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Guest Post: SM0VPO’s 3D Printed 10KV Tuning Capacitor

The completed antenna.

Many thanks to SWLing Post contributor,Harry Lythall (SM0VPO), who shares the following update to his excellent homebrew 20 meter magnetic loop antenna post:

3D Printed 10KV Tuning Capacitor

by Harry Lythall (SM0VPO)

Introduction

You may have seen my 20m (14MHz) loop, or frame, antenna, and the ease of construction with just a bit of wire and a bit of plastic tube. The tuning arrangement is a little primitive, using just a “gimmick capacitor”, comprising two bits of wire twisted together.

The original “Gimmick” capacitor that burns.

This arrangement works very well for QRP, where the average RF power is about 5 Watts or less. If you exceed this power level, then the twisted-wire capacitor tends to warm up and the tuning drifts a little. But if you use more than about 10 Watts of continuous RF power, then things start to burn. Cheap insulated wire also smokes. This is because the impedance at the ends of the coil is so high that you can get many 100s of volts and the insulation, normally intended for house wiring, breaks down.

In this page I will show you how to build a super-cheap tuning capacitor that will tolerate up to 10,000 Volts of RF and allow you to use up to about 100 Watts of RF into my 14MHz (20m) antenna. The capacitor is also tunable so that you can adjust it by hand (when the RF is removed, of course :-). The tuning range is about 8pf to well over 30pf when really compressed. The normal range for the antenna is about 12pf to 15pf.

Construction

My prototype does not look very pretty, and it is not supported on anything other than the connection wires from the antenna. Very few components are used:

two plastic foam pan scrubbers or one thick plastic bath sponge
two pieces of metal 4cm x 6cm. Copper-clad board works fine
one plastic nut and bolt – see text
one heavy-duty 3-pole block connector with centre-pole removed

Components for the 10KV tuning capacitor

The plastic nut and bolt need to be about 6cm long and totally non-conductive. If you want to make my bolt, then do NOT use black plastic because some black plastics use carbon as a colouring agent. You can use a nylon bolt, as used to secure IKEA toilet seats, but you will also need a washer to spread the stress, otherwise the copper-clad board tends to bend under the stress with time. I chose copper-clad board because it is easy to solder – no need for drilling or connection bolts. I made my plastic compression tuning bolt using my 3D printer. I have included the project files for you to download.

Ikea toilet-seat bolt

Note that the connector for the wires needs to be well spaced between the metal inserts, in order to tolerate up to 10KV, so I used a 3-pole connector and took out the centre pole. At 100-Watts continuous there is a very slight warming after a few minutes, but no sign of smoke, sparks or corona.

The connector with the centre-pole removed.

I tried a few different types of plastic dielectric and all worked well, providing they are 100% dry. The best ones those I stole from the kitchen cupboard (when Maj-Lis was not watching). I think it costs about $1.50 for a packet of 10 pieces. Perhaps I should have used a nicer colour? A pretty pink? Heart-shaped? No! maybe that would be going a bit too to far ;-). Here is my finished capacitor using my 3D printed tuning screw.

The assembled 10KV capacitor.

3D Files

Once more, this project is ridiculously easy to make with a 3D printer. The hardest part was to get the pitch of the threads right, then clone/connect the pieces to get a longer thread. They were printed with the bolt vertical, so the slope under each thread is a steeper angle than the upper slope. This makes the printout a lot easier to print. If the angle is too steep, then it may extrude PLA into mid-air. My nut and bolt heads are about 3cm Diameter, and the 10mm thread for the nut was cut using boolean subtraction. I then enlarged the nut by about 3% so it still fits but there is a little slack so that it does not bind. I may have overdone it a little, but not much.

Project rendered in 3D Studio Max

Here are the files:
3D studio MAX file – 3d-cap-01.max
ASCII STL file – 3d-cap-01.stl
ASCII OBJ file – 3d-cap-01.obj
the GCODE file for my Wanhao (Prusa) Duplicator i3 – 3d-cap-01.gcode

Just right-click on the file and select “save as”. Some web browsers try to open ASCII files instead of saving them. I usually export STL files, but on this occasion, I tried comparing OBJ files since CURA slicer will accept both. Although I use a WANHAO replica of the PRUSA Duplicator i3 printer, the GCODE files are rather generic and will probably work on most printers.

My printer settings in this GCODE are:

Nozzle temperature = 200°C
Bed temperature = 60°C
Support structure = brim
Layer height = 0.1mm
Print speed = 60mm/s
Fill density = 40%
Shell thickness = 1.2mm

The printer temperature is 200°C, which is 5°C hotter than recommended for PLA filament. I find that I get a better print at 200°C as it sticks to the bed a lot easier. When printing this screw thread, it may be advisable to start at 200°C and then turn down the nozzle temperature to 195°C after a few layers have been printed.

