VDSL interference – Notches

A quick update on the state of interference to my HF reception from my neighbours’ VDSL services. Almost certainly a result of poor internal cabling and line balance issues.  Very successfully drive me off HF.  The depth of the notches shows what could be achieved if BT would simply notch out the amateur bands, with almost no effect on throughput.

I took some samples at lunchtime of the VDSL noises as received on my 80m dipole (at least 40m away from any houses) to show the notches and relative levels on each band.  Spectrum displays are at

This is from a resonant antenna, so the spectra are very much *not* representative of the real levels outside of 80m and odd harmonics, but the steps are still highly obvious.  If only the infrastructure could notch out the ham bands as well….  Oh wait, it can, but BT can’t be bothered to do it.  Funny, that.

I am lucky in that when I transmit, only a few of the carriers drop out and get replaced with hugely loud pilot tones.  Perhaps I should volunteer to fix the cabling issues for the neighbours and speed up their broken crappy broadband service, but when the uneducated and technically unaware amongst them get a sniff that there is some connection between their crappy performance and my interest in radio, the pitchforks come out.

Sorry neighbours, you are on your own and I’ll come back to HF when FTTP arrives.  Thanks BT, you’ve killed part of my  hobby with your brain-dead technology.




Trombone for VHF NFD 70cm superbeam

In the usual spirit of VHF NDF madcap antenna schemes, Richard G0GLZ (the Dear Leader of the Goole Radio and Electronics Society Mad Antenna Engineering Section) decided to turn several old Jaybeam MBM44s into a monster 70cm megabeam.  Sadly, the proposed 100 elements were not quite achieved, but at least 88 eles were deployed.  My task, if I chose to accept it, was to make a U brace to keep the thing in the sky.  I found a bit of thinwall 50mm ally tube and turned up some bushes, two which were press-fit into the 50mm tube and sliding fit for half of the scrap support strut from one of the MBM44s, the other two were a press fit on the strut tube and sliding fit inside the 50mm tube.  A few grub screws and a pair of locking screws completed the trombone arrangement.  This thing could support a HUGE beam, despite whatever the East Riding of Yorkshire elements could throw at it.

SCAM12 UJ mast base pivot

I decided to make a universal joint base for the SCAM mast I use for portable work.  Milled up out of bits of ally plate and bar, and using 20mm stainless steel pivots in bored holes with plenty of grease nipples.  Works nicely.  Note the wheelie-bin wheels! Perfect for rapid deployment.


Important note:  the sunshade/sail is *not* part of the antenna!

New case for UT5JCW 70MHz transverter

I finally got around to making a proper case for my 4m transverter

Body is milled from solid aluminium bar, the lid is milled from 12mm plate and has a lip and recess. After rechecking the alignment and bias, the two-tone output looks sort-of OK with 5th order IMD about -36dB relative to each tone.

For comparison, here is the same span for the 28MHz Elad FDM-DUO SDR radio driving the IF.  Very clean indeed, as you’d expect.

Just to confirm how good the SDR is, here is a spectrum measured on my Agilent E4406A 4GHz transmitter tester at 0.5Hz resolution bandwidth.

More VDSL interference – The Notches

VDSL interference in upload band U1 up to the notch at 3700-3750, then band D1 from there to 5200kHz

Notch at 5200kHz at the end of band U1, then from 5250kHz upwards in band D2

Band D2 has a sort-of notch around 8450kHz, then the true horror of band U2 starts at 8500 and carries on, wiping out 30M completely.  This carries on up to 12MHz

Nice notch at 12MHz where band D2 starts at 12050, but ths pulsing and pilot tones go on way past 17.6MHz for me at least, but then we have 70Mbps or higher download rates in this end of the village.  I can see pilot tones and pulsing noise way past 35MHz

I can detect huge levels of transmission from various neighbours, I guess they all have rotten internal cabling, but I am not even going to mention it as at least one has already asked if my aerials cause his crappy mobile phone signal.  I blame the education system.  Oh wait, he used to be a teacher.  Hmmmm.

Strangely, my VDSL is connected using a nice short lead to a carefully-installed modem plugged directly into the master socket, and I cannot hear a peep out of it whether it is on or off.  Funny, that.  Hard to blame BT for the inadequacies of internal house wiring.  I’ve moved to VHF/UHF/SHF until FTTP arrives.

