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.
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.
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!
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
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).
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.
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.
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.
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……
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.
Many thanks to SWLing Post contributor, TomL, who shares a recording he made of the Voice of Korea on June 14, 2018. This English broadcast focuses on the Singapore summit and is, no doubt, historic in its content. [Note that we’ve posted other recordings on the Shortwave Radio Audio Archive.]
Recorded on my noisy Lenovo laptop, SDRPlay RSP2, and an unamplified 18.5 foot antenna on the roof of my SUV.
I’m most impressed with the quality of his recording–VOK is not the easiest station to snag in the US midwest:
Wanting MOAR options for my recent amplified whip antenna experiment, I decided to add a second antenna input to the kitchen cutting board (can I call it a “Breadboard”? – Ha, that’s an electronics joke!). The idea behind it came from realizing that I might not want to spend all my time outside at a picnic table or on the beach, especially if it is drizzly and windy. And I still wanted a better ground for the antenna. So, I thought I could use more Trucker Parts and put an antenna on top of the roof of my vehicle so I could listen in the relative comfort and safety of my small SUV (or even a friend’s car).
Here is the crowded “Breadboard” with some extra items added.
I thought of the vertical antenna as a short longwire and had an old, original RF Systems Magnetic Longwire Balun. That device allows for an improvement in signal/noise ratio (in theory) if used on a longwire. Perhaps it works on this, too?? You can see the gray cylinder connected right beneath the trucker mirror mount on the left (this will not be tested at this time, see External Antenna below). The output goes to a greenish Daiwa switch on the right.
A large amplified antenna has the real possibility of overloading the amplifier. With the Magnetic Balun, I am hoping the VHF band is attenuated enough to preclude any problems because its response naturally tapers off past 40MHz. But Mediumwave is well within its bandpass. I remembered an old Kiwa Electronics Broadcast Band Rejection filter not being used for a long, long time, so I connected that right after the Daiwa switch (the metal box with red plate).
This output then goes to an RF choke just before entering the pre-amp. I figure I will be using my SDRPlay RSP2 and noisy laptop and wanted to try to reduce any interference traveling on the outside coax braid before it gets amplified.
OK, now for the other Daiwa switch selection. The external antenna will be connected and disconnected as often as I use it. I attached two right-angle coax adapters to be the connection point for the antenna. This is so that the physical switch threads do not have to handle that wear-and-tear. You can see it as the fuzzy out of focus thing sticking up out of the left side switch position.
The wire going out the top of the Breadboard goes to a Firestik K-11 magnetic mount placed on top of the roof of the SUV. I also wanted this to be connected to a magnetic balun. I just happened to have a nearly unused Palomar Engineers Magnetic Longwire Balun. It has its own ground lug for use with a counterpoise. Temporarily, I left the 18 feet of wire that came attached to the K-11 Mount and attached an adapter and BNC test lead; on the other end is connected the spade lugs to the Balun (red wire to the lanyard nut, black wire to the ground lug). It all fits neatly inside a Sistema 3 liter container.
The magnet and box self-clamp easily onto the roof of the vehicle. I added a new 18 inch section to the Trucker Antenna Shafts creating a full 72 inch antenna, complete with mag mount, ground plane (car body), and magnetic balun. It is very easy to put up and take down and the box helps keep everything contained.
I am pushing things a bit here. Magnetic Baluns are not really meant to be used on vertical antennas. It probably breaks some sort of Cosmic Electrical Law somewhere that causes electromagnetic waves to get very confused and die a horrible, twisted, circular death. But I figure that it is an unbalanced “line” similar to a longwire antenna; it’s just a little short and goes straight up instead of horizontal! I like the idea, so I am going to run with it.
It goes without saying that the Antenna Shafts, magnet mount, and magnetic balun are weatherproof (but NOT lightning proof!). Take proper lightning precautions and take it down. Even so, I might add a small drainage hole to the box since it did rain a tiny bit during testing.
Secondly, this setup is ONLY FOR STATIONARY VEHICLES!!!! DO NOT TRY TO DRIVE DOWN THE ROAD OR HIGHWAY!!!! The magnetic mount will NOT stay on the car and will damage your vehicle and maybe a vehicle travelling next to you!
As you can see from the picture, my new Tecsun S-8800 is getting a workout while connected to the Cross Country preselector (not shown behind it) and to the backpack next to the back seat window. The Tecsun S-8800 is a nice radio. My copy has a couple of quirks that I might have to send it back (the AM band tunes incorrectly 2 kHz lower than it should and the SW SSB tunes 140 Hz higher than indicated and I have to compensate using the fine-tuning dial for these modes–FM seems correctly tuned).
Other than this, the actual performance is really quite good! DSP does have sharp cutoffs to the IF bandwidth (especially resolving strong station interference when selecting 3 kHz vs. 4 kHz). With all my filters/balun/choke, I did not notice any MW or FM breakthroughs and signals on those bands were nicely contained and “normal”. Interference from my cell phone while looking up internet frequency listings was minimal – seems like the cable shielding, choke, and car roof are doing a good job.
The audio output jacks have very thin clearance between the jacks and the housing of the radio. So for the second time, I will not have recordings since the cable I wanted to use has home theater style construction with very thick plug outer connectors and will not fit!
From an RF-quiet “Forest Preserve” (County park), there were a variety of stations received from the 25 through 19 meter bands (Local time 11am-1:30pm). A few stations I have never heard before until now:
Radio Free Asia in Korean on 11985 KHz (Tinian Island)
Radio China International in Esperanto on 11650 KHz (Xian China)
Radio Farda in Persian on 12005 KHz (Wooferton England) – broadcast opposite my direction
Radio Bible BCI in Somali on 15310 KHz (Nauen Germany) – Strange sounding but interesting Christian Somali music
Radio Free Asia in Chinese on 13675 KHz (Dushanbe Tajikistan)
Voice of Hope Africa in English on 13680 KHz (Lusaka Zambia) – had to use ECSS USB to get away from strong interference from RFA on 13675, fairly good intelligibility (including music)! I wish there was a 6 KHz option for SSB mode since the audio was slightly muffled and could not compensate much with the tone controls. That kind of feature usually comes with radios costing 3X more, however.
Voice of Korea in French (Kujang North Korea) being squashed by Radio China International (Kashi China) in English on 13760 KHz
It was so nice not to be on a beach and have people walk by STARING at me with my weird radio/antenna setup. And I was dry and comfy sipping a cool drink while there was a drizzle of rain pelting the windshield. Downside might be that the car setup cannot always be located optimally if I want to be next to that Very Large Body of Water (Lake Michigan) to help enhance reception but this is not a bad alternative. The next test will have to be during early evening when signals are booming into my location and see if performance holds up under those conditions!