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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.
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.
Do you miss Ian McFarland on Radio Canada International?
Yeah, me too.
That’s why I’m always pleased to receive off air recordings from Shortwave Radio Audio Archive contributor, Tom Laskowski.
I’ve just posted his latest off-air recording on the archive and thought I’d post it here as well. Tom note:
Here is another one of my many recordings of Shortwave Listener’s Digest from Radio Canada International, this time from January 4, 1982. This program highlights are: Glenn Hauser with his Year in Review for the previous year of 1981; part 1. Larry Magne with a test report for the Panasonic RF-9000 which listed at $US 3800!!!. The program ends with Glenn Hauser’s second part of his review of the previous year’s highlights. Unfortunately the broadcast suffers from some adjacent-channel interference.
Many thanks to SWLing Post contributor, TomL, who shares the following guest post as a his Backpack Shack 3 continues to evolve:
Backpack Shack 3.0 – Part 3
by TomL
I have now gone overboard since I think bigger must be better. The temptation was just too great and now there is an MFJ-1979 17-foot telescoping whip antenna in my car (with consequences).
MFJ Angst
I have a love/hate relationship with MFJ products because of what I think are useful ideas that are made somewhat poorly. But I went ahead and bought the large whip since I figured they could not possibly screw up something so simple, right?
Wrong. As I excitedly tried to screw the supposed 3/8”-24 threaded end into the nice standard Firestik K-11 magnet mount, I realized I was turning and turning it but it was not going in!!! I even had a small steel sliver of metal sticking into my flesh to prove I was not dreaming. The previous day, it had screwed in very tightly, but it did screw in. So, there I was after a long day of work, ready to listen to some SWL-Nirvana and I could not get the blasted antenna into the mount–? That Firestik mount is a VERY standard 3/8”-24 female thread and the other third-party antenna shafts fit perfectly and easily EVERY time I use them. I hate $60 of poor workmanship and MFJ seems to be the poster child of overpromising and underdelivering.
I was determined to make this work, by force if needed. One of the Trucker antenna shafts by necessity had an extra coupling nut on it to allow the extra 18 inch shaft to connect, so I took it off there and tried to thread it onto the MFJ-1979. It barely moved. Not to be thwarted, I dug out an adjustable wrench and 3/8” socket wrench with ½” socket and grunted and twisted and tightened until the coupling nut was threaded all the way “up its shaft”. That is what I feel like telling MFJ! That coupling nut is never coming off and now that I truly have bought it and cannot return it, I might as well use it.
The stainless steel telescoping rod is extremely thin and feels like it can bend and dent with any kind of mishandling. So it resides collapsed in a 27 inch PVC pipe with plumbing pipe foam inside to baby it when it is not being used. It remains to be seen if I can remember to “Handle With Care” when extending/collapsing it. We’ll see.
Ready-to-go
OK, so using the 18 inch antenna shaft attached to the magnet mount, then the coupling nut on the MFJ antenna, I extended it to a total of about 13 feet. With the DX Engineering Pre-amp turned on, and using the SDR Play RSP2, I was getting many signals booming in. All the usual names we are familiar with – RMI, CRI, Turkey, Cuba, etc. But also the noise level was very high. I know it is summer but I may have been overloading the Pre-amp a little bit. Here is an example, Radio Progresso from Cuba with some very nice acapella music but also a noisy background (plus, a noisy laptop computer pulse!):
So I decided to come back in the morning before my workday started and see if the static crashes would have died down.
Preamp Angst
The next morning I had everything hooked up again in the same spot at the Forest Preserve (located in a suburb of Northern Illinois). I moved the Cross Country Preselector to be directly connected from the roof, then to the antenna switch on the “Breadboard” (see part 2) to better prevent overloading. I turned on the Verizon battery pack and nothing. No Pre-amp light. Switched it on, off, on, off – nothing. So, I thought I must have burnt it out the previous session?
Later on, I found it was some sort of short in the switch and I will have to move the D-cell batteries to a backup battery pack. In the meantime, I had to do without the Pre-amp and was forced to extend the MFJ antenna all the way. With the 18 inch extension attached to the magnet mount, that was a total of 18.5 feet from antenna tip to the top of my car roof.
This was actually fortuitous since I was already concerned about overloading the Pre-amp or perhaps amplifying background noise. This forced me to test it in a more “barefoot” manner, hearing what it would natively hear without any Pre-amp. It was also lucky there was no wind to blow it over! It seems that if one is in an RFI-quiet area with decent view of horizons, the 20+dB Pre-amp may not be needed, depending on frequency band involved.
I have read that “Norton” style 10 dB Pre-amps and custom handmade transformer baluns are used by Dr. Dallas Lankford in his Low Noise Vertical antennas. I don’t want to get into winding baluns so I am using one Palomar Longwire Balun to simulate the “magnetic” transfer. His design uses two, one 10:1 at the antenna and a 1:1 balun at the feedline into the house. For more reading on LNV antennas, see these references:
I purposely monitored Voice of Korea for their news statement on the De-Nuke talks on the 25 meter band and found it came in great, just as many others have heard it. This was encouraging. Examining carefully the Data file from the SDR, here is what I pulled from it. I am pleasantly surprised and happy with the results; some stations I had never heard before and the language and music are very exotic. All of it was a little more than one half hour of recording time (14 June 2018, 1300 UTC). You may have to crank up the volume on the weaker recordings to hear those properly.
Recordings
(Station, Frequency, Language(s), Transmitter site from www.short-wave.info):
Voice of Vietnam, 12020 kHz, English, Hanoi Vietnam (with local UFO noises near me)
Taiwan International, 11640 kHz, Chinese, Kouhu Taiwan (blasting in strongly plus strong echo of broadcast at top of the hour – is a second transmitter signal going around the earth the other way and getting to me??)