Coaxial Low-Pass filter for 23cm

I’ve been making a high-power coaxial low-pass filter for 23cm using a 9th-order Chebyshev design based on the most excellent filter design spreadsheet from Dom F1FRV at http://f1frv.free.fr

I think Dom’s site is no longer active, so I’ve made a copy of the spreadsheet I used here 23cm LPF LP9-23cmv2 and the Internet Archive has the main page archived here

This specific design uses the parameters in the screenshot below.  I used 3mm diameter copper steam pipe with 0.45mm wall for the central rod.  I bored the aluminium block to 30.00 +- 0.02mm internal diameter.  The discs were turned from brass to 26.00 +-0.01mm OD and drilled and reamed with a central 3mm hole to be a tight sliding fit.

Three holes 3mm diameter and 5mm deep were drilled at 120-degree intervals around the edge of the discs.  I turned some PTFE rod to slightly over 3mm OD and pushed it into the holes, then trimmed with a scalpel to 2mm thick using a piece of scrap aluminium machined to 1.95mm thickness and drilled 3.2mm.

The discs were soldered to the central tube, with the spacing very carefully controlled. Dom suggests it is a better solution to use 2mm threaded rod and locknuts as it makes adjustment possible.  Also the thinner centre rod allows slightly improved performance because of the higher ratio of impedances that is possible.

A quick test assembly to ensure the discs and insulators fit correctly, then I shortened the centre pin and turned to to fit the ID of the copper tube, with a spigot 5mm long by 2.1mm diameter

I soldered the disks and the first 7-16 DIN socket and fitted them into the outer tube, then cut the central tube flush with the other end of the outer,

So I could solder the second 7-16 DIN centre conductor in place, I bored a stepped hole in the underside of the body, and milled a 15mm radius curve on the end of a brass bar to make a plug which matches the inside curve of the body. Perhaps not really necessary, but it was a quick and simple piece of milling using a boring bar.

I turned the plug to be a tight press fit into the access hole. If I ever need to get it out, I’ll have to drill and tap a hole in the plug!

The end of the 7-16 DIN centre pin was turned to 2.1mm to be a push fit into the 3mm copper pipe, same as at the other end.  I tinned the pin and the inside of the pipe after cleaning them with IPA and used rosin flux (Weller Electronic Flux T0051383199) then reflowed the joint and pushed the connector into place.

Underside showing mounting holes and the plug protecting the access hole. I’ve fixed it in place with centre-punch marks around the plug.

Finished filter ready to test and install.

For a quick test of the response, I checked using my Agilent E4421B signal generator and HP436A power meter.  At 1296.2MHz, the insertion loss was less than 0.03dB.  At 1500MHz, -7dB and at 1800MHz, -30dB.  I then set a 1200-2900MHz sweep running and the analyser monitoring the same range with peak hold, and the response curve looks reasonable, with the 2nd harmonic down at least 60dB.  I need to do a proper set of measurements up to 3rd/4th harmonics, but that needs the tracking generator setting up.

Initial insertion loss scan.  Second harmonic causes a bump above 2400MHz

Scan of insertion loss from 1.4 to 2.8GHz

Input return loss between 800MHz and 2.8GHz. 5dB/div

Input return loss 700MHz to 1.5GHz 5dB/div.

I did some high-power tests using a DF9IC-design PA running at 280W on 1296.2MHz into a good dummy load with return loss > 36dB.  I used an HP776D 20dB dual directional coupler with a Narda 30dB N attenuator into an HP8481A sensor on an HP436A power meter to measure the response and return loss.  RL at 1296.2MHz was 35.5dB compared with 36.0dB into the dummy load directly.

Multiple tests of insertion loss compared with a female-female 7-16 DIN adaptor show a  spread of values from 0.01 to 0.03dB. That is probably within the error limits of the test equipment.  As that represents under 2W total loss, I think that is acceptable.

Ten minutes key-down at 280W from the watercooled PA with forced-air cooling of the dummy load showed no temperature change of the body or connectors, so I think it is working fine.  I did consider getting the copper and brass parts silver plated, but I don’t think there is anything to be gained.

