 ohm coaxes in parallel on each leg to maintain the match to 50 ohms balanced. If you don’t mind the SWR on the line being 2:1, you could just use an integer number of halfwaves of coax on each output of ANY impedance, as usual.

As the outer shield of the coax is only 0.66 of a wavelength in circumference, there should be no weird effects with asymmetric interaction with the antenna.

It would be quite possible to bond the shields at the input and output, and even coil the balun up inside a metal box or mount it inside the boom of a yagi if the coax loss was low enough to ensure the cable temp didn’t get excessive (says the op who burnt his hand on a temporary length of Chinese RG402 carrying 140 watts on 2.3GHz the other day…)

GRAVES ISS reflections and doppler

I watched the ISS move across the sky to the south using reflections of the GRAVES radar on 143.050 MHz.  Nice classic S curve.  Lots of meteorscatter pings as well. This is today’s curve, started late but managed to catch the tail right until it dipped below the tropo horizon. Line at 12000 is a local birdie, bang on the centre frequency of GRAVES.  Horizontal ticks are at 30 second intervals, time moving upwards.  In the middle minute, the rate of change of doppler was 2.9kHz per minute or 48.3Hz per second.

Max doppler shift was -6150Hz, which means the angle of the track was about 32 degrees to line of sight, which looked about right

This was an earlier capture

Quick-change toolpost locking lever

Inspired by John Mills, I decided it was time I made a locking lever for the quick-change toolpost on my Colchester Student 1800 lathe.  Nut is made from 40mm EN8 steel, tapped 16mm for the toolpost. Shaft is 16mm EN8 with two flats 16mm wide milled across it for a 12mm spanner.

I had a 16mm tap for the post thread, but could only find a half-inch UNC (13tpi) tap for the handle socket. No sign of a die or die-nut that size, not even a 12mm die, so I used a single-point carbide turning tool to machine a half inch UNC thread on one end of the handle. Lucky that the 1800 does metric and imperial without gearwheel changes.

Other end of the handle is threaded 10mm with a die-nut I found in my dad’s old toolbox.  Just my luck though, no sign of a 10mm tap anywhere, so I ground a taper and some flutes into an old high-tensile 10mm bolt.  I made the knob from a bit of 6082 ally bar and used the bodged tap to thread it.  Worked pretty well, although I drilled it to 9mm and I guess it rolled the thread more than cut it.

The threads are good and snug, and I had to use the flats on the shaft to get the ends on.  No Loctite needed!

Ally knob

That rolled thread in the ally doesn’t look too horrible

Had to use a lot of torque to get the thing assembed


Finished item in place on the toolpost.  For scale, that is a 10 inch chuck in the background

Lathe tool height gauge

After watching Youtube vids from Joe Pieczynski https://www.youtube.com/watch?v=1MrjnIcscxI and Rober Witkamp https://www.youtube.com/watch?v=iNhQYFsou1Y, I decided I had better make a lath tool height gauge for my Colchester 1800.

The gauge is made from stainless steel at the top and aluminium at the bottom, with a 20mm x 10mm rare-earth magnet epoxied into the base, inset about 0.2mm.  I used a hydraulic press to fit a spigot turned on the stainless section into a reamed hole in the aluminium. Didn’t feel the need for a roll pin or thread.

Overall height is about 160mm, diameter is 30mm. Using a fingernail, I can now set my tools to within a few tens of micrometres of centre in a few seconds.


23cm QRO coaxial hybrid combiner

As an exercise in milling and turning, I decided to have a bash at making a 0 degree six-quarter-wave 3dB ring hybrid to use with a pair of DF9IC 250W 1.3GHz PA modules. It was huge fun, although if I charged my time I think it would be scarily expensive! The finished thing is about 145mm diameter, with three N sockets for  the two PAs and a dump port, and a 7-16 DIN for the output port.