Eighteen feet of whippy rod can sway in the gentlest breeze (consequences of “bigger must be better”). The described setup has fallen over in as little as a 12 mph sustained wind when fully extended because I had the base in a plastic box. I want plastic under the magnet(s) in order to get it off easily and put away out of sight! Now installed is a larger QUAD magnet mount for better stability:
I am using the flat plastic lid from a 20 gallon tote container under the quad mount and a mover’s tie down strap to the main bar of the quad (I have room for multiple straps if needed). Ten foot fits just fine:
Because the backpack and quad mount can fit inside the 20 gallon tote container, this setup can be attached to a picnic table in a state park or campsite if I choose. The Firestik single magnet mount will be recycled as a VHF antenna mount. I can go virtually anywhere now.
Instead of the 20+dB DX Engineering Pre-amp, perhaps one of those “Norton” 10 dB Pre-amps might be optimal (Kiwaelectronics.com broadband-preamp). And I need to figure out why my Verizon battery pack failed as each Tenergy D cell measured fine. Oh yeah, I have to buy an extra coupling nut, too……
Happy Listening,
TomL
Thanks so much for sharing this latest iteration of the BackPack Shack 3.0, Tom! It seems to me, as you imply, your current setup could be installed pretty much anywhere.
I’m sorry to hear about your troubles with MFJ. I’ve only had good experiences with them in the past, but I suspect the specs on the 3/8”-24 thread were simply incorrect or perhaps metric and mislabeled.
Post readers: Read Tom’s past contributions and articles by clicking here.
Click here to read Backpack Shack 3.0 – Part 1 and Part 2.
Many thanks to SWLing Post contributor, Chris Freitas, who writes:
I recently came across a post on the SWLing Post after I saw Sangean released the WR-7. It recently became available on Amazon and decided to purchase one after seeing your review and Radio Jay Allan’s review.
As I am writing this, I am pleased with the overall quality of this radio. The dark walnut wood is a nice solid build and the light-up dial is a nice touch.
The sound quality is fantastic and beats the pants of my larger iHome “Orb” Bluetooth speaker and sounds a bit better than the Jambox Mini.
I have not done a lot of FM listening yet, but this little radio is pretty sensitive without the wire antenna. It picks up all of the Memphis FM stations with little or no static. With the included wire antenna, I can receive WMAV 90.3 FM in Oxford, MS…about 80 miles away with ease.
The WR-7 is definitely a keeper and will be using it on trips and between rooms in my apartment or outside while sipping on some coffee.
Anywho, I wanted to tell you about my experience with the radio and I have included some pictures of it too.
Thank you for sharing your report, Chris! You’re right: the Sangean WR-7 is a fine little FM radio and Bluetooth speaker. I’m happy to hear how well it’s serving you and even snagging distant FM stations. Later this year, I’m going on an extended trip and will take the WR-7 with me to provide a nice source of local FM music and WiFi radio while paired with my iPad running TuneIn.
From the Isle of Music, July 8-14:
This week, our special guest is clarinetist/composer Ernesto Vega. He will share some of his first album, Venir al Mundo and some of his work in Familia by Arturo O’Farrill and Chucho Valdés.
Four options to listen to the transmission:
1. For Eastern Europe but audible well beyond the target area in most of the Eastern Hemisphere (including parts of East Asia and Oceania) with 100Kw, Sunday 1500-1600 UTC on SpaceLine, 9400 KHz, from Kostinbrod, Bulgaria (1800-1900 MSK)
2. For the Americas and parts of Europe, Tuesday 0000-0100 UTC on WBCQ, 7490 KHz from Monticello, ME, USA (Monday 8-9PM EST in the US). This has been audible in parts of NW, Central and Southern Europe with an excellent skip to Italy recently.
3 & 4. For Europe and sometimes beyond, Tuesday 1900-2000 UTC and Saturday 1200-1300 UTC on Channel 292, 6070 KHz from Rohrbach, Germany.
Uncle Bill’s Melting Pot, Sun, July 8 & Tues, July 10, 2018
Episode 70 explores some musical aspects of Afrofuturism, a movement with incorporates avant garde Jazz and electronica among other things.
The broadcasts take place:
1. Sundays 2200-2230 UTC (6:00PM -6:30PM Eastern US) on
WBCQ The Planet 7490 KHz from the US to the Americas and parts of Europe
2. Tuesdays 2000-2030 UTC on Channel 292, 6070 KHz from Rohrbach, Germany for Europe. If current propagation conditions hold, the broadcast should reach from Iceland to Western Russia, Scandinavia down to North Africa and the Middle East, AND a long bounce to parts of New Zealand.
It helped the BBC broadcast its radio programmes across Europe.
Now the vast site of the former radio transmission station in the west Dorset countryside is set for a new lease of life.
The Rampisham Down site next to the A356 Maiden Newton to Crewkerne road, which extends to more than 180 acres, is on the market with a guide price of £2.5 million.
It includes commercial land, and a huge area for grazing – which could be used for a ‘recreational business’.
Rampisham Down was designated a Site of Special Scientific Interest (SSSI) in 2014 for its special grassland and heathland habitats.
British Solar Renewables (BSR) wanted to build a huge solar park there, but after a lengthy planning battle – in which the decision was ‘called in’ by the government’ – the company shelved its plans.
A site nearby not deemed of high importance for wildlife was instead chosen for the solar park and given planning permission.
This solar park could help to power the new venture at Rampisham Down, it is said.
All but one of the original telecommunications towers, which helped to broadcast the BBC World Service in Europe until the station was decommissioned in 2011, have been removed.
The remaining tower has become a nesting platform for peregrine falcons, as part of work by BSR, in conjunction with Natural England, to restore the land and make it a home for wildlife.[…]