I hope that you find this project interesting. It is a bit small, but it gets me active on 14MHz from my car with this portable antenna. I have a new car and I don’t want to start throwing long aluminium tubes in it that scratch the interior to bits.

Don’t forget to visit my messageboard if you have any questions about this or any other project. I always look forward to receiving feedback, positive or negative ?

Very best regards from Harry Lythall
SM0VPO (QRA = JO89WO), Märsta, Sweden.
EA/SM0VPO (QRA = IM86BS), Nerja, Spain.

Thank you so much for sharing this, Harry! I love both the frugality and ingenuity in this unique capacitor design!

Post Readers: be sure to check out Harry’s website which is loaded with radio projects of all stripes. You’ll easily spend a few hours digging through his tutorials and downloads. Harry also maintains an alternate mirror server located here.

Tidlow explores 1970s CB radio culture through photos

DRM feeds RNZ Pacific relays

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(Source: Radio World via Mike Hansgen)

RANGITAIKI, New Zealand — Radio New Zealand Pacific, the official international arm of Radio New Zealand, is using Digital Radio Mondiale digital radio transmission/reception equipment to feed studio-quality audio to some of its 20 relay stations in the Pacific Ocean region. The others use satellite feeds or web downloads.

The locations being served by DRM include the Cook Islands, where RNZ Pacific’s programs are rebroadcast locally in analog mode by Aitutaki 88FM, the islands’ only broadcaster. RNZ Pacific also serves Tonga, Samoa, and the Solomon Islands using DRM; among others. Previously, RNZ Pacific had fed its relays using analog AM shortwave radio, with that transmission mode’s limited audio range and interference issues.

“When DRM became available to us in 2005, we saw it as a great opportunity to provide high quality audio to Pacific radio stations that relayed our news broadcasts from our AM transmitter,” said RNZ Pacific’s Technical Manager Adrian Sainsbury. “As a platform to deliver good quality audio to remote island FM stations, it has been a great success.”[…]

Click here to read the full article at Radio World.

As the article points out, RNZ has been using DRM as a feed for quite a few years. I think this is a brilliant use of the technology. Of course, those of us in the rest of the world can snag RNZ DRM broadcasts as well.

Hospitals and RF noise: FM and HD radio’s strong suit

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The Sangean HDR-14 AM/FM HD radio

For the past week, I’ve been away from home spending time with my mother at the hospital while she recovers from a surgery. I’ve got a number of reviews and evaluations in the pipeline, but thankfully no shortwave or HF radios on the table this week (although the ELAD FDM-S3 and CommRadio CTX-10 are just around the corner). Listening to shortwave (or even mediumwave) in a hospital room can be an exercise in futility–there are just too many devices emitting noise and the buildings are built like bunkers with incredibly thick walls to attenuate signals.

I’ve had the little Sangean HDR-14 with me, however, and have been very pleased with its ability to snag FM stations both analog and digital. I’ve also had fun discovering a surprisingly diverse FM landscape in this metro area. I haven’t snagged an AM HD station yet, but my hope is one evening I might DX one (fingers crossed and not holding my breath).

The Sangean HDR-14 (left) and CC Skywave SSB (right)

At the end of most days, I’ve been able to catch a little shortwave action with my CC Skywave SSB (pre-production) portable at the guest house where I’m staying. The evenings have been surprisingly peaceful here with only the occasional popup thunderstorm to insert a little QRN in my listening sessions.

Last night, while listening to jazz on FM, I finished reading All The Light We Cannot See by Anthony Doerr (affiliate link).

We’ve mentioned this book before and I know of at least dozen SWLing Post contributors and friends who’ve personally recommended it to me.

It is a superb novel and will, no doubt, tug at the heart strings of any radio enthusiast or WWII history buff. Highly recommended!

Indeed, last night I couldn’t fall asleep until I finished the book around 12:30 AM!

And mom? She’s recovering quite well and we hope will be discharged from the hospital soon.

Internet Radio and Global Time Travel

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Many thanks to SWLing Post contributor, Kim Elliott, who shares this brilliant article from The Paris Review:

On The Radio, It’s Always Midnight

By Seb Emina

“Ultimately, we don’t belong in the world governed by time,” says Michael Cremo, a guest on KNWZ, a radio station in Palm Springs, California. “As beings of pure consciousness, we are essentially timeless.” It is around two thirty A.M. in Palm Springs and around eleven thirty A.M. in Paris, where I am tidying my apartment. Cremo is talking about the end-time, which he thinks could well be imminent, but his point is relevant to the experience of listening to local radio from somewhere I am not. I love listening to radio, but sometimes I don’t want to listen to a particular station, genre, or category. Sometimes I want to listen to a time of day. Which is, of course, entirely possible thanks to the rise of online streaming at the expense of older analogue broadcast methods. If I am feeling afternoony in the morning, I can leave the world that is “governed by time” and join whichever community of radio listeners—in Mumbai, Perth, or Hong Kong—is currently experiencing three P.M. The optimism of a morning show somewhere to my west offers a fresh beginning to a day that’s become lousy by midafternoon, whereas the broadcasts of early evening, burbling across the towns and cities to my east, can turn my morning shower into a kind of short-haul time machine past those hours in which I’m expected to be productive. But for the loosest and strangest of broadcast atmospheres, I am drawn most often to the dead of night, to the so-called graveyard shift. That low-budget menagerie of voices and music is concocted to serve an unlikely fellowship of insomniacs, police officers, teenagers, and bakers—and cheats like me, tuning in from afar to behold radio’s closest equivalent to the Arctic Circle.

“When you listen to radio, you are a witness of the everlasting war between idea and appearance, between time and eternity, between the human and the divine,” Herman Hesse writes.[…]

Continue reading the full article at The Paris Review.

It’s rare that I read something in the press that so directly speaks to my own personal relationship with radio.

While, of course, I still prefer listening to shortwave radio, international broadcasts are designed with a global audience in mind; in a sense, they’re timeless and gloss over our global time zones.

Internet radio streaming brings in the local and actually provides more community context. And, of course, while a country might only have one international broadcaster, it could literally have hundreds of local stations–some catering to very small communities.

I’m certainly guilty of time zone surfing with WiFi/Internet radio, I just didn’t realize others did it too! Even in the mornings (since the demise of my shortwave staple Radio Australia), I typically listen to CBC St. John’s, Newfoundland. They’ve got an excellent morning show and they’re 1.5 hours ahead of my time. When I wake at 5:00 AM, I listen to the world report and the CBC local staff are pulling out some of their best morning programs.

Then anytime between 3:00 or 6:00 PM, I’ll tune through one of a number of New Zealand or Australian stations. Because it’s morning there, the presenters seem to have a spring in their step.

If you read the full article in The Paris Review, you’ll also note that the author Seb Emina and the artist Daniel John Jones created “a perpetual morning-radio aggregator” called Global Breakfast Radio.

You can stream Global Breakfast Radio at globalbreakfastradio.com. I highly recommend it!

Post readers: Any other time zone surfers out there? Please comment!

Not familiar with Internet Radio? Check out the first part of our Internet Radio primer by clicking here.

White Alice Communications System: a precursor to USAF sat comms

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The St Lawrence Island site. Author: U.S. Army Corps of Engineers Digital Visual Library (via Abandoned Spaces)

Many thanks to SWLing Post contributor, Steven Crawford, who writes:

Came across this article about a communications system that was a precursor to sat comms for the Air Force and thought it may be of interest to your followers.

(Source: Abandoned Spaces)

Equipped with up to 80 radio stations, this remnant of the Cold War once served as a tropospheric scatter communications system using microwave relay. Its location, Alaska. It was initially constructed during the 1950s.

The United States Air Force saw great benefit from this communication system for it improved their communication tremendously. Years before this system was installed, Alaska had only the essentials when it came to communication systems.

It wasn’t until Bell Systems came up with their design for the U.S.A.F. that Alaska would receive proper communications links. The name White Alice Communications System in itself is an acronym made up of the following words – White, for it was used in the snow-covered land of Alaska, and Alice, that stands for Alaska Integrated Communications and Electronics.[…]

Continue reading at Abandoned Spaces.

Many thanks for sharing this, Steven!

Radio World features a tribute to the Zenith Transoceanic

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Many thanks to a number of SWLing Post contributors who’ve shared a link to this excellent article by Denny Sanders in Radio World Magazine about the history of the Zenith Transoceanic:

Zenith Trans-Oceanic Radio in War and Peace

This iconic portable receiver was known for durability and quality

They say necessity is the mother of invention. Nothing proves this more than the story of how the iconic Zenith Trans-Oceanic portable radio receiver came into existence.

Commander Eugene McDonald (1886–1958), the founder of Zenith Radio, was a stickler for quality and insisted that any Zenith product represented cutting edge technology and design integrity.

He was also an accomplished yachtsman. During his many ocean voyages, he constantly was frustrated with the inability of any portable commercial radio set to perform reliably at sea. In about 1939, he ordered the Zenith R&D department to come up with a rock-solid, portable AM receiver sensitive enough to pull in signals from great distances. He insisted that the radio be a multi-band unit including shortwave, marine and aircraft bands.

The Zenith crew came up with a gem: the Trans-Oceanic, a gorgeous piece of engineering housed in a robust and dramatic cabinet designed by the brilliant Zenith industrial designer Robert Davol Budlong.[…]

Continue reading the full article at Radio World Magazine online.