Interesting that the theoretical minimum of loss at 6.6GHz is definitely real, with only -3.2dB loss at 6623.7 MHz

I am reasonably confident in the diameter of the discs as I used two good digital micrometers and a Moore and Wright analogue one and calibrated them with 25mm and 30mm reference standards.  Width of the disks is less certain as only one face is machined parallel. The inside bore measurement is a little more tricky.  I used my inside bore gauges, plus I made up a couple of gauge plugs, one undersize, one oversize, which I measured with the micrometers and used as a test piece to verify the bore.

Update 30 July 2017

I had an email from Gary G3TMG, who has been doing some analysis of the design using a 3D E-M solver.  I have removed the recently-posted measurements I made with the Agelent E4421B and HP436A with the 8481D -70dBm to -20dBm sensor as it looks like there is a problem with leakage in those figures.

I am going to re-test using a simple male to male coupler in place of the LPF and use through attenuators instead of a directional coupler to do the measurements.  I may also use an HP33330B crystal detector and set the level on the signal generator to give a specific output form the detector rather than using the power meter for measurements.

Gary also asked about return loss measurements, which I have not had time to do yet, and about my confidence level in the physical dimensions of the filter elements.

I machined the bore and discs using my Colchester 1800 lathe, and tested for parallelism with dial gauges to better than 0.01mm.  I only checked each end of the bore, but I ran the boring bar through three times at the same diameter once I was close to the correct measurement, to reduce any error from side pressure on the tool.  The last cut only showed a faint dust. However, with a 2mm clearance, I don’t think the model would be too much out even if the bore or disks were off by 0.02mm.

The biggest variables are the concentricity and the positioning of the disks.  I should have made up a jig to hold the disks, or perhaps drilled and tapped a grub screw in each one.  All I did was set a vernier to the right spacing and held it in place while I soldered the disk.  

The fit on to the copper pipe was fairly good, with almost no rocking detectable, but that could mean the disk is canted by a small amount, as well as being offset from the ideal position.  I measured each spacing individually rather than absolutely from one end, as I felt that would give the most accurate result for the length of the hi-Z elements.

I hand-filed the excess solder from the faces of the disks and the central tube.  I could have done the soldering much more neatly if I had jigged it up and done it on a hotplate.

The concentricity is determined by the length of the PTFE rods.  I tried to get it close, using a 2mm drill as a test of the spacing, but it is hard to get it better than 0.1mm.  However, the error in the characteristic impedance and effective capacitance for a 5% offset is not great.

There is another potential error in that I machined the pins of the DIN connectors so the step was right at the face of the aluminium block, but there is a 4mm length where the pin emerges from the PTFE insulation where it is inside the back face of the connector, with 4.5mm diameter pin and 16mm ID.  I wondered about making up a PTFE insert or a brass collar to make that 50 ohms, but in the end I decided not to bother.

It would be good to know if the concentricity matters less than the ratio of diameters, and if the spacing is less critical than that ratio, and whether the thickness of the disks needs to be as precise as the diameters.  Also would be interesting to know if having one face of the disk as a slight cone makes much difference.  When parting off the disks, it was very hard to get the parted-off face to better than 0.04mm difference between edge thickness and central thickness.

The capacitance of the PTFE pips is also a bit tricky as they are modelled as simple dielectric patches between the edge of the disk and the outer tube.  Mine are actually 3mm rods pressed into 5mm deep radial holes in the edge of the disks, so I guess their effective capacitance is a little lower than a 3mm diameter x 2mm thick patch.

If I get some spare time, I’ll do some return-loss measurements to see how things look.  I have some decent HP directional couplers, but I can’t go down much below 800MHz with them.

Finger-trap cable support

I use a four-strand finger trap terminated with a four-fold sinnet plait to support long dangling lengths of coax on my pump-up masts. (Sorry to get highly basketmaker-technical, but I am a basketmaker!)

This one was done in a couple of minutes on a bit of 25mm Alkathene water pipe. I use a bigger version of this for a bundle of five big coaxes and various control and power leads. This one is done upside-down really, I would normally start from the shackle, doing a 4-fold sinnet plait, then change to finger-trap weave, then bind and tape the ends at the lowest point where there is almost no tension. This one is designed for re-use, and will slip on and off a pole or pipe or cable very easily by removing the tape

I tested one of these to over 200kg on a scrap bit of armoured cable, and pulled 100m of alkathene through a duct with a big winch without any problems.

Interesting paper on an improved version to fix grafts in place and pull blood vessels and other tissues.