Starting from a piece of 150 x 25 aluminium bar, I turned a giant Polo mint, then used a rotary table on the mill to remove a 17.3mm deep by 19.4mm wide channel.  According to a simulation using ATLC2, that would give 70.7 ohm characteristic impedance (50*sqrt(2)) with a 6mm round centre conductor.

Made up a jig from an aluminium plate so the outside diameter matches the required ID of the ring.  Brass plug turned to match ID of tube and bolted to the edge of the disk.


The ring was soldered using a blowtorch, although I think the 75W iron would have done it OK.

During the build, I snapped off the 7mm pin from the 7-16 DIN socket, so had to drill the pin and the back of the socket and insert a 2mm copper tube, which was then soldered in place.

I milled flats at 60 degree intervals and drilled 4mm clearance holes for the centre PTFE sleeves of the N sockets, and tapped 3mm mounting holes. I made 4mm tapped holes and 16mm clearance plus a stepped indent for the 7-16 DIN socket.

Had to shorten the cap screws by screwing them into a tapped plate, bandsawing them off and touching them on the grinder.

To ensure the centre conductor was exactly in the correct position for drilling and soldering, I made up some spacers, which were pushed in to the groove.  I set up some springy wire to ensure the tube was held in tension and then clamped the spacers with some 2 inch Kant Twist clamps.

Spacer detail – 19.4 mm wide, 6.05 mm slot, 5mm thick, 18.5mm high to fit 17.3mm groove.

Spacer in place, ready to clamp

The 2mm tubes were soldered to the each of the sockets and cut to length.  The tube was drilled through the socket centre hole. The 2mm tubes were pushed through and cut to length, then soldered to the ring.

Should really have filed the solder flat.

I tapped 5mm mounting holes underneath at 135mm spacing.

Top view without the lid.

and with the lid bolted in place

Firs measurements show the balance is OK to 0.3dB from 1240 to 1370MHz. With 15dBm in, I got 12.15 from one port and 11.85 from the other, with -15.6dBm from the dump port

With all three Ns terminated in 50 ohms and +15dBm on the DIN, I saw -35.6dB at the dummy load port at 1296MHz and 26.4dB at 1306MHz.

Input return loss to the DIN port was worse than I expected, at 17.3dB from 1240 to 1370MHz. I need to investigate that more closely.

Removing one of the 50 ohm loads, the level at the dump port rose to +10.22dBm

I have not tested the input RL at the two 0-degree ports yet.

The calculated impedance of the coax cavity is around 70.8 ohms according to ATLC2

More DF9IC 23cm PA heat spreaders

I’ve just finished another six of the DF9IC 23cm PA heat spreaders.  By the time I get good at this, I won’t need to make any more!

Henning’s 2015 Weinheim paper has the details.  More info on his website

100mm x 71.4mm x 12mm copper, with a 0.65mm x 10mm milled slot, 8mm coax link pocket, 3 x 4.5mm mounting holes and 8 x 3mm threaded and relieved PCB mounting holes.

Edges are milled with two-flute fast taper 16mm HSS end mill, slots with 10mm and 6mm two-flute slot drills


Faces are milled flat with an 80mm 6-insert shell end mill on a 27mm R8 mandrel using a flood of cutting oil.

Tapped holes are drilled 2.7mm clear, then relieved on the back with 3.5mm drill to 6mm deep.  The remaining 6mm is tapped using a thread-forming tap from the underside.  I used Westway high-pressure tapping oil.

Close-up view of underside showing three 3.5mm clearance holes with 3mm formed thread in the lower half, and one of the 4.5mm mounting holes.  Also shows the decent finish from that huge end-mill.  Much nicer than the chattery finish from the flycutter I used previously.

Corners are milled to clear the overlap seam in a standard tinplate box.  Holes are deburred using a single-hole countersink bit.  Edges deburred with a deburring tool.  Faces and edges polished with a Scotchbrite rotary mop.

All machining was done on my ancient Bridgeport 3HP mill with varispeed head and soft-start inverter.